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MCROFKiHE REFERENCE LIBWltRY A project of Yoltinteers in Asia .* Soil Conservation: Project design and lmplementation Usina l&ur Inten.sive Techniques By: Bernard Leblond & Laurent Guerin UNDP-iLO/INT/81/844 Published by: International Labour Office IL0 Publications CH-1211 Geneva 22 SWITZERLAND Available from: International Labour Office IL0 Publications CH-1211 Geneva 22 SWlTZE,RLAND Reproduced with pormission. Reproduction of this microfiche document in any form is subject to the same restrictions as those of the original document. iJNDP-ILCl/INl-/“8l/LWl Interregional Project for Implementation and Evaluation of Special Public Works Programmes ONSERVATION PROJECTDESIGNAND IMPLEMENTATION LJSINGLABDLIR INTENSIVETECHNIQIJES UNITED NATIONS DEVELOPMENT PROGRAMME \ / INTERNATIONAL LABOUR OFFICE Bernard LEBLONO Laurent GUERIPI Geneva 1983 UNDP-lb3/INT/81/044 Interregicnal Project for lmplemenlation and Evaluation of Special Public Works Programmes ScllL CONSERVATION PROJECTIDESIGNAND Il’IlPLEMENTATICIN USINGLABOUR INTENSIVETECHNIQUES Bernard LEBLOND, Doctor in Earth Sciences Laurent GUERIN, Adviser in Civil Engineering Emergency Employment Schemes Branch International Labour Office l Geneva Copyright 0 International Labour Organisatio,: 1982 Publications of the International Labour Office enjoy copyright under Protocol 2 of the Universal Copyright Convention. Nrvertheless, short excerpts from them may be reproduced without authorisation, on condition that the source is indicated. For rights uf reproduction or translation, application should be made to the Publications Branch (Rights and Permissions), International Labour Office, Cl-l-1211 Geneva 22, Switzerland. The International Labour Office welcomes such applications. lS8N 92-2-103395-3 First published 7983 Second impression 1988 The designations employed in IL0 publications, which are in conformity with United Nations practice, and the presentation of material therein do net imply the expression of any opinion whatsoever on the part of the International Labour Office concerning the legal status of any country, area or territory or of its authorities, or concerning the delimitation of its frontiers. The responsibility for opinions expressed in signed articles, studies and other contributions rests solely with their authors, and pubiication does not constitute an endorsement by the International Labour Office of the opinions exprossed in them. Reference to names of firms and commercial products and processes does not imply their endorsement by the International Labour Office, and any failure to mention a particular firm, commercial product or process is not a sign of disapproval It0 publications can be obtained through major booksellers or IL0 local offices in many countries, or direct from IL0 Publications, International Labour Office, CH-1211 Geneva 22, Switzerland. A catalogue or list of new publications will be sent free of charge from rhe above address. Prmted by the International Labour Offlce, Geneva. Swltzetland %ef ace Soil conservation, of course, occupies an important place, often the most important, in special public works programmes in developing countries. Such programmes aim tr? increase employment and raise the ,ncome of the poorest, essentially through the creation of an infrastructure in rural areas that will improve sgricultural and forestry production, develop communications and generally improve the quality of life. In diew of their employment objectives, the programmes use the available workforce to the maximum, putting the accent on labour-intensive techniques. Such an approach is particularly weli suited to soi! conservation work from, for example, the construction of anti-erosion terraces or banks, to re-aforestation, itself related to the establishment of nurseries. It has been observed that the labour component in such projects is, on average, 70% of the total cost. Community participation is another important aspect that is shared by soil conservation activities and special public works programmes. Soil conservation is not only limited to isolated, individual interventions; to be effective, it must be undertaken at a community level, developed on the largest scale possible and, consequently, involve all the people concerned. Participation, which assumes grass-roots agreement, Is one of the fundamentals of special public works programmes, along with the crucial role played by local Administration representatives and the technical services, in mobilising the workforce. Finally, the place set aside foi so!’ conservation in the special programmes fo’,L:ws from the fact that the latter deal, by definition, with disa,vantaged groups. The zones covered by the programmes tend to be the poorest in natural resources, where ?he soils are progressively degraded as a result of deforestation and erosion. Ir, this context, special programmes can be seen as contributing, not only to reconstituting and preserving small-scale farmers’ capital, that is the land, but also to the protection Df ttie environment. ***********tttt** The present document, prepared by two experienced consultants, Mssrs Leblond and Guertn, is part of a series of technical documents published by the International Labour Office within the framework of the UNDPllLO interregional project for the planning, organisation and execution of special public works programmes. Soil conservation is a very wide and complex subjec: irvhlch has been fully explored from all points of view. The authors have, therefore, limited themselves to a reminder of the basic thinking concerning soil erosion and an analysis of the labour-intensive techniques employed in the battle agains? erosion. Justifiably, then, a large part of the study is devoted to the presentation of a methodology for establishing a soil conservation project model that meets the criteria of special public works programmes. Finally, the authors precisely and succinctly set out the rational organisation and successive stages of a labourintensive operation. *t*.******.t.tt*t This manual, wi;h its emphasis on a practical approach to the subject, has been written mainly for the national planners, senior engineers and technicians who must establish and execute the programmes in developing countries. Interest in the social and economic aspects of the programme is becoming more intense daily, and the international community contributes active support in the fields of financial and technical assistance. Long-term in nature but urgent, soil conservation reflects the wish of the poorest communities to preserve and improve their !and heritage, so thai iheir own and future generations’ basic needs can be assured. That, at least, is the desire expressed here and that should be rewarded by wide distribution of this remarkab!e work. The authors have drawn on Iheir practical experience and pay all due attention to the socio-economic conditions sf the populations concerned, and to the provision of appropriate techniques and instruments. Maurice ldoux Senior adviser Division for global and interregional Projects United Nations Development Programme (UNDP) e phenomenon of soil conservation is no new. Plato, as early as four centuries before ou age, deplored mountain erosion in Greece, bu the phenomenon has now reached preceden ted proportions, and, should it tinue at the current rate, one thhr arable land will be depicted twenty years. servation schemes in PROGRAMMES advantage of responding to mic imperative and of prouse of unskilled labour for uctions for the rts of erosion a standard ice: 20 Swiss francs ISBN 92-2-103395-3 Page INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . CHAPTER A. 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..I.... GENERAL PRINCIPLES 3 A.I. THE DIFFERENT FORMS OF SOIL DEGRADATION . . . . . . . . . . . . . . . . . . . . . . 3 A-2. RAINFALL EROSION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 2.1. 2.1.1. 2.1.2. 2.1.3. 2.1.4. 2.1.5. 2.2. 2.2.1. 2.2.2. A-4. A.5. . . . . . . . . . . . . . . . . . . . ..I...... 3 Rainfall ................................ . ..*.. Nature of the so?1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..I Slope of the land . . . . . . . . . . . ..*......*...... . ..I... .. Vegetation . . . . . . . . . . . . . . . . . . . . . . . . I.................. Man . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...* . . . . . . _ . The effects of rainfall erosion 5 5 6 ........................ 6 Mechanical effects ................................... Chemical effects ..................................... Integration of rain 11 3.1. Wind erosion 3.2. The effects water factors of wind erosion factors 6 7 SOIL EROSION BY WIND . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..I 11 OTHER FORMS OF SOlL DEGRADATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 4.1. Excessive humidity 4.2. Excess 4.3. Unsuitable 4.4. Socio-economic of toxic erosion 11 ............................ .............................I....... 12 .................................. 12 ...................... 12 salts agricultural practices ............. 73 LAND USE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 aspects of soil degradation 5.1. Land employment . . . . . . . . . . . . . . . . . . . . . . . . ..I............. 5.2. Land classification . . . . . . . . . . . . . . . . . . . ..I.............. 5.2.2. B-1. erosion 7 5.2.1. CHAPTER B. in rainfall .............* 2.3. A-3. Factors 13 13 Classification system developed by the Soil Conservation Service of the US Department of Agriculture . .. ... .. . .. ". . . . . .. .. .. . . . . . . . . .. ... . "S....,...... Other classification systems . . ..I........ EROSION PROTECTION TECHNIQUES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . CONSERVATION OF ?ROTECTED LAND . . . . . ..I....................... 1.1. 1.1.1. 1.1.2. 1 .1 .3. 1.1.4. 1.1.5. 1.1.6. 1.1.7. 1.1.8. 1.1.9. Forests 13 17 21 21 ................................................ Role of the forest in soil conservation mechanisms ............ Main forest species used in afforestation Preparing a reafforestation plan ..................... Ground preparation .................................. Seed preparation ..................................... Afforestation using the direct sowing method ......... ............................ Afforestation by planting Maintaining the afforested area ...................... .............................. Plantation conservation . ... 21 21 22 27 28 ;: 32 33 40 --Page 1.2. 1.2.1. 1.2.2. B.2. .............................................. 43 ..... of pastures in soil conservation mechanisms R31e Techniques for the creation, conservation and ............................ improvement of pastures 2.1.1 2.1.2. 2.1.3. 2.1.4. 7.1.5. 2.1.6. 2.1.7. 2.2. 2.2.1. 2 >.2. 2:;.3. 2.3. 2.3.1. 2.3.2. 2.3.3. 2.4. 2.4.1. 2.4.2. 2.5. 2.5.1. 2.5.2. 2.5-Z. 2.6. 2.6.1. 2.6.2. 2.7. 2.7.1. 2.7.2. 2 .7.3. Cover 44 .................................. procedures Biological 44 ............................................. Mulching ........................................ Crop rotation ............... ....................... Mixed cropping Lea crops ............................................ ........................ Rotation field-strip cropping ............... organic reserves Increasing the soil's Farming practices ...................................... 46 Contour ploughing .................................... Contcur or slightly inclined ridging ................. ............................ Subsoiling and chiselling Defence networks terraces Terraces Terraces Drainage 3.1.1. 3.1.2. 3.1.3. 3.2. 3.2.1. 3.2.2. 3.3. 47 49 49 The role of defence networks and the ........................................... Main types ........................................... Dimensions Bench 47 ....................................... systems used .... ......................................... constructed const.:ucted works at a single progressively go .................. ................. ......................................... Characteristics of gully erosion and the role of control works ................................... ....................... Determining run-off conditions ........................... Main types of construction Bank, channel Stabilising Protection Correcting 49 50 54 60 and gully protection banks with vegetation of banks by construction the slope of water 64 65 69 70 ..................... courses .................... works ............ 70 70 .................. 73 Role of constructions ................................ Type of work ......................................... Prlnclples of calculating structure dimensions 73 73 78 WIND EROSION PROTECTICN TECHNIQUES ........................... 3.1. 45 45 45 45 45 45 46 .......................................... CrOpS Q3 43 CONSERVATION OF CROP LAND .................................... 2.1. B-3. Pastures Dune stabilisation ..................................... Conventional techniques for stabilising maritime dunes ..................................... Techniques for the stabilisation of continental dunes .............................................. Dune stabilisation by a coating of bituminous ........................................... products Crop land protection ................................... ........................................... Windbreaks Other techniques ..................................... Pasture protection ..................................... ....... 81 81 81 82 83 83 83 83 04 Page REHABILITATION TECHNIQIJES FOR WATERLOGGEDSGILS ............. CHAPTER C. 85 C.I. AIMS OF DRAINAGE ............................................. 85 c-2. DIFFERENT TYPES OF DRAINAGE .................................. 85 2.1. Open ditches ........................................... 85 2.2. Underdrains ............................................. 86 2.2.1. 2.2.2. 2.2.3. 2.2.4. 2.2.5. 2.2.6. 2.2.7. 2.2.8. 2.2.9. c.3. 2.3. Mole drai:lage 2.4. Subsoiling i; 88 ............................................. 89 ....................... Water movement in a drained soil 3.2. Determination ............................ 3.2.4. 3.2.5. 3.3. 3.3.1. 3.3.2. 3.3.3. 3.3.11. 3.3.5. of basic data 91 Causes of wetness .................................... Optimal water table level ............................ Permissible submersion time .......................... .......................... Type of soil - permeability Intrinsic flow rate of network ....................... 91 92 ....................... 95 Calculating the drainage network ;'3 94 Network design ....................................... Drain depth .......................................... Drain spacing ........................................ Drain diameter and length ............................ ............................ Calculation of collectors 95 96 97 101 101 PRELIMINARY STUDIES .......................................... 1.1. Aims ........................................ 1.2. Situation 1.3. 1.3.1. 1.3.2. 89 89 METHODOLOGY FOR SETTING UP A SOIL CONSERVATIONPROJECT ...... 1.2.1. 1.2.2. D-2. i; 87 .......................................... 3.1. 3.2.3. D.1. i"6 87 a7 DETERMINING THE CHARACTERISTICS OF A DRAINAGE NETWORK........ 3.2.1. 3.2.2. CHAPTERD. ....................................... Fascine drains Stone slab drains .................................... ........................... Drains with stone backfill Wooden box drains .................................... Peat drains .......................................... Earthenware piping drains ............................ .................................. Concrete drainpipes .......................... Bituminous fibre drainpipes ................................... Plastic drainpipes 103 . .......... .................................... analysis the extent of possible measures .Damage assessment .................................... ................................. Envisaged investment DRAFT PROJECT ................................................ 2.1. Aims ................................................... 2.2. Analysis 2.3. Hypothesis of basic data ................................. of improvements ............................. 103 103 The physical context ................................. ........................... The socio-economic context Assessing 103 103 106 .............. 106 106 107 108 108 108 109 Page 2.4. 2.4.1, 2.1!.2. 2.4.3. 2.5. D.3. ..................................... The expenditures Income ............................................... Balance sheet ........................................ Selection of improvements 110 110 110 111 ~................~............ 111 3.1. Objectives 111 3.2. Field 3.3.1. 3.3.2. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . studies ................ ... ..... ....... ........... Topographic work ..................................... ....................................... Survey borings Project document 111 111 112 . . . . . . . . . . . . . . . . . .._................... 112 Written documents .................................... Drawings ............................................. 112 112 PROJECT IMPLEMENTATION ...................................... 113 GENERAL SITE ORGANISATION .................................... 113 1 .I. 1.1.1. 1.1.2. 1.2. 1.2.1. 1.2.2. 1.2.3. 1.2.4. 1.2.5. 1.2.6. 1.3. 1.3.1. 1.3.2. 1.3.3. 1.4. 1.4.1. 1.4.2. 1.4.3. 1.4.4. E.l. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111 3.3. E.l. analsysis THE PROJECT . . . ..-...-......................=................. 3.2.1. 3.2.2. CHAPTERE. Econo;nic Survey of local conditions and site 2.7.1. 2.1.2. 2.1.3. ..... .................................. Natural constraints ................... Technical and economic constraints Project planning ....................................... ............................... Objectives of planning ............. Forms of planning and their requirements Planning procedure ................................... Bar chart or GANTT chart ............................. ..................... Time and location chart planning .................... Network or critical path planning Setting up the site .................................... 113 113 114 115 ?15 116 117 118 119 121 124 Construction of access roads ......................... ....................... Constrtiction of site buildings Plant installation ................................... 124 125 125 management . . . . . . . . . . . ..*.......................... 125 Site ................................. Management technique Personnel management ................................. Incentive system ..................................... Role of project management ........................... TOPOGRAPHICALSTUDIES . . . . . ..a...... 2.1. reconnaissance Topographical survey . . . . . . . . . . . . . . ..-.......- . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..- Planimetric survey ................................... Levelling ............................................ Composition of a topographic survey team and survey standards ................................... 125 126 130 131 132 132 132 133 134 Page E-3. Preparation of the ground plan 3.2. Preparation on the ground .............................. 3.2.1. 3.2.3. 3.2.4. 3.3. 4.2. 134 Main staking out ..................................... ................................... Additional staking ....................................... Staking report ..................................... Displaced stakes Personnel requirements and work 135 135 135 135 ................. output Clearing the the works Stripping over the area covered . .._.........._........................... land by 136 ................................. - suhsoiling 5.1.1. 5.1.2. 5.1.3. 5.2. 5.2.1. 5.2-Z. 5.2.3. 5.2.4. General comments 137 Tools ............. Labour 5.5. Earth--moving 141 141 141 141 142 . . .._._........................................ 142 .................................... organisation methods 144 ....................... and outputs ..................... '.'rench digging and spoil removal .......................... Loading and reworking spoil Spoil haulage ........................................ .................. spreading and compacting Unloading, Bank grading ......................................... Examples of earthwor'cs ................................. of Construction 144 144 146 147 148 149 149 a canal or ditch ..................... of earth dams ........................... Construction 138 138 138 139 ...................................... tools loading earth .............................. ............................... soil haulage ............................ soil compacting Trenching Tools for Tools for Tools fcr 5.4. 5.6-i. 5.6.2. and handling . . . . . . . . . . . . . . . . . . . . . . . . . . ..-..................... Manpower 5.6. type 138 .................................. Land classification Bank gradients ....................................... .............................. Bulking - consolidation 5.3. 5.5.1. 5.5.2. 5.5.3. 5.5.4. 5.5.5. on soil 136 136 FXECUTION OF EARTHWORKS...................................... 5.1. E.6. 134 GROUND PREPARATION . . . .._............_.-...................... 4.1. E-5. ......................... 3.1. 3.2.2. E-4. 134 MARKING AND STAKING OUT THE STRUCTURE .-..............._...... 149 151 IMPLEMENTATION OF REAFFORESTATION WORK . . . . . . . . . . . . . . . . . . . . . . . 151 ........................................... 151 6.1. Nur-sery 6.2. Planting 6.3. Construction 6.4. Plantation 6.5. planting Exampies of a plantation: lines .............................................. 6 -6. work Productivity ............................................... of forest maintenance norms tracks 152 .......................... 152 ................................. ..................................... along 153 contour 153 154 -x- Page E-7. .._....e IMPLEMENTATION OF DRAINAGE WORKS .............. 7.1. Installation 7.2. Tracing 7.3. Calculating 7.4. Trench 7.5. Trimming 7.6. b-din 7.7. Manpower and output E-8. E-9. of a drainage out total trench trench laying ..................... network 155 ......................... width 155 ............................... depth trench bottom 156 157 ............................. and backfilling the in manual trench drainage ................ work ............ CONSTRUCTIGN OF MASONRY WORK ................................. of masonry ........................ 158 160 160 160 8.1. Differert types 8.2. Dry-stone masonry 8.3. Normal masonry work .................................... 162 8.4. Gabion masonry work .................................... 163 work work 155 155 ......................................... digging the ....... ................................. CARRYING OUT CONCRETE WORK ................................... 9.1. Characteristics 9.2. Concrete 9.3. Shu%tel,ing 9.4. Steel 9.5. Installing of concretes preparation ........................... ................................... the .................................... reinforcement bars 167 167 167 168 .............................................. reinforcement 160 ...................... 168 169 APPENDICES I. II. STANDARD PLANS ..................................................... 171 BIBLIOGRAPHY ....................................................... 205 INTRODUCTION The soil is one of the-essential elements for plant, animal and human life, It changing eilvironment which, .under normal conditions, is is a complex and constantly the result of an equilibrium between the forces of soil formation and erosion. In the humt;s removed from the soil ia usually reconregions covered with vegeta:ion, The natural development of soil with a covering of vegetation is slow; stituted. that around L5,000-12,000 years are required for the development of it is estimated . 30 cm of soil. Man extracts from the soil the main part of his food production but increasing demographic pressure forces him to extend land culti:ration, intensify production and use techniques that are not always in line with the maintenace of soil Fertility. of man's mishandling, the equilibrium is disturbed, erosion Under the effect whole regions are hard hit by this degradation accelerates and, in mar&y countries, disastroua consequences for the physical and and the fall in soil yield - !;ith economic environment. conservation and related afforestation Soil This is due to the fact that, in the importance. has assumed alarming proportions. There is need ecological balance which, in Of and maintenance the taining increased food prod1otion and, indeed, world. schemes have assumed increasing developing countries, soil erosion for environmen'al preservation itself, is very necessary for mainvery living conditions of the such soil conservation schemes hold a Being reiatively more labour-intensive, potential for high labour absorption during the construe tion phase as well as contributing to the development of land infrastructure for increased food production. The relevance lies in the high iabour-intensity, quick wage employment generation for unskilled idle rural labourers and resultant infrastructure development. Soil conservation is the outcome of a balance 'between the satisfaction of It is a national problem in view of its social and current and long-term needs. of which far exceeds the technical and oolitical imolications - the solution . Implementation of a soil conservation 'financial capabilities of the farmer alone. programme, if it is to be successful, must be understood and supported by the populaThis also presupposes technical innvoation and collective awaretion as a whole. ness which will come about only after information and education campaigns. The present document does not claim to replace the very abundant literature on this subject to which the reader should refer should he require a deeper knowledge engineers and senior technicians of the subject. Its more modest aim is to assist It contains: responsible for planning and implementing soil conservation projects. a review of the basic data on different types of erosion and control measures making rjtindard methods for the design and planning of intensive use of unskilled labour, operationlaW supervision. projects and, finally, guidelines for site oranisation, Appendices contain a series of standard plans which, although they may, in many are no substitution for the specific . cases, suffice to define the work in hand, It should be emphasised that the studies that should be carried out for each site. plans given are not necessarily the only possible solution to a given problem and that in all cases they have to be adapted to local conditions, customs an,! traditions s which should always be especially with regard to the maximum use of local resource systematically pursued. -3- CHAPTER A GENERAL PRINCIPLES THE DIFFERENT A.l. Soil degradation excessive humidity, The most which poor reshape the main basis agricultural of fertile away the fine The wind may also covering with (low-lying Other rivers their forms of soil's chemical countries, periods been climate salination rendered is result fallowing which The soil's the degradation salts for may also brought in Factors in cultivation Atmospheric rainfall In run-off that erosion are nature the soil but this not renewed and fertilising build up in nevertheless case, is the may be due to of soil not in soil of fertile the is irrigation the it In many regeneration of hectares effects production. pressures. of poor and resultant agricultural In or structure with natural result conditions rainfail the by irrigation this of of an arid resulting land in have been way. RAINFALL EROSION erosion i, c the precipitation surface be the Thousands A.2. 2.1. are The sources of degradation demographic allowed nutrients and alkalinisation. unfit of high components. ti GibL.a~~ce and thus spectacular forms the fertile. aeration economy. These fertile excessive less is by carrying degradation. to continue degraded. which soil to topographical soil agricultural may carry soil main degradation more insidious, the same effect plains, are and wind previously and due either of wind, techniques. tnem ai of alluvial effect rainfall top the were defective region's that of This where the the cause impossibility are as the reduced. sufficient. the the depositing which into over including another components, degradation content exploitation is and varied or strip soil, the due to off may have The consequences for excessive totally regions soil soil, farming are running particles, overflowing yields soil as devastating and even the chemical low just have in irrigation. of water The wind may be numerous on cultivation, Rain sand deposits humidity over-generous degradation away larger the or unsuitable of an unprotected carry from soil production. land), leaching components sterile humidity of relief. particles Excessive practice forms ground run-off rain irrigation spectacular away the this may be due to FORMS OF SOIL DZQRADATION has considerable of the soil, main cause destructive the slope, of rainfall force. vegetation erosion Other which factors and human activity. produces affecting -4- Rainfall 2.1.1. The main and the characteristics Rainfall striking the it to leads The the soil compaction influence with of Annual rainfall 2.1.2. :%1ature of The readily important matter of by means intensity wash smallest of to still difficult which refers to are to particles The of a recording have in in soil raindrops rainfall intensity; rain gauge. increasing soil covered by a film rain an effect quantities erosion of assess soil depends since it humidity, of water run-off. on soil loss water will and years rf soil. on the soil's depends nature on nllmerous texture,chemical structure, the are proportion clays international and system of the different largest classifies soil and is parameters, composition a particle size range less - silts with a particle size range between - fine sands - coarse sands - gravels with Structure separate The of with size a particle a particle refers to aggregates cohesion Henin's a particle of the the of the and organic- structure - the mean percentage of - the fraction of dispersed - the fraction of coarse size stable clay between USA, it erosion than gravel. 0.02 mm and 0.2 0.2 and 2.0 2.0 mm individual "structural the The soil. mm mm particles in the soil and shape. factors stability" of which can be determined by are: aggregates, plus loam, sand. Recordings in Madagascar have shown that 1.5 mmlmin are rarely erosive, whereas rainfall The figure of 2 mmjmin is the always erosive. occurs. In Arkansas, (6 per cent), 0.02 these the mm and between of or index", 0.002 greater arrangement or in as follows: 0.002 range range different "instability 1 range size size than particles stones texture with with size are clays slope with intense - use intensity factors energy with the content. Texture into rain, aggregates. more larger of important increases A soil also most kinetic increases have away the amount soil susceptibility is of the the energy tion variations the Erodibility kinetic and will produce of frequency. precipitation heavier most rainfall one of result measured rainfall more the of ar'e is and demo1 is increasing disaggregate is amount intensity precipitations intensity erosivity soil; Rainfall i.e. Rainfall frequency. erosion. of rainfall intensities cut-off is estimated that on uncovered, occurs as soon as the rainfall intensities of over point above of less than 2 mm/min are which erosion loamy soil with a slight reaches 2.5 mm in 5 minutes. It is expressed by stability Stability is together sand linked to due in and the equation + loam" dispersion "clay aggregates - 0.9 coarse stable Structural the is an important particular alluvial complex of factor humic the in and clayey water also plays structure nature a role in in reduced results run-off colloids The chemical particles. absorbant Disaggregation to sands the and of soil bf the which bases structural soil erosion. hold which are 1 stability. permeability and porosity. -.C'ope 2.1.3. of The speed erosion of the With of the nature - vegetation - precipitation 2.1.4. increasing rate and the ine a given - is on soil increases run-off amount with increasing slope, and soil speed. of particles carried away also vary in relation in several slope. angle of the soil of slope, erosion intensity will depend on: cover intensity Vegetation This run-off witn flow length land rainfall increases Run-off to the - a major factor in controlling soil degradation and acts ways: - By protecting the soil from the direct impact of water drops. When rain water is intercepted by the plant covering before it reaches the soil, the height of its fall is reduced and, consequently, its kinetic energy and destructive effect are smaller. - By intercepting some of the evaporates without increasing - By inhibiting ground vegetable matter. - By enriching porosity. Roots soil and, the in particles. surface. infiltration. soil with particuiar, Their Dead water roots Organic effect increase matter rainwater which then the ground run-off run-off due organic material the is the matting even to of greater porosity resulting matting which fine if of from the remains volume. the leaf of roots improves roots they on the soil the densely surface decomposition and and accumulated structure increase grow foliage and cohesion close and to promote improves of the water soil structure. 1 stability; Calcium and sodium magnesium ions allow ions cause dispersion flocculation and structural and, consequently, disaggregation. greater -6- The effects Forest Forest growth soil has the on the greatest 2 tr, 3 per covering of vegetation. effect Cent may also type have in protecting soil OrgaIIiC material. a significant effect of from water provided the erosion. plant coverage dense. Fallowing involved increased hoed also (forest Crops plays a conserving fallowing, have a less density of crop role fallowing, conservational bare on the Resistance pasture type of vegetation fallowing). effect. Forage crops. depending offers better to erosion increases protection than with cereals or crops. Orchards trol do not constitute a sufficiently dense vegetation to effectively con- erosion. Research pastures in carried with relation out slopes of the density to - 100 per - 40 to 60 per - 20 per 2.1.5. cent around 20 to 36 per cent of vegetation covered cent Lake Aloatra, Madagascar have shown 0.026 covered covered soil the l), on the following Aristida topsoil losses coverage: soil cent (ref. t/ha/year 4 +/ha/year soil 12 t/ha/year Man Man is often at a prime the Abusive applies factor how it of is 2.2. of erosion. use of forests of are water the rurl-off. soil may lead soil on very organic irrational to their steep material use of the the destruction; slopes, unsuitable is from lost soil the is same crops, soil and particles erosion impact of water Primary or tlsplashl' which breaks and 1.50 increases The finer pension. rainfall due to on the di-aggregation capacity, the by wilich since effects 60 cm vertically These degradation replaced. These raindrop of mechanisms Mechanical rain soil and pastures clearing the The effects 2.2.1. in origin to abusive ignorance of depending Contains A grabs is vary soil with the pores excess up the water rain are in runs on the erosion soil cm horizontally. particles block and the droplets is particles soil due to and the the impact and may project The energy released dispersed by the erosive by soil of force the them up to particle intensity. more readily the off soil surface carrying decreasing away the fine "splash" the soil particles effect. absorption in SUS- -7- erosion "Splash" absorbs a large Water which the streams. Gradually, of the by modifying small stability processes fields in As erosion erosion erosive and transporting and the gullies process is allowed are of to sediment can cause great productive -Chemical Added to particular in the loss, mineral salts. of there anastomosing centimetres are erosion not it mm fertile this breaking in of 0.2 and most processes rills adequate, down into or channels and wider soil forming cultivation however, when gully of water land small occurs, to the crops processes into sediment finally, the off. erosion The erosion danger If or biological the its become gullies ravines. may be stripped dams or, where The rills increased. Dividing of are is sea. and silt fields more carried by Local deposi- up drainage and flooding. erosion significant Drainage cultivated A rainfall per slope. is the loss losses of of soil and destruction effects soil case top land, small damage to growing of uncontrolled 1 capacity. down the may reduce such the required. the the soil. lakes, increasing The end result 2.2.2. are and rivers channels, its erosion; measures If place, the a few practices in top takes streams irrigation of rill collective gullies, erosion to retain reduces extensive tion gully the runs a danger. deeper all which up to sand particles strip enormously progressively sufficient significantly are to continue or no longer collects vegetation in minute grooves components by levelling run-off water small size. no longer powers become out of and biological is of on a watershed may progressively or aggregate continues, Where rill hollow may be employed which soil a sheet away fine of erosion by a covering energy. the forms streams Cultivation soil mechanical or may carry soil. kinetic through diffuse these type or prevented raindrop's filter is whicrl This layers the run-off and depth diameter. of does not Initially, width can be reduced part land water and up to 10 mm on a soaked may contain 150 g in soil components, up to 50 g of the may carry fertile off case of bare 5-15 kg of 2 in calcium nitrate land. this fertiliser hectare. Integration 2.3. of An attempt Wischmeier's at rain water an integration erosion study factors of rain water erosion factors is given in formula. 1 To give an example of the proportions that this type of erosion can attain, it with a total surface area of 3.3 million km2, 1.4 milis estimated that in India, lion km2 are subject to significant soil loss and 6,000 million tonnes of soil are lost each year from a surface area of only 800,000 km2 (UNESCO Courrier, May 1980). 2 appear In India, each year, it is i.e. estimated more than that 6 million is applied in tonnes of fertile components the form of fertiliser. dis- -8- Fi g. A-1: Diagram of the main natural DECISIVE ! ’ ACIIWE 4 \ FACiOAS in rUn-Off erosion MAIN SURFACE TYPE RAIN GAUGING LESS INFILTRATION PARAHEIFRS I--- factors WATER NASS I OURATION OF FLOW FOOD REQUIREHENTS FLOW RAIE VARIABLE PARAMIEAS 6 OECISIVE PARARETERS \ FOOD AND HEATING RE&JIREHENTS I SOIL ?ELIEt I 1 FA'IOURAGLE FACTORS I 1 1 ANIHALS SLOPE HAN 1 DEGREE OF DES'RUCTION OF THE VEGElAllUN BAD SOIL OESTRUCTION OF THE SLOPE #ANAGEHENl VEGETATION I VARIABLE PARAMETERS 8 AESULl CLODS iARIAl3LE PARAHETERS t RESISTANCE FACTORS /~ 1 /T DECISIVE PARAHETERS ANORjiiR”E -2___2-) OflSlACLES ROOTS PERCOLATION SlRUClURE TEXTURE ~,~soIL STALKS AN0 T Jrs LEA”f I PERMEABILITY AND CAPACITY COHESION FOLIAGE I I UNDERGROUND AGAINSI PRECIPIlAlION \ AGAINSI RUN-OFF OBSTACLES After This formula, when applied soil 10~s and determine that erosion does not Wischmeier's what erosion exceed formula to a specific the is threshold methods at which Deloyeand makes it region, control H. should it H. Rebour possible to ensure dangerous. as follows: IA=R(h.LSCP) A is the R is the orecipation It soil loss in for a mean annual For a given per erosivity can be calculated Generally, tonnes acre' factor a rainfall rainfall (1 American or "rain or for index is the short ton over used. rainfall: R = Eg x Im 100 where Eg = kinetic energy Im = maximum intensity 1 1 acre is equal of the of rain the to approximately in rain feet/tonne/year; in 30 min in 4,000 m2. = 0.907 kg); index". rainfails inches/hour. 1 to estimate be implemented becomes 1953 (ref. a given period. - 9 - To calculate recording in the which to establish kinetic the rain is is given it rain, is necessary down into segments energy a rainfall of to have equal a hyetogram intensity in order ratio. between by the of br.r,ken a duration-intensity The relationship intensity energy kinetic of (of regular intensity) an{ formula: Eu = 916 + 331 Log Ih In which: Eu = unit Ih kinetic = intensity The energy which have energy in feet/ton/year in mm/h in segment Eh is equal to Eu multiplied The energy fallen during the segment. In order to calculate section of the curve Im, it is necessary in which the largest K or the resistance is a dimensionless to erosion. These "soil of index" a soil L.S or the "slope index" the angle and length of C or the "cultivation under well--defined where C = 1 (fig. is the index" conditions A.3). factor values A.2: Wischmeier's universal L.S as a function of lo equation the length which measures the relative are obtained experimentally. factor.; A.2). it indicates the of earth loss of cultivated a continually worked fallow P or the "water and soil conservation index" conservation is practised field in which soil along the line of maximum slope (fig. A.4). Fig. of millimetres to mark cn the recording the 30 min number of millimetres of rain fell. a dimensionless slope (see fig. is the ratio to that of by the number is cumulative. is the ratio to that of (graph giving and percentage of earth a cultivated the values of of the slope effect land land loss on a field the factor (ref. 23)) * 8 Length of slope in metres of - 10 - Fig. ~.3: coefficient Crop values - C C Type of crop In the United States non-hoed crops plant covering, fallow and 0.6-0.8 (rice-cereals) green 0.3-0.6 manure depending on the condition, iocation, climate 0.3-(1.5) In Tunisia: bare earth - bare fallow 1 land orchards 0.90 wheat 0.71 rotation with 0.40 cereals 0.47 fodder rotation with improved pastures For mechanised of 1.3-1-a. Fig. A.4: Value of Slope % l-l2.0 2.1 7,o 7.1-12.0 12.1-18.0 18.1-24.0 0‘01 cultivation, Erosion P factor: 0.15-0.23 fodder reduction water these values coefficient and soil should of be subjected soil conser vation conservation index to a coefficient remedies in % Contour line cultivation L value to be considered: length of field slope Rotational field strip cropping value of L to be considered: length of field slope Terraces the earth in the channel is considered lost Value of L to be considered: distance between channels the earth in the channel is not considered lost Value of L to be considered: distance betwee.. channels 60 50 60 80 90 30 25 3c! 40 45 60 50 60 80 90 30 25 30 40 45 11 - A-3. Wind erosion 3.1. SOIL EROSION BY WIND factors The erosive effect of wind varies The ability of wind to move soil At soil level, depending on the nature of the vegetation and soil. particle size. a few millimetres increases It is is that Medium-size particles the of surface the avalanche effect. land number Vegetation and reduces The soils particular, fine soils the offer is wind. is the best during the cattle hooves wind dry tends Crops of the soil laminar as the for distance to move the finest on particle size. suspension mm diameter "creep" in of the air a height from soil of the soil particles ana may form dust storms to break are particular, in of wind. soil the plays pastures. area force of the process. texture a role regions of which has an the the coarse and semi-arid - part this saltation of effect greater breaks the those along; the it also arid of since humidity vegetation grazing carried limiting effect action The saltation increases are of a bouncing surface. wind erosion level up the the thus in saltation. as they land, to the due to over called against and light carried phenomenon to wind resistance in are along in motion exposed the increase soil texture danger Wind erosion by loss of to erosion and the and, in and dry in which disappears The constant action totally of surface and make it more susceptible to is encountered on sandy areas absence whereas roughness of wind fine soil sufficiently cultivation (ploughing, dense techniques banking) have coastal vegetation. Repeated season which tend maintain a protective workto or effect. erosion has a deleterious structural of also techniques can also cause erosion. soil fragmentation during the dry of erosion surface The effects leads wind climates, and variou? farming soil and excessive increase - in The soil's period season required depending protection of occurs, temperate due to the 3.2. or susceptible dry is and erosion. In ing area sands. a major flow intensity height. by a phenomenon roll the Wind erosion there 0.05-5 ground lowest on wind distances. to most of carried of particles the increases speed The amplitude exposed speed varies are particles increases wind of the zero, logarithm wind considerable The larger wind of particles move over of the depends speed is wind the The effect The smallest over of estimated 15 km/h. that wind and thereafter as a function particles components, degradation effect on the and fertile and a reduction soil: components in water in particular, retention which capacity; - 12 - - by moving the coarser components which build form dunes which can cover and make sterile .. '.'ind erosion also effects vegetation Excessive itself. humidity Excessive humidity in the soil Water is an essential component have disadvantages due to: reduction rihich of chemical prevents the reduction - its action and which - the difficulty - reduction in crop Excessive humidity Excess leads for to degradation soil and plant and bio-chemical oxidation - 4.2. of soil and certain temperature of yield micro-organism as a result may range salts gradually build tions harmful to crops. may occur, penetrate promoted !rnsuitable up in flow is not where the agriculture soil in passing they soil sufficient will to leach not be harmful due to vaporisation There to degradation crop loss. the up of toxic quantity of soil, to plant until they i.e. life, reach take thess concentra- of soil structure and reduced soil permeability. and by suitable draining practices and failure of its plant measures since labour-intensive tion, leads agricultural Intensive 1 to total may lead to the build brings with it a certain phenomena can be controlled by leaching the water with a high dissolved-salt content. of evaporation; by drainage. These to evactiate the surface deeply into the soil by the high humidity; fall and this only deficiency life; components exhaust oxygen the phenomenon of alkalinisato salt concentration (Salinisation), calcium ions are repalced by sodium ions in the absorbant soil i.e. In addition 4.3. from quantities salts If the water mineral salts. the salts down to lower levels tion excessive of excessive from a slight and semi-arid climates, irrigation Each new irrigation flow the soil. in fertility. wet soil; can be controlled toxic but resulting on plant roots which can no longer often suffer from parasitic disease of cultivating and reduced life action In arid salts and OTHER FORMS OF SOIL DEGRADATION A.4. - obstacles Windborne sand particles have effect on grass and crops. The wind increases vaporisation and tends the soil's usable water content more rapidly. an abrasive ~6 ext;aust 4.1. up behind various entire regions. nutrients. for to apply This remedying this adequate amounts of fertilisers is noted form of soil degradation' do not come within the framework work. are other forms of degradation such as sedimentation or soil acidifica- .. 13 - Socio-economic degradation -1,o. aspects of soil The main effect of soil degradation The harm to agricultural that result. returning the land to its fertile is the damage to agricultural activities land may be irreversible, or the cost of state may be so high as to be not economically viable. The farmer's expenses must be sufficient such as iertilisers, invest poor profits in soil seeds, conservation This situation soils than to allow him to live and pay other They must also be sufficient to etc. fuel, and improvement. is of course more difficult on largehoidings with the As erosion progresses, and less profitable and finishes the deleterious effects finally to achieve in smallholdings with good soils. farmer's work becomes more difficult, more expensive At the regional level, by becoming impossible. undermined the total structure of social and economic., life. In order to avoid encourege conservation in work of value such situations measures which it becomes necessary, wherever possible, to bring together the largest number Of farmers to the collectivity. A.5. LAND USE Land employment 5.1. Depending categories: on its production and protection land is used for Protection land usually role in the The balance country's level economic than land Production ma:or land characteristics, between of production of development classified the forest cr pasture cultivated land and protection and may change land depending that will vegetation is and plays situated vary on technical, a downhill. depending social on the and conditions. Although production protection it land, land is of less signficance from has a decisive role in maintaining the economic point of view the country's biological Land classification 5.2.1. two large land. balance. 5.2. into cultivation. has natural conservation is normally Classification system developed by the Soil Conservation Service of the US Department of Agriculture The classification is divided - capability units; - capability subclasses; - capability classes; see fig, into: A.3, crop coefficient values. - units The capabilit:r prdouction and respond in group soils about ttle 14 that - have about same way tc the the same influence management on crop requirements of common crops. The subcla; wetness hazard, eq 1.group capability and climatic The capability of risk classes to erosion having similar limitations (erosion limitations). describe and limitations The simplified units progressively, in eight stages, the degree of use. classification is as follows: Simplified classification - Unit Subclass Class Land I suitable Land with 1 for annual cultivation cultivation I Land that can be permaulently is treated normally rotated, These are lowlands for which not necessary. II Less fertile land with lower yield, often on a slight has already taken place by slope i3 %), where erosion Moderate conreducing the depth of the arable land, servation practices necessary. Land with intermittent cultivated and which, when with fertiliser or lime. conservation practices are cultivation III The soil has to be reconstituted a vegetation covering to occur, place only from time to time. land with slopes of IO-16 %. periodically by allowing and cultivation takes This is the case of eroded IV Land where the slope is steeper Is suitable seriously eroded. limited cultivation. than only Land II requiring permanent in class III for occasional cover V Land not suitable for Suitable to erosion. careful exploitation. VI Land of the above type but which has poor erosion resistance due to physical properties or topography. can be used for pasture with conservation techniques required from time to time. VII III VIII Exhausted stituted pract:ces. and more or ploughing but for permanent relatively pasture. resistant Requires It Pronounced erosion that can be reconland. by grassing or planting with total conservation Non-produc.tive land Soils tion, VIII etc. of class forests, are suitable Shouid not only for be cleared. natural vegeta- Fig. A.5 - !. ! I! -A- r. ,‘> _ i : Land classification to run-pff erosion in tl,e LISA) Ilt".:~ _. (J~l.ilI :; . on the basis of susceptibility (based on a drawing published ii L!v a III: e d stage ,b_ . . ..r. -- of erosion (Upper Volta) Examples of advanced stage of erosion Niger Upper Upper ‘~01 ta Volta - 17 - Other 5.2.2. classification 5.2.2.1. Classification by degree 5.2.2.2. Classification of see fig. is the Diagramatic of run-off A.7: Apparent erosion or Stage I. stability of II. Stage of insidious erosion III. Stage of initial apparent erosion by s lope of in erosion tropical (ref. ll), Africa BEEK and BENNEMA :FAO 1972) a new classification into run-off of see fig. (ref. A.7. l), A.8. developing Current potential state land The classification 5.2.2.3. Fig. systems _- system countries. It technical effects incorporates capability classification erosion designed (ref. 28). more specifically social This for and economic factors classification. of degrees of Soil characteristics Types of remedy Insignificant, clear run-off water. No apparent erosion Fiat or almost flat land or very slight slopes less than 3 % High permeability: 20 cm/hour (cf. 3.2.4, Ch. D) Dense cover of vegetation Good fertility Good soil cohesion Land well supplied with humus All types of possible No treatments Slight run-off of turbid water at very low speed during. heavy precipitations No apparent erosion or traces of rills Slight uniform slopes of less than 3 %, or very intersected slopes Gf 5-8 % Good permeability: 12-15 cm/hour Light covering of vegetation Moderate fertility Relatively good soil cohesion Land with a relatively good humus content All crops on contour lines Well planned rotation From time to time, bench terraces Reduced grazing density Uniform slopes of 5-8 %, or intersected slopes of lo-16 % Small collection basins (1 or 2 hectares) Moderate permeability: 8-10 cm/hour Very sligilt cover of vegetation Moderate fertility Poorly coherent soils Land with only slight humus content Alternating crops on contour lines with % in annual cover crops Bench terraces often necessary Reduced grazing density Run-off already quite pronounced with moderate precipitation; muddy water flowing at moderate speed Appearance of light patches and stones on surface Shallow gullies appear especially after the soil has been broken UP, but do not hinder machines Slight fertility reduction in crop , - 18 - Fig. A.7 Current potential state (cpncl.) Apparent erosion or IV. Stage of intense erosion Stage V. dangerous erosion of VI. Final stage of erosion effects of Soil characteristics Types of remedy Heavy run-off of muddy water with moderate and heavy precipitation, speed quite high to very high Increase in the number of patches and stones Deep gullies which begin to impair mechanical cultivation Uniform slopes of lo16 % or intersected slopes of 20-30 % Watershed of several hectares Low permeability: 2-5 cm/hour No vegetation covering Alternating permanent grass and cereal cultivation, for example cover crops must dominate in the rotation system Terracing essential Back-sloping terracing necessary on slopes over 15 % Ploughed crops are possible between terraces Low grazing density * Pronounced run-off at the slightest rainfall Water carries gravel or aggregates moving at high speed in the event of heavy precipitation Deep gullies preventing the movement of heavy machines Land carried away in blocks Uniform slopes of 2030 % or intersected slopes of 45-65 % Large watershed Very low permeability 0.5-l cm/hour No vegetation cover Mediocre fertility Unstable soils "Algerian terraces" essential Cover cultivation everywhere, with permanent grass cover on large surfaces Pasture, woods very light grazing density with periodic prohibition Top soil stripped Very steep slopes Very large watersheds Virtually zero permeability No vegetation cover Fertility completely lost Unstanic. soJ~c' Diversion channel above and below to protect cultivated areas Prohibition of grazing Trial tree-planting u:l back-sloping Lerraces entirely away Observations 1st column: A given area of land may go successively commencing with the stage of described, through stability. the six stages 2nd column: There is a constant danger that the features may deteriorate, but they can regress if the treatment given in column 4 is suitably applied. it would be necessary to Where the proposed remedies prove inadequate, immediately apply the remedies of the next, more serious Stage. 3rd column: This is merely in combination; 4th there is listed a nubmer of treatments by order For each category, the choice should be made depending on the stage to of effectiveness; Obviously, general remedies (workwhich the erosion has progressed. Various etc.) should be applied at all stages. fertiliser, ing. the erosion In general, stages may coexist in a single plot of land. starts at the lowest part (watershed effect) and moves progressively upwards. column: a list of different features, it is not necessary however, some of which must exist that all be present. - 19 - Fig. A.8: Classificaticn (in tropi Reference Slope % Various 3-12 Crops nating grass of land Africa) crops alterwith cover on the basis of slope 1 Possible methods Land use o-3 :dl cultivation Crops to be avoided Protective measures Mechanlsed tion cultiva- - Mechanised tion cultiva- Precautions to be taken for bush-type culture on bare soil Absorption or diversion network Contour cultivatjon 12-25 Crops grassland woodland Manual cultivation Animal-drawn machines Bush-type crops on bare soil Anti-erosion networks Terraces 25 Pasture forestland Manual All Anti-erosion ditches cultivation - 21 - CHAPTER B EROSION B.l. PROTECTiON CONSERVATION TECrlNIQdES OF PROTECTED LAND Forests . Role of the conservation Depending increasing on the open forests, - light - tropical forest The role of forest the formations forest the the organic rich flows, increasing ground-water forests, conservation protects the mechanisms is against water soil roots) porosity which of the protect the and permeability. due to: soil impact thus structure; soil against This organic run-off matter is substances. against run-off, their regions, 1 level. A forest forests duration it may help also play an important and reducing peak flows, reserve role which in evening limit out the to reconstitute to its and a recreation protect influenc the role e on the relative since they region's humidity of help to lower Plant climate. the in some air; precipitation. addition Land conservation necessary have a corrective to increase increases in Finally, logical forests may have a significant transpiration drained of effects. In marshy 1 may be made by order and disaggregation (leaves, structure, action soil effect matter in nutrient pernicious in wt.ich splash In their regions a distinction formations. the forest and improve water zone, or deciduous vegetation reducing (b) climatic cover: - (a) forest in so mechanisms is this protective area role, for on continuous cover against During the nineteenth by a vast programme it where it has a role as a bio- man. based or supplement a forest the is vegetation attack non-existent cover. It of man and animals is therefore or to or inadequate. the Lande region of Gascony in France century, of reafforestaticn using maritime pines. was the - The main are not that objective 22 of afforestation incompatible but the side - may be production of afforestation The two or protection. dealt with in this publication is of protection. Main forest afforestation 1.1.2. Choice 1.1.2.1. These should species are used of species selected on the be made to give basis preference The varieties requirements. in of to used the over-all types for of soil However, objectives. trees that conservation attempts can also meet production may have the following objectives. - conservation - reccvery - improvement - stabilisation of of soil land selected be suitable - offer of the The choice objective of of the coastal soil: sand, continental sand, the operation; conditions: difficulties. Review used of moving environmental no cultivation 1.1.2.2. degradation; should: objectives for erosion; ground; and protection The variety - run-off has undergone of marshy etc. meet the soil that windscreen, - against in of the main afforestation species tree varieties (ref. 13) to be used afforestation depends programme on the which climatic conditions may be either timber and the production main or conservation. The selected soil condition or intended for are suited well testing The main humidity the level in certain the and, susceptibilities planting should shrubs. rilthough it provides rnc roachmen be given new varities depending Wherever to local prove on the a project is varities which necessary, prior out. varieties that playing has a patchy zone. should preference situation be carried have can be used a predominant are categorised grass forestry construction cover and there production materials below on the basis of the role. Steppe regions with rainfall between 200 and 500 mm(Sahelien and subdesert This since may also conservation, to factors (a) the soil should climatic varieties is zone) are only it small, relatively is and firewood few bushes of vital economic and helps in and importance controlling desert t . Plantation techi.iques counter the competition carried out to concentrate should for water be designed from and to ensure natural to exploit natural vegetation. infiltration of water. conditions Earthworks and should be - 23 - The main varieties - Acacias used are: (mimosoideae) Numerous varieties are widely encountered in dry and very dry zones They comprise numerous subspecies and varieties and Australia. have very specific ecological requirements and are well adapted conditions which they in Africa which often to the severe encounter. They include: - Acacia laeta: sandy - this and rocky Acacia has very soils albida: or to clay this is resistance good a tree to drought, is suitable for subsoils. without spines, the trunk of which may grow up to 1 m in diameter in good soil. It provides firewood and may be fed to animals. forage seeds, and the fruit This variety is par-. titularly useful for soil conservation in view of its deep rcots and the micro-climate vicinity - - provided Acacia dunes; cyanophylla: this variety it is used together with Acacia raddiana: this from Mauritania tion - by the vegetation of the driest Acacia it produces in the is a large acacia It is found in a variety the most arid used for is a small It spiny tree which is a good variety of arid zones. grows the regions reafforesta- in very dry regions it on sandy, which Prosopis juiiflora (mimosaceae): this is a small spiny is used by cartwrights and for the production of posts. planted stabilising of regions. this Senegal: firewood; - that is used in particular for tamarisk for wind breaks. to the Sudan. soils and on clay subsoils. results in the reafforestation - cover of crops. gives stoney excellent tree; the wood It is an excellent stabiliser for sand. is an excellent Genus euphorbia Euphorbia balsamifera: this is a shrub of 2-5 m in size from the very dry regions of the south Sahara. It propogates easily and is widely used for stabilising sands in arid regions. (b) Dry-climate Sainfall is between The grass again the Savannah cover limiting 500 and 1,000 is more dense factor. than The species mm with 7 to 8 months in the preceding selected must of dry case but be suitable season. water for is once these conditions. ---_1 The main ap~~:les - Azidarachia areas ,ndica of India used are: in the dry this is a tree which is widespread It provides ve1.y Sudano-Sahe lian climate zones. (meliaceae): and in the - 24 - it good firewood, source requires it of water; is deep topsoil a light, poorly suited with a relatively impermeable to clay, close soils subject to flooding. - Anacardium - occidentale Dalbergia sissoo it tree; adapted to since is it is poles and for Acacia variety Euphorba - Acacia - Terminalia - Acacia albida. - Acacia Senegal. - Prosopis a moderate-size, and for soils but and vigorously is for used in out the is must be well throws used It firewood. they It soils. misshapen and control new shoots It of windbreaks. drained erosion production well and suckers. of firewood, can be planted only soil. is grows well widespread in does well a shrub turicali: in heavy soils on a dry used for Senegal and in which the Sudan. to subject are The flooding; the ground. hedges 1 i nes. and in mearnsii. tomentosa. juliflora. Semi-humid The rainfall - to clay construction this variety - (c) the healthy sdstringens and poor suited is and stakes (caesalphinicaea): scorpioides, pubescens poles deep rooting siamea rich, for stoney not Cassia in - used sandy, it deep: - is this (papilionaceae): tropical is between The principal species Teak (tectona grandis): supply Savannah and should 1,000 used and 1,300 3 to 6 consecutive dry months. are: this be well mm with requires deep, fertile is an excellert It drained. soil with an adequate wood for water shipbuilding and cabinetmaking. - Gmelina this arborea: reafforestation and the young Propagation needs for deep are which curtains, very hardy The wood is easy. alluvial a species protective plants is is soil and is is suited lines of and cattle used for will general susceptible to i ndividual etc. trees not The tree graze on them. The tree joinery. to asphyxia from stagnant water. - Cassia siamea. Genus eucalyptus: numerous tests .-. +:o semi-humid tropical climates. suitable formed. for forests prOteCtiVf? ?'hey provide clsed to build lines excellent of trees have been Numerous since they general and shelters. carried out on their adaptability species are available. They grow purpose rapidly, are robust wood and firewood The species used are and well and can be include: - 25 - tereticornis Eucalyptus with the exception Eucalyptus micro-corys Eucalyptus robusta It is urophylla, E:-lcalyptus camaldulensis. Eucalyptus deglupta. are suitable for calcarota. - Callitris glauca. - Chlorophora Casuarinil loams, soils. and healthy soils. salty coastal marshland of waterlogged on heavy ground. work. etc., of groundwater is this used numerous The wood is also is tree but is which is and and the used for a source a remarkable It as a windbreak. low-altitude level a iarge conditions of resin. suitable joinery. (filao): sandy, varied are soils. and it this also in very The species and light (iroko): and is grow land. does not It tolerate tree is for widely requires stagnant dune used for a relatively surface water. Bamboos - Gxyienanthera for - and in also used Cd’, Tropical In view of those these in pulp which climates, large has good resistance soils; it does not like clayey, number of species the for that can be grown, on a large scale Cmelina arborea: PlanWd on a large scale Planted on a large scale Acrocarpus Cassia fraxinifalius siamea suitable and paper- and salty soils; it it is possible to qualitative and quantitative point of view. soil conservation is not usually a requirement. Planted falcata is pulp Savannah grandis: Albizia for and papermaking. are best from afforestation adorata: used compact Tectona Cedrella to drought, is maintenance. this and humid the this and ferruginous land Bambusa vulgaris: is select abyssinica superficial dry, making In genus and external reafforestation higher this woodwork, equiselifolia stabilisation less reafforestation mediocre excelsa cabinetmaking moist and sandy good soil. the in purpose Callitris SOilS and superficial on loamy, afforestation - for and dry alluvial etc. species for and general rich in more or for Eucalyptus lumber - grows salyna. majority soils requires Eucalyptus a good choice - acid useful the relatively preferentially grandis, Genus pinaceae: are of Euchlyptus soils. - for - 26 - Araucaria In mountain eunninghamii: Chlorophora excelsa Chlorophora regia Eucalyptus camaldulensis Chlorophora rostrata Eucalyptus tereticornis Eucalyptus umbellata Eucalyptus citriodora Eucalyptus cloeziana Eucalyptus deglupta Eucalyptus saligna: More difficult and soil Eucalyptus grandi.s: Very similar same species Eucalyptus pilularis: Planted on a large scale Eucalyptus propinqua: Planted on a large scale Eucalyptus paniculata: Planted on a large scale Pinus merkusii Pinus kesiya Pinus elliottii: Pinus insularis Pinus taeda Pinus caribaea var. Pinus caribaea bahamensis Pinus oocarpa: of preceding view tree, in madagascar of if rainfall not the at medium At low altitudes At medium altitudes altitudes Pinus aptula: At high altitudes Temperate specific point caribaea At high is the the successful pseudostrobus: There climates a large (Europe) number of species used and these should sycamore, bean be matched to the objectives. They include tall trees nettle the following: (horse tree, elm, intermediate - chestnut, plane, or filler oak, poplar, trees deciduous (service hornbeam, maple, mollntain - to Pinus (e) - from Has proved altitude Pinus oocarpa, var. ochoterranaF: - climates tree, beech, maple, lime for tree. ash, wild tree); cooses; judas tree, walnut, elm, alder, birch, horse chestnut, false acacia, plum, Willow, laurel, etc.); ash); - evergreen shrubs for filling (arbutus, out holm-oak, the lower holly parts of windbreaks and wooded strips: cherry. - 27 - - - deciduous (hawthorn, tamarisk, blackthorn, evergreens conifers for (laurel, and thuya for evergreen wi_ndbreaks Japanese cedar, SeqUOia, - Lambert cypress, pines. Preparing dogwood, syringa, elder, thorn, cotoneaster, privet, purslane); larch a reafforestation objectives of when choosing on poor for soils; windbreaks on the sea coast; plan the plan "ne species have most been established, suitable are rainfall is the related main to factors the to climate and soil. As far as annual number choice of rain species dates (superficial of study is year soil, a predominant a sufficient is also and also guide necessary the factor. in The determining the to make allowance regularity of for rainfall, so as to hazards, may be necessary to concentrate can also be used rainwater to limit run-off vaporisation mulch). survey not It techniques the as a physical not relevant Suitable of possible, the soil auger is concerned samples and where should be taken a complete to determine any factors. The potential rooting soil, the measures working limiting is rain and the zone, points. pedological the of throughout In an arid As far concerned, to be propagated. planting to certain is climate of millimetres distribution specify for may be the a high level The presence of water and thus Certain root development proximity of of ground water. of an upper reduces species will can be used as pines, is a rocky layer substrata, only to recolonise of (e.g. vaporisation grow an essential is in compacted sand) which a limiting layers of of Obstacles clay, limits factor a deep layer thin factor. topsoil layers capillary in light dense to arid movement climates. other, topsoil; where of the such underlying rock fissured. The soil nature of the preparation method a will arable portion. The chemical of rose, hedgerows: - consider guelder rose); thyme, cypress Once the is bush - 3 -1.3. the bladder-senna, species chemical often (presence requirements be a valuable soil content of is guide of the soil also will calcium, acidity, often inadequate, to selection. vary also depending on the thickness be a determining salinity). and the Knowledge results of and factor in of species' local the the experiments choice may - 28 - Ground 1.1.4. preparation Methods 1.1.4.1. These vary considerably the species twography, economic conditions. In certain of proper The main clearance, - preparation - staking-out, - preparation - measures - erosion with introduction is of service of planting can also 1.1.4.2. local is (terraces, socio- a prerequisite embankments, contour out by hand are: holes, run-off water, to use manual deep ploughing be stripped the type planting; - labour - the ground l's not wet ground). for vegetation plentiful this labour; which it is by mechanical stripping are used, - On grass-covered measures vegetation, B.2. can be carried possibie methods is anti-erosion section existing to be used and the is techniques under the soil, paths, Vegetation methods rf of work. always or of to accumulate control subsoiling Manual that designed not Vegetation be described type how it Anti-erosion operations - It the reafforestation. will on the to be planted, cases, etc.) ditches, and depend is necessary for example the to use powerful means or even by the case machines. use of chemicals. by manual in particular, cover requires when: only slight modification stripping (very prior to and cheap; suitable sites, for soil meachnical preparation in certain slopes, cases, very be unnecessary. In other - cases, the vegetation may be removed: --.------.---.--. -----.------in narrow strips 1.5 m wide along either here will be picks and hoes; may, steep the contour or by removing vegetation in a radius of This will be done with picks and shovels. --._.-_. -- -._. -_..- ..-._ .-__-.._-.__--.--- 50-70 On brush-covered will Forest-covered clearing forests. operations sites, sites are soil do not usually preparation usually intended present to replace cm around be highly a soil The tools lines. the labour protection ex?sting forests planting used holes. intensive. problem and by productive - 29 - Preparing absorption 1.1.4.3. These methods used in zones order increase the water allow the roots to (60 x 60 x 60 cm). storage capacity of take, deep ploughing the or With of holes, rainwater the technique (see and Other direct water planting to: on pre-existing soil the vegetation; by undercutting the use of large planting these techniques soil to holes are not intensive. Another earth digging of in - exception action water eliminate the destructive arid improve - labour the are the ground to and retention it planting called Appendix, collects Standard around ridges laid towards large the the out in planting "steppe" method Plan 1). planting No. used The water on sloping is land retained fashion or collector are used gullies to collect can collect by a ridge (‘ r I k-3 tl’J>ays along .vatt’r in t~he terraces; be built the 1::.zI!it kin 3n npiSroxirilatcly c <.ri : 3 c:I‘ i rr‘cCulari+ i ie- .J. > in on the the location basis of of collector drains, paths, are touched they etc. mind: crests ilonstant or ridges so that they spacing between a crop not strip by by q’corrccting” the - 59 - Fig. Terrace B.lO: lengths Direction United and horizontal slOpeS (ref. 1) ) of flow: States I 0,O.s % 122 m , ' ' 0 122m ,I heavy rainfall areas Commonly adopted lengths: 0.16 % , 0.25 % , 0.33 % total 400 ’ 122 m ! I 122m 122 m on very good absorbant soils 0.08 % I, 0.16 % 1, 0.25 % I total 488 ’ 0 , for low-rainfall climates 0.08 % , 0.16 % 152m ’ 152m ’ total I 152m This rule can be applied, with precautiun, m m 456 m in TROPICALAFRICA Zaire Heavy soils and rainfall 0.00 % , 0.17 % , 0.33 % , 0.5 % I I I 180 270 360 0.04 % ,I 90 0.08 % , 0.33 % ,I 0.16 % ,I 0.25 % I 1 120 Permeabl~4~oila, low-inT ensity raiz EL 0,ll , 0.33 % , 0.16 % , 0.25 % i I I I 120 240 360 480 0 0 -1 480 formula 1 formula 2 formula 3 , 0.08 % , 0.16 % I I 150 300 450 @ 0.33 % is an optimum WESTAFRICA (frcn L: Fournier) 490-550 UPPERVOLTA: L: 200 Virtually ALGERIA: m, slopes: m, slope: 0.01-0.02-0.3 uniform % 0.2 % (ditches) uniform slope 0.5 % and 0.3 %, dry zone Longitudinal slope 0.5 %, length 400 m TUNISIA: MADAGASCAR: Contour channels and terraces 0.5 % - advisable: Fig. B.11: Terrace Determination as a function length for Nichols' of slope standards of ground slope and the length Sandy ground Maximum slope per thousand a ground slope of: for 0.2-0.3 % terraces of the terrace Clayey ground Maximum slope per thousand a ground slope of: 5% 10 % 15 % 5% 10 % 15 % m 0 0 0 0 0 0 m 0.2 0.4 0.6 0.8 1.0 1.2 120-210 m 0.6 1.0 1.2 1.6 2.0 2.2 210-300 m 1.0 1.4 2.0 2.4 2.8 3.2 300-390 m 1.2 2.0 2.6 3.2 3.8 4.0 390480 m 1.6 2.4 3.2 4.0 - 0- 30 30-120 for - 60 - Bench 2.4. terraces1 Bench This forms. which off terraces process was too for wate,, of saturation of There are a series to recover of for They will flat or nearly cuitivation reduce flat land or completely plat- on slopes eliminate run- infiltration. requires upper soil two main homogeneous, layer of deep relatively layers types terraces constructed: into possible an impermeable the land utilisation. construction The presence sloping makes it steep and promote Their convert near and cause at land the a single permeable surface slips depending terrace, constructed and sufficiently is which on the likely soils. to cause may be catastrophic. way in go and terraces which they are constructive pro- gressivcly. 2.4.1. Terraces constructed at solution usually entails retaining walls. It soil proved This have is used a single go massive only ineffective. earthworks when other Terraces and the procedures of this construction for type are of enhancing and conserving justifiable only on good soil. The main types - earthwork lip; - terraces - stepped terraces - irrigation channel at - of terraces with terraces termites terraces are (see Standard in which the bank is grassed dry stcnework on slopes Stepped slight stakes less terraces the top by a small earth than with which and wattle, depends are on the exceptional channel at the top inadvisable Terrace slope. cases, in 1.50 countries height They m. snd a drainage are where is usually usually 20 %. dry-stone walls and grassed-banked terraces will resist run-off. In ileavy sequently at of at 12): walls; terraces which have an irrigation the base with a slight back slope; The _width of a bench terrace equal to or less than 1 m and, in built protected No. banks; with made with abound. Plan the slope the base of draining Fig. volume B.12 rainfall regions, terraces are the next the water shows of earthwork 1 the built higher run-off with a slight to a collector required relation Bench terraces have been used in the Far East and South America. areas of high population where there the slope with terrace Con- construction. a drainage may itself channel have a slight drain. cross-section in back The whole terrace. a typical may degrade of CLo the a terrace natural built at one go and the the Mediterranean, in mountainous land. slope. since ancient times around They are used, in particular, is a shortage of agricultural - 61 - Fig. ! ~----.-- Terrace -- B-12: ..._. ____ built _.. .__ at ~. ..___~ one -~.-I- go (Burundi) .- .__. --~-- .- .~ _. ----.-, Construction at one Terraces of a terrace go (Lesothoj at one go (Lesotho) - 62 - Terraces Typical in dry cross-section stones (Gard, of a bench France) terrace 63 - Fig. 8.13: Guide 50 design with 1 m vertical Slope of land Width of bench available for cultivation Total width of bench terrace No. of benches per 100 m of slope Maximum depth of cut Area of benches available for cultivation per ha Slope area of riser per ha of benches Volume of cut per ha of benches 2.4.2. -Terraces and construction interval of bench terraces 5 % 25 m 18.50 8.50 5.17 3.50 2.50 1.83 1.36 m 20.00 10.00 6.67 5.00 4.00 3.33 2.86 m 5 0.47 10 0.45 15 0.42 20 0.40 25 0.37 96 0.925 30 3s 0.35 0.32 0.850 0.775 0.700 0.625 0.550 0,475 m2 919 1 838 2 758 3 667 4 596 5 515 6 434 m3 1 175 1 135 1 077 1 02c 963 903 847' constructed progressively These are constructed in the same soil snd climatic conditions as those desprrvic,usly but since tneir progressive construct!on makes use of agricultural they are much less labour intensive and, consequently, the costs are techniques. cribed lower. Construction is carried by placing obstacles future riser. out: horizontally or on a slight slope at the point of a In the farming that is carried out on the strips marked out in this way, soil is progressively moved from uphill to downhill by continuous downhill ploughing or pickaxing. The land accumulates behind these obstacles and progressively increases in heig;It to form a terrace with a slope which is sU?ficiently slight not to erode. Terraces obtained in this way are not usually horizontal but have slight downhill slope. Two types filters (a) of natural and complete Filters. part of the too severe vegetation The filters obstacles solid These break can be used: obstacles. the erosive soil carried in it. and can be controlled cover, etc.). force of the running water and hold back a. They are effective when the erosion Is not by farming techniques (ridge may be mads of: - piles of stones - contour - lines - contour - rows of fruit or crop bunds protected of dense rigid residues; by stabilising grass; hedges; trees or vines. plants (ados); cropping, dense - 64 - (b) Complete solid stabilising in the plants and which are result earth Drainage 2.5. progressively either or banks bunds or initially converted into of ridges, earth such on which as those to constitute horizontal trees described defence bench or networks terraces as a to prevent rill slippage. works of control The construction gully are intended paragraph, Characteristics the role of 2.5.1. are planted, preceding of These obstacles. gully erosion works of waterways and for rain run-off is intended and erosion. As the watershed and deepen (from as the 0.20-2.00 flow and water m in Channels cross-section this, natural of speed develop the water vegetation run-off and then depth), and gullies the the increases, collects gullies (with into establishes rills a depth erosion and the anastomise they form greater which an equilibrium itself which Subsequently increase. by regressive course into than tends channels 2 m). to stabilise cross-section; after cross-sectional state is retained. Regressive Regressive result in erosion all the cross-section necessary soil is Even when the modified. in start the likely flow rate to develop concentrating water installation of not to failure. destined Effort general plan construction if the methods. this of in these drainage may finally away before the this form cultivation of surface soil, reached, the of of pasture erosion; at it is erosion. conditions cropping equilibrium this erosion the watershed result in the water point is are an increase courses cross-section. channels natural natural networks channel the the a watershed has been and the diversion flow diversion to control for again a new equilibrium Consequently erosion. of forests and renewal of stripped cross-section to start in to conserve progression of downhill being order equilibrium The construction of In Destruction run-off moving of a watershed reached. to prevent nevertheless gradually erosion waterways if run-off must one wishes erosion over terraces) and this waterways and gully of (ditches, be arranged to ensure require the results whole that the also has the in renewed prior these to the efforts development watershed, effect using are of a suitable - 65 - The two basic (a) run-off (b) the nature section). data flow and shape tion of the run-off Run-off 2.5.2.1. for the development beds for draining In the of peak run-off case of peak C = the run-off catchment basins with are: water (drainage cross- surface The concentration (see a duration area of time This area rational less method than 200 ha, given by the concentration time determinaequation: C I A in m3/s coefficient for slope. this a surface can be made by the flow I = rainfall average plan rates rate Q = the A = the a drainage conditions Q = 0.00275 where of rate; Determining 2.5.2. essential is fig. equal to the ,the basin depends given B.15) in ha. on the by the basin's total length of the basin and the formula: t ITc = 0.018($ o'77 I where T c = the concentration L = the maximum water S = the slope, The rainfall determined duration return Fig. (or to their ratio of the a period larger ouration are shown Rainfall Climate patterns of for figure in Africa Mean annual rainfall (mm) equal small selects 500 108 Sudano-Sahelian 600-l 300 114 T = return 500-3 duration 000 the time be made between rainfall intensity and a return intensity Intensity-duration 200-l COO must concentration the is return of duration values in relation and Madagascar Mediterranean 1 000-3 the below. of T = 10 y Madagascar to Rainfall B.14 length. constructions, Rain Guinean-equatorial to A choice constructions. in fall time one usually 10 years for in ratios. period), to in netres difference intensity/duration equal minutes distance for 50 years B.14: flow intensity recurrence duration equal to from in time 66-120 120 characteristics 30 min T=50y Rain T=lOy of in mm/h 60 min T=50y 60 150 75 - 90 42 108 - - 66 - Peak run-off conservation completely case of possible where Fig. leaving out of consideration any soil the concentration time or reduce run-off have been instal!ed, run-off may be for Topography it is is formula - Table Forests: . flat, slope 0.5 % . undulating, slope 5-10 % . hills, slope lo-30 % necessary to use the with and vegetation greater than 200 ha, it to use analytical Reference out. hydrology should be textbooks. possible basins area has been carried hydrology it a surface methodology. survey catchment Rational B.15: with rational no hydrometric In West Africa, suitable basins peak run-off, to specialised 'made here is catchment to use the In determining methods be determined halted. In the not should measures which may increase Where absorption networks coefficients. is rates ORSTOM de RODIER-ALJ?rRAY method a surface area of up to 200 km2 (ref. which 31). of C values Soil texture Very sandy Laamy clay Compact 0.10 0.25 0.30 0.30 0.35 0.50 0.40 0.50 0.60 0.10 0.16 0.22 0.30 0.36 0.42 0.40 0.55 0.60 0.30 0.42 0.52 0.50 0.60 0.72 0.60 0.70 0.82 30 96 impermeable surface 0.40 0.50 50 96 impermeable 70 96 impermeable 0.55 0.60 0.65 0.80 loam clay Prairies: . flat . undulating . hills Cultivated: . flat . undulating . hills Urban zones: . flat . undulating 2.5.2.2. Drainage The simplest channel way cross-sections and most widely cross-sections is the used method of MANNING-STRICKLER calculating formula flows as follows: in which: Q is the flow rate S the cross-sectional R the hydraulic where in cubic area metres of the radius p = the perimeter in m per second drainage way in m2 and drainage - 67 - i the longitudinal K is the These Fig. slope coefficient values are K values B.16: of for the shown in in the the water course surface figure texture of the drainage way wall. B.16. MANNING-STRICKLER formula K Characteristics Very smooth Sand/cement mortar rendering, sheet metal without planks, walls: Smooth mortar Planks quarry Smooth walls: . . . . . . . . . . . . . . . . . . ..s........... rendering without tiles very smooth; planed protruding welds . . . . . . . . . . . careful joints, ordinary ..*.......*..............a................. 85 rendering, 80 Smooth concrete; concrete channels with numerous joints ................................................. Ordinary Irregular roughly Rough walls: Very rough Very with walls: extremely masonry; regular earthwork earthwork, rough or old built masonry ..,.....,..... irregular rock beds Earthwork in condition; 70 concrete, old or ................... 60 rivers regular the velocity in the channel of water course is equal to the rivers ..,... with Completely abandoned earthworks, torrents large blocks . . . . . . . . . . ..**...............*............. The water 75 ......... earthwork with grass; . . *..,.....e....................... poor 100-90 pebble beds 50 .. 40 carrying 20-15 ratio of the flow rate to cross-section: For a given type MANNING-STRICKLER formula on the basis - the flow - the drainage-way - the slope to be given to cross-section are known. In erosion water velocity. channels. rate, makes it of cross-section protection Figure B.17 channel the gives the K value the flow where the main parameter the has been determined) the to calculate: a known drainage-way where the work, (where possible permissible rate flow cross-section and slope rate and slope; are and drainage-way to be controlled threshold known; velocities is the in drainagethese - 68 - Fig. Permissible B.17: Original Fine, surface materials non-colloidal sand Sandy, Loamy 3 Volcanic velocities in unlined Water transporting colloidal alluvium Water transporting coarse alluvium: sand, gravel, pebbles 0.45 0.75 0.45 0.60 colloidal mud 0.60 0.90 0.60 alluvium 0.60 1.05 0.60 loam 0.75 1.05 0.67 0.75 1.05 0.60 0.75 1.50 1.12 1.12 1.50 0.90 1.12 1.50 1.50 1.12 1.50 1.50 1.20 1.65 1.50 1.20 1.80 1.95 1.50 1.65 1.95 1.80 1.80 1.50 very colloidal Alluvial colloidal Mixture of colloidal gravel clay and pebbles; mud mud and pebbles and non-colloidal Pebbles and stones Schists and volcanic Table of permissible Types of vegetation crusts muds or plates velocities in grassed channels Permissible Clays Bermuda grass Kentucky blue - Cynodon grass Blue gramma grass gracilis Buchloe d?-tyloides dactylon - Poa pratensis velocities and loams (m/s) Sandy soils Good vegetation Moderate vegetation Good vegetation Moderate vegetation 2.40 1.59 1.50 0.99 1.65 1.11 1.20 0.81 1.65 1.11 1.20 0.81 1.65 1.11 1.20 0.81 0.75 0.51 0.45 0.45 - Boutelous - Medicago The means of sativa reducing drainage water velocity - Reduce the surface smoothness the roughness of the walls of cover. - Reduce the hydraulic radius R by increasing this may be done using by preference, for channels which are large and shallow. - Reduce course. the m/s) Clear water without debris 0.75 Mixture of loams non-colloidal Alfalfa (in 0.52 gravel Coarse channels loam ash Compact, drainage colloidal Muddy, colloidal Ordinary compact Fine water longitudinal slope are: characteristic K. This may be done by increasing the water course, e.g. by means of a vegetation by placing the figure for the water perimeter; waterways of equal cross-section, weirs transversely along the water - - 69 - types Main 2.5.3. These are a larger may lead which of the to an increase soil waterways if by giving erosion constructions the characteristics resultant These are watershed to protect roads, They are similar th-m do not inter- waterways. This erosion. advisable to make them suitable One may increase crop into the protection zones against a waterway. Ilaths, to the natural renewed is which improve for these carrying permissible water or by installing anti- or retention to protect tiracks, conservation sections. protection desi,gned into it bed a grass at critical and divert it undertaken, erosion. When soil to make constructions and cause is water-course Diversion, trenches 2.5.3.2. flow rate natural the run-off. and concentrate work without velocity run-off in collect may be necessary conservation their away the water naturally it undertaken, part Before the waterways waterways are being cept constructi.on Natural 2.5.3.1. works of run-off They also from upper include reaches ditches of designed etc. ditches, tiers, and terraces St-own in Standard Plan No. 9. 2.5.3.3. These the water are Grassed channels usually resectioned velocity does not They are slightly - the bank slope - the total - the channel 1s very is waterways the very small No. which i3) have been grassed to ensure that value. a very being depending Plan threshold and have slight, varies depth Standard exceed convex width (cf. flattened trapezcidal cross-section: 4/1-6/l; on the flow and varies rate; depending on the critical velocity and slope. Grassing natural grassing When the allowed is the bunds requires channel to grow. American as shown careful is not sufficient. is very large, When the "flood-way" in sowing the diagram climatic The grass be carefully a natural transverse in and favourable which must vegetation slope is of very the bed width conditions shrubs slight, is when supervised. and bushes another restricted can be system used by two parallel below. Parallel bvnds Natural "Flood-way" land - 70 - 2.6. Bank, channel and gully prOteCtiOn Bank cave-ins, meander formation and gullying may reduce the area of cropped Control Concave gully banks are undercut by water and gradually retreat. land. measures include the protection of the bank by natural vegetation or by various obstacles or by displacing the cutting power of the current towards the centre of The techniques employed vary considerably depending on the the water course. force of the water, the extent of the phenomenon and local resources. 2.6.1. Stabilising banks with vegetation The most simple approach is to allow natural fire and other it. against the ravages of cattle, vegetation deleterious t,o grow by protecting elements. Protection can be provided by fencing off the zone on each side of the gully. If the banks are too steep, The fenced zone should be 3-8 m wider than the gully. slope - a minimum of l/l earth-moving work may be necessary to provide a shallower or better still l/2, on which vegetation can obtain a footing. The most resistant and least demanding plants are what are generally called "weeds". They help followed by shrubs and bushes. These will appear the first. to prepare the soil and are Where humus loss is high, the process of vegetation development promoted by covering the soil with branches, straw and leaves. can be When natural. vegetation does not. suffice to cover the banks with sufficiently dense growth, a planting programme may be called for, using trees, bushes, creepers, being given to local species. brambles, etc., with preference Grassing over thr bdnks may ensure good protection. However, this requires well prepared seed and a relatively fertile soil. A variety such as Bermuda grass (cynodon dactylon) which flourishes under difficult conditions and stabilises the soil well, Inay be used. Use may also be made of trees such as willows and poplars in wet soil, false acacias in dry soil and privet, xiid blackberry and plum, elder, poplars, acacias, etc. A number of grassing techniques may be used; seed broad-casting; sowing or plants of selected species in furrows on t,he upper slopes (‘i/2); turves, placed in holes. Where possible, use should be made of seeds or plants taken from close by. Precautions should be taken to protect recently sown area against water eroslon. Wire-link fencing of 1 x 3 cm mesh will catch floating debris and ensure protection in this way. 2.6.2. t?-otection 2.6.2.1. of banks by construction -_I_- works Summary protection The methcds used vary considerably and depend mainly Examples are sncwn in Standard Plans Nos. 14, 15 and 16. on local resources. - These measures may include, and reeds out - alia: - branches - anchoring - made up of tree trunks “jacks” or “parrots” base by stones and retained by a chain; - pallisades stakes laid inter 71 intertwined steel by stakes; wire; splayed out, anchored at their fillin Protecting and wattling staking '2 6.2.2. with banks and anchored made from wood and wattling. Pebble L. on the Stonework prctection wi t I1 this technique, the bank is 1s iajd out at. random on the bank. a low bank by with pebble filling (Standard Plan reprofiled No. 1’7) to a slope of 273 or l/2 and stone The mean Jiameter of the stones should be calculated to ensure that they (Izba formula 1. The stone facing may be cannot be aarried away by t6.e current s.ij-1 .‘i? m d3ep and should be at l-east 1 .5 times the mean diameter of the stones ::r:et! . iGn e n t ii e b An k has recently been filled or where the soil is crumbly, a iiiter layer of gravel should be placed between the bank and the stonework. This 15-20 cm thick. layer 5 l-i5 u 1.A b P ‘~~.-Concrete- ;.c +t=L-, Einh water -, : Level Stonework Filter -*;Yank protection by stonework pf1 .. I@-7 - 72 - the a footing at low ater, l- 1.50 m deep and l-2 If the base of the bank is accessible stonework base in a trapezoidal trench is installed at m wide at the base. The advantage of the stonework is that it can be laid at random. It immedi-. ately combats any undermining which takes place at the base and thus ensures however, frequent refilling is required at least during the excellent protection; first year. 2.6.2.3. Bank protection Slope with fascines protection and stonework with fascines Fascines are used to produce boxes which are then filled with rocks. The fasrines which are iaid out along the contour lines are held in place by stakes made of freshly cut wocd which start off the shrub growth on the bank (see Standard Plan No. 16). -Steel wire Retaining stakes Pebbles Bed lined with interlaced reeds ' Bank lining 2.6.2.4. Bank protection using gabions (see Standard Gabions are wire mesh packages filled with stone Chapter E). They provide excellent bank protection, mould to the shape of the bed as and when undercutting When the fill behind the gabions is protection by a screen of reeds, rushes, Plan No. 17) and linked together in particular since occurs. it is advisable crumbly, etc., or a gravel filter. to provide (see they - 73 -. Bank protection 2.6.2.5. by masonry work This type of protection is seldom used for crop land since it is much more In addition, it is much less flexible expensive than random stonework or gabions. than the latter and more liable to undercutting. Where the water carries pebbles of over 0.10 m in size, the masonry work should be made up of blocks approximately 0.60 m in depth and the joints should filled with bituminous material. be Bank protection by groins (see Standard Plans Nos. 18, 19) 2.6.2.6. Groins are constructions, anchored to the bank, designed force of the water towards the centre of the water course. to divert the erosive the height of the groin is restricted to the section of water in In gullies, which alluvial drift occurs. The top of the groin rises in steps to the anchoring The groin is directed downstream at an angle of 10-50,’ to thee point on the bank. direction of flow. The length of the groins should not be more than to ensure that the water flow is not hindered. li3 or l/4 of the bed width The Standard The groins may be made of a variety of materials. and 19 give some examples of the most commonly used types of groin. - groins - stone rubble - gabion groins; - masonry work groins; - groins made from logs or fascines; with groins; a concrete superstructure. The gabion groins are simple in design but nevertheless extremely In addition, their construction requires larger ‘quantities of unskilled 2.7. 2.7-l. Plans Nos. lb These include: Correcting the slope of water effective. labour. courses Role of constructions These transverse constructions are intended to reduce the water energy regressive erosion. steeply sloping water courses and to control in The principle behind these constructions is to produce waterfalls by creating obstacles to the solid materials transported and to reduce the longitudinal slope between each fall and at the same time the water velocity. A distinction may be made between provisional constructions which are intended and permanent constructions. to allow the growth of stabilising vegetation, 2.7.2. Type of work 2.7.2.1. Stabilisation by vegetation The direct stabilisation of the longitudinal profile by means of VegetatiOn It can be carried is a system used in the US for small or medium-sized gullies. out as follows: (a) The shrubs are planted By shrub barriers planted across the flow line. there may be a row of stakes 30 cm close together in rows and, in addition, Tree barriers reduce the water flow velocity and allow loam to downstream. accumulate behind the dams. (b) By planting grass turves across the flow line This is an expensive head of small gullies. soil does not make direct grassing possible. where erosion starts at the process which can be used when The flow rates must be low. Small temporary dams (see Standard Plans Nos. 20 and 21) 2.7.2.2. 'These are intended the growth of vegetation to reduce the water upstream. retain velocity, fine soil and promote These works should: -- have a height - should be spaced relatively close a minimum (virtually zero slope); - be anchored - be supplemented by overflow collectors water during the period of use. -- of 30-45 cm; at an adequate depth to each other to reduce .the water speed to ,- in the base and banks of the gully; of adequate capacity to evacuate flood A very wide variety of materials are used for the construction of these small steel wire, rubble. In temporary construction.?., the materials dams: earth, piles, need not be as resistant as in permanent constructions and the construction need Damage caused by heavy flows of water can he reoairsd cdsily not be so precise. at law ,,ost. The ma&n types (a) of small temporary dams are: Darth dams A simple low-height (30-45 cm) earth dam is built across the g1.Y::: to maintain A transverse overflow gully earth and moisture at the bottom of the water course. It should be grassed rover to is laid out laterally to evacuate flood water. provide protection against erosion. These are suitable for small flow rates. (b) Branch weirs Weirs made of branches small floK rates. gully and they are adequate for In the case of small gullies (2-3 m wide), straw is packed at the base of the and held in place by branches fixed LU the banks with piles. It filled are easy and cheap to build is also possible to use a series with packed straw. For larger gullies, the arrangement of pi !es laid out across shown in Standard the gully Plan No. 21 and can be used. The water should flow over the cer.tr e of the weir which is kept lower than the sides to ensure that the water joes not 1'1:~ *round the outside and over the banks. - -75 - Transverse cross-section Pile (cj and straw weir Wire mesh weirs Wire mesh may be used instead of branches to retain the packed straw for the weir. (dj Dry-s tone weirs These are used for small and medium gullies in adequate quantities. are available on slight slopes anti when materials life lcnger than that of the other These constructions usually have a service They are also more flexible and can be more readily constructions mentioned above. that may occur below the weir. adapted to ciiarges in the land by filling up hollows Example The most soild wire Where the mesh. stones are those of a dry-stone made from stone are more irregular slabs or rounded, weir placed they edge to edge. may be held Dry-s+one weirs are usually less than 60 cm in height, but, under In such cases it is essential circumstances, may be as high as 1.00 m. The downstream footing that the f:sti.ng downstream is well protected. equal to at least 1.5 times the fall of Irater. in place by exceptional to ensure should be 76 The overflow be lo-20 cm below can be calculated This threshold. lip should be located in the centre of the water course and should The width of the overflow lip the maximum height of the weir. for flow over a solid from the maximum flow rate using the formula rectangular or trapezoidal shape. lip may have a dished, Witn rectangular overflow lip (e) _Cphion weirs for dry-stone weirs when the stone available The?r can be a good substitute Gabion check dams have good resistance to water flow on site 1s of poor quality. and have the same flexibility as dry stonework. superstructure Footings of gabions can be combined with a dry -stone dams to form a base so that the height of the fall can be increased. Small 2.7.2.3. permanent Structures of this type are built than those mentioned above and in view should be taken .in their construction. longitudinal from materials of their higher The factors used are into entering (1) the size 12) the stability (3) infiltration fissures is structures this is have a slope the deciding slope is too steep, the number of structures will be excessive and, secondary check dams can be installed between the main structures, the spacing. The main materials It cross-section which are more resistant cost, special precautions The principle is to ensure that the bays between the on which the water wil.i not build up an erosive velocity; factor in spacing the structures. -- small weirs These are intended to permanently stabilise the of a gully when vegetation growth is not adequate. When the in this case, thus increasing in of the dry the stone, masonry, calculation gabions and design and concrete. of such structures are: spillway; of the structure; of water downstream. therefore - the peak flows - the nature structure. below necessary the foundation which to have data to be evacuated of the structure for leaks and on: throughout land may create the a depth structure's life; of at least the height of the - 77 - If there be constructed is an impermeable on this. in other cases, trickles of water. Plan. layer in the subsoil, the dam foundations sufficiently deep trenches should Examples of this type of structure be constructed to divert are shown in the Standard In this type of dam, the flow occurs over the crest or trapezoidal spillways. of dished, curved, rectangular (a) Spill structures (sills, These are intended to halt event of a natural ledge. guide of the structure Small earthwork by means channels) regressive erosion in a channel or gully these structures may be temporary In the case of small channels, In larger gullies or channels, they will dry stonework or branches. masonry or concrete. with a chute, made from gabions, (b) should in tht and made from be structures dams Establishment of water reservoirs behind small earthwork ?,ams can help in controlling gully erosion by halting water courses near to their point of origin. These are merely small dams with a height of no more than 3 m and a reservoir The design of larger dams is beyond capacity of several thousand cubic mebres. the scope of this document and reference should be made to specialised manuals. The operating principle of these structures is not erosion protection creation of a temporary or permanent water reservoir for use in irrigation, watering of cattle or fire fighting. The construction following data: of small dams requires consideration to be taken but the the of the --- rainfall patterns; - vegetation the nature of the feeder zones: surface, soils to determine run-off coefficients and discharge - the size - the characteristics (drill samples). of solid flow which may fill of the land for the retention cover, nature volumes; of the basilIs; the dam foundations: depth, permeability --- The dam should he located In a nsck of the valley downstream from a hollow The ratio between the reservoir capacity order to minimise the dam dimensions. and the dam volume should be at least 3. The dam comprise; a barrier, a s.pLllway and intake In the case of an earthworks dam approximately construction specifications are as follows: and discharge 3 m high, structures. the most common in - - slope of upstream bank: - slope sf downstream - height of dam crest - height of the dam crest - crest - stripping of earth all the topsoil; 70 - - l/2-1/3; --- -- bank: l/2; --v--e- above highest --.p____ water ________ 0.30 m; (freeboard): ----0.80 m; water level: level above the normal -- 1.20 m minimum; width: over the foundation to a depth of 0.20-0.30 m to remove ---.building of a trench 1.50 m wide along the dam axis down to the impermeable This trench will be filled with compacted impermeable earth; substratum. -------_ -/pill of impermeable clay earth containing no more than 30-40 per cent clay. The fill is laid out in thin layers of 15 cm thick and compacted; -a-the upstream wall of the dam is protected against wave impact by grassing or, a lining of gravel or dry stonework. if this is not possible, -- The size of the spillway should be calculated to evacuate floods of ten-year This may account for a major part of the total dam cost. The most intervals. simple are natural grassed spillways which must be sufficiently large, shallow Where construction and of gentle slope to evacuate flood water at moderate speed. the cross-section usually employed is trapezoidal with work requires excavation, a slope of I/4. Where adequate grassing cannot be obtained or the discharge to be evacuated is too large, it will be necessary to construct an artificial spillway in Such constructions may' not be justifiable for gabions, masonry or concrete. small dams. A drain channel is provided for exploitation made of a concrete tube held in place by concrete undermining. of the water and this cbllars and protected is usually against The sluice pipe is used to evacuate small quantities of excess water without it being necessary for them to flow over the main spillway. It is similar in design to the drain channel. 2.7.3. river Principles dimensions - of calculating Given below are a few simple structures (weirs, dams). 2.7.3.1. Calculating Two cases should "dry". structure formulae for spillway discharge be considered depending calculating on whether the dimensions the sill is "wet" of small or - 79 - Dry sill (a) It is assumed that The flow is the sill calculated is by the dry when: following general formula: IQ E m J;;;.H3'".Ll in which: Q = m = H = L q the spillway flow in m3/s the coefficient depending on the shape of the the hejght in m from the crest of the spiilway length of spillway in metres. the In practice, - profiled - thin wall sill water spill): - thick water (b) is following spillway sill: q 0.46; wall is less than sill (the thickness m = 0.38. spill): of the sill is greater the thickness or equal of the to that of the Wet sill + 0.2 is applied -g) 3 d- to the preceding = : = = Length 2.7.3.2. is possible The length of the stilling to use the of the stilling This formula. coefficient ; coefficient for dry sill total height of water upstream from the sill the height of the sill the difference in level between the upstream the spillway H = height of the head of water on the spiilway. It sill m: of the ml c m (1.05 m D S Z m are used for (thickness m = 0.40; A reduction coefficient calculated as follows: where values spillway and downstream edges basin Rehbok, basin Schoklitsh and MCD formulae. must be one or two times the height of the chute. 2.7.3.X. It is classification Face slopes for earthwork dam possible to empirically height) (slope =baSe- of a small adopt the slope values given by Terzaghi's - 80 - Types Upstream of soil Bomogeneous soil of wide-ranging particle sizes Homogeneous coarse silt Homogeneous silty clay Sand and gravel with clay core 2.7.3.4. Protection against under the dam 2/T 113 215 113 water Lane's rule can be used to check a danger of leakage. LV + Lh 3 slope Downstream l/2 512 l/2 215 seepage that seepage under the dam does not present m.h In Lv Lh h which: = the length of the vertical path = the length of the horizontal path = the difference in head between the upstream and downstream the dam the values for which are given below: m : Lane's coefficient, Type of foundation Value Very fine sand Fine sand Medium sand Coarse sand Fine gravel Medium gravel Coarse gravel Gravel and sand Medium clay Compact clay Hard clay 8.5 7.0 6.0 5.0 4.0 3.5 3.0 2.5 2.0 Gully correction slope of m 1.8 1 .lj structures in Cape Verde sides of - 81 - 8.3. WIND EROSION PROTECTION TECHNIQUES Dune stabilisation Dunes are large sandy areas which can be moved by the wind when they are not When moved by the wind, these dunes can cover agristabilised by vegetation. block traffic routes, damage housing and cultural zones, render them sterile, result in the abandonment of entire regions. when carried by the wind at soil level, forms dunes when confronted with branches, hedges. These move in the direction of the dominant various obstacles: wind and may take on a typical crescentic shape with their tips pointing downwind with a gentle slope facing towards the wind and a much steeper slope facing away from the wind. There are two major categories - Maritime dunes - are found sandy coasts with regular - Continental destruction of dune: maritime dunes and continental to some degree or other throughout winds blowing in from the sea. dulies form in arid by cattle grazing. regions Conventional techniques -- for maritime dunes and are often the world the result dunes. on low of vegetation stabilising - The most widely used technique was developed in Europe in the nineteenth century and was used successfully to reafforest the Gascogny Landes in France with pinus pinaster. The technique comprises three main phases: - the creation - stabilisation - reafforestation of the stabilised is highly labour intensive. (a) of an artificial "coastal of the dunes behind Creation of the coastal strip"; the coastal dunes. strip; This dune stabilisation technique strip The objective is to reduce the quantities of sand carried and deposited by the wind by creating a slope which would form an obstacle to the progression of sand particles. The first above the high wooden stakes wooden stakes or sheet piling; cases. step in establishing the coastal bar is to lay out along the beach water mark, a wattling between 0.75 and 1.00 m high made out of Where sunk into the sand and intertwined with close branches. it is also possible to use sheets of fibro-cement are not available, however, this is expensive and can be envisaged only in special When the mound of sand that has accumulated behind the wattling has reached a height of 0.50-O-75 m a second wattle barrier is placed on top of the first and is continued until an equilibrium profile has been obtained, i.e. until the This may be reached within a sand grains can no longer pass over the obstacle. few years with a slope of 30-140 per cent and a height of s;;::zi:rtcly !C m. - 82 - When the dominant wind is not perpendicular to the beach, groins constructed in the same way are run out from the main wattle fence intended to halt the movement If a single coastal bar is not sufficient, a number can of sand along the barrier. be built parallel to each other. (b) Dune stabilisation This is intended to create conditions which are favourable This may be carried out with hardy perennial reafforestation. fences or by covering the soil with branches. Cover plants must ensure gooc soil They can be sown to burial by sand. of plants should be resistant to both be carried away by the wind and it is or protect the seeds by covering them cuttings often gives the best results. Some of arenaria in pescaprae in tenusissima, for subsequent plants, wattle coverage, have rapid growth and be resistant rows The first or propagated by cuttings. Seeds may the wind and wind-borne sea salt. advisable to sow during the least windy season Propagation by with straw and branches. the species that have been used with success are marram grass (ammophila a member of the convolvulaceae family, ipomea Europe and North America), Madagascar and, in the North Cameroon, stylosanthes gracilis, melinis digitaria unifloris, cynodon dactylon, pennisetum clandestinum. Rows of small wattle windbreaks made of cut branches, bamboo, palm leaves, These small windcover is inadequate. reeds, etc., are used when the vegetative Depending breaks of 0.5-2 m high are laid out in a network of 2-40 m in dimension. on the situation, these windbreaks can also be made up of plants such as saccharum as in Tunisia. aegyptiacum, Another process which can be combined with sand with dead branches or other plant debris. (c) the preceding one, is to cover the Reafforestation The reafforestation of stabilised dunes is carried out by means of nursery The planting techniques are the same as those described in grown plants. The species used should be resistant to the effects of wind and section 8.1. salt and one cannot expect trees to grow suitably in a strip 200 m wide from the a network 1 x 1 m on the wind The plants should be close together: coastal bar. exposed side and 2 x 2 m on the sheltered side. In an arid compete closely climate, grasses which have been planted with the young plants fcr water. In certain very favourable climates tures, it may be possible to plant trees preparation. 3.1.2. Techniques continental for the stabilisation dunes with long rainy directly without to stabilise dunes will periods and hig:l temperaany other method of of The principles of stabilisation may be the same as those used for maritime dunes but the climatic conditions in arid zones do not always make it possible use them; this is the case in the Sahara, for example. to - The procedures preventive together - fascines - straw covering - planting 3.1.3. that with 83 - can be used to reduce soil pasture and track control. (palm leaves exposure to the wind are mainly The other procedures used are: in the Sahara); and grassing; of windbreaks. Rune stabilisation by a coating bituminous products 0:‘ This is a modern technique which has been used for afforestation Kuwait, India, Pakistan and, recently, in Libya and Tunisia. in the US, A bituminous emulsion is spread over the sand surface and penetrates to a depth When the emulsion dries it forms a crust which provides complete protecof 2-3 cm. The emulsion is spread mechanically: a bulldozer-drawn tion against the wind. It seems that. In Libya, each vehicle covered 4 ha per day. tank and a spray gun. eucalyptus) if the product the substance has toxic effects on certain trees (acacia, If it is applied before planting, planting work seriously is applied after planting. damages the protective layer and reduces its effects. This technique planting procedures. 3.2. 3.2.1. requires only small amounts of labour, except for conventional crop land protection Windbreaks The objective of these is to reduce wind speed to less than 18-25 km/h at which The windbreaks are composed of trees and level the wind loses its erosive effect. shrubs. The distance protected is proportional is blowing perpendicular to the windbreak, of up to 20 times the windbreak height. to the windbreak height. the wind speed is reduced Wide windbreaks are not necessarily more effective The best results are obtained when the windbreak height as its width. When the wind over a distance than narrow windbreaks. is approximately the same windbreaks reduce vaporisation. By reducing wind speed and increasing humidity, They may create a Dense windbreaks are more effective from this point of view. favourable environment for afforestation in a semi-arid zone. An example of a dense windbreak used in the US uses a number of different species in order to create a dense foliage over the total height of the windbreak. The central zone is made up of Shrubs are planted densely on the windward side. tall trees and the intermediate zones contain trees which are somewhat less tall. The whole has a triangular cross-section which forms a dense vegetation barrier over its total height. 3.2.2. Other These are cropping techniques resources and employ little labour. which are carried Examples are: out with normal farming 3se of cover - leaving - parallel-strip - tillage which maintains too t'ine a tilth; - green manuring. 3.3. Pasture crops in rotation - crop residues or in fallow in place land; as long as possible after harvesting; cropping; the soil in clods, by avoiding tools which produce prote5iion This is a problem which is encountered particularly in extensive semi-arid Gcats and sheep are the wind's best helpers regions such as the Sahelian regions. in preparing land for wind erosion. Control is mainly by limited grazing and ba.lning certain zones. - 85 - CHAPTER C REHABILITATION TECHNIQUES FOR WATERLOGGED SOILS c.1. AIMS OF DRAINAGE The aim of drainage is to evacuate water in excess of vegetation and soil Certain soils have a natural drainage system others do not; requirements. these latter require the installation of an artificial drainage system. Drainage creates in the soil the conditions required for good plant development It improves the soil's physical and promotes soil aeration and root penetration. qualities and makes it possible to reclaim zones which were previously considered unsuitable for cultivation. In irrigated zones, drainage is linked to leaching techniques to prevent accumulation of toxic salts in the soil and also to prevent gullying and soil erosion. Drainage techniques are suitable the good of the community. c-2. 2.1. for labout-intensive work in activities for DIFFERENT TYPES OF DRAINAGE Open ditches They are suitable for a wide These are the most simple and most widely used. range of flow rates with very slight or zero slopes provided they are of adequate cross- section. Tney are most effective from the drainage point of view since they intercept Their construction and maintenance are both ground water and surface run-off. highly labour-intensive when no machines are evailaole. Their main disadvantages are their take up on crop land and the hindrance animals and men. Where the drainage between ditches, the land loss and the will be given to underground drainage. low density networks and when they are size in view of the large area that they they cause to the movement of machines and requirement entail= only short distances hindrance become excessive and preference Open ditches are therefore used mainly for also required to evacuate surface water. Channels are a type of small open ditch or gully designed to evacuate surface water. on pasture land or when the agri-.Channel networks are usual y installed The cultural value of t!ie land does not justify the installation of underdrains. - 86 - channels are usually 40-50 cm deep, O-C.20 m wide at the They can be dug by hand or by machine. at the surface. they require relatively frequent maintenance. effective, base and around 1 m wide If they are to remain to m 0 to 0.20 Channel Farming techniques can achieve wide and 0.50 m high are constructed a ditch at the base of the slope to 2.2. buried drain Beds 8-30 m the same objectives as channels. or furrows and ridges cut down the slope with collect the water. Underdrains These are collectors in the soil. made of drainpipes or They can be interconnected a drainage ditch or into surface water directly. a natural other piping material, which are between each other and run into They cannot be used to evacuate collector. This type of drainage is expensive and is suitable only for soils with a It does however have the advantage of requiring minimum high commercial yield. maintenance and to have a long service life if it is correctly installed. Another advantage is that the land over the drain is free for cultivation. A very wide range of materials are used for under-drain construction. Certain are traditional materials and can be of interest in regions where modern drainage materials are difficult to obtain or expensive. The main 2.2.1. types Fascine of underdrains are: drains A trench is fascines leaving trench backfill. dug to the required dimensions, a space for water drainage. the bottom The fascines is filled with are covered with Backfill / Fascine 2.2.2. Stone A cavity slab is drain drains produced using stone slabs placed at the bottom of a trench. the - 87 - Drains 2.2.3. with stone backfill These drains are made with round stones They clog up easily. the gaps between them. so that the water can run off through Wooden box drains 2.2.4. They are made using a wooden These are suitable for soft and marshy land. They are used only box structure with an internal dimension of 7 cm or more. rarely. Peat drains 2.2.5. These are used in peaty soils in which the peat itself is used to form the drain. 2.2.6. Earthenware piping drains These have been used since the beginning of the nineteenth century. They comprise fired clay pipes between 5 and 15 cm in diameter and usually 30 cm in length. have flat ends and consequently they may be moved out of alignment Also, they usually To overcome this disadvantage, if the earth settles. interlocking drain pipes have Fired clay pipes are manufactured in brickworks been produced in the Netherlands. They should be of very good quality if breakage is to be in the same way as tiles. avoided during laying and if they are not to crumble under the effect of water. All defective drains should be eliminated before laying. 2.2.7. similar Concrete drainpipes These are used where fired dimensions. clay pipes are difficult to produce. They are of Concrete drainpipes ar e manufactured using special machinery and drainpipe quality depends on the method of manufacture and the quality of the aggregates used. These aggregates should be permeable to snsu-e good drainpipe porosity; however, the pores are rapidly blocked by fine soil particles in the water. In the concrete is attacked by sulphates in the water. acid and peaty soils, 2.2.8. Bituminous fibre drainpipes These drainpipes are made of fibre impregnated with bitumen and moulded under They are pressure. Holes or slots are made in the pipe to allow water to enter. lighter than clay or concrete pipes and are more resistant. 2.2.9. tural Plastic drainpipes The use of plastic drainpipes underdrainage work. is becoming widespread in all types of agricul- They are made of rigid or flexible non-plasticised polyvinylchloride (PVC). There are slots at regular intervals in the pipe walls to allow the passage of' water. The diameters used are between 30 mm and 100 mm. The characteristics of rigjd PVC drainpipes are shown in fig. C.l. - 88 - C.l: -Chara,,:.‘ristics .._-_ Perforated PVC drainpipe _--- Fig. of r;gid PVC drainpipes --_ Exterior diameter in mm Weight in kg Wall thickness in mm 32 40 50 63 0.110 0.138 0.195 0.273 0.8 0.8 0.9 1 .o Rigid PVC drainpipes are produced in 6 m ?engths and, in special cases, The pipes have slits of 35-40 mm long and 0.5-O-8 mm wide perpendicular 9 m lengths. The of 20-50 mm depending on the type of s'oil. to the pipes' axes and at intervals The drainpipes are joined together by a sleeve. slits are staggered. Slit: Rigid length 40 mm width 0.5 mm PVC drainpipe Flexible PVC drainpipes are supplied in ZOO-250 m lengths on a drum and are The pipe itself is corrugated, which ensures intended for mechanical laying. The main characteristics greater resistance to crushing and improved flexibility. are as follows: 0.7-1.0 mm Wall thickness: Connection by T's, sleeves, etc. 1.5 mm diameter Perforations - circular: 1 x 1.5 - 3 mm No. of perforations/m of drain: 600-700 Surface area of perforations/m of drain: or rectangular lo-25 cm'. These drainpipes are easily blocked by loam and roots; they are also vulnerable to clogging by ferric hydroxide. During storage before laying they should be protected from sun and weather which causes ageing of the plastic and a deterioration of the mechanical characteristics. 2.3. Mole drainage Mole drainage is used in plastic and low permeability soils. The technique consists in digging a drain in the soil, without any external support whatever, using a ripper blade fitted with a "mole" - a pointed cylindrical metal tool - which digs a circular drain 0.05-0.12 m in diameter at a depth of 0.60-0.80 m. These mole holes act as drains but must be closely spaced. They require very special conditions: the soi: must be sufficiently plastic, the clay content should - 89 - not exceed 30-35 per cent and at the moment they are dug the water content should not be greater than the plasticity limit (ATTERBERG's Plasticity Index). The aggregates should have excellent stability (stability index lower than 1). Th%erequirements for mole drainage are: -- - a drain length of less than - a slope between 0.2 and 5 per cent. Mole drainage costs very (from three to ten years). 2.4. 80 m between two outlets; little but it -has a relatively short service life -Subsoiling Subsoiling is used to mechanically break which reduce water penetration into the soil. up hard pans or ploughing compaction Subsoili;lg rapidly changes hydrodynamic characteristics, in particular soil's hydraulic conductivity and makes it suitable for underdraining. It root penetration. loosens and aerates the soil which promotes better c-3. 3.1. the also DETERMINING THE CHARACTERISTICS OF A DRAINACE NETWORK Water movement in a drained-- soil Water movement in soil It is presumed that is not drained naturally. is shown diagramatically the soil is permeable, in fig. isotropic C-2. and the deep water Between two rows of drainpipes, the deep water table is convex in shape, height of the table being at its maximum halfway between the two rows. A drop of water which infiltrates the soil's surface, into the non-saturated soil under the effect of gravity. table, it is no longer subject only to gravity and it will the drain. Fig. C.2 shows diagramatically their distance from the drain. the route taken table the first descends vertically When it reaches the water flow in the direction of by water streams (iepending on - Fig. C-2: Water movement in drained --Input *.-. 90 - soil (Precipitation or irrigation) ._. Permeable laye Impermeable layer - The water drop 1 which falls directly gravity and descends vertically until - Water drop 2 reaches drain after travelling --- - above the drain is subject it reaches the drain. only to the water a short table clrse to the drain and finally reaches the distant: in the saturated soil. -. travelled in the water table increases. For water drops 3 and 4, the distance Following predominantly vertica:L movement in the upper part of the water the liquid-flows curve towards the drain and may finally reach it via table, an upward movement. -----Water drop 5 lands the furthest from the drain and its route first descends it then moves and ther curves when the impermeable layer is reached; horizontally and the final section ascends towards the drain. On the other side of the yy' axis, the situation is symmetrical. _-_--_-- When the depth of the permeable layer is small in relationship to the distance in the segment of soil above the between the drains, the flow diagram shows tnat, below the drain level the movement movement is predominantly vertical; drain level, The flow diagram differs depending on the depth of is predominantly horizontal. the impermeable layer; if this is at the drain level or only slightly below it, If the of the drain will predominate. horizontal movemen; in the direction of vertical movement will be impermeable layar is at greater depth, the proportion greater The shape of the water table surface between two drains varies depending and the permeability and thickon the input rate, the distance between the drains, ness of the permeable layer. When the input rate is zero, the water table surface will tend towards the It may also descend below drain horizontal to reach an equilibrium at drain depth. If the shape of the water table surface is level as a res,Jlt of capillary losses. it will tend to come closer to the soil's surface the more the distance more convex, betweeli the drains is increased or if the thickness or permeability of the draining layer is liecreased - The shape dependent Fig. the the the the of water the table 91 - surface oetween two drains is, therefore, upon: input rate q layer depth of the permeable of the drained permeability distance between the drains D; layer K; L. C.3. The optimum level of the water table (p-h) is the basic. datum in drainage problems and it is dependent on crop requirements. The other data are input rate flow rate" or "characteristic q, the depth D and the permeability K of the drained layer. The interval 3.2. 3.2.1. between Determination of Causes wetness of the basic drains is determined on the (a) (b) Cc) (d) (e) causes of soil of these parameters. data Before trying to solve a problem of soil waterlogging, mine the causes of the wetr.ess. These causes may differ should be adapted to each :ase. The main basis wetness it is necessary to deterand ;.he control measures are: basins above the zone in question. water from catchment it is necessary to intercept this run-off water and divert or man-made collectors (cf. Chapter B); ~-------river water overflowing into an allub.ial plain. In such a case, it is necessary to protect the zone by embankments or to dig drainage ditches to evacuate the flood water in a minimum time compatible with the type of crop; inflow of run-off In such a case, it into rlatural a high underground conditions; water table due to specific topographical or geological --. a high level of rainfall producing excessive rise in the ground water level In an or the development of saturated horizons in low permeability soils. arid zone, the high level of irrigation required for soil leaching may drainage techniques In this case, conventional produce the same effects. are used; inadequate capacity of such a case, increasing the problem. natural collectors. the size of the This collector may trap the water and, in may be sufficient to solve 1 - 92 - Optimal 3.2.2. water table level The ability of the top layer of plants and for good agricultural slose to the surface of the soil, which is adequate too low capillary may also have water supply of soil to dry out is essential for gaod rooting If the water table is permanently too yield. good rooting will not be possible. a water table an unfavourable during the dry effect season. since it will not provide s rooting water table varies depending on the plant In a light soil, the water table may be hi gher than The optimal depth of the depth and the soil texture. a heavy soil in which capillary supply is in greater. In the case of leguminous crops for which the roots do not penetrate deeply, (30-60 cm) to ensure the water table should be relatively close to the soil surface This is also the case for grasslands. optimum yield. Cereals require Fruit species. trees a water require Permissible 3.2.3. table a lower :;ubmersion Crop submersion between water 60 and table of 80 cm below the soil's surface. m or more depending l-l.50 on the time covers: - partial submersion which - total plant submersion stalks. when the Total submersion may cause Crop damage caused affects the plant's root water level rises above greater by submersion damage than depends system; soil level partial and affects the submersion. on: -.---.---- - submersion - the point - the type 7-15 time; at which of plant submersion being occurs days loss. Fruit siderably trees are highly depending on the In project retards planning, growth cycle; development. Submersion of times of l-2 months prior to the vegetative submersion for 15 days may reduce yield by produce is very sensitive in a reduced yield which are particularly Maximum loss plant's -- Grassland may withstand submersion During the vegetative period, 30-50 per cent. period. Cereals formation. submersion. the grown. In general, submersion of l-3 days may result in total crop Market garden mersion may result in to will submersion and even one day's subvary depending on the species. sensitive to submersion curing flowering and grain may be as high as 20 per cent following a three-day sensitive species. to submersion one may use the following although permissible the results submersion vary con- times: - The figure Fig. below Effects of the C-4: 93 some figures gives - for of submersion on yields optimum harvest) the effect (maximum of loss submersion in on yield: percentage -- Submersion time 3 days 7 days 15 days Grassland Autumn cereals Spring cereals Maize Perennial fodder Potatoes Sugar beet 20 20 20 10 50 50 100 IO 20 50 40 80 40 100 50 Sunflower 10 40 - 3.2.4. Type of soil Permeabiliuy is significant parameter porosity on texture, A number of these are: - the of - the the ability of water to drain down into for calculating a drainage network. and organic-matter content. formulae relate permeabi .lity 100 100 100 100 the Soil It soil. permeability is a depend The mos t important to soil texture. are same diameter SCHLITCHTER formula soil particles the D; the value HAZEN formula I Km/s D,O is of the the soil diameter which permits made up of particles Use of however, it the will formulae will ensure not give a sufficiently tion 100 - permeabix- This applies when all the R is related to the porosity; of 100 Direct measurements of permeability This can be done in a laboratory The most simple soil in situ. called the "auger-hole method"; = 0.01 Dfo 1 cent passage of 10 per less than DlO. the correct accurate is using whole and reliable the principle by weight order of magnitude of result for practical preferable to the samples or by the method is that of is as follows: use of of the por- the results; purposes. formulae. direct measurement on ERNST which is also - 94 A hole layer is made in or a depth the soil with - a 4-5 cm auger down as far as the impermeable of 2 m. As much water as possible to reduce the ground water water rises in the hole is is removed level by at measured at from least 5-10 the hole 40 cm. s intervals. by means of a ladle The speed at which so as the ground Where: H is the depth of the hole under the ground water level in cm S the depth of the impermeable layer below the bottom of the hole in cm r the diameter of the hole in cm' of the water Y the mean distance between ground water leve 1 and the level Y over a period of time T K the hydraulic conductivity in m/day. The hydraulic (a) K=( (b) K- Fig. C.5: H+20 conductivity 4 000 r2 r ) (2 - Y w 3 600 r2 (H + 10 r) (2 - Y R ERNST's is '1 Y) 'At X calculated if gif S> S as follows: l/2 q H 0 or if S ( l/2 H Y) method Measuri .n stick Depth Water-bearing Impermeable 3.2.5. __ Intrinsic This flow rate of --.-.-.- indicator land land network IS the flow that the drainage relation to a unit of the land's surface mined by the following equation: network area. must collec+, The intrinsic and evacuate flow rate in is deter- - where sible time 3.3- 95 - qc = the intrinsic flow re;e in llsiha of evaporation (a dimensionless e = the coefficient i = critical rainfall intensity in mm/h. The critical rainfall submersion time for T depend on agricultural is the rainfall a recurrent time and eccnomic -theage Network 3.3.1. number lower for a time 6 corresponding The values for time T. factors. than 1) to the permis63 and recurrence network design The design and layout of the drainage network depend primarily on topographical The first task is to locate the thalweg (middle line of a river) and the factors. The main collector drains should be located along Che thalweg. crest lines. Minor drains should empty into the main collector drains withcut ever crossing a crest line. The minor slope, slight drains Fig. drains should be laid out at angles i.e. more or less parallel to the contour lines. (less than 0.003), they may be laid out parallel angles to the ploughing are also p 1 aced n: right to the lines of greatest Where the slope is very to the slope. The minor direction (see fig. c.6). c.6 The collectors main collectors the secondary are always are situated thalwegs. in laid the out along the main thalwegs The layout of drains and collector!drain and should therefore be adapted terrain, layouts are shown in fig. C-7. tne right line of greatest and the secondary combinations may vary to each specific case. The slope. collectors in depending on Some typical - 96 - Fig. C.7: Layout of a drainage network Intermediate Transverse drainage Run-off Intermediate collectors water ~~zzJ$E+~~ Collector 5elt drains (protection against external run-off water) 3.3.2. Drain Drainage land of variable Draining of waterlogged permeability pocket depth The depth 2nd spacing of drains are two closely linked parameters. As d .ain 2"5r !I is increased, spacing between drains may also be increased. In homogeneous land, deep drainage has a certain number of advantages over superficial drainage since: - 97 - - the - drains number - drains them. table is lowered giving further, may be more widely spaced, and drainage cost; Iare in this way protected hand, deep drains season. On the other in the dry table the water the permeable soil not to put the drain of the two layers. the into overlies the results in against root irvasion tend to cause study a very impermeable a too should impermeable layer but If the impermeable land lies above a permeable drain as deeply as possible in the subsoil. For practical A depth of 0.60-0.80 as deep drainage. On very drain purposes, m is considered depth varies small; aeration; a reduction which rapid of be carried locate subsoil, between 0.70 a depth of over water to determine is at to clog the out it it drain tends fall subsoil, to in preferable the one should borderline try to locate m and 1.50 m. 1.20 m is considered land, the drain depth will depend on drainage slope It may be necessary to have the drain shallow at the and the depth of the outlet. start and deeper as it approaches the outlet to ensure a sufficient slope for the water to drain away effectively. 3.3.3. Drain 3.3.3.1. put: slightly soil which a pedological In heterogeneous land, most suitable drain depth. If better spacing Selection A distinction the permanent A large sloping number for the permanent Europe and anywhere In irrigated drain water input regime formulae. of methods of calculation should be made between two regimes regime and the transitory regime. of drain of water formulae have been developed for calculating These presuppose constant input. regime. They else where rain is of long duration zones and in regions of high intensity is not constant and it is therefore and low input drain and out- spacing may be used intensity. in and short duration rainfall, necessary to use transitory Calculating the spacing between drains is easier for the permanent regime than use of the permanent regime formulae is often moreover, for the transitory regime; especially when precise knowledge about justifiable even in the transitory regime, food conditions and hydrological constants is not available. - 98 - 3.3.3.2. (aj Calculating drain spacing the permanent regime in The HOOGHOUDTformula Drain spacing is given by the I L2 fcllowing formula: = 8 K2 d h + 4 K, h2 9 4 I where-: L = q = K2 z K1 = h d the spacing between drains in m the intrinsic flow rate in m/days or m3/m2 of the drained zone the hydraulic conductivity of the layer below the drain in m/day the hydraulic conductivity of the layer situated above the drain in m/day = the height of the water table above the drain level halfway between the two drains (in m) = the depth of the equivalent layer. A value which is a function of spacing between the drains L, the drain radius r and the depth D of the impermeable layer be;ow the drain (see Fig. c-8). Use of this formula layers is at drain level, carried out by successive L is already known. Fig. C-8: Values of d in presupposes that the boundary between the two permeable which is not always checked. The calculation is approximations, with d not being known accurately until the HOOGHOUDT formula D 049 --0.49 0.49 149 0.10 0.m aho, D UmQ%-- a71 0.60 0.63 a69 0.71 a73 074 (171 a75 a75 a76 a.79 1.~067 (~7s am on 0.89 a91 0.91 4x94 au a9d a96 1.n am au 049 I.(1D I.03 1.0s 1.12 1.1) I.14 I.14 I.15 l.n 2¶0 1.75 N I.01 I.YJ I.41 I.11 I.34 I.59 I.U I.? 1.u I.63 I.43 I.67 IN 1.71 IJO I.73 I.11 1.n 1.11 1.n 1.M Q7I (LPI I.14 I.9 the the I.5 1.49 I.99 1.m 1.n I.1 1.91 I.97 202 2a 215 229 I.57 I.U I.Y 1.10 I.69 1.76 I.91 I.94 l.W In I.Y 1.99 i.5l I.% 203 211 1.97 20) 216 227 201 119 2% 213 211 2% 2SJ 241 217 211 244 254 222 2s7 251 2u 231 250 263 m 2Y 2% 27129s 241 2b5 2M 100 2U2792922ln224 234 291 1.01 3.24 149 224 ZJI I.72 196 202 211 229 242 234 zm 2.71 2.17 ID1 I.15 24J 191 1.U 2% 1.U I QPI a97 a.* 099 Q99 Q'H 2 1.n l.u, 1.11 I.90 I.92 I.94 1 aA9 249 296.2722w a.11 c 7 I 9 10 12.9 II 17.3 m a U la 24) 2% I.64 274 I >.I 4.14 cu 4.57 4.74 SDl 4.n 4.u 4.95 s.23 I.47 J.R 6'10 a2 I.44 6rn 643 7.M 497 xs7 ait 6.63 70) 1.Q 3.1s %,I LU 7.00 733 1.a a.84 9.64 3.m 6n7.n Ia4 5.30 644 n.ao 9.47 660 LY 9.97 11.1 - (b) 99 - The ERNST formula The general equation is as follows: where: = the height of the water table above the level of the between the two drains in m flow rate in m/day q = the intrinsic = the thickness of the saturated layer above the drain DV of the thickness multiplied KD = K, D, + K2 D2 = the product permeability of the various layers in m2/day h the D2 D, = the q thickness mean flow Ln Do/ n K u = thickness DO calculated U the = wettened Depending layers of q of the section Rr = radial of the layer perimeter of on the different lower layer in m of the upper layer with resistance which is a function for the radial the which drains in m by the permeability of halfway K, drain resistance position has been drain. position of the permeabilities, drain the in relation to the boundaries between formulae to be used are as follows: 2 h=B~+$+,n$ Kl Homogeneous soil Do < $ L Drain at the boundarv of two lavers different Permeability Kl& K2 9L2 (KlDl + K2D2) + & K14K2 2 DV h=~+$&+&&r+ h=R KS >) K2 + Hooghoudt formula Ln " Drain in the upper layer h q Dv 9L2 9L gf?i- + B(KIDI + ~2~2) + rrK1 In $ of Calculating the transitory 3.3.3.3. - 100 drain spacing regime in - GLOVER-DUMM formula (ai - t/j ht = ho x 1.16 e if $ < 0.80 0 j = VL2 10 KD D = Do + ho + ht 4 L2= ~71.16 2) where: J V hO ht t = reservoir coefficient = the effective porosity = the the = the q load load time above the above the expressed The effective porosity permeability expressed in (b) This as follows: BOUSSINESQ formula applies when the (in in days) per cent drain at drain at in days. V is given times times t t approximately by the square permeable layer root of the cm (1903) drain is located on the and is expressed - 101 - Drain 3.3.4. diameter The choice and the amount Drain drain and length of drain of water diameter increased with is diameter and iength to be evacuated. usually drain constant length, The flow rate drained multiplied by the intrinsic is i.e. Where the length to be drained Large diameter urains inildequate. length may be longer than for a function over the the surface equal to the flow rate. Q = S.qc is = L.E. length whole area product of flow flow in this surface area drained becomes too may therefore small rate and the to be drained large, drain capacity becomes be used; in this cast the drain but the cost is also higher. drains, diameter in drains are to select the type that can be used.) between and collectors drain Collectors 5 and 8 cm. depending of on gradient diameter. Calculation 3.3.5. the plot .qc The commonly used diameters of underdrains however may have diameters up to 30 cm. C-9 shows the drained. the (The simplest and the most widely used method is and its diameter and then calculate the maximum length Fig. and drain of of collectors drains The flow rate in collectors which feed into them. (see Collector diameter Fig. C.9). Fig. C-9: Flow rate is equal may be variable in drainage to the sum of as a function collectors (in the of l/s) 13.z: II -rt 1.55,5..c 8.: !I Il.05,I1.1 I.1 ;I .:1.03,1b.! ‘.I b':11.55,1..1 1.I I':1L,,.lO : .4 I! ::‘.o-‘zo.l I.! .:.I al ‘.I ‘.I ‘.( ,.! ,,! .J .I 1.1 1. 8.: 1.: .( I.( I.. .I 1.1 ‘.I #.I 1.1 I.! ‘.I1.5 I.( ‘.! I,! !,I 0. 1.X '.U :.U .5. 1.0: 1.0: !.I! ).?I !.li i.l 1.5' I..! 1.1: i.O! L.?! l.OI i:;: l.il ! 0.25 0.’ dn I Ii 1c lb 116,1 1‘ it 111 zz I 11 z lb rI z* 011 ,,*': 1 ,I I 11i!bl : II 5’11 Ill4 !‘U 4% 5hl I 35 +I I to z 6: I G, 0I bb II4z 5 ‘I I .5 ,I:, b bb5p L is O-Z& : r(5 I :1 bl¶Uz SE I. a 1 1.5 1 1.s I I. s L 1.5 t.5 :5 I* 1.5 I 1.s , 5 cz 1. flow flow rate rates in the and gradient minor - 103 - CHAPTER D METHODOLOGY FOR SETTING UP A SOIL CONSERVATION PROJECT D.1. 1.1. PRELIMINARY STUDIES Aims Setting up a soil conservation project comes within the framework of an over-all strategy which involves numerous disciplines and follows a logical process in decision making. The first phase of the process is the analysis of the situation from the physical, human and economic point of view. This should provide the necessary factors for deciding on the value of tne project, its limitations and advantages and the economic fundamentals from which it is possible to evaluate the significance of any measures that might be envisaged. Preliminary studies are the first stage to help in aimed at defining the aims of coherent and well-founded to be employed in achieving these aims. 1.2. Situation This analysis analysis will cover successively: tics of picture data in the soil degradation and the impact The analysis should in the region. order to arrive at a first estimation 1.2-I. The physical 1.2.1.1. Data the physical and hydraulic characterisof this degradation on the socio-economic comprise a review of al.1 the available of the measures that might be envisaged context_ collection The data may be obtai..;ed The type of the services. supply this information are (a) the decision-making process development, and the means follcwing basic data consultation with the relevant national to be collected and the services that may as follows: Cartography - The most widely used scale is I/200,000 or I/IOO,OOO. maps. -topographic Certain regions may be covered by maps using a scale of I/50,000 or I/20,000 In the case of numerous countries (North Africa, West Africa, Madagascar, maps may be obtained in France from the National Geographic InStAtUte etc.), (Institut geographique national); - Fig. D-1: Over-all flow chart for 104 - drawing 1 Preliminary up a soil studies conservation 1 Possible Aims Analysis of basic data Hypothesis of improvements Land I studies project Economic Project meatures Choice of improvements components - - - aerial photographs. on black and white satellite information 105 - Many countries panchromatic have film at been covered a scale of approximately These photographs (LANDSAT pictures). at the regional level. by aerial photograph: l/50,0005 may provide interesting (b 1 -Geology Geological maps at a scale 1/1,000,000, usually available from the national geological ties, etc. (c) or I/500,000 services I/200,000. (BRGM in These are France), universi- Pedolx General pedological maps, where they exist, often have a small scale (1/1,000,000). For cer'ain projects, pedological maps may be available with and they can be obtained from geological or agricultural services. larger scales, (d) Climatology Usually, all daily precipitation. countries have a precipitation Climatologicai stations make more detailed intensity, temperature, evaporation, hygrometry, These data can be obtained tural departments. (e) from meteorological observation data network collection wind speed services, which covering measures rainfall and direction, airports etc. and agricul- Vegetation Data about the type and distributicn of natural vegetation and the density main species that can be used for afforestation purposes can be obtained from cultural, water, forest and animal breeding services. (f) Hydrology Characteristics Type of hydrometric 1.2.1.2. and agri- Data of the hydrographic observations carried network out. and the hydrological regime. analysis During the pre1iminar.y to determine the magnitude incidence end intensity cf Soils may be classified type of erosion to which damage caused. study, utilisation degrad ;tion of soil the : actors behind of the data should make it phenomena by highlighting the degradation. according to their erodibility they are sloject and the incidence while specifying and intensity possible the the of the - - 106 - 1.2.2 : The socio-economi_c_ 1.2.2.1. Data The main data -.. (a) context collection to be collected deal with: population in the zone in demography: number of working people, trends; type of farming (Family, industrial, (type, yield, costs), agricultural (b) farming: production (c) soil utilisation: urban zones; (d) (e) animal agriculture, policy, 1.2.2.2. development which should sectors plans, may be damaged or disrupted permanent assets such as land, the infrastructure tion networks), - seasonal depending (flooding, - economic activity of communication make cultivated Probable growth trend 1.3.1. areas breeding, Forest, current legislative be made of agricultural which might be affected - 1.3. etc.), income; population, farmed, industrial or measures. Data analysis An over-all review order to specify all items animal agricultural breeding; agricultural in question, activity by soil and soil utilisation' degradation. may be classified agricultural infrastructure of economic activity under three headings: (buildings, (roads, etc.); irriga- assets such as crops which may be damaged to different degrees on the intensity and period of occurrence of the phenomenon crop destruction, etc.); uhich may be perturbed, due for example to the destruction water run-off or by wind-borne materials which may routes, land sterile or seriously compromise a region's industry. economic growth rates should be estimated in order in the value of these assets over coming Years. Assessing the extent of possible data about to determine the measures Damage assessment 1.3.1.1. Collecting Damage that may occur in classified into two categories: the damage absence of soi 1 conservation measures may be (a) the main one being the loss of land capital under the same Capital losses, heading as other infrastructural assets such as farm buildings, irrigation In estimating capital losses, it is possible to use as a bas iS networks, etc. These should be re-evaluated estimates of damage that occurred in the past. to bring them in line with current monetary conditions. (b) Production losses which may result From reduced soil fertility, flooding, Production loss may be a deposition of wind or river-borne sterile soil. variable phenomenon related to the intensity of the destructed phenomena, the main ones being, precipitation intensity in the case of rain erosion and wind In this case, the updated cost of annual force in the case of wind erosion. damage should be used for calculation. Mean annual 1.3.1.2. The methods and related that to available damage may be used for determining mean annual damage are numerous statistics: where a number of values exist calculated from the arithmetical for annual mean of damage, the damage data; mean damage should be when only a single value for annual damage is available and the incidence of that damage is proportional to this damage is not known, it may be hypothesised With a knowledge of the the intensity of the phenomena which cause it. relationship between the phenomenon behind the damage and the incidence of it is possible to deduce the cost of damage for various this phenomenon, assess the mean annual damage cost; incidences and From this where a number of damage incidence combinations to draw a "damage incidence distribution curve" Mean annual damage is Future 1.3.1.3. the area bounded are available, it (see fig. D-2). by the co-ordinate 1.3.2. damage damage will vary depending on the forecast economic The calculation for mean annual damage should therefore economic growth rate. Envisaged possible axes. Future region. Forecast is growth rate For the be weighted by the investment prices is the sum of envisaged investThe amount of future damage at current be higher where the planned improvements are ment. The sum invested may, however, expec!.ed to result in an increase in the value of agricultural land which, after The level of land value increase have higher crop yield. conservation work, will may be assessed from the amount of increased revenue, at current prices, indexed by the economic growth rate for this type of income. Fig. D-2: Dama; Incidence curve , Damage curve Incidence occurrence of phenomenon Damage costs - - DRAFT PROJECT D.2. 2.1. 108 Aims Once the draft study has made it possible to assess the the draft servation project and the envisageable investment, the means to be employed to achieve the set aims. value project of a soil should condefine The draft project should define the various possible improvements From both the to permit decision making as to which project technical and economic point of view, These objectives are often contained in an will best meet the final objectives. for the development of a catchment basin. They over-all land improvement policy may be economic, social, political, ecological, etc. 2.2. Analysis of basic data This analysis should cover all the data relevant to the establishment of the The basic data collected should usually be supplemented by field draft project. studies and surveys and then subjected to detailed analysis so as to form the basis for assessing possible improvements. The analytical methods that can be used are outside the framework of this book. We will, therefore, consider only the main data required in analysing a soil conservation project. Climatology: - monthly - exceptional daily annual precipitation; - rainfall rainfal, Morphology - relief; - gradient - erosion basin. and annual precipitation studies duration of the precipitation: and statistical statistical analysis with and determination of relationships for various probabilities. catchment analysis; of maximum between rainfall and basin: characteristics: forms: mean values classes eroclibility of slope; classification of various sectors of the Hydrology: - surface area of catchment - estimation - determination of flow rates from man-made - determination of flow rates from collectors. of run-off basins - boundaries of sub-basins; coefficients; structures; Pedology: - general reconnoitring - auger soil the scale samples with of the project of terrain; sample (usually density depending one sample per on land variations 5-10 ha); and - 109 - - collection of soil samples ties and permeability; - determination - evaluation and mapping for analysis of crop of physical suitability and chemical proper- categories; of earthworks. Topography: - drawing up of plans suitable for the direct surveying or by photogrammetry; - survey of land cross-sections - types - crop - yields; - animal (l/10,000 profiles along the main emissaries for the main features. .. animal Agriculture project with to l/20,000) preparation by of husbandry: of crop; rotation; type a*nsity, of grazing, use of passage routes. Socio-economics: 2.3. - industrial - agricultural - market Hypothesis structures; income; and rentable value of land. of improvements G&en the basic data have been assessed, the project leader may present one or more hypotheses for improvements that are both technically and Financially acceptable giving justification for the principle behind the project and the main technical arrangements. This hypo. sis for improvements should be accompanied by the following docu- ments: - location plan at a scale of l/20,000 or l/10,000 locate the siting of the main structures; - the main technical and: - arrangements - characteristic - cross-section - layout - emissary improvements, - standard plans - spacing - layout - requirements over-all improved flow a.4 together and type of forest assessment hectare. for the relevant it is possible standard plans, rates, spacing of defence of main and secondary for with From which and drainage networks, collectors, the main structures, of plants, roads, and location of improvement of nurseries, costs with etc.; an indication of cost per to - 110 - 2.4. Economic analysis For each draft in current account, 2.4.1. project prices, The expenditures it is necessary For each of the preliminary investment: tion compensation, etc.; - maintenance valid 2.4.2. and expenditure measures. include: - studies, and operating These expenditures economic footing. to draw up an income planned conservation cost of improvements, land purchases, evic- expenses. should be discounted so that they can be compared on a Income covers: - the main component in a soii capital appreciation, the anticipated additional discounted income; - As a result of the protection measures, damage will be reduction of damage. totally eliminated in the reduced For a given phenomenon recurrence time, i.e. In the damage incidence distribution curve case of small recurrent periods. 0.3, this will be seen as a reduction in damage costs For shown in fig. By comparing this curve with the damage curve as It given recurrence time. the reduction in the mean annual damage is the was prior to the improvements, These curves may have a different shape area between the two curves. and their comparison is essential For depending on the type of improvement, For example, we have selecting between different types of improvement. The curve shown in fig. D.3 two types of curve that followed improvements. Curve Cl shows damage distribution incidence Following afforestation work. to an absorption network in an area where there are C2 applies, for example, embankments may be broken with damage which rainfalls, heavy, low-incidence would be greater than the initial damage. Fig. D.3: Damage incidence distribution conservation project cost - Damage Co before measures - Damage C, after measures defence defence Income R = Co - C, Mean annual Incidence 1 being return - Balance 2.4.3. Ihe viability - sheet balance sheet of income and selection The main criteria used it possible to define are is viable rate, profit profitability the if difference there is which is is zero. rate between a profit incorn at and expenditure. current prices. the value of the actualisation If the improvement is financed should not be greater than the rate for by a loan rate. - The relative receipts. projects. - Cost which is equal to the quotient gain at discounted prices, This makes it possible to choose between various independent benefit Selection 2.5. makes are: which Internal profitability which the discounted the internal loan, interest and expenditure criteria. Discounted profit, The improvements - 111 - of ratio. of improvements The draft projects make it possible to define the alternative improvements intended to achieve a stipulated objective and which are technically and however, They also provide the factors for making a choice; economically feasible. this will not always be guided by the economic factor alone but may also take into account other criteria, e.g. social, political, etc. D.3. 3.1. Objectives - Once the draft project has the project itself improvement, for the implementation. The project and drawings. 3.2. 3.2.1. - / THE PROJECT Field covers additional permitted the selection of a possible type covers the drawing up of all the necessary field studies and the preparation of of items documents studies Topographic preparation intervals - longitudinal of l/2,000 - cross-sections of work of plans at a scale of 112,000 or l/5,000 with contour lines at 0.25 III for slight gradients and up to 0.5-I m for steeper gradients plofiles of natural water outlets and main collectors for the horizontals and II100 or If50 for the verticals; of outlets and all salient points at a scale of at a scale l/100 or l/50. - 112 - 3.2.2. Survey borings- The technique them will vary and depth depending For plantation on the work, it is difficulty of digging planting depth every 10 ha or so. For cut-and-fill and the sampl e borings type of work envisaged. of analyses neces sary to determine by carrying out holes, earthwc-k, trenching soil to type, a sample difficulty should be carried humidity boring of recessnry to determine soil type and in works, it i Soil analyses are carried out on samples taken with an permeability. depth greater than the maximum drain depth tind at a rate of one sample it is also necessary to dig a trench every 50 ha to make a descriptive 3.3. Project These They 3.3.1. a descriptior of the m in particular auger every soil at a 10 ha; survey. to dig one or more trenches height of the finished F+-uc- project and assessment of expenditure. comprise: Written documents: (a) an explanatory memorandum describing cal arrangements; (b) a descriptive out and the (c) specifications Cd) an analysis (e) a quantity 3.3.2. and the 1.00 documents comprise the project principle and estimate which enumerates and describes the origin, quality and preparation of materials; for the work and breakdown estimate which map at a scale to of be carried out work by a commercial a location !g) a map of (h) a longitudinal (i) drawings the of defence network profile the of structures of the expenses for each item l/20,000; at the at main a scale of l/2,000; collectors and the a suitable scale emissaries; (l/20 or l/50). techni- to be carried firm; Drawings: (f) the prices; calculates on be assessed. For drainage For the construction of small dams, it is necessary along the axis of the structure to a depth equal to the ture in order to determine soil types and permeability. out of work; CHAPTER E PROJECT IMPLEMENTATION' This priority skilled conservation work sites in which chapter describes the operation of soil is given to labour-intenstive procedures using untrained manpower, little a large number of simple tools and few machines. labour, E-1. 1.1. Survey of local conditions site reconnaissance Before opening tion to survey the decide on the work Natural 1.1.1. GENERAL SITE ORGANISATION and the site manager should make a prior visit to the locaa site, techaical and economic constraints so that he can major natural, procedures and conditions. constraints The main constraint is climate which may make it necessary to halt work at the either because of frost which prevents masonry site for some period during the year, and concreting work being carried out or due to excessive heat which may inconveniThe rainy season may also prevent certain types ence workers and reduce output. of work being carried out or be an impediment to haulage or materials supplies. Water flow should be investigated before work is carried out on water courses. PPvine or gully rectification and dam construction should be carried out at the lowest water level or the work may be severely damaged or even totally destroyed Nevertheless, if the during the construction period by the arrival of flood water. work cannot be carried out at any other period or if the flood calendar is not additional protection such as the installation of coffer dams sufficiently regular, tr,is type of additional protecHowever, with water diversions, should be planned. Economic optimalisation should be sought between the tion is ustially expensive. 1 This chapter covers only the general principles of site organisation and Soil conservation work does not, in fact, necessitate any project implementation. types of work are applicable special organisation and the guidelines for other Reference should be made to Training Course No. III without any major adaptation. which has been produced as part of "Project design, implementation and evaluation", the Inter-regional Project for Planning and Administration of Special Public Works Projects. - 114 - cost of additional For building. tion of is the not protection and the danger of the structure being destroyed during the small structures described in this document, additional protec- ususally structure necessary being destroyed The sanitary conditions in particularly important for the Information should be obtained safety and control measures can the hygiene of epidemic disease, particular attention. 1.1.2. Technical and economic and would during be too expensive in relation to the danger building. the region are another natural constraint, and are setisfactory progress of labour-intensive wrrks. about the risk of any epidemic so that the necessary Since water is a major vector be taken on site. of the water being used should be the subject of constraints During the site reconnaissance? note should Site access should be assured. taken 3n the condition of existing roads and tracks, their distance from the constructlon site ,nd any need for the construction of new access routes. be be established by examining the potential of Supplies of materials should the quality of the materials they supply, delivery capabilities existing quarries, Employment can be increased when use is made of local materials which and prices. are available clcse by and can be exploited without great difficulty using rudimentary techniques. At the project design stage, it is essential have suitable are available in adequate quantity, cessed at a rate which meets site requirements. for materials exploitation, processing and haulage a decision is taken on reliance on local materials. to ensure that characteristics these materials and can be prothe manpower required In addition, should be assessed carefully before In this way, a reinforced concrete weir would require large quantities of sand, gravel and cement and wculd ideally be constructed using capital-intensive methods. In such a case, it would its replacement by a stonework weir might be envisaged. be necessary to ensure that the necessary quantity of quarry stones was available, that the stone would be of sufficient hardness and that the haulage distances would not be excessive. If these requirements were not met,'the advantage to be drawn from using these materials would be eliminated by the high cost of vehicles for t-,h e I r transport. price Frr non-quarry factors should materials such be examined. as wood, fuel, cement and steel, the supply and Supplies of equipment. It is necessary to assess the number of tools and machines required on the site: .onditions for the pu-chase of new equipment, local Supplies of new resources and the condition of available second-hand material. equipment may take a long time in arriving and it is advisable to make allowance for this when plans are prepared. The possibility of manufacturing certain simple tools and having equipment maintained and repaired by local craftsmen should be considered. If local resources are inadequate, plans may be made to train local craftsmen or equip the site with Its own resources (small workshop, etc.j. - Local manpower and supervisory - local availability of unskilled - local availability rf skilled - availability - working - cash wages of in - should be selected taking into account: workers; workers: working and payment walkers workers supervisory conditions: 115 hours, such as blacksmiths, foremen, social topography structure, public masons, technicians, etc.; etc.; holidays; kind. Consideration should also be given to seasonal variations which may be very and be a major constraint for work which requires continuous marked in certain cases, in river bank pl'otection work, which is normally carried For example, progression. a marked the time factor may be of particular importance; out during the dry season, seasonal shortage of labour may make it necessary to change the type of protection work or the haulage technique (use of haulage vehicles in operations for which they Completion of the work in stages or segments might be a are particularly viable). possible compromise where there is a temporary labour shortage. The lack of skilled workers may also be a deciding factor in the design of the A gabion or stonework retaining wall would require lesr skilled workers structure. On the other hand, it would be necessary to than a reinforced concrete structure. Consequently use much larger quantities of materials to achieve the same strength. the structure will probably take longer to build and its commissioning will be delayed. 1.2. 1 .2.1. -Project planning Objectives of planning Operational planning is an essential management tool which will make it possible to pursue a strategy or tactic, to keep control of the numerous factors that may The objectives of affect final cost, and ensure thr best use of available labour. work preparation and organisation, planning are those stipulated by task sequencing, These entail the drafting of various documents, and work control and follow-up. the type and precision of which are directly related to the type of work being managed. With these documents it should be possible to have a precise idea of the abstract factors in a site, to highlight interdependence between various activities and pick out those operations which demand the project manager's greatest attention rainy season, in relation to the numerous constraints that have been foreseen: religious holidays, harvesting, etc. Depending on the size and complexity of the project, it may be necessary to operations and basic tasks, the repetition of break down the site into activities, Planning is which over a given cycle calls for specific attention and analysis. therefore a task for a technician with a thorough experience of building sites, men, working methods, the various presentation techniques and with a good mind for analysis and review. It is a task for a specialist. - Forms of 1.2.2. It will of which planning - 116 planning and their requirements be necessary to select between existing planning procedures, all In deciding the suitability of the three factors should be taken into various have certain advantages and weaknesses. procedure for the project in qUeStiOn, consideration: The criterion affecting the choice of presentation system Project complexity. The majority of works involved in a is that of the complexity of the works. relatively simple and the interdependence betsoil conservation programme are ween the various tasks or operations is direct and relatively uncomplicated. Experience has shown that a choice of one of the following three types of presentation would meet the requirements of nearly all programmes: (a) (b) - bar chart - time - network or and location or Exploitation skill of the (c) GANTT chart planning; chart critical planning; path planning. The programming technique will depend on the It will therefore differ depending on whether the staff responsible for charge of the project, of the workers document. using it. the user is the engineer in the works programme accounting, supervisor Type of works. This - work covering - linear work - work carried - labour - material - plant - general - repetitive - general - financing will also affect a large area (reafforestation, (ravine The factors which programming technique: or the correction, out at a given are being choice erosion point planned the of of programming dune control (small may, foreman. stabilisation,etc.); ditches, dam, weir, course, technique: also etc.); etc.). have an effect on the requirements; and tool rotation supplies; and maintenance; works organisation; cycles of progress basic tasks and supervision schedule, f of the project; etc. Clarity and ease of interpretation are prime requirements in planning and an effort should be made to present information simply, by giving a self-explanatory and precise picture of the problem being analysed. The highlighting of interthis makes it dependence between various operations is a fundamental requirement; possible It to pick out critical tasks which should be given major be noted that planning is merely a hypothesis, for scheduling the various tasks in a project. fore be possible to correct it and, if necessary, modify or adapt change of work plan. realistic attention. should also possible, although the mOSt It must thereit flexibly to any 117 1.2.3. -Planning procedure All planning commences with the basic technical data: ing - assessment - volumes - probable tions, - of available labour and quantities for performance in particular; - special site - various constraints - allocation - labour, - availability for in each each various the (rainy items project of in formation and calculat- zone; task; basic task storage, requirements: of collecting season, in water, view of access harvesting, specific road, religious climatic condi- etc.; holidays, etc.); funds; equipment, of tool requirements; equipment, tools, etc. The various tasks involved in the completion of the project are then placed in Any form of depiction can be used: a logical sequence. arrows, rectangles, bars, out in parallel etc.; then one adds to the list those tasks which can be carried and the ordering tasks (ordering of supplies, rotation of vehicles, requests for topographic studies, for example). In this way, one obtains an initial picture which highlights the task sequencing. The diagram will be supplemented with information on the basic times for each activity, the labour and mechanical A resource.; to be employed and all the constraints which can be seen so far. number of diagrams are drawn up in this way in order to identify the most economic programming. The sequencing of operations is the most delicate part maximum attention. Once these studies have been done, the presented in the most suitable form. of planning and requires work's programme can be In the following paragraphs we will examine in sequence the planning of a single construction site using the three most commonly used methods (bar charting, time and location charting and critical path planning). The example in question embankment combined with the general organisation of a site of 8 km of protective an irrigation Fig. E-1: channel Cross-section (see fig. E-1). of channel and embankment Grassing Stone facing bank is - 118 - The list of the main operations is Direction of advance Install site Strip vegetation Excavation Stone - 8km Gang No. Gang size Duration (weeks) 2 200 12 facing This lists of zero duration comprehension. The type The ordering operations, tasks. have been omitted place ctinstraints, to the ordering of stones, cement, the main operational but nevertheless These may related Bar chart on the as follows: of which are to facilitate etc. or GANTT chart chart proposed by GANTT is easily One lists, producing it. read and requires on the left-hand little side skill of the part of the person the various operations to be carried out in usually from top to bottom, their logical order of execution and on the horizontal scale the time schedule against each operation, are drawn rectangles, Horizontally, to a convenient scale. )ropcrtional in length to the time the operation will take. or lines, 2C workers 0 100 wor!tcrs JJnskilled I.abour requirements - 119 - To the lower indicating labour, made by joining gang movements section tool, of the planning equipment, etc, up the various during operation chart it is requirements. operations by arrows of the site. The major disadvantage of this type relationships that exist between It is also difficult complex projects. relationships of the tasks are (linked GANTT chart) to show of diagram is that it does not show the different tasks, especially in the case to supervise project progress since the various of spatial possible to add a diagram Improvements can also be not shown. this type of diagrammatic presentation is very useful and is Nevertheless, summarising detailed scheduling usually used for planning simple construction sites, and for displaying to a large number of that has been analysed by other methods, and in particular the workers themselves, the results of these analyses. people, 1 .2.5. Time and location chart planning However, it can be used This is an extension of the GANTT-type bar chart. bank protection works, iinear soil only for linear and narrow sites such as: conservation work-, forest tracks, embankments; or for tall and narrow structures: retaining walls, weirs, small dams, which entail successive repetition of the same activities. The basis for drawing merely plotting on a chart and an X co-ordinate (the up a time and location chart points against Y co-ordinates chainage of the project). is very simple and entails (time in a convenient scale) A Y Time (weeks) I 0 0 1 2 3 .-x. 4 km project chainage > Using this technique, points A and B are defined in both time and space. Point A is therefore located at the 2 km chainage at the start >f the third The line week; point B is at the 4 km chainage at the start of the fifth week. joining A and B therefore represents a two-week task being carried out between the 2 km and 4 km chainages of the project (an erosion control ditch, for example). The direction of advance of this task is of course defined as going from left to of the fourth right. If a check is made on the advance of the work at the start week, it will be necessary to ensure that the 3 km chainage (point M on the chart) One can therefore see the value of has been reached as foreseen in the diagram. since one can detect at an early date this method in site supervision and control, any delay or anomaly and rapidly make the necessary organisational changes. - It is construction 120 - section of the work under possible to add to the chart a longitudinal on which are marked the main features which may act as constraints: This river to be crossed, cemetery, etc. X axis above or below the chart. diagram should be drawn parallel to the Any isolated point on the chart will show a task of 0 duration at the specific This will be an order task (for example, ordering the supply point of the site. of steel, cement, etc. to the site). A vertical line represents a task being carried out at one specific point of the site (construction of a weir, for example), and the duration of the task will be shown by the distance covered by this line on Two parallel lines indicate operations carried out the selected Lime scale. simultaneously b;r two teams at different locations: Day No. 16 Day No. 10 __ I 0 1 2 3 Tall and slender projects are shown using the ease in reading the planning chart, the time scale and the height of the structure on the Y co-ordinate Fig. 4 I 5 6 7 km same principle; is however, for shown on the X co-ordinate as is shown in fig. E.3. E-3. - 5m 4 6 VI ti H’ Cabi . !: -.-- -I :t's logic &hart; the required resources are not yet taken into consideration. The second step is to deploy these resources (manpower, time, materials) between the various operations so as to optimalise production time and costs. When the network has been completed, the t'critical" activities identified so that particular attention can be paid to them during the work. It is usually advisable to present the results in the chart which is more easily read and used by the workers themselves. Developing a network diagram requires important of which are given below: the use of special should be the course of form terms, of a bar the most - a chart Network or graph: usually orientated. lines, 122 - comprising peaks joined Requires beginning or end of an activity. Event: is shown by a circle in which are indicated: the earliest and latest finish dates. the Earliest Task or activity: actual time required sequence. Logical which The event chronological number, a strategic the network main linkage number time this is a sequence of interdependent is an event commence. at tasks, the order several activities finish or from which It is may initiate a number of operations. In the case of large, complex projects, the work. more or less extensive charts starting from these stage in the course of can be broken down into points. Earliest event time: starting from the initial of event at time the successive on a single zero, the tasks are event, this event a backward path is Latest event time-: final event and taking successively the If a number of of the shortest paths path. Free float: event time. is Dummy activity: dependence and linking zero duration. Network construction converge the on a given difference made over the network starting from duration of each of the basic tasks. this event will be given the time event, between this is a broken line two events in different construction method site comprising the (practical following the latest event time and the example): activities: i.e. a weir foundation Immediately job preceding Job Duration (days) A B Set-up site Marking out Construct shuttering Prepare steel reinforcement Excavation Install shattering Install steel reinforcement 2 Commencement. of 1 2 A A A 2 B 2 Place 2 C-E D-F Conclusion concrete earliest joining two events showing an interA dummy activity is sequences. No. G H of which A node sequence of the arrows is traced and the durations added together. When a number of paths coincide will be given the time of the longest path. C D E F and be changed. this Node: several activities the curved Latest event time J5?c the length of which is independent of the shown by a line, This line ends in an arrow which shows task to complete it. event sequence: cannot cr no resources. event's Event <- by straight 4 1 of site site of - 123 - This which Fig. list forms of the jobs links will in be classified the network by order (see fig. of sequence in the table below E-5): E.5. stage Stage-2 1 / Stage 3 --/ Stage 4 ~~ Stage 5 Stsge 6 f Install steel reinforcement shuttering ----A Set-up -c Pour concrete H- \ sit 1e Prepare steel reinforcement =I== I will The network will then be added the earliest The I'floats" for critical path ( -H) be laid out using the symbols described and latest times for each event. each event (latest event time minus will be the one which passes through above to which earliest event time). the events with a zero float In this way it will be seen that job D can suffer a four-day delay (8-4) without any repercussion on the final completion of the project. On the other hand, any delay in the completion of job C will have repercussions on the final completion date. It will be seen resources but indicate D have been completed. that jobs 4/5 respectively and 617 are "dummy" activities. that jobs F and G cannot start They require no until jobs C and - 124 - path method for the The same principle is used below to show the critical The stage classificaconstruction of the 8 km embankment and irrigation channel. that for this type of site, It will be noted, however, tion method will be used. They are as follows: certain constraints have to be introduced. - job B starts one week after the start - job C starts two weeks after - job D starts two weeks after the - job E starts after completion of - job F starts after completion of' job the of start job of start B and Stage the tasks 1 Stage Fig. 1.3. 1.3.1. Diagram E.6: Setting C C ing in each job and that part which by distinguish of the others 2 Stage 3 C (2 days) B Stage 4 C (2 days) B D Stage 5 F E development up the Construction job by stage: A (2 days) B A (1 day) B D. Allowance can be made for these constraints that part which must be carried out independently has to be carried out simultaneously to others. Scheduling job of jobs A site of access roads When access to the site is difficult, an access road will be necessary for supplying the site with equipment and materials. The works supervisor should ensure, dcring the course oi' the work, that the necessary signs are in position protect the road construction workers and to avoid accidents. to - Construction 1.3.2. of site 125 - buildings accommodation for the The buildings (offices, sheds, etc.) should be located at a relative mess room, workers, the access road to avoid noise and dust. The workshop avoid expensive and store Plant Plant possible; be built Site 1.4. (a) as possible to the site to a dry place for storing the cement. be stored plant and fuel from which away dust and direct sunlight is not muddy or sandy. it-: the shade from stores shou'.,l be kept on the site accommodation clean to avoid and the fire technique Site supplies Good site planning necessary equipment the work of a gang always an increase depends especially and materials. Delays in supplies the whole site; and his may affect in costs. being well supplied with a stock of the most common spare parts At the same time, and this should be managed to ensure security of supplies. Quantity the may make it necessary to halt the consequence of this is From the opening of the site, the works supervisor should have a schedule This should be kept supply requirements for the various stages of the work. date as the work advances and steps should be taken in advance to ensure that suppliers are able to meet deadlines and supply the necessary quantities. (b) if management Mana ement 1.4.1. in should be stored where it is protected preference should be given to an area a : ;-und The hazards. as close installation Fuel reserves should storage area. plant be built and transport. A shed should 1.3.3. should works supervisor and distance from the site should of up to be established measurements It is necessary to measure the work actually completed !and the quantities of In materials used and to compare this with the estimates made p!?ior to the work. the case of earthworks, these estimates are based mainly on measurements of surface areas and lengths intended to calculate the volumes involved. in Quantity measurement also covers cost calculation: fuel consumed, These estimations may be entrusted estimation transport of all distances, to a quantity the other etc. surveyor. factors involved - 126 - (c) Productivity This involves individual improvement improving gangs being To achieve gang the foreman capabilities the workers' - the gangs - the best workers or the best - the most suitable tools are - the workers' Daily should ensure and competence - (d) and organising the various tasks to avoid idle. this, are output that: are used to the best effect; homogeneous; welfare Works progress progress is kept, Fig. E.5. are encouraged (incentives, bonuses, etc.); used; looked after (housing, food, etc.). reports reports The planning should the site supervisor gangs should be drawn up and signed if the be updated each week; should be informed immediately. by the planned foreman. tempo has not been m Excavation volume 8 000 __ 6 000 __ 7 1.4.2. Personnel (a) I 14 I 21 Time in > days management- Recruitment of workers Special labour-intensive public works programmes require special personnel management which are matched to the specific social objectjves of these programmes Voluntary worker participation can be obtained only by suitable prior preparation, and make use of public information and which may be of an administrative nature, awareness campaigns. When the project exceeds a certain level of workers and time, it is necessary the volunteers themto organise a volunteer recruitment and handling programme; selves should be classified by specific criteria to the jobs they will be expected On recruitment, each to carry out taki.;g into account their special abilities. worker should receive an individual work card which may be based on the model shown below. - Fig. 127 - ~.8. Work card Group (E) Name ................................. ...................................... Sex ..................... Age ........ ................................. Place of residence .................................. No. of dependants Profession ......................................... Qualifications No. r L Photo I ..................................... ..................... issued on ................ RECTO Work periods End Work details Date Project I Post 25.2 SEGOU reafforestation 17 No. Checked Signature II Date 12.4. checked Signature VERSO A weekly attendance bearing only his family special capabilities), worked for calculating this tally of the site, himself. E.9) should be drawn up for each worker, name and address (with, where necessary, any checking off the number of hours and days Depending on the size pay (or related benefits in kind). foreman or works supervisor will be kept by a tallyman, sheet (fig. name, given designed for - Fig. 128 - E-9. Site ............................................. Work attendance Name ._.................................... Category ............................... Gang . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Week From From From From From From ..... ..... ..... .,... ..... ..... Worker's to to to to to to Personnel card No. Wages/h No. of rations per day applied The regulations (lilt*ar;lsl? Hours Days Cash Kind payroll regulations The provisions legislatior:. - Wage Work done designation Weekly working Week's total Work supervisor's initials Worker Name - 19.. ..... ..... ..... ..... ..... ..... - Site Sun. sat. Fri. Thurs. signature 1 Registration No. (b) sheet . . . . . . . . . . ..--------.-----..---.-...Hourly wage . . . . . . . . . . ..-_............. Month . . . . . . . . . . . . . . . . . . . . . ..--.. Wed. Tues. Mon. . . . . . . . . . . . . ..-..---.--.. NO. should should specify: working week; iicu~~s; of tile be in line with loca customs and current Remarks - - method of - possible payment; sanctions (for absence, Regulations may be established (c) remuneration Worker The level 129 - of remuneration late in should arrival, consultation etc.). with workers' representatives. be set by the competent authorities Since these are community work. remuneration may be lower than into account the specific nature of the ment programmes carried out by volunteers, minimum legal wage or paid in the form of benefits in taking developthe kind. Payment should be carried out at regular intervals on a daily or weekly basis. The works supervsior should ensure that the necessary funds are available in order In the case of payment in kind (food rations, for to avoid delays in payment. care should be taken not to diverge from local tastes which might be a example), source for (d) discontent. Transport To the extent when the however, to organise on the work (e) their site Worker On small aimed primarily and quartering efforts should be made to recruit manpower locally; possible, site is at a distance from the workers' homes, it is advisable or if necessary their quartering transport to the workplace, lodging, food and adequate sanitary facilities. by providing training sites at and on short-term work, worker training may be rudimentary the correct handling of the relevant tools. and In the case of larger and long-term sites, this training should be backed up by training aimed at providing workers with the knowledge necessary for them to take over responsibility for the correct operation and maintenance of the completed retraining of local craftsproject (introduction to modern agricultural techniques, and writing, etc.). men, reading (f) Human relations, conflicts The works supervisor should ensure that volunteers are fully informed of the purpose of the work entrusted taking also be divided up into gangs, where necessary or community structures. the in special public works programmes, ticn usually leads to collective self-discipline. heterogeneous social groups (different ethnic period under artificial conditions may result The works supervisor of the project. To this and that a good community sanctioned accordingly. should assert end, he must spirit exists receive a warm welcome and Volunteers should to them. into account existing social involvement of However, origins) living in conflicts. the population the coexistence together for in quesof a limited his authority and ensure correct progress ensure that the site regulations are observed Any infraction should be on the site. - (g) they Health, safety and working 130 - conditions Suitable working conditions will be laid will have an effect on work productivity Sanitary distance from Safety facilities should be related the workers' homes. and health requirements should to down by practice and continuity. the size be applied. of and legislation; the For it is cuttings, trenches and ditches, banks which are higher than 1.20 m; - for excavations, stability of any - for work carried out in water, it is necessary to ensure and harmless and avoid risks of drowning and cave-ins. site and the example: necessary Every isolated site employing more than ten workers should medical kit for first aid which may be carried out by a volunteer prior training or by the works supervisor himself. that to the ensure water 1.4.3. Incentive In employing more than to provide a small 250 workers. clean have an emergency who has received In the case of work sites employing more than 50 workers, there should nurse or health worker who can provide first aid and treat common diseases (malaria, headaches, dysentery). In the case of sites 12 months, it is necessary permanent nurse for every is the be a 500 workers or work lasting more than mobile dispensary with a bed and a system all firms employing workers, incentives are desirable to encourage better results. Recompense is therefore given for good results obtained either individually or by the gang as a whole. Praise or rewards may be made and additional holidays given. However, such procedures may prove unsuccessful especially in the case of It will therefore often be necessary to turn to: short-term work. - the - piecework (a) flnish-and-go system, payment. The finish-and-go system This system is particularly attractive to workers, such as small farmers, agricultural workers, etc., who see the special public works programme as a supplement to their main job - agriculture. The interest for them is to finish their woric early and return home so that they can possibly spend the rest of the day on this main job. The system also suits agricultural workers who are not usually accustomed to regular working hours. It is possible to determine a task which lasts for several days, although this is not usually desirable, nor always permitted by legislation. There is a danger that this practice may lead to five daily attendance cards being given for three days' actual work and this is clearly open to abuse. Usually, preference is given to daily tasks; since the result is extra work for skilled workers, the tasks are essentially simple ones, such as for example, digging a certain length of trench of uniform cross-section. - (b) Piecework 131 - systems Where local legislation permits, piecework systems offer numerous advantages, in particular where a standard wage rate is fixed for each task and the worker (or gang) receives a fixed amount for each unit of work carried out. The advantages and disadvantages of these systems are as follows: Advantages Disadvantages the worker is able to earn more than the normal daily wage in return for higher output; in attempting to achieve higher the worker may neglect quality safety; his output will be higher since his wage depends directly on the effort he puts into his work; if the rates are too low, the incentive to work hard may be lost, or the worker may work too hard to the detriment of his health; these systems promote discipline and require vision; individual less super- the worker himself attempts to improve the efficiency of his working method; output, and differences in earnings between harder workers and less hard workers may sometimes cause resentment; these systems are more complex to apply than a daily wage or a finish-and-go system. when, ir.stead of an individual, it is a gang which is doing piecework, the gang will organise itself to ensure maximum effectiveness and the most active members will encourage the others to work harder; in certain cases, where the situation allows, the workers will be able to stipulate their own working hours. 1.4.4. Role of project management The project management The main tasks are: - work programming; - work implementation; - supervision is responsible for the correct implementation of planning. and control. Work programming requires an over-all long-term view of the project and should Carrying out the work requires be the responsibility of the project manager. daily supervision which will be the responsibility of the works supervisor. Responsibilities - of project manager programming budgeting and budget control forecasting recruitment of assistants training - research public relations selection of main items of equipment wage negotiations project control Responsibilities - of works supervisor implementation of work quality and productivity control recruitment and payment of workers purchasing of materials discipline transport and organisation of manpower supervision of equipment and plant maintenance of buildings and roads - TOPOGRAPHICAL STUDIES E.2. Topographical projects on hilly '132 - studies may prove necessary in numerous ground or which cover a large area. cases, It is diffjdult, within the framework of this document, methodology and the type of topographical studies that should projects within a characteristics of the soil conservation We will therefore not do programme may vary considerably. general principles for topographical studies and invite the numerous specialised documents in this :ield. 2.1. Topographical A survey (a) the (b) survey which survey in picks out the altitude which the various natural to refer features of these natural to of features carried out by the measurement of the the is distances and angles. measurements - the - the measuring rod in wood or metal of 4-5 m in length fitted level for checking horizontals and a plumb line for measuring distances on slopes greater than 2 or 3 per cent; - a folding I Angie chain surveyor’s 2-m rule (20 m in for length); measuring the details of the with a spirit horizontal structure. measurements the most commonly used being goniometers, plates one of which is graduated to permit 1 reader survey The planime%ric survey is The instruments required are: I to specify precise be foreseen since the special public works more than indicate certain or a structure; Planimetric Distance for comprises: the levelling measured. 2.1.1. particular survey planimetric terrain in - surveyor's each other; squares - optical - a type of goniograph plane table, relative position of a number of - compasses azimuths. square which for locating replaces points the the theodolite; the measurement and features surveyor's with which points; these comprise of angles; at an angle of 45 ' two to square; it is possible to measure the I I with which it is possible to measure differences between magnetic 2.1.2. Levelling Two procedures - direct - indirect Direct are levelling usually for levelling levelling horizontal for is used: sites; inclined carried sites. out using a level and a levelling staff. Since the level can be used only for horizontal sights, it is to carry out a series of levelling sights. steeply sloping ground, in fig. E-10. called change-point levelling, is il lustrated Fig. necessary on This procedure, E.lO. the difference in level between two points is calculated In indirect levelling, by measuring the angle formed between a straight line linking these two points and This method requires the the horizontal, and the distance between the two points. use of theodolites or clinometers. The latter are particularly well suited to rapid reconnaissance l/l,OOOth. The indirect Fig. E.11. surveys leveling and allow method gradients is to be measured illustrated in fig. with E.ll a precision below. of around - 2.1.3. Composition and survey of a topographic standards The composition and difficulties - topographic - one or two staff - two chain - two labourers, is the terrain. - team survey team will vary A basio team will usually depending comprise: on the different surveying slope surveyor; men; men; to The density scales survey of a topographic of 134 of carry levelling the levelling points stakes. generally accepted for as follows: Surveying l/10 No. of scale points per ha 1 000th l/5 000th 2 l/2 000th 6-10 l/l 000th 15-30 l/500 levelling th 20-100 E.3. MARKING AND STAKING OUT THE STRUCTURE Marking out a structure consists in indicating the axes and the external dimensions on the ground. This is done by means of stakes which should be precisely The marking out operations should located in relation to fixed reference points. be carried out in the presence of the project manager and the works supervisor. 3.1. Preparation of the ground plan The structure should be depicted by a ground plan and by elevations sections. The ground plan is drawn up by indicating on a tracing paper axes and the measurement of angles and altitude. 3.2. Preparation on the and the main ground The transverse or longitudinal axis of the structure should be indicated by two fixed reference points which would be either boundary stones set in concrete or concrete boundary posts in which a nail is set in the upper part to act as a reference point. The number and altitude of the reference point should be marked on the boundary stone. The fixed reference points should be located outside work and should be sufficiently visible not to be damaged work. They should remain in place throughout the duration co-ordinates and altitude are usually incorporated in the of the area. the external limits of the during the course of the of the work and their general levelling survey - Main 3.2.1. staking This is - for works - for earthworks: the location Staking used out define: engineering the of is level of the route, the axes; the longitudinal profile, the curves, cross-sections. carried out using hard-wood, square or circular cross-section the stakes should be driven home with a In loose soil, land, they should be cemented into holes made with a jumper The stake heads should be painted to ensure that they are stake should be numbered and referred in plan and altitude reference above construction: axis the stakes, 50 cm in length. on rocky sledgehammer; bar. each - out to of 135 clearly to the visible, fixed and points. The stake head should be set at the exact measurement of the if this is not more than 30 cm higher or lower, or an exact future number ground of decimals or below. Additional 3.2.2. staking . This is carried out from the main stakes and indicates the boundaries of These stakes are not levelled such as the edges of trenches or banks. works, they should be painted in a different colour to that of the main stakes. Staking 3.2.3. the and report This document should be drawn up in should indicate the number of the profiles, reference points. the presence of the and the position works supervisor and altitude of who the Example: No. of Part of structure stakes Position of stakes Slopes and sections Alignments and curves Distance between stakes I 3.2.4. Displaced Before the Height of earthworks Comments I stakes work is started, the main stakes which covered by the works should be displaced at a constant of the structure. This displaced staking should also axis stakes as shown in fig. E.12 below: are located within the area distance outside the boundary be levelled in relation to the - Fig. 136 - E.12. L- Axis Personnel 3.3. requirements The personnel and work required for the output marking the basic topographical survey team as listed team should have an output of 4-6 km per day. Material required - a level for - a goniometer - two levelling - a surveyor's - stakes; - cement, - paint marking determining for out Clearing covered should comprise, be provided by ground, such a as a minimum: altitudes; the marking out chain water marking (or steel proper; for the tape); the installation of the main fixed reference points; stakes. E.4. 4.1. work out will On normal above. staffs; sand, for for anti staking the land over by the works the GROUND PREPARATION area In soil conservation projects, land clearance will be limited to preparing areas of work such as channels, embankments and earth dams with a supplementary for the movement of workers and plant. Land clearing consists in progress of the works or might eliminating any vegetation lead to their deterioration. For manual work, the tools used will vary considerably country. The main types are the machette, hatchet, axe, saw, scythe, sickle and hand winch. Output norms are related primarily to the density of types may be classified into various prolips. For manual gories can be distinguished as shtiwn in fig. E-13. which would depending hoe, fork, hinder the margin the on the pick, the vegetation. clearing work, pruning Land five cate- - Fig. 137 - E.13. Category Characteristics higher than sparse shrubs Manual output Observations Between 200 and 300 m2 per day/man Output depends on climate and site organisation -- Grass Savannah No vegetation 1 m; a few Shrub Savannah Scattered bushes; thorny vegetation; land covered with perennial grasses; numerous shrubs between 100 and 200 m2 per day/man An alternative a chain pulled two tractors Wooded Savannah Semi-arid tropical dense vegetation; brush; numerous thickly wooded; large trees Between 50 and 100 m2 per day/man Tree stumps and trunks and brush are left in place; they are burnt on site 3 weeks after having been felled Forest Hard and soft wood; large diameter trees forming a vault Unusual work within the framework of soil conservation Six men and a winch 10 trees fell about animal haulage day; might be envisaged regions; thorny shrubs; some ; projects 4.2. in Stripping is to use between can per ' - subsoiling The stripping areas which are operation entails to be filled. the removal of Normal trenching topsoil methods over a depth are used. of 20-30 Subsoiling is a deep scarification (up to 60 cm) intended to increase soil permeability or break up a hard pan. It is used in arid zones to prepare the prior to reafforestation work. It requires powerful mechanical plant (tractors 35-70 hp for subsoiling to a depth of 60 cm). area Instead of subsoiling the to be covered with ridges soil may also be loosened with a pick in which trees are to be planted. The following productivity data for light stripping in the Philippines. The soil is loosened by a plough and the stumps removed by hand. - Soil Non-cohesive soil; fields of and burnt, and in which there roots per square metre Method Four successive ploughings; ing roots by three passes handpicking with axe under and subsoiling were before being shovelled cm soil of the collected clear sugar cane which have been cut remains two or three stumps or followed by removal of the remaina bamboo scraper and three times - 2 ploughs, Gang composition: - 2 labourers 1 scraper, 165 m2 per Over-all: Productivity 138 hour handpicking per gang Per operation: first ploughing: subsequent II bamboo scraper: handpicking: r General comments and handling EXECUTION OF EARTHWORKS type Land classification 5.1.1. The type of - the working - the choice - output cuts on soil land method determine: to be used; of equipment to be used; consequently, and, Soil type and fills. will the has a considerable A distinction - loose soil - rocky soils which Loose soils may b,e classified Light which Heavy soil Very heavy soil Rocky ground Very 5.1.2. topsoil, hard rock without and the previous shape to be given to loosening; topsoil, sand, as follows: dry sand, fine gravel firm soil mixed with sand, wet sand, compact compact clayey fine gravel, large gravel, peat Firm soil mixed with stones, clay gravel, clay, marl, broken rubble soil, Wet clay, compact marl, brittle slate, faulted rubble, soft quarry decomposed rock Soft Hard rock earthworks. effect on trenches may be made between: may be classified rock of may be loosened. Moist clayey soil cost may be excavated Dry soils Ordinary Soft plough plough scraper q E.5. 5.1. 250 m2 per hour per 490 m2 per hour per 490 2 per hour per 250 m2 per labourer : limestone, consistent limestone, compact clay, large stone, as follows: chalk, sandstone, Granite and gneiss, hard Granite and compact gneiss, compact slate, congolomerates limestone quartzite, syenite, basalt Bank gradients The slope of a bank should always be less than the angle of repose that would It is defined by the be formed in a bank abandoned to the action of weathering. base and i the angle of the ratio H tgi where H is the height of the bank, B its B bank to the horizontal. q Fig. E.14. Banks cut ground Type of soil Dry Hard rock Soft or fractured rock Rock debris, scree, pebbles Subsoil mixed with stones and topsoil Clayey soil, clay, marl Gravel, non-clayey coarse sand Non-clayey fine sand Bulking 5.1.3. Soil, three are - in land natural Banks Filled Waterlogged land cut in banks dumped soil Dry land Waterlogged 5/l 5/I l/l l/l 312 312 l/l l/l l/l 4/s Ill 415 I/l l/2 213 l/2 415 II3 2/3 l/3 213 l/2 213 l/3 l/2 l/3 II2 l/3 - consolidation when extracted, types: the coefficient been extracted: increases of initial of persistent in bulking volume. F which This is is measured called bulking. when the There soil has just F = K+ - the coefficient land bulking F' wnich is measured after consolidation: - - the coefficient of soil which of newly Fig. of consolidation has consolidated extracted 140 - which is completely the decrease in relation in the apparent volume to the initial volume soil: E.16. Walked soil before consolidation Soil Fig. Values E.17: Type of coefficients of bulking after and consolidation Coefficient consolidation of soil j ffiz;y';" ~rsistant I F I F 10-15 % l-l.5 x 8-l.? % 15-20 '$I 1.5-2 % 12-15 % '-Heavy soil mixed with sand Clayey soil Clay Marl Very compact clay and marl Scree Hard core tine) 20-25 % 2-4 % 15-17 % 25-30 % 4-6 % 17-19 % 30-35 % 6-7 $ 19-21 % 35-40 % 7-8 % 21-23 % 40-65 % 8-15 % 23-30 % 30-40 % 8-15 % 17-18 % 40-65 % 25-40 % IO-IS % out sand Knowledge earthworks. bulked vo= of coefficients To produce a fill volume (before v 1 + F. of with a final consolidation) bulking and consolidation is important volume V', it is necessary to of V VI and excavate an in l+T q of soi T Gravel Topsoil, consolidation in carrying place a provisional situ volume of Tools 5.2. The tools mechanical used manual earth moving are either hand tools or portable tools. Trenching 5.2.1. for The main tools manual tools are: This is made of steel and has a point at one end and a blade The pickaxe. the wooden shaft is about 1 m in length. at the other; The point blunts rapidly when used in hard soil; it can be repaired by welding on a new spike. A smith and his mate can repair 30-40 pickaxes per day. It is necessary to expect that about 20 per cent of the stock of pickaxes will be in fcr repair. The pick. This The wedge. break fissured a pickaxe with made of steel This is rock. A steel The crowbar. Portable is mechanical lever tools Pneumatic spades which have an output of 5-10 from a small mobile m in 1.20 Tools 5.2.2. for loading For loading - the are used in times higher which a steel blade A shovel load loaded - 5.2.3. end for use in soft with a 2 kg sledgehammer used to break fissured rock. to rock. heavy than break pick, soil the soft of compact clay hand pickaxe. or wedge fissured rock. and crowbar and receive their or marl. which compressed They They have an they replace. air supply compressor. earth excavated shovel length each include: by one man alone motor at and struck Pneumatic picks which are used to output 5-10 times higher than the They can be handled a point earth, use can be used weighing usually 1.0-1.5 weighs is made of: for excavating very loose earth. It is made of kg fixed to a wooden shaft 1.20 m in length. 2.5-3 kg; lumps weighing more than 5 kg are by hand; the --fork by size. Tools which for has teeth soil Depending on the means are employed: - wicker - hoppers - 2-man - 40-60-litre 3-4 and is used to grade stoney materials haulage country, local baskets; carried cm apart on the bamboo stretchers; wheelbarrows. back; resources and haulage distances, the following - For larger equipment - is pack capacities used, saddle and haulage such with 142 - distances a capacity loo-150 of: 120-150 and cart l,OOO-1,200 - sledge with - shaft, piece Example of kg for a pair of 100 m), animal-drawn of side an ox, a mule, a donkey, a camel; 400 kg for of: an ox, donkey of 2 m2 and 2,000 which the kg for animals are it may be necessary not available, motor barrows, dumpers, etc. is equipment: or horse, oxen; 400 kg for wood to each haulage haulage of a capacity a capacity Where animal mechanical with for kg for kg for kg for kg 60-100 single-axle than as: TO-120 - (greater 7 m2; harnessed. to use light productivity HAULAGE AND UNLOADING OF HOMOGENEOUS LOOSE MATERIALS (by ox cart) 4 On level ground cework d on 0 iO0 200 Distance The upper curve vision, with payment under poor supervision, Mean cart 5.2.4. Tools capacity: for soil 300 in 400 500 metres shows the outputs attainable with good organisation by piecework. The lower curve shows the outputs with daily rated payment. 0.315 and superattained m3. compacting The simplest soil compacting tools are hand tampers maniptilated by a single worker and pneumatic tampers also operated by a single worker and supplied with compressed air by a small motor compressor. 5.3. Manpower Earth moving is ideal for the employment of large and meets perfectly the objectives of labour-intensive utilisation is a solution to the economic problems of L numbers of unskilled workers work in which manpower a developing country. - 143 - there are However, and the factors affecting conditions and limits working conditions for climate; - workers' - nutritional - local customs which govern periods of work, of certain types of work (work with feet in - skill - the conditions - the way in manpower in this way, include: health; status; in the of certain use under which workers LOADING, grading is or project buffalo-drawn soil. pil,:s (e.g. soil tipped in small piles (e.g. scraper maximum haulage capacity: differences in and supervision. output that can be bamboo scraper) The scraper by ploughing. Average output: can also by a buffalo cart): be used 5 m3/hour by a dump truck): 25 m3/hour 0.05 m3 15 10 5 0 20 Distance 40 in 60 metres curve shows the outputs that are obtainable with good organ isat ion The iower curve shows outputs attainwith payment by piecework. supervision and daily rated payment. The special factors - age and condition - proximity - soil - slope of water affecting of for type; of out; 20 0 The upper and supervision, able with poor carried being illustrates the of organisation loose large cl s a ff; .5 m c +.A 2% 3 u 803 a is paid. loosened spreading in inacceptability HAULAGE AND UNLOADING HOMOGENEOUSLOOSE SOIL previously tipped the are (using The soil job distribution, water, etc.); tools; which The example below clearly expected depending on the level soil use of and output - fcr the haulage route. buffalo; buffalo; productivity include: - '144 - Labour 5.4. organisation A manual earth-moving water labourers, leaders, and a watchman of for the There is the work. usually The basic gang - either - or two men: - or three site comprises: carriers where supervisor, necessary, four a topography men or gang team, a tallyman equipment. one gang leader for every 20-25 men depending on the difficulty comprises: one man with a pickaxe one with and shovel; a pickaxe men, with two and two shovels pickaxe a site and the other pickaxes and a shovel in softer ground. with for a shovel; very hard ground, or one on the means employed. The way in which manpcn pr is used may vary depending Either the earth moving can oe done entirely by manpower or by manpower in combinaThe type of soil tion with local resources such as animals or mechanical plant. Where the and haulage distances will often decide the solution to be employed. it may be better to excavate and load by hand and haul haulage distances are la?ge, and ripping Certain work cannot be done by hand, such as subsoiling mechanically. manpower is a back-up here. certain earth-moving Finally, applies to ditching and banking in Earth-moving 5.5. 5.5.1. Trench methods digging jobs are suitable for piecework payment; this particular. and outputs and spoil removal trenches may be dug directly with a shovel; In very loose ground, in ground, the soil is first loosened and then removed with a shovel; pickaxes, wedges or crowbars may be required. in harder hard ground, The work may advance along the axis of the trench using two or three workers: the first loosens the soil with a pickaxe while the second (and third) follow and This is the procedure usually used for shovel out the s%oil or deepen the trench. narrow trenches. to Fig. For wider trenches, the trench axis. E-18. the workers may be arranged in a line and work perpendicular - 145 - Trench digging methods Fig. E.19: Terrace construction Fig. E-20: Construction Deep trenches are of should shown in a protection the diagrams below: ditch be dug as follows: Construction ditch with downhill Output in earthworks and trench size. Soil other such Cohesion pickaxe is and adherence as soil the considerably water measure content of soil depending are on soil the main factors may also play a role. hardness and directly characteristics influencing affects the output; output of the man. Density Soil Worker sufficiently way. Under follows: density cohesion, factors varies of a protection spoil thrown affects adherence the shoveller's to the tools output. affec'ts the output of both pickaxe and shovel men. Workers should be deployment also has a great effect on output. spread out (about 2 m apart) so that they do not get in each other's optimal conditions, maximal output of workers using hand tools is as - 146 - Fig. E.21: Output in manual IType of soil earthworks Daily I 3.5-5.0 2.5-3.5 l-O-2.5 .In 'ihe case of narrow ditches, about 20 per cent if the trench is Some common outputs in fig. E.22. Fig. E-22: Common values and the for (m3/md;l 5.0-7.0 Light soil Ordinary soil Heavy soil Very heavy soil shown output workers are hindered less than 1.20 m wide breakdown output in between trench digging and output or over lowered by 2 m deep. and spoil removal are digging - Soil type Topsoil etc.) (loam, Cubic metres excavated and thrown 1 m in an a-hour day Breakdown of 8-hour Excavation (hours) Spoil removal pitching or loading (hours) 3.0 5.0 3.0 2.0 5.3 2.7 1.5 5.7 2.3 1.5 5.6 2.4 1 .o 53 2.2 sand, or clay soil, moderately compact Hard, compact soil Chalky soil Marly Waterlogged soil Moderately hard laterite Very hard laterite Soft rock worked with pick or wedge 1.0 6.7 0.8 7.0 0.6 7.0 1.3 : 1.0 1.0 day - FIR. Maximum E.23: Type shovelling output Hourly of operation shovel throw a wheelbarrow Loading Loading Loading a truck a cl^rt a lorry Spoil 5.5.3. in loading output Light Simple Loading 147 - in soil or picking m3 of bulked up soil soil Ordinary soil Heavy soil 1.5-1.0 1.0-0.8 0.8-0.6 2.5-2.0 2.0-l-5 1.5-1.0 l-O-O.8 2.0-l-5 1.5-l-O 1.0-0.8 0.6-0.4 l-5-1.0 1.0-0.8 0.8-O-6 0.6-0.4 1.0-0.8 0.8-0.6 0.6-0.4 0.4-0.3 Very soil heavy 0.6-0.4 haulage The main means of haulage are shown in section 4.2. The simplest means are usually used for haulage distances of less than 100 A wheelThe wheelbarrow is the most common and has a capacity of 40-60 litres. barrow pusher can complete a return journey of 30 m in each direction whilst a shoveller is filling a 50-litre barrow. Animals or mechanical equipment Animal haulage requires flat ground empty return being uphill. The theoretical output for are used for or the haulage transport haulage should equipment is q . of more than 100 m, be downhill with the given by the formula: 1 cu x v Rt = 2 in which: Fig. Fig. E.24: Cu is V is the the E-24 gives Theoretical payload haulage examples capacity speed in of output Equipment theoretical of cc various outputs. types 0.075 of haulage equipment Remarks RI = 1 CuxV 'z (t) (a) Wheelbarrow, 50 1 Cart - 1 horse, 750 1 Cart - 2 horses, 1 500 1 Motor barrow, 300 1 Small dumper, 350 1 Small dumper, 600 1 km/h. Small haulage equipment 0.12 1.125 1.7 2.250 3.4 0.450 1.12 0.525 1.3 0.900 2.7 1 3 wheels, 4 wheels, 4 wheels, 3 hp 4 hp 8 hp - of Wheelbarrow O-5 per cent Fig. 148 - Output and gang haulage: (good haulage track). composition for natural gradients E-25. Volume of No. material of workers (m3) Haulage Spreading 2.0 1.0 1.0 13.5 1.5 1 .o 1.0 8.5 10.5 1.0 1 .o 1.0 6.5 8.0 1 .o 1.0 0.5 5.5 7.0 0.5 1.0 0.5 In situ Bulked Loading 13.5 17.0 10.5 A buffalo-drawn steel scraper unloading of homogeneous loose soil of 1 gives good results for for haulage distances The diagram below shows mean outputs 0.075 m3 on flat ground. for the Compaction loading, haulage of up to 100 m. scraper which has and a mean capacity 25 20 Output in m3 for an a-hour day 15 10 Distances 0 5.5.4. Unloading, spreading in q etres 5 40 20 60 80 100 m and compacting manual spreading can be carried out at a rate of 2-4 m3/h/man. After unloading, The work varies however depending on whether the spoil is normal or compacted. Normal spoil is usually encountered in the type of soil conservation work described here, with the exception of fill for dams. Ordinary spoil vegetable debris. being compacted. should be cleaned of mud, running It should be tipped over the total soil, peat, sods, stumps and height of the dump, without Compacted soil should be prepared in the same way, then evened out in Since horizontal layers 15-25 cm thick over the total height of the platform. compacting is often inadequate on the edges of banks, the structure may be 20-40 m oversized on each side. Each layer should be dampened and then carefully compacted with hand tampers or drawn rollers to the specifications laid down. Before each new layer is spread, the base should be scarified, then dampened. The gradient of the bank slope r,houid be checked using a jig fitted with a spirit level. Output for manual compacting higher using a drawn roller. is 1.2-i-5 m3/h using hand tampers and IO-15 times - 149 - Bank grading 5.5.5. The definitive every Workers gradient I 5.6. is is trimmed are given to of the bank by cuts made with a pickaxe about mark and stake out existing excavate a trench (e) dig (f) stretch pickaxe; a line (g) roughly dig (h) give a trench levels check the bottom. at between intervals two of about The 1.5 m. cuts. or ditch stakes with vertical with between these shape with the the 30 cm wide out the as follows: width and, sides the and mark out level; with to planned banks; a spirit of the using banks; to the cross-section are canal; stake soil, (d) check to line a line involved the between final a horizontal of a canal operations a line trench; top earthworks The various run the along from by stretching Construction on the working placed checked Examples 5.6.1. (b) is 10 m. The slope I gradient a jig. ditch; a pickaxe, the mark the finished depth shape of the the edges of ditch, the boundaries of of the ditch; every 5 m; ditch with a - 150 - Fig. E.26: Ditch digging techniz Staking out the final dimensions of ditch Phase / 1: Staking out the final width and vertical excavation down to specified depth Phase 2: Excavation of a trench to final width of the ditch Phase 3: Shaping Phase Checking the section with spirit level 4: crossa Construction 5.6.2. The various set of operations mark up site, construction strip dam, additional a coffer dam for from foundations in insta as follows: reconnaissance; temporarily diverting the water course; basin; foundation are: a; ground and emptying filtration 11 foot from to natural take-off bring are zone; topsoil install involved and trees borrow remove fill of dams out vegetation prepare earth level; pipe ; materials; drains; fill; civil engineering work for take-off civil engineering work for flood cover banks; install hydraulic finishing work. Reafforestation principle working 6.1. Nursery This escape; equipment; E-6. the and feedback; IMPLEMENTATION techniques methods have been and outputs. OF REAFFORESTATION WORK described in Chapter B. Given below are work comprises: - nursery construction; - seedbed construction; - sowing - pricking - watering; - plant in seedbeds, out into plant plant beds beds or or pots; POtSi transport. The tools lines, stakes, used are: hoes, pickaxes, watering cans, secateurs, spades, shovels, rakes, spirit polyethylene sachets, etc. levels, Work times depend mainly on Nursery work requires large amounts of manpower. A basis can be the type of plants being grown and the type of care being given. for various plant spacings, gives the following taken 0.3-0.6 hours per plant which, work times: - 152 - Plant spacing No. of (m) Work times plants/ha Minimum Maximum days/ha days/ha 3.0 x 3.0 1 100 47 94 2.5 x 2.5 1 600 68 136 2.0 x 2.5 2 000 86 172 2.0 x 2.0 2 500 107 214 1.5 x 2.0 3 300 141 282 1.5 x 1.5 4 400 188 376 Planting 6.2. Preparatory preparation. work includes marking The tools used for planting pickaxes, hoes, lines. are out, the staking out, same as those clearing used in and soil trenching, i.e. shovels, The basic gang comprises two workers: second transports the young plants in pots, holes and completes the planting operation. may do the whole operation himself. The workers advance in leader being slightly ahead of materials. Planting should A foreman should to ensure that from the tip. be sufficiently first digs the planting hole, the sacks or baskets, places them in the In certain cases, a single worker straight or diagonal Other workers ensure the line; a constant gang supply and deep to take the whole root Roots should not be bent system without crushing the roots against each other. and should be fully covered with earth. To in towards the centre of the hole, hold it at the earth mark whilst ensure the pl.ant is covered to the right depth, Soil should be placed around the plant with care the hole is filled with soil. and trodden down when the hole is half full and then trodden down once again when the hole is completely full. plants pulled holes a more or less of the rest. the wide follow up the line of planters they are vertical and do not to check a proportion come out of the ground of the when Under the best working conditions, when using hand tools, gangs c?.n plant 100-500 trees in an R-hour working day. This level of output will vary depending density of vegetatioil cover, type of on the diffic:Alties of the site (topogra;:,y, scil, soil preparation, etc.). 6.3. Construction This comprises of forest basically tracks earth-moving According to the FED productivity requires 7.6 hours of work per cubic work. norms in Rwanda, forest metre of earth moved. track construction - 153 - 6.4. Plantation maintenance Plantation maintenance years (replacement of plants regularly thereafter. Maximum output Clearing for around requirements are considerable that do not take, weeding) weeding young is 200-300 plants requires maintenance In Mohanda (Burundi), callitris plants/ha have been estimated days pel hectare per year. 6.5. Fig. Planting along the first decrease two m2 per man-day. a man-day for requirements for at 0.3 h/plant, 200 plants. a plantation of 3,000 i.e. 128 T-hour working planting Examples of a plantation: Song contour lines E.27: during and then contour lines_ #-B -w---c, -7zg-z L.ine of stakes With an interval d between plants, a line to the slope and spaced at d from each other. in A first length. row of trees is planted along the of stakes contour is line inserted using however, Subsequent rows are planted in a similar manner; between the trees is less than 0.8 d, a new row is started. When the inserted until distance between rows is 1.6 d or more. an additional the spaoe between the rows falls to 0.8 d. perpendicular a spirit when the level d distance row should be Planting in an arid Arrangement designed concentrate surface zone. to run-off Productivity 6.6. norms Manpower requirements in man-?ays/ha with a density covering nursery work, planting and maintenance: ranging in Burundi. Some typical - filling - pricking - planting: - hole - holing: outputs sachets: out digging (Rwanda): 0.03 sachets: 100 plants man-days per plant; 0.01-0.02 man-days per T-hour and planting: number of holes in rocky soil: in stony soil: in deep soil: care: of 2,500 plants/ha, from 272 to 478 man-days per plant; day; 300 plants per man-day; per man-day 24-30 30-45 40-50 - plant - creating a windbreak by direct sowing: 24 man-days for 100 m comprising staking out, tilling, sowing, mulching, separation, three thinning outs, separation or natural regeneration followed by thinning out; - creating a windbreak by planting: rows and a spacing of 2 m. 200 per man-day, i.e. 300-400 m2; 22 man-days for 100 m for plants in three - IMPLEMENTATION E-7. Installation of The collectors The drains Fig. which marked are mark out the mark out distances a line equal mark out a second the a drainage are first 155 - network out in grouped drain OF DRAINAGE WORKS the in in parallel relation perpendicular to the drain same way as the to to the spacing; perpendicular lines line can fixed ditches. be marked reference first drain, which is out as follows: points; and this staked out is in staked the out at same way as first; trace a line between the points staked this line the specified drain length; out stake stakes out E.28: the whole Installation drain of network a drain with on the perpendiculars, every marking on 30 m or so. network Perpendicular Stakes indicating drain spacing e--e ----- 7.2. Tracing 0.30 Drainage and 0.70 out trench width trenches intended m at ground level The trench A line marked with total is marked between the a pickaxe. out on the pickets The line is subsequently and a second groove is marked The line itself is for laying of underdrains vary in and 0.06 and 0.20 m at the base. marks ground the by a line trench of string external attached at stake. 7.3. Calculating trench The mean depth The level is depth and the obtained minimal using drain a dumpy gradient level. are between pickets. along axis; moved sideways to the along this line. a 30-m length of width known. this limit each a groove of the end to is trench a wooden - 156 - A 20-cm wooden there is no rapid A surveyor's staff The elevation The depth uniform of is hammered into is each placed alongside stake is approximately every 50 m if from stake. a single then calculated depth is written surveying point. whilst maintaining on each marker. used may vary slightly depending used for manual trenching: an ordinary - a fork - a draining spade which has a blade arranged in direct prolongation of are curved slightly inwards; a relatively garden spade and pickaxe for for removing excavating the country. the sod and the top layer 40-50 the on the cm in shaft. of In general, topsoil stony the layer; soil; length and lo-15 The long edges cm in of the width, blade a scraper which is used to finish the trench bottom and produce the correct It is made from a piece of curved metal 30-35 cm in length, shaped slope. and sharpened at the end. The shaft is 2-3 m in length and is at 40-50 0 to the blade. Blade diameter should be matched to the diameter of the drainage pipe. It Fig. each noted - uphill level digging The tools following are - soil in gradient. of the trench is and the relevant slope, Trench 7.4. stake change is necessary to start digging so that any excess water will E.29: Longitudinal Digging a drainage cross-section the drain at the drain away as the lowest point, work advances. working trench Strip 1st digger topsoil Soil surface _---me---- Second diaffer Swcified - Plan Spoil view Transverse cross-section bottom --- from 2nd digger Topsoil Spoil from 2nd digger - 157 - The first digger cuts placed on the right of way for the pipe layer. up the the sod and removes the trench about 30 cm from This topsoil is the first This tOPSoil. the trench to be used topsoil is edge leaving a pathwhen the trench is backfilled. The trench is The spoil then dug with a draining is thrown onto the left 50 cm. spade to a depth of approximately bank of the trench. The second digger works approximately 3 m behind the first digger. deepens the trench but to a smaller width and throws his spoil onto the He digs the trench an additional 50 cm approximately. of the trench. the trench is further deepened by a third digger i&o works in the sav, as the second. The trench for 7.5. are the obtained in this way will be slightly than that. specified drain. Trimming the trench For this operation mounted adjustable bottom one uses rectangular the scraper crosspieces and wooden ("boning The object here is to connect these points to dig the trench to by a uniform gradient. the Using the drainage pipe. the smoothed The trimming Fig. shallower He left bank 2f necessame way E.30: scraper, process Trimming trench is a trench shown bottom is depth diagrammatically or steel rods"). marked staffs on the and compacted on which markers to receive below: bottom n A Specific bottom Measuring rod and the - 158 - At point A marked by a stake, boning above rod adjusted the bottom of places a fixed the boning rod At point marker B where the depth hb above the at a height C, one places A, C and B. line At point between drawn To trim the trench at height H. The correct mark on the the On flat to have the assistants. laying The tools the trench such uses the that it scraper a long-handled - a steel pick laying tile hook to are in piles pick cutting brought of 30-35 or concrete up the pipe or holing to form on which one of a a sight a mark has established been between and it is preferable an operator and two trench earthenware hammer for The drainpipes placed trench. for one places will is reached when R sight line is and the crosspieces A and C. the is known, The height ha above the ground. is equal to Pa + ha = H. of a length worker and backfilling used - They are from the Pa of Pb of the trench is also known, ground so that hb + Pb = H. the depth shaft depth to height the trench a marker depth, the ground, checking the gradient is more difficult work done by a topography survey team comprising Drain 7.6. trench scraper where the site pipes pipes and lay the pipes on a trailer every are: it in the when being trench bottom; laid. or on stretchers. 10 m or so and at a distance of 3-4 m The pipes are laid starting at the highest point. The pipe layer picks them up with the tile hook and places them on the trench bottom, striking them sharply several times on the edge with the iron hook to seat them together properly. When it pipes at the is necessary to turn the pipes an angle with the pick hammer. When placing together drainpipes covered with earthenware crocks. Long drains sections on the (6 m) are put trench bottom. Long concertina-type of together PVC drains round di_fferent outside machines which dig the trentih; - machines which dig the trench backfilled alongside first the The trench is was laid separately out by shovel using the remainder not this diameters, the and also designed ditch is the done by cutting joints and then laid are in whole manual laying and are In certain countries: laid directly on the trench bottom by drainage machines. They do not come these machines have almost entirely replaced manual labour. It should within the framework of this report on labour-intensive public works. merely be noted that there are two types: - are a curve, lay the for drainage pipes. by covering the drainpipe with trench. Subsequently, backfilling of the spoil and then lightly topsoil which is carried compacting. - Fig. E.31: Main drainage 159 - tools II III I 3 Y-L -W Footpiece I4 Drainage spade Scraper n IL Tile hook -. I -===I II I T Pick hammer Small spade drainage - Mrnpower A,. 'i -7. and output A drainage - a skilled - site site in manual usually work supervisor workers and 4-8 whose jobs 2-4 workers depenciing on the 1 worker; pipe transport: 1 worker; pipe unloading: 1 worker; pipe laying: 1 or 2 workers. backfilling the trench: a good (loose earth), trench per hour. a gang comprising a site conditions, Output of only half drain per day. types divided up as f'o depth; drainage worker can advance supervisor and four this may nevert,;eless diggers be con- CONSTRUCTION OF MASONRY WORK E.8. Different are tlanch earthworks: Under these can lay 250 m of sidered normal. 8.1. drainage comprises: Under very good conditions m of ordinary 0.8 m depth 6-8 160 - of masonry work The term masonry work covers all types of structures made from stone. for the construction of gully Masonry work is used in soil. conservation projects or ravine erosion protection work when the necessary materials are found in adequate quantity close to the site. One may distinguish between dry-stone - normal ceme,lt masonry or lime work in mortar; which the gaps - gabion sage. masonry work which the stones Dry-stone When they made of blocks work masonry in in which types _ 8.2. masonry three stones of are masonry fitted between are are subject to of a sufficient the held without stones are together in the action of water, dry-stone weight to resist the action on the basis of current Vmax = 0 .qpiyT--A which: Vmax is the maximum current n is the rock l3 is the acceleration Ps - P = density P together binder; filled with a metal mesh work Stone size may be calculated IZBASH's formula: in work: speed at high Water (9.81 m/s21 diameter of of the gravit.y material under water. of facings should the current. speed using, for be example, - 161 - the thickness of the dry-atone In principle, it When atones are placed on erodable material, stones and the bank a filter made up of a gravel Mean output for stone breaking Mean dimension breaking facing is I,40 11 mm 0,IO the dry-atone - shape and compact - place their the stones by laying them greatest length is in the - insertion of approximately facings is least 30 cm. between the thickness. as follows: per m3 30 mm 40 mm constructing cm in IO-20 0,8Om3 0,75 a3 0,70 m3 0,55 m3 of to place mean output of 8 hours Approx. man-day after 45 mm The method be at by hammer may be estimated 50 mm m3 as follows: bank; perpendicular thickness of larger atones, called 20 cm into the bank, Stones these voids should be fitted together can be filled with atone Fig. Construction E.32: necessary bed of mm 135 should of a dry-stone to the the bank facing; surface which are anchored headers, at a distance of one every with the fragments. minimum of voids; that at a depth m2. if faca \20 30 cm necessary, of - 162 - The table below shows facing constructed manually the number of at thicknesses Man-hours Material Normal per 20 cm Brick Sandstone/limestone 0.55 0.60 0.60 0.65 Granite 0.65 0.75 masonry that the immediate stones, vicinity in thick the required of the site. sizes, to running are bound together with cement given below presuppose of the site. that Wall thickness in cm mortar when the the materials cm in the specialised 20-40 is structure are Mean thickness of 15 cm between been is exposed water. The outputs immediate vicinity It have work The atone used for this type of work may be natural stone of largest plane or cut atone of various dimensions. Stone cutting wc:-k and cut stones are used only rarely. The stones dry-atone m2 15 cm thick These figures presuppose supplied from a quarry in the 8.3. m2 of man-hours required per of 15 and 20 cm. 4.0 mh/m2 60 7 mhlm' 5.5 90 10 mh/m2 8.0 mhlm' mh/m2 mason and the For a mortar-bonded - manual mixing - mortar jointing Approximate of the time per square metre is divided take into labourer. it wall, cement as the quantity required or work is lime necessary mortar: advances: of mortar Thickness of wall in cm also 1.0 to man-hours 8.5 man-hours per per account: m3; m2. required: Mean thickness 15 cm 30 0.07 60 13.00 90 20.00 the 30 cm 5 mh/m2 that to stones 30 can be taken the brought of stones 30 cm m3 m3 m3 0.05 10.00 15.00 m3 m3 m3 equally - 163 - 350-400 kg of cement per cubic The cement content of a mortar is as fOl10W3: If the sand is very fine and made up of grains of less than metre of dry sand. 0.5 mm, the quantity of cement should increase by 20-25 per cent. The amount of mixing water (in litres) should be 25 per cent of-the weight of the cement plus 6 per cent of the weight of the sand, i.e. 220 litres of water of water for a mortar for mortar containing 350 kg of cement/m3 sand and 23g litres containing 450 kg cement/m3 sand. Mortar and plastic, not fracture. is checked not stick Constructing The ball of mortar should be firm it by hand. of 20 cm, should and, when dropped from a height by modelling to the akin a mortar-bonded wall comprises the following - ccnstruction of concrete footings crete for small structures, steel - laying - settling filling - laying headers good bond with - cleaning - pointing; in which the joints are cleaned to a depth of 3 cm before the and then finishing them with a fine sand mortar with a cement mortar sets, content of 600 kg cement/m3 sand. Gabion 8.4. stones on a thick layer at the base of the reinforced concrete operations: of mortar the bricks against each other the voids with atone fragments the 40 cm long the bank; joints masonry (bed of mortar); by striking them with a hammer and without moving the larger atones; at intervals by removing structure (ordinary confor larger structures); of approximately mortar 1 mc to ensure a runs; work Characteristics Gabions are boxes made out of metal mesh filled A distinction is made between: cage gabions and footing gabions (0.50 body work of structures; of foundations which may be subject to deformation. Fig. E-33: Common sizes Footing gabions Width (m) 1 43 111 I 1 2 1 5 6 Cage gabions (1 carefully aligned stones. m high) used for the main m high) used for the body work of gabions Length (m) Type with 1 1 '5 6 1 1 Height (ml 0.50 Weight (mesh in kg 9.8 120-100) 0.50 0.50 0.50 18.2 14.0 22.4 26.6 1 1 1 1 1 14.0 19.6 25.2 30.8 36.4 - 164 - Weir made from gabions and dry-stone The gabion walls are made of galvanised The most common mesh sizes hexagonal mesh. the size of the smallest filling of this size, and the weight of the atones around 5-10 kg. T'I ensure sides zre held that the gabions do not together by steel wire. Method of manufacturing mesh is the wire cut (b) the walls (c) the gabion (d) the edges are bound (e) the bottom soil; anchored (f) the stone (g) fit in the (h) continuation necessary: (i) closing adjanent folded is located is filling The tools in its to the wire - pliers; - jumper - wooden blocks - a sledgehammer fig. adjacent E.34) flat; are tied together; position; gabion; ground using steel stakes rammed into the trusses; whilst and tying adjusting the upper the include: stake; to wedge the to drive stones; home the jumper internal edges together cutters; bar or steel when stacked, commenced; of filling - final to the adjacent internal required out (cf. in to form a box and the edges is the cover gabion. shape gabion to shape and laid (Cape Verde) steel wire, with a double twist are 100 x 120 mm. When using mesh stones should be at least 160 mm, their a stone-filled (a) are lose work bar. trusses with those as and when of the - 165 - Fig. E-34: Assembly and installation of wire . .63 .. mesh gabions . . 6s,. Fold walls to form a box open at the top Wire mesh cut and laid out flat Tie edges together Detail truss Fill Using a jumper bar align the sides with stones Fill centre of gabion with small stones if not sufficient large stones Trusses Cabion 4xlxO.5D Plan Plan .. .. .. .. l3IIl m-1 Elevation of a wire Diagram of trusses Trusses Gabion 4x1~1 Plan Elevation Gabion 0.5 m high used for footings Gabion 2x1~1 Plan Kiilg m Elevation Elevation Gabions I m high used for the main body of the structure on the footing or foundation Tying the edge of the cover with adjacent gabions Tying the cover Unit weight Gabions can be installed objectives of labour-intensive using unskilled projects. labour and they meet perfectly the - 166 - Weir in stone Front view Detail masonry - Characteristics of concretes Concrete is obtained by mixing and water which act as the binder. There are numerous - CARRYING OUT CONCRETE WORK E.9. 9.1. 167 sand categories of and gravel (called aggregates) lean concrete aggregate; with a cement content of 250 kg cement per - rich concrete of aggregate; with a cement content of cement - normal concrete aggregate. a cement To obtain good quality to avoid the use of friable adhesion between aggregates. The choice of content concrete, materials, materials plays it 450-600 of kg of 350 kg cement cubic per per metre cubic cubic Concrete A simple mix a 50 kg sack - two 70-l - a 70-l - 20-30 an important role in concrete quality. dimensions where same be sharp. (by of barrow barrow volume) and contain or sulphates less than 2 g/l of dissolved which attack ordinary cement should of concrete is as follows: load of of gravel; sand; depending normally shovelled in first, followed then water is added. Mixing concrete. category a batch cement; loads 1 of water for on the Another way of proportioning with a known capacity. A crate capacity of 0.1 m3. Lower of preparation - Concrete metre is necessary to use suitable aggregates and reduce voids to the minim.um and ensure good The water should be clear, odourless salts. Water containing organic matter should be avoided. 9.2. of metre The aggregates should be correctly graded with components of various to minimise the size of voids. It is advisable to use well-known quarries the aggregates are always produced using the same materials and under the conditions. The sand should cement concrete: - with with concretes ambient temperature. concrete constituents is to use bottomless 60 cm on each side and 28 cm deep will have The aggregate is a concrete mixer. the mixture is first turned dry and cement; two minutes and should produce a homogeneous be mixed by the lasts crates a in can be mixed by hand in small quantities. - 168 - the concrete is usually transported sites, The time between mixing and placing should On small wheelbarrow. in moderately to the not placement exceed site by 20 minutes warm weather. from a height during placing since this The concrete should not be dropped When it is placed in wooden shuttering, the cause segregation of aggregates. watertight and wetted prior to placing. When a large latter should be clean, it should be poured in successive layers. height of concrete is to be placed, The concrete should improve its consolidation. suitable for shallow Sodding is carried rodding or vibration to be compacted by tamping, Compacting can be done by hand tampers and this is then layers out (20 cm). by thrusting Vibration is the most effective compressor and a vibrating needle. a steel rod it procedure; into is after-care is Once the concrete has been placed, to ensure that the water does not vaporise too rapidly; After-care may be carried out by covering the concrete constantly moistened Shuttering may sacking, the carried required this with placed out concrete. using a for around two weeks is called curing. a curing agent or with etc. may be removed after 2 1 days of drying. Shuttering 9.3. usually These are moulds, is placed and the concrete is suitable for their task. ment cient least which the steel be constructed reinforcement robustly and be Usually planks of 2-3 cm thickness are used and the distance of the reinforcerods between themselves and from the walls of the shuttering should be suffito allow the correct placement of the concrete and leave a free space of at the maximum size of the aggregate. A distinction of the structure used for exterior 9.4. made from wood, in poured. They must Steel is made between the which are underground walls. reinforcement ordinary or not shuttering which is visible, and high-quality used for parts shuttering 4 The main purpose of the steel reinforcement in reinforced steel concrete The is to accept the tensile stresses to which the concrete may be subjected. strength of a reinforced steel concrete structure depends not only on the quality of the concrete but also on the correct layout of the steel reinforcement bars. Two categories - smooth of reinforcement mild limit steel of maximum relatively bar are usually used: bars: elasticity: 2,400 permissible working poor adherence; kg/cm2; load: 1,600 kg/cm2; - - high-adherence steel high-adherence bars whilst the majority 4,200 kg/cm2; 2,800 kg/n,m2. steel they are 75 per of cases. bars cent stronger, scarcely it is for plain high-adherence round bars. A reinforcement plan designed circumstances be carried out with Steel bar steel reinforcement - the rods - loose - the - the individual components are rust rods Bending are is are radii mild is cut to removed bent to (R) of prepared the shape round in lengths Installing the rods should by the reinforcement annealed not exceed steel wire the fol:.owing Steel of assembled values: R =50 R=20 bars to ensure maintenance of Steel reinforcement too The bars oxidise and this walls are Since these awaiting Steel reinforcement which is left to be bent and then unbent, intentionally fissures in the rods, especially those brittle. it is advisable Consequently, steel. Storage plan; and the To ensure that the distances between the rods and the shuttering maintained during concrete placement, use is made of concrete wedges. wedges are embedded in the concrete, they must be rot resistant. sections no phases: The main concern in installing reinforcement bars is the distances between the steel and the shuttering walls. close to the wall is poorly protected against corrosion. results in blistering which may spa11 the wall. Steel reinforcement installation under R=30 steel reinforcement following bars bench: using should steel plain round former in brush; on a bending rods more expensive than advisable to use the required boxes, hoops, hairpins in mild steel 9.5. the a wire assembled steel plain high-adherence are by means of rods are labelled. steel - bars: limit of elasticity: maximum working load: Since 169 on the site awaiting installation is likely or otherwise. This is likely to cause of high-adherence steel which is relatively to have such reinforcements made from mild reinforcements reinforcements should be stored away from moisture. it is necessary to avoid contact For prolonged storage, that during storage rods of different diameters and different mixed up, which may result in subsequent errors on the site. with the strengths Ensure soil. are not - 171 - APPENDIX I STANDARD PLANS Standard Protection terrace ditch cross-sections Ridges Progressively constructed Bench terraces Grassed channels Bank stabilisation Bank protection Gully correction Stone dams Weir Small earth dam Drainage techniques terraces 5-6-7-8 176, 9 180 10 181 11 182 177, 178, 179 187, 188 194 12 183 13 184 14-15-16-17 185, 186, 18-19 189, 190 20-21-22-23 191, 192, 193, 24 195 25-26-27 196, 197, 198 28 199 29-30-31-32 200, 201, 202, 203 - -L 172 - 5 00 Ill - 173 - cm . - 174 - 1 .. . . =!itzLz~ AJ7- ._. 1, ..i. - y& ,~i -1,’ .\; .;F-T - 176 - 1 - 177 - I=rh c =v&h 3 z vaho tn. - 178 - - 179 - 6 4a L 6.50 m I 44s a-/, l.50 2.00 12.00 400 / 550 0.50 IO.60 0.60 IO.70 6.50 17.30 1.03 1 0.95 0.50 IO.60 1.7 5 1 1.80 - QamYlON 180 - blha4 cs 1 123456789 10 I40 I50 145 1.40 Ia 125 I+hdi~+diU#+di~[~) 0.5 1 0.4 1 03 1 07 1 02 101s 1 0.1s 1045 015 1 h= 100s 2.00 m. 1 = lOOr 5.00 m h 0.70 m. = -lmlGl -ct$Qace: w-k uldrk. 5.2 w6 ______*------ - PaqpSS ~VELY ccdwocrf2 3mi I~J’&, af+hlU &¶je 182 - COAJSTRUCTED ,L@xx ’ na?Qec~~: ’ - WlTll 183 - - 184 - - 185 - - 186 - .-- -1 V @Wi l=lu&g) WATTt.Eb 6ejctSS WITH EAeql- - gobion lad - cage L gobion +ob% I Cc- -- lboc q-i- ------.-.. ___ - 190 ‘- - GlLCY ~RRccnoN rholwcq 191 - - 192 - (hz 6 OAOm.) . ---‘e’ ----23 E - 195 - - 196 - IS.00 m -T I - 1.W 5 .oo 197 - 2.00 - 198 - - 199 - . . . . ..:* *..: , ,.. . . . -. .: . - -_ _.. _- 29 - 201 - . . .:. .. . - 202 - 2OoWmm eYKT--@:,= ~~- ---- si .---- - . . . . . . . . 203 - I -.-_----we OtJrhEkT Of AmzA\N \NTO a cnus cm2n mm=~ 32 :P-.B .- -mm-- - 205 - APPENDIX II BIBLII!GRAPHY -..--Reference 1 CTFT. Conservation des sols au Sud du Sahara. rurales en Afrique, No. 12, 1969. Collection 2 Soil Conservation Service of the US Department of Soil conservation manual, Washington, 1950. Agriculture. 3 Ministkre de la Techniques 4 BRGM. 5 FAO. 6 FAO. Cooperation. Memento de l'agronome. rurales en Afrique, 1974. Drainage agricole en rapport Bulletin du Bureau de recherches Section III, No. 2, 1978. avec l'irrigation geologiques Techniques Collection et la et minieres. salinite. Drainage of salty No. 16, Rome, 1973. soils. Irrigation and Drainage Paper, Drainage of heavy 6, Rome, 1971. soils. Irrigation and Drainage Paper, No. Earthmoving by manual labour and machines. Series, No. 17, Bangkok, 1961. 7 United Nations. Flood Control a FRO. Methodes et machines pour poterie et autres materiaux. Bulletin No. 6. 9 de 1'Agriculture: Agriculture et Ministere March-April d'information, NO. 347-348, A manual on establishment February 1973. le drainage Energie techniques 10 FAO. II La conservation M. DELOYE and H. REBOUR. Collection des techniques agricoles 12 FAO. Soil No. conservation for par canalisations en et machines agricoles, Bulletin forzt. 1980. in man-made forest, de la fertilite mediterrankennes, developing des ~01s. 1953. Soil countries. technique Bulletin 30. 13 Ministkre de la Techniques 14 GOOR et al. arides. Memento Coope?ration: rurales en Afrique. Les methodes FAO, Rome, de plantations Reboisement PONCET A. 1974. eionomiques, 16 L. GUEFIN/P. 17 A-1. FRASER. 18 W.F.J. Some monographs for van BEERS. International Institute spacings. Improvement. Bulletin No. 8. 19 M. POIREE and Ch. OLLIER. Eyrolles, Paris, 1973. 20 Paul A manual forestieres en zones 1964. 15 GARNIER. Collection du forestier. Guide pratique on the des chefs planting de chantier. of man-made forests, et Bit, agricole. terracing steep Editions slopes. 1981. FAO, 1973. the calculation of drain for Land Reclamation and Assainissement New methods of bench JACOBSON. scs. USA, December 1962. techniques Aspects de protection. - 206 - Reference 21 L. SACCARDY. Note Terres 901s. 22 L. BUGEAT: 23 Centre 24 Ministere de 1'Agriculture. francaise, Paris, Cours sur le calcul des banquettes Algerie, 1950. et eaux. de conservation des sols, de restauration Tunis, 1957. de recherche du genie rural en Tunisie. L'gquation Caicul universelle des pertes du sol, par M. WISCHMEIER. climatiques et utilisation parametres agricoles, pedologiques, SCET. CSOP, 1963. de l'equation. Retenues collinaires. des La Documentation 1963. 25 Techniques SOGETHA. Ministere de la 26 A. LENCASTRE. Manuel Paris, 1976 27 R. BenoTt de COICNACQ. travaux. Polycopie, 28 BEEK, K.J. and BENNEMA, J. Land evaluation use planning; an ecological methodology. Wageninghen. FAO, dot. 8 B-22. 29 H. DIENER. Programme pilote de travaux h haute intensitb les d'oeuvre dans la province de Muramvya (Burundi): reboisement. BIT, Ceneve, 1979. 30 Les petits ouvrages Afrique. 31 J. 32 Guidelines for the Organisation of Special Programmes. ILO, Emile COSTA, Sunhi' Nguyen T.B. THUY and Aime FARDET: 33 Introduction aux programmes speciaux de travaux Ph. GARNIER, T.B. THUY, L. GUERIN. BIT, 34 des rurales en Afrique. Cooperation, 1965. d'hydraulique gendrale. Travaux de DSR. Algerie, 1958. en gabions. Les ouvrages en gabions. Editions Ecole for Collection Eyrolles, des conducteurs agricultural Landbour Techniques de land Hogeschool, de maintravaux de rurales en RODIER and C. AUVRAY. Estimation des debits de trues decennales pour des bassins versants de superficie inferieure a 200 km2 en Afrique occidentale. ORSTOM, 1975. Labour-Intensive GUHA, Ibrahim publics. Genkve, Works HUSSAIN, 1981. Normes pratiques de rendemer pour les principaux travaux executes par des techniques h ha,ute intens iti de main-d'oeuvre (titre provisoire BIT, a paral"tre.