water management ; oadworks *esign in t he sa...
TRANSCRIPT
WorldiUt* £ Africa Transport Policy ProgramThe World Bank and the Economic Commission for Africa
Water Management in; oadworks *esignin t he Sa he SSATP Working Paper No. 29
VOLUME ONEREPORT
-44 ~ ~ -
Knowledge,Information andTechnology Center (KNIT)Africa Region/The World Bank
-- _J. ' ' , .' _'~~~~~~~~~~~~~~~~~~~~~~
Pub
lic D
iscl
osur
e A
utho
rized
Pub
lic D
iscl
osur
e A
utho
rized
Pub
lic D
iscl
osur
e A
utho
rized
Pub
lic D
iscl
osur
e A
utho
rized
Pub
lic D
iscl
osur
e A
utho
rized
Pub
lic D
iscl
osur
e A
utho
rized
Pub
lic D
iscl
osur
e A
utho
rized
Pub
lic D
iscl
osur
e A
utho
rized
THE WORLD BANKSub-Saharan Africa Transport Policy Program
SSATP WORKING PAPER NO. 29
WATER MANAGEMENTIN ROADWORKS DESIGN
IN THE SAHEL REGION
VOLUME 1
REPORT
April 1997
Knowledge, Information, and Technology Center, KNITAfrica Region
The World Bank
II I WATER MANAGEMENT IN ROADWORKS
This report was written by Messrs. Jean-Luc Frejacques and Jean Perrin, of the Consulting firm BCEOM, Societe
Francaise d'ingenierie, and put into its final format by Jean-Marie Lantran, Senior Construction Industry Specialist,
Task Manager for the Infrastrucure Division, West African Department, of the World Bank. The mission carried
out by the two experts was financed by the Norwegian Trust Fund under the Sahelian Operations Review. The
experts benefited from interviews and visits in Burkina Faso, Mali, Niger, and Togo. The report has also drawn on
inputs and comments from Mmes. and Messrs. Aline Cabal, Bernard Beck, Cynthia Cook (AFTES), Christopher
Hoban (TWUTD), Michel Ray (EMTIN), Pierre Henault (AF5IN), Jaffar Bentchikou (EMIIN), and Seykou
Hardara (Direction de l'Hydraulique, Mali). Messrs and Mmes Escoffier, Pontier (BCEOM), Amy Champion,
Caroline Moisson have participated in the editing, drawing, and desk top publishing. Maria Perisic and Vince Mac
Cullough have drafted the summary, Alex Greene has translated the original in French into English. Mmes and
Messrs Peter Watson and Alberto Harth were the successive Chiefs of the Infrastructure Division, and Catherine
Marshall and Jean-Louis Sarbib the successive Directors of the West African Department, African Region, were
responsible for this study. Final editing and production by Lawrence Mastri.
In order to reach a wider audience, this document is being informally published by the World Bank's Sub-Saharan
AfricaTransport Policy Program (SSATP) Working Paper Series under the direction of Mr. Snorri D. Hallgrimsson.
The findings and condusions expressed in this paper are entirely those of the author and should not be attributed to
the World Bank, to its affiliated organizations, or to members of its Board of Executive Directors or the countries
they represent.
I III
Contents
Chapter 1: Overview ............................... 1THE MISSION .................................................... 2
Chapter 2: Description and Purpose of the Works ............. ...... 3TYPOLOGY ..................................................... 3COMMONLY USED WATER MANAGEMENT WORKS ................................................... 3COUNTRY OR REGION SPECIFICITIES ................... ................................. 12SUMMARY OF CHAPTER 2 .................................................... 13
Chapter 3: Determining the Design of the Works .................. 15ROAD TRAFFIC .................................................... 15TOPOGRAPHY AND SITE SELECTION .................. .................................. 15RAINFALL AND HYDROLOGY .................................................... 16SOLID LOAD .................................................... 18EVAPORATION .................................................... 19INFILTRATION .................................................... 20WATER QUALITY .................................................... 20WATER BALANCE AND FLOOD ATTENUATION .................................................... 21SUMMARY OF CHAPTER 3 .................................................... 24
Chapter 4: Choosing Technical Solutions .............................. 27CONSTRUCTION COST .................................................... 27TYPE OF WORKS .................................................... 29CONSTRUCTION SPECIFICATIONS .................................................... 32FLOOD SPILLWAY .................................................... 34SUMMARY OF CHAPTER 4 .................................................... 36
Chapter 5: Economic Choice andJustification ................ ...... 39VEHICLE OPERATING COST .................................................... 40CONSTRUCTION AND MAINTENANCE COST .................................................... 40ASSESSMENT OF SOCIOECONOMIC BENEFITS .................................................... 41SUMMARY OF CHAPTER 5 .................................................... 43
iv I WATER MANAGEMENT IN ROADWORKS
Contents
Chapter 6: Development and Management of the Works ....... 45ORIGINS ........................................................... 45DESIGN AND CONSTRUCTION .................. ......................................... 46MAINTENANCE ........................................................... 48WORKS MANAGEMENT ........................................................... 48SUMMARY OF CHAPTER VI ........................................................... 49
Chapter 7: Summary ..................................... 51DESCRIPTION AND PURPOSES OF THE DIFFERENT WORKS .................. .............. 51FACTORS AND PARAMETERS ........................................................... 53MANAGING WATER-RETENTION WORKS ........................................................... 60
Annex 1: Bibliography ....... 63
Annex 2.' Model Terms of Reference.... 67
Annex 3: Terms of Referencefor the Mission ....... 71
Annex 4: Photos of the Works ......... 73
Foreword
U ntil recendy, road designers in the Sahel considered water a nuisance. It eroded theirbuilt-up structures, it penetrated their pavements and weakened the soils under.neath. It also washed away the gravel pavements that made the surface acceptable to
passing vehides, and, occasionally, would flush out entire segments of road, cutting surfacecommunications to areas behind the break.
But a new approach is gaining ground. It builds on the realization that new infrastructure ofany kind - roads, buildings, market structures, telecommunication installations - must bedesigned with, not against, the environment. Traditionally, road design in rural Africa wouldhave minimized the future damage by water to the road structure; however, this paper showsthat design can turn the road into a retention structure for preserving and managing scarcewater for people, livestock, and agriculture.
Volume I sets out the principles and practices of this approach to road design in the Sahel.Volume II offers road designers a practical manual on dealing with the real life problems setforth in Volume I.
These two volumes will help rural infrastructure designers in the Sahel to stabilize theenvironmental balance in construction areas, demonstrating that environmental concerns gohand in hand with economic development.
/1'S
Kevin M. CleaverTechnical DirectorAfrica Region
vi WATER MANAGEMENT IN ROADWORKS
Glossary and abbreviationsAADTC Annual Average Daily Traffic CountALG Autorite de developpement integre de la region Liptako-Gourma
(Liptako-Gourma Region Integrated Development Authority)CIEH Comite interafricain d'etudes hydrauliques
(Inter-African Water Works Studies Committee)DNHE Direction nationale de l'hydraulique et de l'energie
(National Directorate of Water Works and Energy, Ministry of Industry,Water Works and Energy), Mali
HDM3 Highway Design and Maintenance Standard Model(Investment and road maintenance economic calculation software, developedunder World Bank aegis)
HV Heavy vehicleKP Kilometer pointLB Left bankLV Light vehicleNT Natural terrainMTV Mass Transit VehicleNGO Non-Governmental OrganizationNR Natural reservoir, as in the expression "NR elevation," i.e., upper sill level
elevationOHVN Operation de la Haute Vallee du Niger (Upper Niger Valley Operation),
Development office with headquarters in BamakoONBAH Office national des barrages et des amenagements hydroagricoles
(National Dams and Agricultural Water Works Office), Burkina Faso
ORSTOM Office pour la recherche scientifique et technique Outre-Mer(French Office of Overseas Scientific and Technical Research)
RB Right bankRN Route nationale (primary road)
vii
Executive Summary
n many developing regions with a short rainy season, millions of gallons of rainwater-a precious resource - are lost through ill-designed roads and road drainage systems.
Nowhere is this more evident than the Sahelian countries. On major highways, rainwateris considered a nuisance, not a resource to be captured and used. Many low-traffic and ruralroad projects (with technical standards imported from other countries) often include expen-sive and unnecessary works to protect against water.
This report provides roadbuilders with advice on how to include appropriate water manage-ment into the design of the project. This will save scarce water resources or avoid the over-design of works on low-traffic roads and trails. A companion manual provides guidance forcarrying out technical design and cost-benefit analysis. Although the focus is on the Saheliancountries, most of the conclusions apply to other countries with short rainy seasons.
The Report
The survey was financed by the Norwegian Trust Fund and conducted by the World Bank aspart of its Sahelian Operation Review. It reviewed water management schemes in roadworks inBurkina Faso, northern Togo, Niger, Mali, and Mauritania. These included:
* water storage works-ponds, raised fords, road-dikes, bridges with spillway sills, and dams;and
* works protecting the road against water-absorption wells, fords, and bridges.
The report describes different roadworks used for managing water in the Sahel. It also evalu-ates the social and economic benefits of water retention works and provides guidelines formaintenance and operation.
The Findings
There were significant differences in the number and type of water-retention works. Some ofthese differences were due to natural conditions and population needs, but some road agenciesare not fully aware of the benefits of managing water efficiently in roadworks.
The largest number of water-storage roadworks were found in Burkina-Faso. This is a result offavorable natural conditions, as well as the high awareness among the road agencies of thebenefits of these works.
The road agency in Mali - while aware of the benefits of water-retention works - is reluctantto develop them for fear of high construction costs and increased risks of water damage toroads.
viii I WATER MANAGEMENT IN ROADWORKS
Executive Summary
In Niger, heavy solid deposits hinder the construction of water-storage works.
Most existing works in Mauritania, are movable sills, which store water for agricultural pur-poses.
In Togo, the only water retention road work is the road dam at Kabou.
Technical recommendations
In designing water retention schemes in road projects many technical factors must be consid-ered, including road traffic, topography, rainfall and hydrology, catchment basin, evaporation,infiltration and water quality, balance, and human, cattle and irrigation needs.One of the major recommendations of this report is that where road traffic exceeds 30 vehicles/day, the road specifications should remain the same along the entire road length. If traffic islower, then the road may be downscaled on specific sections.
Other recommendations include: dikes constructed as simple road fills may be used to storewater depending on the quality of soil and water retention height; slopes should always beprotected; a stilling basin should be used, even where the difference between retention anddownstream level is low; and special attention should be paid to the flood spillway.
Socioeconomic recommendations
The socioeconomic evaluation should take into account: vehicle operating cost on the seg-ment of the road with waterworks; construction costs; operation and maintenance costs; typeand size of agricultural production; water supply for humans and livestock, and environmentaland sanitation impacts and benefits. Using these criteria, road designers can assess the sound-ness of including water retention schemes in road design.
Managerial recommendations
Road and water resources authorities must be involved from the start of any project. Theoperation of waterworks must be within the capability of users, local agencies, and NGOs orother organizations. Maintenance and operations must be a major concern for designers sinceonly well maintained projects are likely to last. This means designing robust projects and usinglabor-based construction methods, at least on rural roads and tracks.
Road authorities should also prepare and disseminate operation and maintenance manuals orinstruction brochures to educate users, increase benefits, and reduce long-term costs.
Chapter 1: Overview
\W ater management in roadworks in the Sahel is often poorly designed forthe following reasons:
* On the most heavily used roads, road designers see water as detrimentalto the road, and try to move it as far away as possible as quickly aspossible. Project designers sometimes forget that water is a preciousresource in the Sahel which they could use in building and maintainingthe road. Moreover, people living along the road would willingly takeadvantage of it for farming and herding, even for only a short period ofthe year;
* On low traffic roads and rural trails, designers use the technical standards ofcountries which have different climates and road traffic conditions fromthose of the Sahel - water control projects are oversized and roads aresometimes built on costly and unjustified embankments.' p These standards areparticularly ill-suited for
dirt roads and low-trafficBecause of these shortcomings, the World Bank commissioned a case ternagineer ouhtt thoeaim
study, financed by the Norwegian Trust Fund, as part of the Sahelian Opera- more at making traveltions Review. The study was assigned to two experts of the BCEOM Socit pgasrnteei thn atfficfranfaise d'inglnierie, Messrs. Perrin and Frejacques, who visited several Western speeds.African countries in June-July 1992. On the basis of this report the Bank's studyreview committee had two documents prepared:
* Report* Manual
This report is directed to road departments and road project design-ers in the Sahel region as well as to national environmental agencies and tech-nicians and economists from donor agencies or NGOs which fund roads inthe Sahel. The report (a) describes various roadworks designed for water man-agement in the Sahel; (b) shows how to evaluate the social/economic benefitsof water retention schemes (permanent or seasonal) upstream of a road; and(c) provides guidance to designers and project managers on maintenance andoperation of the works.
The report focuses on roadworks projects - waterworks structures designed tohold back or discharge water are secondary. In other words, the road is the mainproduct and water detention is a by-product which merits only a minor part ofthe total resources allocated. Projects whose main purpose is to hold back water- with a road or trail as an ancillary feature - are not addressed in this report.
2 1 OVERVIEW
Chapter I
From a hydrological point of view, the Sahel is limited by 200mm/year and 850 mm/year isohyets. For this review, the zone has been extended southward to indude areas with morerainfall, but which suffer from severe seasonal droughts since the zone does not keep groundwater because the bed rock is close to the surface. These areas have been induded in the zoneunder review which is roughly limited to the 200 and 1 200 mm/year isohyets.
The report tries to describe the broad range of options and the criteria involved inmaking a choice It examines the impact of the various choices and provides a methodology foreconomic appraisal. Chapter 7 shows the institutional setup required to promote, follow, andmanage road projects which include water retention schemes. Details and technical issues aredealt with in the accompanying manual.
THE MISSION
The Terms of Reference stipulated that a team composed of a road engineer and a damspecialist would visit several Sahelian countries to have discussions with local authorities incharge of roads, agriculture, and hydraulics at the village level, and review existing schemes inorder to draw lessons and guidance. The mission took place in June and July 1992 in thefollowing countries:
* Burkina Faso from June 22-30th* Northern Togo July 1st* Niger from July 2-7th* Mali from July 8-14th* Mauritania from July 15-18th.
The mission met with local authorities, donor agencies, consulting firms, construc-tion contractors, nongovernmental organizations, and research organizations (see Appendix 3).On the sites of the schemes, the mission also met with the village communities using the water.The mission received a warm welcome and useful information.
The major issue facing the mission was obtaining accurate older data. In most govern-mental agencies, data on projects more than five years old are missing (except in Mauritania).There is a high turnover of personnel, and there is little collective memory of past projects.Much of the data was supplied by consulting firms and contractors; but they too experienced ahigh staff turnover.
Chapter 2: Description and Purposeof the Works
TYPOLOGY
There are a great many kinds of roadworks incorporating water management schemes in theSahel. A typology of these works will distinguish:
* between (a) works that allow continuous road traffic (bridge, box culvert) and (b) thosethat require some interruptions (paved ford);
* between (a) works that allow a river or wadi to cross a road without affecting its integrity(bridge, ordinary box culvert or paved ford), but do not indude any provision for waterretention, and (b) those that allow regular and peak water flow and also create means ofpermanent or temporary water storage;
* between (a) works that involve fixed components and (b) those that involve movable gatesproviding better upstream water level control;
* between works that offer (a) high and (b) low water retention levels.
The mission found an array of retention levels ranging from 1 to 10 meters. There isno sudden jump in retention level but a gradual transition from 1 to 10 m; but, clearly, theconstruction requirements for 10-meter retention works differ appreciably from those for small,1-meter works. Other less important distinguishing criteria include unidirectional or bidirec-tional discharge and the existence or absence of water offtake works.
This chapter first describes the most common types of works, draws attention to theiradvantages and their drawbacks, as well as precautions and constraints involved in their use.The chapter then indicates the regional specificities.
COMMONLY USED WATER MANAGEMENT WORKS
The most commonly used works are:
(a) pond (storm basin)
(b) absorption well (drainage well or cesspool)
(c) stream-level paved ford
(d) raised paved ford, with or without conduits/box culverts, without sluicegates
(e) raised paved ford, with box culverts and sluicegates
4 | DESCRIPTION OFTHE WORKS
Chapter 2
(f) combined road-dike structure with raised box culverts
(g) bridge equipped with fixed or movable spillway sill
(h) dam upstream of the road
(i) other types of works.
Pond (Storm Basin)
Description. This is the simplest kind of project to design and execute. It consists of digging ahole which - although it may vary in size (in the hundreds of cubic meters) -is deep enoughto penetrate the water table, at least as it is during and immediately after the rainy season. Theusual layout is rectangular, with the long side parallel to the road, the width being equal to oneor several times the width of the excavator used. The edges of the pond must be improved,using a motor grader or bulldozer, or manually to facilitate runoff of the water to the pond andto prevent children and livestock from drowning. At least one side is graded to a gende slope tofacilitate access. The storm basin is a variant of the pond, whose purpose is to receive the waterdischarged from the road during storms and route it either to a watercourse or into the watertable.
PondCross-sectionalview
Context. A pond can be dug almost anywhere, regardless of the road traffic, provided the watertable is not too deep (at least at the time of year when it is highest) or if the soil is impermeable.If the soil is permeable and the water table deep, a 50-cm thick impermeable compacted layer(day) will be added in order to conserve water.
Cost. The excavation cost is usually moderate (US$2-4/m 3) provided an excavator is availablenearby. The cost is zero or insignificant when the pond consists of final upgrading of a borrow-pit of material used for building or resurfacing the road.
Benefits. If the pond is dug before or during construction, maintenance, or rehabilitation ofthe road, it can supply water to the works contractor, thereby saving high costs of transporta-tion. Roadside residents then use the pond over a period of at least several months to water
livestock or perhaps to irrigate anearby garden.
Rubble-Lined Trench (Cross-section)
Constraints. A pond serves no pur-pose on permeable soil. In somemosquito-ridden areas it is inadvis-able to dig ponds or leave pondsundeveloped after roadworks havebeen completed. It is unwise to lo-cate the pond close to the road be-cause of the risk of polluting thefoundation course or the road base,
and also because it can create an additional hazard for vehicles going off the road. In the case ofa storm basin, one has to carefully dimension the ditches that receive the water at the foot of theroad and the discharge device that leads it to the basin. These ditches must be protected againsterosion - for instance, laying quarry-stones will limit sediment washing away- to maintainthe ditch sides. There should be similar protection for evacuating excess water from the naturalstream.
Absorption Well (or Drainage Well or Cesspool)
Description. The absorption well carries storm water to the water table as quickly as possible. Itconsists of a capture device (gutters or ditches), a reinforced concrete accumulation tank, and adevice for discharge to the water table (usually a length of perforated conduit surrounded by
gravel or pebbles). Thevolume of the tank de-
Absorption Well or Drainage Well or Cesspool (Cross-section) pends on the amount ofwater likely to be accumu-lated after a heavy rain-storm.
Context. The absorptionwell technique is used inareas where drainage by
6 1 DESCRIPTION OF THE WORKS
Chapter 2
gravity by pipes or by ditches is not cost-effective and where the soil is permeable at low depth.Water flows slowly from the well into the water table. The water basin, which stocks the watertemporarily, must be proportioned to the area concerned and its permeability. In urban areas,land constraints require that the storm basin be made of a concrete tank with an outlet. Thisscheme is costly. In rural areas the storm basin performs the same function.
Cost. An absorbing well with a 24-m3 tank (3 x 4 x 2 m) costs around US$5-8,000. A stormbasin, without the collector ditches, costs US$2-4/m3 excavated in friable soil, with a dumpnearby. The cost rises substantially with the depth and with the difficulty of excavation.
Benefits. The object is to clear the water from the highway to allow the passage of vehicles.Because of its cost the scheme is used in urban areas. It avoids or reduces flooding of nearbylots.
Constraints. This scheme should be used with caution. The water table is often used for humanor animal consumption, and discharging polluted runoff water into the water table can carryoil, grease, toxins and detritus of various kinds which stagnate on the highway. This type ofproject has no effect when the soil into which the water is to be channeled is impermeable.Moreover, it is necessary to maintain the water absorbing capacity of the natural terrain in thevicinity of the discharge conduit; otherwise the pebble/gravel filter becomes saturated and thewell no longer functions.
Stream-Level Paved Ford
Description. The road follows the natural terrain, so as not to impede the flow of the water, anddescends into the thalweg, where it crosses the wadi. Consequently, the longitudinal sectionmay include fairly steep gradients. During and following a rainstorm, travel is halted by theflow of water on the road. The highway structure, and that of the shoulders, must be resistantto erosion during passage of the water. The pavement structure should resist erosions duringwater flows. It may be flexible (bituminous concrete on gravel stone) or rigid (concrete pave-ment). Concrete or gabion-type cutoffs are installed upstream and, especially, downstream.
Context. This kind of project isfeasible only on low-traffic roads, Stream-Level Paved Ford (Cross-section of the road)trails and in semiarid climateswhere traffic restrictions are nei-ther too frequent nor too long.The size of the storm basin is alsoa factor since it affects the dura- . ... - _tion of discharge of the water. Aneconomic analysis would deter-mine whether this scheme(which does not provide a per-manent service) is acceptable.
Gablons protect the downstream side of the paved ford
Cost. A concrete highway 6 m wide and 20 cm thick - laid on a 50-cm pebble/gravel base,with shoulders 1 m wide with the same structure, protected by two reinforced concrete cutoffs1.5 m deep and 15 cm thick- can cost between US$800 and US$1,800 per linear meter. Thecost of maintaining the ford after each rainy season, as well as the economic cost of trafficrestrictions, must also be considered.
Benefits. This technique is economical provided it is properly done. It is particularly "ecologi-cal" in the sense that it imposes the minimum constraint on natural runoff of the water.
Constraints. This type of project is inadvisable when the water conditions entail long (over 12hours) and frequent (over 10 times a year) interruptions. This project is also ill-advised wherethe soil is non-cohesive and thin. This is because of the difficulty of protecting against erosion,making it more expensive than a traditional structure. And it is dangerous for traffic because ofthe steep access slopes to the wadi and to the water flow (speed, height).
Raised Ford, with Ungated Conduits/Box Culverts
Description. The ford is no longer at the stream level of the thalweg but at a higher level. Underthe highway, above and close to the stream level, conduits or box culverts allow the water toflow.
Raised Paved Ford (Perspective) Context. This structure is used onsecondary roads and on wadis withlong permanent flow. Except foroccasional overflows, it keeps thetraffic on a dry road.
Cost. When the wadi has pebblesor rocks/stones this structure costsaround US$2,000 per linear meter.'Where the soil is a thin material andstone quarries are far away, the con-crete or rockfill protection maydouble the cost.
IL\ / Benefits. Raising the ford abovestream level reduces the frequencyand duration of traffic interruptionsand also makes the longitudinalprofile gender in the ford's vicinity.
8 1 DESCRIPTION OF THE WORKS
Chapter 2
Constraints. During overflow periods, this scheme is subject to rapid whirls and waves whichoffer appropriate protection; the pavement should be concrete; upstream and, particularly,downstream sides should be protected by rockfill or concrete; this protection should extendwell beyond the pipes and protect all the flowed area.
Raised Ford, with Gated Conduits/Box Culverts
Description. The scheme is similar to the former, but pillars and abutments are equipped withgrooves in which mobile slabs can be inserted to allow maneuvering of the water level up-stream. Box culverts are simpler to equip than pipes, which are not recommended.
Context. This work is usual on secondary roads in inhabited areas. Farmers maintain andoperate the scheme, and regulate the water level, in particular in rice-cropping fields, or for lateseason gardening.
Costs. The cost is similar to that of ungated raised fords because the need for protection againsterosion is the same. The slabs cause an additional cost, generally moderate because they aremade on the spot.
Raised Ford with Box Culverts and Sluicegates
|...Th; IT
' I
Benefits. In addition to the benefit of the former scheme, this one allows some cropping, ormakes farming more productive.
Constraints are the same as for the former scheme; but the installation of gates increases the riskof overflow. The protection should be designed and built carefully.
Combined Road/Embankment Structure, with Raised Conduits/Culverts
Description. The road is on an embankment, above flood level. The small lower structures(conduits, box-culverts) are well above the bottom of the thalweg, but are large enough to dealwith large overflows. The structure works as a dam and keeps a reserve of water upstream of theroad. Box-culverts should be preferred to pipes, because they work better against the edgingwave of the major overflows and are less subject to erosion.
Context. This type of project is much more expensive than the preceding one because the roadfill is higher and generally much longer. It should be used on major roads, where interruptionsin traffic flow are not allowed, and higher road standards are imposed. The material for the fillshould be impermeable, which limits this structure to where this material can be found. Sucha large conservation of water would be beneficial in populated areas. This scheme is commonon main roads in Burkina Faso.
Cost. The costly part of this structure is the fill. If this is necessary to keep the water off theroad (as in a large valley with a wide minor bed, as is sometimes found in the Sahel), buildingpipes or box-culverts above the bottom of the river does not increase the cost - on the con-trary, they are shorter and less expensive because they are placed on the level of the naturalground, on the side of the valley.
Benefits. Traffic on the road is not interrupted during the rainy season. The water held by thegates can be used for a somewhat long period depending on the size of the reservoir.
Combine Road/Embankment with Elevated Box Culverts
(Cross-section of the valley
Constraints. The fill core must be impermeable and must be protected against waves upstreamand sometimes downstream. The channel through which the water flows has to be properlymaintained, and sometimes must be protected with rocks or gabions. Also, the creation of alarge water retention scheme in tropical areas calls for monitoring sanitation.
10 I DESCRIPTION OFTHE WORKS
Chapter 2
Bridge Equipped with Fixed or Mobile Sill
Description. The device consists of a sill installed under a road bridge or upstream. This can bea wall, thin or thick, or a cofferdam or a set of gates. The bridge approaches must be protected,and the access fill core must be able to bear the proximity of a permanent mass of water, andmust be protected against damage from ripples and waves.
Context. The road is a main road, and a bridge must be built. The area is populated, and waterneeds are important, for people as well asfor cattle, and farmers are hoping to growflood recession crops. The flood area cre-ated in this way includes neither dwell-ings nor lots where flooding could causedamage. This scheme with fixed sill iscommon in Burkina Faso. Mobile l_sluicegates are frequently used in Mali andMauritania for temporary water reten-tion.
Cost. Here again, the absolute cost(bridge + access fill + gates) is high but inlarge part necessary for the road. Therelative cost is limited to the building ofthe sill, the fitting of the gates, the pro-tection of the abutments of the bridge,and the precautions for execution of theaccess fill.
Benefits. The water is available for subsidence crops for a relatively long period, depending onthe reservoir volume. The ideal solution would be creating a permanent water reserve.
Constraints. As in the previous example, this structure requires impermeable material for thefill core and must allow for monitoring sanitary conditions. In addition, mobile sluicegatesshould be permanently monitored during overflow periods.
Dam Upstream of the Road
Description. In this structure the road is separated from the dam which protects it from thewater flow. The road, which is affected only by the surpluses not absorbed by the reservoir, canbe kept clear of water or include a ford section, depending on traffic density.
The dam is comprised of the following structures:
an embankment of impermeable materials which may or may not be protected againstslap and/or overflow;
the flood discharge de- Dam Upstream from Road (Cross-section also showingvice, consisting of a the spillway)
The ~~~~~spiliway, which may beThe thin sill (profiled in thin or thick oay cf
concrete or masonry, thin or tlck, or a cof-metal sheet piles) oper- ferdam, a channel,ates in the dewateredstate (flow does not de- where the water speed ,- -pend on the water level is moderated, then adownstream); the thick sill(earthworks shell, pro- narrower chute wheretected by a concrete or it increases, terminat-masonry curtain) is usedif the spillway has to op- ing in a reinforced con-erate submerged (flow is * c r egoverned by the water crete converger leadiglevel downstream). to a stilling basin
which slows the waterbefore returning it tothe river.
Context. Two cases can be identified:
(a) the road is heavily traveled and justifies large investments, and construction of a damupstream is the most economical technique to rid the road of water; and
(b) the road carries little traffic, the dam has been built for non-road reasons, and it ismore economical to have the road pass downstream of the dam than on it, if toonarrow. As far as the road is concerned, this second case was dealt with implicitly, inthe previous examples (ford, raised ford, embankment). Constructing a dam for non-road reasons is outside the scope of this report.
Cost. The cost varies with the section of the valley. In case (a) it is usually higher than that ofa bridge and can be economically justified by the value of the water. In case (b), on the con-trary, it allows crossing even a large valley at the cost of an ordinary road.
Benefits. The dam can be less expensive to build than a long combined road/embankment anda bridge. For main roads, this road/embankment requires an 8-10 meter wide platform whenthe dam requires a 4 meter platform only. Moreover, it can be more economical to separate theroad from the dam because the dam's core materials (day) may be inadequate for the road'sconstitution. Separating the two structures allows each to be executed at a lower cost. Finally,the dam makes it possible to store the water for agricultural uses.
Constraints. Impermeable materials are needed to build the dam, which should be protectedfrom wave erosion. The water storage, which is rather large, must be monitored for sanitation.
12 | DESCRIPTION OFTHE WORKS
Chapter 2
Other Types of Works
There are many other works, combining two or more of the types described above. Theyindude, for example, dikes provided with a raised ford and box culvert laid for an elevationsuch that, as a rule, only exceptional flood waters cross the ford while all regular normal peakflows (one- or two-year flood) are absorbed by the culvert. Traffic on such a road is practicallycontinuous.
At Ouagadougou (project 1) a fixed sill (ford) is combined with a bridge located up-stream of the sill (see photo 18). Vehicles pass over the bridge all year round; pedestrians useeither the bridge or (except when water flow is high) the ford.
At Tabalak, in Niger, the Tahoua Arlit road crosses a permanent lake. The necessarystructure, a long embankment, had to be accompanied by a bridge to make it possible tobalance the water level to the right and left of the road, reflecting the storm zones. Under sucha bridge the water circulates in alternate directions (photo 20); both embankment slopes havehad to be protected.
COUNTRY OR REGION SPECIFICITIES
While small roadworks and conventional bridges are distributed fairly evenly in the Sahel re-gions, the density and types of water-retaining roadworks differ significantly between countriesand between regions. While this is due primarily to natural conditions, which differ fromregion to region, some differences are explained by historical or administrative traditions.
By far, Burkina Faso has the largest number of water-retention roadworks. They aremostly fixed-sill works with retention heights of over 1.5 m. While these characteristics are dueto favorable natural conditions, they probably also reflect the keen awareness of the benefits ofthese works on the part of Burkina's road agencies, which since 1982 have asked designers totake them into consideration. The reservoirs are often permanent and serve more for humanwater supply and livestock watering- as well as for fishing- than for the organized develop-ment of farming activities. Note, however, that the mission inspected works chiefly constructedon main roads; it did not have time to inspect a great many works built at the initiative ofNGOs on rural tracks. Consideration of such works might well modify the foregoing conclu-sions.
- In Niger, the mission received the impression that solid load and meander are themajor problems. Heavy solid-load volume has hindered the construction of many water-reten-tion roadworks. The existing works are of the fixed sill type and are designed mainly to serveroad needs (by creating water reserves for road construction). While the road agencies areaware of the socioeconomic benefits of such works, they have done little to promote themexcept on the Dori Tera road.
In Mali, the road agencies, while understanding the benefits of water-retention works,are unwilling to develop them, citing both their high construction cost and the increased risk
13
of dosures and even destruction of the road. The water resources agencies are behind most ofthe water-retention roadworks built or planned. The works are generally meant to improveagricultural production, and they include adjustable sills; moreover the frequent availability ofrock quarries favors widespread use of stonework in construction.
In Mauritania, water-retention works are essentially to allow farming. Human waterneeds are supplied by tapping the water table or deep resources; without these, there are nopeople, and soil permeability, rainfall or evaporation prevent the creation of permanent re-serves. On the other hand, cropfarming in Mauritania is only possible in bottomlands, wherewater accumulates for several months and disappears through evaporation or infiltration. Con-trolled, movable-gate water reservoirs make it possible to create moist zones artificially, permit-ting off-season cropfarming. For this reason, the works in Mauritania are the movable-sill typefor agricultural; no reservoir is permanent. Mauritania also has works located in the main bedof the Senegal River which make it possible either to control the interior lakes or to lengthenthe submersion time of the high areas of the main bed of the river. These works are the sametype as the preceding ones (movable sills) but have to allow discharge of the water in bothdirections. Although the mission found works of this type only in Mauritania, they probablyalso exist in Mali (Segou-Mopti region) and Chad.
In Togo, the mission inspected only a road dam at Kabou, intended essentially forsupplying water to villages and watering livestock. According to the Lama-Kara ProvincialDirector of Public Works, this is the only example of a water-retention road project in Togo. Itis probably representative, however, of works that exist or could be built in the dry zones,which lack a water table and are located between isohyets 800 and 1200 mm. This also appliesto northern Benin, northern C6te d'Ivoire and northern Nigeria.
SUMMARY OF CHAPTER 2
* There are many types of roadworks with water-management features; the main ones were re-viewed in this chapter. Some of them include provision for water storage: pond (storm basin);raised ford without box culvert; raised ford with box culverts and sluicegates; combined road-dike structure with raised box culverts (or conduits); bridge equipped with fixed or movablespillway sill, and dams.
* In contrast, other works are designed to keep water as far away as possible from the road:absorption well; stream-level ford; raised ford with conduits and ungated box culverts; conven-tional bridge.
* Significant differences were noted among the countries visited in terms of numbers and pur-poses of water-retention works.
* Burldna Faso has by far the most water-storage roadworks; they are mostly fixed-sill works witha retention height of over 1.5 m. While these characteristics are due to favorable natural condi-tions, they probably also reflect the keen awareness of the benefits of such works on the part ofBurkina's road agencies, which since 1982 have asked designers to take them into consideration.
14 | DESCRIPTION OFTHE WORKS
Chapter 2
* In many cases the reservoirs are permanent and are used mainly for human water supply andlivestock watering. They are also often used for fishing.
* In Niger, heavy solid load has hindered the construction of many water-storage roadworks.The works built are of the fixed-sill type, designed mainly to meet road needs (by creating waterreserves for road construction).
* In Mali, the road agencies, while aware of the benefits of water-retention works, are reluctant todevelop them, citing their high construction cost and increased road destruction and dosurerisk. The water resources agencies are behind most of the water-retention roadworks built orplanned.
* In Mauritania, water-retention works are essentially to make farming possible. Cropfarming isfeasible in Mauritania only in bottomlands, where water accumulates for several months anddisappears through evaporation or infiltration. Controlled, movable-gate water reservoirs makeit possible to create moist zones artificially, permitting off-season cropfarming. For that reason,the works that exist in Mauritania are of the movable-sill type and used for agricultural pur-poses. There are no permanent reservoirs.
* In Togo, the mission inspected only a road dam at Kabou, intended mainly for village waterand livestock watering. According to the Lama-Kara Provincial Director of Public Works, thisis the only example of a water-retention road project in Togo. It is probably representative ofworks that exist or could be built in the dry zones, which lack a water table and are locatedbetween isohyets 800 and 1200 mm. This also applies to northern Benin, northern Cote d'Ivoireand northern Nigeria.
1 15
Chapter 3: Determining the Designof the Works
T his chapter studies the various factors that define the advantages and determine thedesign of roadworks projects that involve water-retention facilities. The purpose is (i)to facilitate from the outset proper choices in a given region or situation, and (ii) to
provide data on the importance and magnitude of the essential parameters.
ROAD TRAFFIC
This parameter affects the economic justification for works projects and governs in particularthe choice between the solution of a box culvert or bridge (ensuring continuous traffic) andthat of a submersible ford or box culvert with traffic interruptions. It also affects the value ofthe benefits afforded by a better longitudinal section or a better road alignment.
TOPOGRAPHY AND SITE SELECTION
Topography dictates site selection. Areas of slight relief are common in the Sahel (longitudinalprofiles of rivers - generally intermittent - of 1-5 in 10,000 and very flat valley floors).There are also hilly or plateau areas with much steeper slopes. The existing roads or new roadprojects generally follow ridge lines. This makes it possible to reduce the number and cost ofthe waterworks without affecting, in the flat areas, the longitudinal section of the road, whicheven following the ridges never presents any but low gradients. While most water-retentionroadworks are located on such terrain, the road naturally has to cross watercourses - eitherbecause even the ridge alignment cannot avoid them except by a long detour, or because theroad has to link two points located on either side of the watercourse. Note that the best roadcrossing point and the best dam site generally coincide: this is the point where the watercourseis narrowest and/or most stable (rock sills, for example). There may be a conflict of locationbetween the road objective and the water objective. This may happen when the most recom-mendable crossing point for the road does not meet the conditions for a reasonably-sized reten-tion basin; whereas a larger reservoir could be created at an upstream or downstream site, whichwould involve lengthening the road slightly. Although the mission did not systematically studythe projects from this angle, it did not see this as a frequent problem.
Another important factor is that of valley cross-section gradients. If the banks aresteep, a bridge is generally the best solution. If, on the other hand, the valley is broad and flat- and the longitudinal section of the river creates frequent and durable valley submersion (acommon case in Burkina Faso) - a raised road (dike-road) is necessary and makes it possibleto install a water reservoir upstream from the dike-road at no great incremental cost. Thispackage of topographic parameters determines the size of the project, the cost of the works, thewater resource benefits (storage volume, permanent or nonpermanent reservoir) and economicjustification.
16 DETERMINING THE DESIGN
Chapter 3
RAINFALL AND HYDROLOGY
The hydrodimatic data affecting the works projects relate to:
(a) precipitation(b) average annual water flow(c) peak flow volumes(d) catchment basins.
(a) Precipitation
The annual rainfall statistics give average rainfall volume, deviation from this average, andpossible year-to-year variations. In the Sahel region precipitation is concentrated between Juneand October and ranges from 200 mm/year at the latitude of Timbuktu to 850 mm towardOuagadougou and 1200 mm in northern Togo and northern Cote d'Ivoire (see Figure 1.1).The figures for instantaneous precipitation (maximum quantity of water falling in a givenspace of time - ranging from a quarter of an hour to 48 hours) over a given period (5, 10 or100 years) - give information on the peak flows expected in small catchment basins (water-sheds). The instantaneous precipitation data generally correlate with annual precipitation.Finally, the monthly rainfall data help determine what crops can be grown and the correspond-ing irrigation water needs.
(b) Average annual water inflow
The hydrological data are much less precise than the rainfall data for small catchment basins.The annual average streamflow ratio is defined as the volume of water carried away by a river ata given site, expressed as a percentage of the quantity of rainwater received by the catchmentbasin. It varies according to rainfall (and therefore from one year to another for a given catch-ment basin), plant cover, slope, geology, and pedology. It can vary widely from one year toanother and between two neighboring catchment basins.
An example of interannual variation is afforded by the case of Lumbila, Burkina Faso(catchment basin area 182 km2 ), whose annual average streamflow ratio was:
12.10% in 1961,6.60% in 1962,0.96% in 1963.
Combining these streamflow ratios with the rainfall data - which can also fluctuatewidely - we find greatly differing flows from one year to another. Thus, in northern C6ted'Ivoire, flow from Bou to Sirasso (catchment basin area of 500 km2) can vary from 1 to 300million m3 , depending on the year. Estimating annual average flows into a reservoir thusinvolves a large risk of error.
|17
(c) Peak flows - design flood
Studying peak flows is essential to dimension box culverts, bridges, or flood discharge workscorrecdy. The methods to determine peak flows are discussed in the manual. Peak flows are notdirectly proportional to catchment basin size. Moreover, ten-year 24-hour precipitation fallssharply with decreasing annual rainfall, and from north to south. Peak flows are thereforeproportionally lower in the strictly Sahelian regions of the northern part rather than in thesouthern part of the study area.
The design flood is the flow by which the project is dimensioned for total operatingsafety. The ten-year peak flow is generally taken into account in the case of small catchmentbasins, when submersion, or even destruction, does not entail loss of human life or substantialmaterial damage. In the case of the most common works, a 100-year design flood is taken intoaccount. Where observations are lacking, the 1 00-year peak flow is usually deduced from the1 0-year peak flow by multiplying it by a factor, which for the Sahel regions is 2 or 3. However,it must be remembered that the values found are no more than orders of magnitude, andprudence is still essential.
The peak flow discharge structure (sill, guide channel and stilling basin) often ac-counts for 30 percent or more of total project cost. It is clear how important it is to calculatethe design flood as carefully as possible by reducing the uncertainties. Finally, it will be seenthat, in the case of a water-retention structure, the maximum peak flow discharged is notnecessarily equal to the arriving flow, which we have called the design flood, since the reservoircan exert a flow attenuation effect.
(d) Catchment basin
Catchment basin size is a vital basic parameter for assessing water inflow and peak flow vol-umes despite the wide variation in streamflow ratio. Road water-retention works, which arealways small reservoir projects (compared with large waterworks dams), never store very largevolumes of water. Generally, a very large catchment basin means high floods and a high-costproject, resulting in a project cost which is out of proportion to storage capacity. Cost per m3stored rises rapidly with increasing catchment basin size, and the most cost-efficient projectsare those for catchment basins of moderate size with annual flows of the same magnitude as thewater needs.
The mission found projects pertaining to catchment basins ranging in size from 4 toover 5,000 km2 . Once a catchment basin exceeds 500 km2 , the water components of theproject, if it is designed to retain the water, become very large. The incremental cost in relationto that of a conventional bridge is justified only if the socioeconomic benefits deriving from thereservoir are themselves also very large. In other words, the roadworks that are a priori mostlikely to allow economically efficient water retention are those with catchment basins of 4-500km2 .
18 | DETERMINING THE DESIGN
Chapter 3
SOLID LOAD
This subchapter reviews:
(a) the importance and magnitude of solid load(b) the solid load deposition mechanism(c) methods of dealing with solid load.
(a) The importance and magnitude of solid load
Solid loads, caused by erosion of the catchment basin by meteoric water, are often large in the
Sahel and have to be factored into the works calculations - they help determine the useful life
of the project and hence its economic justification. Solid load depends on many factors, par-
ticularly soil degradation and plant cover: volumes of 1,000 t/km2/year are by no means excep-
tional in the northern part of the region but can sometimes be practically zero in the south.
(b) The solid load deposition mechanism
Solid loads consist partly of sandy or stony materials -which are dragged along the bottom of
the watercourse and deposited as soon as the current speed slackens, i.e., upstream of the reser-
voir - and partly of very fine mud partides, carried in suspension, which in contrast tend to
settle at the low points of the reservoir, i.e., downstream against the dam (see Figure 3.1). The
mission frequently encountered this kind of silting of the reservoir floor in contact with the
dam in old works.Solid Load Deposition Mechanism
Deposition of Sand _ -
and Stones
Deposition of Mud
(c) Methods of dealing with solid load
Large solid loads can doom a water-retention project or at least restrict it to a certain type of
works; for example, a diversion sill for rainy-season irrigation instead of a storage reservoir for
off-season irrigation. Methods of combating solid loads include protection of the catchment
basin, by sowing grass and reforestation. These methods are complicated, expensive, and slow
1 19
to produce results. It is better not to count on reduction of solid loads in designing a project.Other methods include: reservoir desilting by flushing or bottom draining, dredging, or theuse of construction equipment after draining the reservoir. These methods are always costlyand produce limited results and can be applied at a reasonable cost only to small volumes.They should not be used or considered in projects.
FLOATING OBJECTS
Floating objects - such as trees or branches, large clumps of grass, and animal carcasses -poseserious risks to works with low overhead clearance, particularly low conduits and box culverts.Floating objects generally appear following the first large storm of the rainy season. With thefirst curling wave, an incredible quantity of detritus of every kind is borne along in a torrent offoaming water. Some of these objects snag on the structure and trap others, which obstruct allor part of it. Even if the project has been correctly calculated, the flood puts the fill underpressure and may break through or overflow, destroying all or part of the road.
Sahel region engineers have long known that it is always the same projects, year afteryear, that are obstructed in this way. These projects are generally located in upstream catch-ment basins, with substantial populations, where sedentary or itinerant farmers fell trees orpractice slash-and-burn agriculture. Systematic clearing of debris before the rainy season andafter each large storm unfortunately remains mostly theoretical (this is mentioned more as afinding than a criticism). Another solution is to install a device that can stop these floatingobjects 50-100 meters upstream of the project. The best method is that of reinforced concreteposts (see photo 12). The manual describes this in detail.
The mission observed that many works which appear a priori to be vulnerable tofloating objects have, to judge from their age, performed well. It seems likely that a reservoirupstream allows many floating objects to come to rest there or lodge on the banks of thereservoir. The most vulnerable works are conduits or box culverts laid at stream level of theriver. Submersible fords, on the other hand, easily allow floating objects to pass. A few floatingobjects sometimes lodge on raised fords without endangering them Raised fords with boxculverts or conduits have to be cleaned out to avoid prolonged submersion of the ford.
EVAPORATION
Evaporation has a direct impact on a reservoir's water balance. It is usually expressed in termsof millimeters of water evaporated in a given time. Evaporation depends on water or soiltemperature, air temperature and humidity, wind, and plant transpiration. A number of meth-ods for measuring evaporation are described in the manual. However, they are generally diffi-cult to correlate with each other and with actual evaporation over the very large water surfacepresented by the reservoir. It is obviously this latter type of evaporation that is of greatestinterest to the project hydrologist.
In the Sahel, actual evaporation typically exceeds 1000 mm/year and can often reachor exceed double that figure. Attempts to combat evaporation have been made, particularly on
20 | DETERMINING THE DESIGN
Chapter 3
Ouaga dam no. 3, by spreading a protective liquid film. These expensive trials have produced
a 20 percent reduction in evaporation. Embanking of the reservoir, by greatly reducing its
evaporative surface, has proved more effective.
INFILTRATION
In a water-retention project infiltration can occur at three levels:
in the worksin the reservoirin the foundations.
Infiltration is never zero. In a water-storage dam it has to be fairly low to allow the
creation of a water reserve. In all cases, it can jeopardize the structure through leakage. Infiltra-
tion quantities therefore have to be assessed precisely. The manual contains some information
on this point.
Even if infiltration is high and prevents the reservoir from filling completely, this does
not mean that the project has no benefit. It raises the water tables upstream and downstream of
the dam and facilitates water supply to human settlements. For example, the Ifida Laba project,
a dike-road with raised lateral box culverts, has never experienced water entry into the culverts
but constitutes a permanent reservoir. The catchment basin area is 7.4 km.2 The water disap-
pears by infiltration, evaporation and human use.
WATER QUALITY
The quality of the surface water varies, and its chemical composition does not usually present
a problem - the real problem is sanitation. Even in the absence of animals, warm water is
subject to fermentation and rapidly becomes foul when it stagnates. This degradation acceler-
ates as the volume of the pond shrinks under the combined impacts of evaporation and infiltra-
tion. This can be aggravated by pollution from animals drinking there. In this way the reser-
voir can rapidly be transformed into a true "culture broth."
In every waterworks project the uses and management of the project need to be de-
fined at the design stage in order to plan construction and ensure proper use of the resource -
e.g., clearing the flooded area of brush and trees, operations which are rarely carried out (see
photos 8 and 15); and sheer-finishing the banks.
For human water supply, using wells in conjunction with the reservoir and filtering of
the water is recommended. For agricultural needs, water quality is not so crucial.
1 21
WATER BALANCE AND FLOOD ATTENUATION
(a) Water balance
The project water balance depends on the following:
reservoir water input (inflow - spillway discharge)losses (infiltration, evaporation)solid loadexpected water consumption.
The balance is prepared month by month, using the longest possible hydrologicalseries. In the case of a new project, when monitored long-series hydrological data are notavailable, the balance is calculated by reference to a series of simulated monthly flows based onthe figures for monthly average flows and their standard deviations. These calculations aregenerally performed with the aid of a PC equipped with a spreadsheet or specific programs.
The crops and production supplements can be determined more precisely using thedesign balance set commonly calculated by agricultural engineers for agricultural dams than byusing just one average annual balance. Studying the water balance provides a better picture ofthe proper size of the project and, in some cases, can result in raising the spillway sill andincreasing the reservoir volume. In the case of projects associated with farming perimeters, thewater balance also helps the authorities define the reservoir management rules more precisely.
(b) Flood attenuation
Because of its volume the reservoir acts as a plug, causing maximum flow at the flood dischargedevice to be lower than maximum flow at the reservoir entry. The reduction in the floodbetween upstream and downstream of the project depends on the fill status of the reservoir atthe time of arrival of the flood. Flood attenuation is usually characterized by the ratio:
inflow volume - outflow volumeinflow volume
One hundred percent attenuation corresponds to an outflow volume of zero; 1 or 2percent attenuation corresponds to an outflow volume practically equal to flood volume up-stream of the project.
The reservoir's potential flood reduction capacity is highest when it is empty and fillsfrom flood inflow. When flood volume is very low, all the water remains in the reservoir andattenuation is equal to 100 percent. However, attenuation is calculated assuming the mostunfavorable conditions - i.e., with the reservoir already filled to its normal level at the time ofthe design flood's arrival. Since inflow volume is known, in the form of a flood hydrogram, theduration and maximum volume of flood downstream of the project can be determined bycalculating the water balance by time step. The method of calculating attenuation is demon-
22 1 DETERMINING THE DESIGN
Chapter 3
strated in the manual. The following table presents figures for small catchment basins which
illustrate the gready varying importance of attenuation in reservoirs.
COUNTRY LOCATION CATCHMENT INFLOW OUTFLOW ATTENUATIBASIN AREA VOLUME (m3/s) VOLUME (%)
(Km2) (m3/s)
BURKINA TOUGAN 9 18 0 100
NATIABOUANI 73 115 106 8
BOURA 125 54 57LEOUPO 22 149 143 4
GOUNIANA 23 149 107 28YAKO 50 169 31 8
LOBI 120 180 145 19
OUAGA2 73 349 235 33
TOGO KABOU 4 58 13 78
The significance of attenuation depends on local conditions, essentially topography.It is usually significant only with large reservoirs located in wide-mouthed basins, and for small
catchment basins with low flood levels. In the case of large catchment basins, the construction
of a dam by a substantial reduction in flood levels could not be justified.
HUMANS AND LIVESTOCK
(a) Human needs
For a project designed for human needs, the target population must obviously live in the vicin-ity. Indeed the population often instigates the project (as, for example, in the case of Kabou,Togo), requesting the road project authorities to indude various improvements. Water needs,other than for farming, are estimated on the basis of the nearby human population and live-
stock numbers.
For the human population, two basic factors must be considered:
the number of inhabitants that lack access to an adequate water resource through any
other means of supply (spring, dug well, tubewell, and so on);
the distance from the resource (existing or potential): except in unusual cases, thedistance from access to the resource should not exceed 5 km. It is therefore necessaryfirst to estimate the population living not more than 5 km from the proposed reservoirand then to assess the existing resources. The minimum need per person is 10 liters/day, actual consumption can vary from 10 to 40 liters/person/day, depending on ac-
l23
cess to the resource (40 liters can represent a shallow well - possibly with pumping-located in the center of a village). For this forward calculation, an average of 20 liters/person/day should be assumed. And needs will rise steadily in response to populationgrowth, apart from any other factor. A safety margin therefore needs to be factored in,of the order of 25 percent of estimated current needs, using a 10-year perspective.
(b) Livestock needs
For livestock, unit daily needs are as follows:
cattle: 30-40 liters/day (50 liters/day per adult animal under Sahelian conditions dur-ing the hottest times);
small ruminants (sheep, goats): 3-4 liters/day.
CROP SUITABILITY, IRRIGABILITY AND WATER NEEDS
(a) Crop suitability
Soil crop suitability is determined by its inherent nature, characterized by many factors such asdepth, texture, structure, pH and chemical content. Depending on these various characteris-tics, a given soil will be better suited to one crop than to another. Characterizing and interpret-ing these factors is a matter for experts, namely, agro-pedologists who can reconnoiter andstudy the land. However, observing cropfarming areas in the vicinity of the proposed site aswell as the crops traditionally grown there will often furnish useful information. Vegetablecrops are often highly valued, especially near villages. These highly labor-intensive crops cansatisfy the family and marketing needs of the greatest number of plot-holders.
(b) Irrigability
A soil's irrigability is determined by applying the same criteria as used for soil suitability classi-fication, ranked in priority and supplemented by others that take account of the factors thataffect water circulation and its availability to the plant (permeability, porosity, useful waterreserve, and so on). Of the supplementary criteria, slope is no doubt the most important: a lowgradient (less than 1 percent) is best to ensure good water distribution. On the other hand,because irrigation and drainage are always interlinked, a minimum gradient is necessary tocarry away surplus water (if only that resulting from exceptional downpours).
(c) Water needs
Agronomic research determines, for each crop, the quantity of water needed to ensure normaldevelopment of the plant during its growth cycle (for example, a total of about 500 mm for
24 | DETERMINING THE DESIGN
Chapter 3
sorghum) as well as the distribution of these needs over time. If rainfall cannot be expected to
supply the right amount of water month by month, irrigation must be considered. Crop water
needs are calculated by augmenting the plant needs (called evapotranspiration needs) by a
given percentage to take account of irrigation efficiency (main supply and distribution losses,
uneven distribution within the parcel, deep percolation). If the soil is naturally (or may be-
come) saline, or if the quality of the water supplied entails a risk of salinization, it may also be
necessary to add water to leach out the salt.
The manual details the calculation of water needs through the following stages:
calculation of crop-specific evapotranspiration: this is generally based on a formula
that considers the climatic parameters and a coefficient specific to each crop;
monthly calculation of average evapotranspiration per hectare, in light of the pro-
posed distribution of the crops;
calculation of effective rainfall (that which neither runs off nor evaporates);
the difference between these figures gives water needs at the field; this is then multi-
plied by the irrigation efficiency ratio indicated above.
We mention below some orders of magnitude of water needs under Sahel conditions.
These needs have to be augmented by the network efficiency ratio (at least 30 percent under
optimum conditions):
Maize or sorghum: 5-6,000 m3/ha
Vegetable plot: 4,000 m3/ha
Rice: at least 10,000 m3/ha for dry-season cycles. This thirsty crop will hence be
generally unsuitable for a perimeter located downstream of a reservoir; on the other
hand, it can be very suitable upstream of a project with a movable sill allowing good
water-level control.
SUMMARY OF CHAPTER 3
This chapter discussed the factors that enter into the choice or design of water-management roadworks.They are:
Traffic
Topography: The works studied are usually located in extremely wide valleys, suggesting aroad-dike or a streambed ford.
I 25
Rainall and hydrology- Generally, much better data are available on rainfall than on hydrol-ogy directly affecting the project; nevertheless, rainfall data can determine flood hydrology andhelp assess irrigated agriculture needs.
Catchment basin: This is the basic parameter used to assess the magnitude of water throughputand flood levels.
Solid load: This is often heavy in the Sahel region. Solid load can by itself doom a project in theabsence of cost-efficient measures to guard against it.
Floating objects
Evaporation: In the Sahel this generally exceeds 1000 mm/year; it has to be taken into accountin the project water balance.
Infiltration
Water quality Although it is advisable to sheer-finish reservoir banks and dear inundated areasof brush and trees, such action was rarely observed.
Water balance: The water balance (net reservoir input minus evaporation and infiltrationlosses and consumption) is monitored monthly or in some cases every two weeks.
Flood attenuation is generally studied on an hour-by-hour basis. In the case of large catchmentbasins (> 20 kiM2), flood attenuation has practically no effect on the discharge device, whichmust be able to handle the flood water entering the reservoir.
Water needs depend on local population size (within a radius of about 5 km) and herd size.
Irrigation water needs are determined by specialist agronomists. Vegetable crops, the onesmost highly valued, require about 4,000 m3/ha/year under Sahel conditions.
--- -- - -
I27
Chapter 4: Choosing Technical Solutions
G ood technical solutions depend on the parameters reviewed in Chapter 3 and the unitprices of the various kinds of works. The best project is, of course, the one that offersthe best cost-benefit ratio. Because of the many possible projects - as indicated in
the preceding chapters - we will first summarize our observations and the research findingson the available technical solutions (civil engineering, agronomics, waterworks), and then dem-onstrate the economic cost-benefit calculation required to identify the optimum solution.
This chapter reviews the basis on which the choice among technical options should bemade, including:
(a) construction cost, to determine whether they differ widely between countries or re-gions and, if they do, whether the "sound" solutions are likely to vary from country tocountry,
(b) types of works selected, in light of local parameters and environmental considerations;
(c) constructional specification, to ensure the safety and longevity of the project.
CONSTRUCTION COST
Capital cost is an essential criterion. It includes the cost of
studiesworksworks supervision.
This paper examines only the cost of the works, which accounts for most of the total cost. Thiscost can always be augmented, as a first approximation, by studies cost (3-5 percent) andsupervision cost (6-8 percent).
Executing a construction project calls for mobilizing a group of production resourcesspecific to that project alone. Actual cost will not be known until the works have been com-pleted. The natural conditions at the work site will determine cost, including
the quality of the soil affects earthworks costs (need for anchoring trenches, permeabil-ity of fill materials, availability of filter or slope protection materials);
the geometric characteristics of the basin affect water storage capacity for a given reser-voir height. Similarly, the cost of crossing a thalweg is greatly affected by overall fillvolume, which is a function of the square of total height.
28 | CHOOSING TECHNICAL SOLUTIONS
Chapter 4
The unit prices of works done under contract are calculated by the contractor based onthe information contained in the tender documentation - supplemented by the contractor'sown knowledge of the terrain. Final cost price, however, depends on many factors:
project volume: the larger the project, the better the amortization of fixed charges;
geographic location of the project: prices are boosted by transportation costs from thesupply areas;
workforce skills;
suitability of the techniques to locally available supplies and know-how.
AVERAGE UNIT PRICES IN THE SAHEL(1992)
Construction Cost Unit CFAF USS
EarthmovingCleaning, brush clearingSite preparation ml 30 0.1 Add 10-20 percentDike fill ms 300 1.2 for site facilites.Impermeable core material m' 1.500 6 PriceordersofDrain sand m' 2.000 8 magnitudeDrain laterite M boe n19Dry-wall slope facing Min 8.000 32 (US$1 * CFAF 250).Mortared-stone slope facing m 6.000 24 (aln20)Protective riprap Annual main-Excavation and deposit nance cost (routineExcavation using ripper m
2 8/15.000 32/60 + perlodlc mainte-Excavation using explosives ms 6.000 24 nance) esmated at
m3 800 3.2 5 percent ofM., 2.000 8 construction cost
m3 6.000 24
Paving m3 2.800 112
Laterite road surfacing m2 1.200 5
Grass sodding/sowing m 2.000 8BIDDIM 300 gr/m'Gabions m3 25.000 100
Concrete/stoneworkSroncwork m' 60.000 240
Concrete (350 kg cement/m') ms 40.000 160Concrete (250 kg cement/m3) m. 30.000 120Reinforced concrete (200 kg m' 140.000 560
cement/rn') Ms 80.000 320
Excavation in loose ground m3 50.000 200
Excavation in rocky ground m' 2.000 8
Formwork m., 8.000 32
Rubber waterproofing fill ml 2.000 8
Concrete reinforcing steel m2 6.000 24ml 30.000 120kg 600 2.4kg 800 3.2ml 30.000 120
129
The contractor calculates total project cost on the basis of work quantity estimates, theproduction resources to be employed and an assessment of indirect costs (indirect site costs,overhead, contingencies and profit).
The procedure is similar for work carried out on force account. The estimation oftotal works cost is based on an overall, case-by-case project analysis. In the preliminary designstage, it is always possible to estimate total cost, on the basis of unit price magnitude. A sum-mary assessment can be made based on the approximate average unit prices (see table) observedin a number of Sahelian countries on the basis of works contracts or price analyses performedduring the studies. While more precise data will need to be gathered whenever a study isundertaken, the prices were relatively homogeneous from country to country.
TYPE OF WORKS
This chapter offers advice on how to select the works (among those listed in Chapter II) thatmerit consideration. The discussion covers the following choices: (a) continuous or intermit-tent traffic works; (b) works with or without water-retention facilities; and (c) road laid on ordownstream of the dike.
(a) Continuous or intermittent traffic works
Crossings for which no neighboring substitution project exists are the most frequent cases inthe Sahel.
The essential considerations are:
vehicle traffic (volume and HV/LV/MT distribution)
duration of traffic interruption
value of vehicle time, and operating cost
cost difference between a continuous traffic and a submersible works project
detour length (where applicable).
Because these parameters vary significantly from case to case, they must be examinedon an individual basis. Nevertheless, findings available from a study carried out in a Maghrebcountry to determine investment thresholds bring out a number of important points:
A traffic interruption can be dealt with by creating a detour (where feasible), replacingmechanized transportation with animal-drawn transportation (where possible), or simply waitingout the interruption.
30 | CHOOSING TECHNICAL SOLUTIONS
Chapter 4
(a) Creating a detour is usually preferable to the economic cost of waiting time bya submersed structure - even when the value of travelers' time is discounted.A detour, even of 100 kin, is preferable to a half-day interruption or of 400 kmto avoid a two-day interruption.
(b) Replacing mechanized transportation with animal-drawn transportation canbe considered in the case of a water-saturated clay road. If a ford structure isflooded, animal-drawn vehicles have no advantage over mechanical vehides.
(c) Assuming, as is often the case, that a ford structure of length L meters can bereplaced by a non-submersible bridge of L/2 meters, and that the incrementaltrip distance is 100 km, we obtain the following figures:
MAXIMUM BRIDGE LENGTH PREFERABLE TO FORD STRUCTURE(Meters)
Annual Average Daily Traffic 1 5 1 5 30(AADT)
Average traffic interruption (numberof days)
2 ford ford ford 104 ford 7 10 24
10 ford 8 25 50
Once again, only a specific calculation can give a reliable result. The table gives onlyorders of magnitude in average cases. However, it shows dearly that under 15 vehicles a day abridge is rarely recommendable, and with traffic counts of up to 30 vehides/day, a ford struc-ture must be considered.
(b) Works with or without water retention, or road-dike
The parameters affecting this choice are:
(a) water needs(b) dam height(c) permeability of fill foundation(d) availability of materials to protect upstream slope(e) flood attenuation capacity of the reservoir.
| 31
These parameters are discussed below.
Water needs. Understandably, studying a water-retention project is ruled out if there are nopopulation centers (or none that lack adequate water supply) in the vicinity of the crossing orif, according to the information supplied by the agricultural agencies, irrigation would notgenerate a significant agricultural production surplus.
Dam height. Dam height determines fill cost: fill volume is proportional to the square ofheight. If fill height for a road crossing is h meters, it becomes h + R with a dam of R
abrm the highest meters, the berm being assumed to be the same with and without water retention. For aexpectd water level. normal cross-section of 8 m width and batter slopes of 2/h = 2/1, fill volume (in m 3/ml)
doubles when the total height h + R rises from 4 to 6 m (see figure).
140 ..............
120 --------------------
l 00z .... ......... f ....... R-0-'-- -0-- Ri1
< 80.* ; '' .- *-- Rm2
>o s> 60 OR-3
40 .........Heigt of
20 ...................... Rtrvoir R (m)
ol -2 3 4
Heih of Rad FiI without Rervir R (m)
Permeability offillfoundation. For a water-retention project to be efficient it must be possibleto store the water without excessive infiltration losses. While the presence of permeable mate-rials at the surface does not present difficulties in the case of a road fill, it calls for the use ofspecial techniques to prevent water from making its way into the foundation. Relatively im-permeable fill materials can be used, placed in a trench and properly compacted. This tech-nique is usually limited to depths of 3-4 meters. Sheet piling curtains or even linings of plasticproducts (such as bentonite cement) can also be used to make the fill completely watertight. Inthat case the costs of these arrangements quickly become large. Such procedures are generallyruled out in water-retention roadworks.
32 | CHOOSING TECHNICAL SOLUTIONS
Chapter 4
Availability ofmaterials to protect upstream slope. Protection of the upstream slope is indispens-able in order to prevent slap erosion of the dike core materials. The areas to be protected arelarge and increase linearly with the height of the fill to be protected. Where the local naturalmaterials are unsuitable, the cost of this protection is a major component of total works cost.
Flood attenuation capacity of the reservoir. The attenuation capacity of the reservoir affects totalconstruction cost. The total cost of a water-retention project includes:
the cost of the sealing fill and its protection;the cost of the flood spillway, which can account for half of the total cost.
In the case of zero attenuation (a situation often encountered when storage volume isnegligible in relation to stream volume), the flood spillway has to be dimensioned for maxi-mum flood volume. If, on the other hand, attenuation is high, reservoir outflow volume ismuch lower than inflow.
The dimensions of the spillway, and also its cost, are then reduced.
(c) Road laid on and downstream of the dike
Where no dam exists, the designer has to decide whether, in order to create a water-retentionfacility, it is preferable to build a narrow dam and a road downstream of it or a wide dam withthe road laid on it. Construction of the road downstream of the dike is justified if linear fill islarge, if the level of the road above the natural terrain downstream of the dike can be kept verylow, and if the cost of the works to cross the flood spillway remains low. These requirements aremet if attenuation is high or if the spillway millrace is steeply sloped, making it possible toreduce the crossing works section.
CONSTRUCTION SPECIFICATIONS
To define construction specifications, two elements have to be considered: (a) the road and(b) the dike.
(a) The road
For traffic safety and a uniform itinerary, the same arrangements and standards should apply toboth the road structure and the works outside it. The roadway and shoulder cross-sections willgenerally be kept unchanged for the entire length of the dike. The crossing of the spillway, ifthe latter takes the form of a bridge or box culvert, should have the same cross-section charac-teristics as in other road projects without water.
Where traffic is low (under 30 vehicles/day) the roadway width may be reduced to3.50 m (a single traffic lane) on the crossing works, particularly if the project offers goodvisibility over each bank of the stream. A detailed economic calculation will show the trafficlevel above which this narrowing no longer yields benefits. In some cases, there may be the fear
of weakening the roadway foundation strata when the water level upstream of the dam is at its
peak. This risk is real only if the roadbed soils are water sensitive. The precautions normally
taken (minimum berm of 50 cm and choice of materials with zero or low sensitivity) can
eliminate this problem.
The other construction features of the dike-road are exactly the same as those in roads
with separate waterworks. Further information can be found in the work Les Routes dans les
Zones Tropicales et Desertiques (BCEOM-CEBPT).
(b) The dike
General design. The manual depicts a number of dike cross-sections characteristic in the tech-
nical specifications. If maximum water retention height is low (under 1.5 m) and the terrain is
not too permeable, the dike can be constructed as a simple road fill placed on the natural
terrain after stripping off the topsoil. The risk of leaks and leak-tunneling is then nonexistent,given not very permeable terrain or low water pressure. The only special construction feature
to be added to an ordinary road fill is slap protection of the upstream slope. The downstreamslope is protected in the conventional manner by grass or drainage gutters; supplemental pro-
tection is added at its low part if the downstream discharge inundates the foot of the dike. On
the other hand, if the terrain is permeable and prone to wash erosion, or if the water retention
level is high, the dike must be designed as a true dam with anchor core and, where appropriate,
a tightness curtain or core. The manual offers information on these devices.
Upper part of dike. The mission encountered dikes in which the crest (which often carries the
road) presents a single transverse slope upstreamward in order to reduce the means of protec-
tion of the downstream slopes. The upstream slopes are less vulnerable since they are almostalways faced with slap protection.
In other cases - for example, when the dike crest carries a road with a wide cross-section-the cross-section is tectiform and the downstream slope has to be properly protected. The
system of a downstream slope protection wall with the water concentrated exactly opposite the
stonework water drainage gutters can be a wise decision, especially if the body of the dike
consists of a fine, easily eroded soil; the entire system must be correctly calculated (see photo
16).
Dike slopes and theirprotection. The slope gradients vary but generally fall within the range h/
1 = 1/1 and 1/4, the gradient 1/2 being most common. The slopes are subject to erosion due toslap and sometimes violent currents which occur in their vicinity, notably near flood spillways.They have to be protected. The protection consists of a concrete curtain, mortared-stonefacing, dry-stone facing set by hand or riprap (see photo 19). The latter two forms of protec-tion have the advantage of being better suited than the others to settling or subsidence. The
observations made during the mission, however, showed that all too often - especially if theriprap is not of continuous grain size or the dry-stone pitching has worked loose - slap and
infiltration erode the dike body between the stones. Concrete protection and well-executedmortared stonework (with, in particular, drainage channels and proper footing foundations)
CHOOSING TECHNICAL SOLUTIONS
Chapter 4
are dearly the most reliable methods (see photos 9 and 20). Generally, filters should be placedbehind the protection to ensure the transition between the fill body and the protection, and toavoid the carrying-away of the fine particles of the fill material, wash-erosion under the blocks,and destruction of the works.
In the case of works built on low or swampy ground or projects of the mobile-sill typewith discharge in both directions, the downstream slopes always have to be protected. If thewater retention level is high, the downstream foot of the dike must receive special protectionsince it is the outlet for the dike infiltration water. A filter therefore has to be provided. Protec-tion by means of riprap or concrete slabs is further recommended if the lower part of thedownstream slope is submerged following stream floods.
FLOOD SPILLWAY
General design. There are many types of flood spillways. They differ not only in the nature ofthe spillway sill but also in the series of works that channel the water from the sill to the old bedof the river downstream of the structure. Most, though not all, flood spillways consist of thefollowing components:
* fixed or movable spillway sill;
* millrace which is a continuation of the spillway, generally with a lowgradient; guide channel which connects to the thalweg, generally with asteep gradient;
* stilling basin downstream of the millrace for return of the water to the river;
* offtake and emptying works.
Flashboard Panels(Chamfered andLined)
Linings
Submersible Ford Structure and Downstream Scouring
Fixed spillway sill. Spillway sills can be shaped, thin or thick (raised culvert or spillway ford).Shaped sills are made of concrete or stonework (see photo 9). Thin sills are made of metalsheet piling (see photo 1 0). We did not encounter any thin sills of reinforced concrete, thoughsuch a device is possible and sometimes even recommendable. Thick sills generally consist ofan earthworks mass simply protected by a concrete or reinforced concrete curtain; the mostcommon cases are raised fords and raised box culverts (see photos 18 and 2).
Movable spillway sill. This consists of fixed vertical rails incorporated into the structure to carrythe flashboard panels. The fixed rails are sometimes replaced by simple rabbets made in theconcrete. The flashboard panels we saw were long, low structures, often 2 x 0.30 m, of woodor hollow metal panels (welded metal sheets) manufactured locally. Their shape is generallythat of a rectangular parallelepiped. The flashboards formed in this way are not completelywatertight. They are sometimes made watertight by using chamfered panels provided with awatertight felt lining.
Millrace and guide channel This part of the spillway is located between the sill and the stilingbasin. Sometimes there is no guide-channel, the millrace running directly into the stillingbasin. Water velocity is generally low in the millrace. In the guide channel, water velocity (atdesign flood) is often high, and these components have to be erosion-resistant. Unless they arecut into the rock, they have to be lined with concrete (usually reinforced concrete) or gabions.Beyond the thin sills, the three components (millrace, guide channel and stilling basin) aremerged to form a single component which is most akin to a stilling basin.
Stilling basin. The stilling basin is a device well known to dam builders but, according to ourobservations, much less familiar to road engineers. The latter simply place protective riprap -all too often inadequatc - downstream of the spillway. The stilling basin provides the transi-tion between rapid discharge of the water into the guide channel (or just after it leaves the
36 CHOOSING TECHNICAL SOLUTIONS
Chapter 4
spillway sill) and its calm discharge into the river downstream. Nearly all the high centralspillways of works with high water level (3 m or more above the bottom of the valley at thepoint opposite the works) that we met showed a true stilling basin (see photo 14). For lowerwater heights, however, or for certain lateral spillways, this component is sometimes reduced toa simple riprap carpet which requires careful maintenance (see photo 7). Even where thedifference in level between the dam and the water line downstream of the structure is small, astilling chamber is recommended. The case of submersible ford structures laid at river-bottomlevel is significant in this connection: a stilling chamber forms naturally downstream of theproject even if it has been protected with riprap (see photo 1).
Stilling basins should be provided for during construction. The manual shows how tocalculate them accurately. As we were often able to confirm, the lack of a stilling basin gener-ates regressive erosion activity culminating in destruction of the entire project.
Offlake and emptying works. Offtake works are calculated on the basis of the projected use ofthe reservoir (human water supply, livestock watering, irrigation or other uses). They are oftenomitted from projects, thereby enhancing the risk of pollution, though they usually accountfor only a small share of total project cost. They comprise:
an offtake tower (see photo 19) or a simple manhole upstreamone or more sluicegates:- upstream- downstream (see photo 3)- upstream and downstream.
The offtake is located slightly above the bottom of the thalweg to avoid it being silted up bysolid loads. It often serves as an emptying gate, although it does not allow the reservoir to beemptied completely.
Stream-levelfords. These works must be designed carefully, because they may create risks ofdeposits or erosion. To work efficiently, they must be shaped according to the stream bed andtheir slope along the stream should not be steeper than 1 percent, if the soil is erosion-sensitive.
SUMMARY OF CHAPTER 4
The unit prices observed in the various Sahelian countries (shown in Table 4.1) are relatively homogeneous.Of course, they have been and will continue to be affected by the devaluation of the CFA franc.
Intermittent-traffic works (ford structures) are recommended for traffic volumes of 2 vehicles/day or less butmust be avoided for traffic levels in excess of 30 vehicles/day. Between these two limits, an economiccalculation has to be done, taking into account construction prices and traffic holdup times.
The table induded presents a general analysis of the impact of the various parameters that decide adoptionor rejection of works.
A number of sound constructional arrangements are described in "Construction Specifications."
The main points are:
* Where road traffic exceeds 30 vehides/day, the road specifications (particularly cross-section) shouldremain unchanged over the entire length of the project. Where traffic is lower, it may be decided,based on an economic calculation, to downscale the road from two lanes to one.
* In the case of water storage, the dike can be constructed as a simple road fill, given good soil andlow water retention height (under 1.5 m). For larger reservoirs, devices used by dam-builders(tightness and anchoring core) should be considered.
In all cases, protection of the slopes against slap, flood currents and rainfall is necessary.
* Peak flows exert the maximum erosion and stress effects on the works. The trickiest component inthe series of flood discharge devices is the stilling basin or pool, which provides the transitionbetween rapid discharge of the water and its calm discharge into the river downstream.
A stilling basin is recommended even where the difference between retention level and down-stream river level is low (under 3 meters). Such a basin tends to form naturally behind a simplestream-level ford; it should be included in the construction plans. The manual shows how tocalculate the size of this part of the structure.
The design of the offtake or emptying works reflects how the reservoir will be used. It includes anofftake tower and gated conduits. They usually account for only a small share of total project cost,but omitting them enhances the pollution risk.
Chapter 5: Economic Choiceand Justification
T he underlying principles of the economic justification for a road project of the water-retention type are the same as for any other project. The "water-retention project" -as we shall call it to distinguish it from the road project as a whole - is limited to a
relatively short road segment, AB. Within AB the "with water retention" and "without waterretention" approaches differ from each other; but below and above this segment, the roadproject remains unchanged.
Two or more variants of the "water-retention project" on road segment AB are pos-sible, the economic benefits of which have to be compared. As in every road project and everyagricultural project, these benefits fall into several categories:
(a) vehicle operating cost on segment AB; the variants are always characterized bythe same alignment or longitudinal section between points A and B;
(b) works construction cost for the alternative studied and subsequent maintenancecost of the works;
(c) socioeconomic benefits, such as agricultural production, human water supply,livestock watering, and other impacts (such as sanitation benefits).
The first step is to identify and quantify costs and benefits year by year. The next stepis to calculate the total discounted cost of each alternative over the useful life of the works andselect the most economical one.
Precise information on methods of economic appraisal of road projects and calcula-tion of benefits will be found in bibliography references 1 and 2. The rest of this chapter willfocus on specific guidance on calculating costs together with comments on the main benefitsoffered by roadworks involving water-retention facilities. Useful life, which is generally takento be 20-25 years for conventional roadworks, poses a special and unavoidable problem inwater-retention projects - that of solid load and silting-up of the reservoir. If the reservoir siltsup in less than 25 years, or even later, it is important to have a clear picture of the likelyevolution of the reservoir and the decline in water resource benefits compared with those itwould generate immediately following construction. One can always adhere to a useful life of20 or 25 years provided one weighs, beyond the point of silting-up, both the advantages (rais-ing of the water table or other water surfaces, where applicable) and the disadvantages (risk ofdestruction of the works).
40 ECONOMIC CHOICE AND JUSTIFICATION
Chapter 5
VEHICLE OPERATING COST
Vehicle operating cost on segment AB for a given variant can be calculated in the conventionalmanner using model HDM3, taking into account the precise length, gradient changes andcurvature data for segment AB. The procedure can also be performed manually by first calcu-lating the cost of movement of the expected traffic over unit length (e.g., 1 km) of paved roadin good horizontal and rectilinear condition (using the vehicle operating cost submodels ofHDM3 or some other model), then taking account of the longitudinal section, alignment andany slowdowns, using the equivalent lengths methods (bibliography reference 1). In the case ofa short and well defined segment, this approach is more accurate and less cumbersome than thecomplete HDM3 approach. One problem is the possible closure of the road during spillperiods. In particular, this problem arises when the designer hesitates between a spillway silland a series of box culverts that allow continuous traffic. This difficulty is addressed in themanual.
CONSTRUCTION AND MAINTENANCE COST
We must distinguish between: (a) the cost of building and maintaining the works and (b) thecost of maintaining the road.
(a) The cost of building and maintaining the works
Construction and maintenance costs are grouped together under a single heading. This is be-cause there is a strong interaction between the construction cost of the works and their subse-quent maintenance cost. The mission's on-site inspections demonstrated clearly how harshlythe structures - whether spillway dams or conventional bridges - are attacked by water at thetime of the Sahelian floods and how the designer must plan to protect them. Assuming that theproject is well designed and the flood volumes have been calculated correctly, the structure willnot be carried away. If, however, the protection is not first-class - even to the point of appear-ing overdone - annual maintenance expenditure can be very high. We cannot overemphasizethe economic value of total protection: here protective measures are taken, the cost of main-taining the works is very small. It is, however, common practice to provide reasonable butincomplete protection; and the mission was unable to obtain reliable average annual mainte-nance cost figures for these cases.
Maintenance costs should generally be calculated in the same way as conventionalworks and those incorporating water retention are calculated. In certain special cases -namely,a water-retention project justified by uncontrollable meandering of the Kori and the periodicnecessity to rebuild a bridge - the annual construction and maintenance expenditures seriesmust include a prior project study that details the likely evolution of the works over the next 20or 30 years.
41
(b) The cost of maintaining the road
In well-designed works, road maintenance costs are the same whether the road is paved orunpaved. The mission found nowhere that the presence of the water table closer to the surfacein a water-retention project than for a bridge access had led to more rapid wear of a paved road.On the other hand, overflows onto the roadway (as, for example, in a slightly under-dimen-sioned, water-retention project) undoubtedly weaken the roadway strata and accelerate its de-terioration. In an economic calculation, however, the design must be assumed sound, and werecommend that the cost of maintaining a paved road be treated identically to a conventionalroad and one incorporating water storage. Similarly, no difference was noted in roadway per-formance for dirt roads laid on non-spilling dikes.
ASSESSMENT OF SOCIOECONOMIC BENEFITS
(a) Introduction
The great advantage of water-retention road projects is that they require the user to bear therelatively insignificant incremental development cost of adapting the road to perform its watercontrol function. That cost comprises (i) development of the ford structure with movable sill(generally consisting of flashboards), (ii) flood spillway, (iii) any strengthening of hydraulicprotection (notably the banks), (iv) water control structures (in the case of submersion), (v)mains and distribution works (irrigation), and (vi) water offtake works (domestic water sup-ply). This cost has to be set against the benefits offered by the scheme, such as the agriculturalvalue added deriving from new crops (or the incremental value added from existing crops, as isusually the case), the value of water supply for people and livestock, fish-farming (where it isfeasible), and other benefits.
(b) Agricultural uses
Basically, two options are available. To engage in subsidence (floodplain) cropfarming up-stream from the road in the reservoir area, and to store water in the reservoir in order to irrigatea cropfarming perimeter downstream. Costs and benefit of those alternatives are reviewedbelow:
Subsidence cropfarming in reservoir area
Costs. Incremental embanking cost as compared with roadworks that allow the water to passunhindered.
Cost of water control works:- sill with movable section (flashboards or gate);- flood spillway.
4-2 ECONOMIC CHOICE AND JUSTIFICATION
Chapter 5
Any necessary protective devices (especially near the flood spillway). For all thesecomponents, the best way to calculate the portion of the cost difference chargeable tothe agricultural part of the project is to estimate the total cost of each of the twovariants: road only, and road-dike plus agricultural water facilities.
Other works associated with agricultural use of the water. The calculation should ofcourse also take into account the cost of waterworks for floodplain crops, such as smalldikes to improve water retention, spillway sills to move water from one cultivationblock to the next, and so on.
Maintenance and operating costs. Only those that relate strictly to the agriculturalportion of the project will be taken into account here.
Added value of any cropfarming previously carried out in the reservoir area.
Benefits. These comprise the value added of new crops grown in the reservoir area, consistingof the gross income obtained from the crops (main products and by-products) less the cost ofinputs and other production factors (seed, fertilizer, treatments, equipment, tractor or team inthe case of mechanized cultivation, and any product packing expenses). An example of thiscalculation is given in the manual.
Downstream irrigation
Costs taken into consideration are those relating to the upstream reservoir (those indicatedabove, except of course the mobile still section, which does not apply here) and the followingadditional components:
gated water intake, with main supply pipe across the dike to the head of the irrigationdistribution canal(s);
the water distribution infrastructure within the irrigation perimeter;
the value added of crops existing prior to the project.
Vegetable-growing yields the highest income, is the most labor-intensive and can utilize theproject works to the best advantage. Vegetable consumption is currently rising steadily through-out the Sahel region and therefore offers farmers good financial prospects.
(c) Use of the water by people and livestock
Costs. Since animals can never be barred access to the reservoir (unless crops are grown in it), wecan assume that no particular means need to be provided for livestock watering. But from thehealth standpoint, people cannot be allowed to draw water from the reservoir, especially whenanimals have free access to it. Arrangements for drawing water must therefore be factored intothe design and cost of the project. These arrangements can range from the simple to the
complex, from a simple dug well in the vicinity of the reservoir, providing minimum waterfiltration, to a more sophisticated well, lined and equipped with coping and finished approachesto facilitate access and eliminate risk of pollution. (We are disregarding here more sophisticateddevices, which could include a closed well equipped with a pump and main supply pipe.) Witha system finished with coping, human water supply can easily be combined with livestockwatering, troughs being added nearby where needed.
Benefits. It is difficult to assess the benefits derived from a water supply for human and animalconsumption. They must at least indicate the following (a) the number of people'served bythe scheme and the quantity of water reserved for their use; (b) the number of animals to bewatered and the quantity of water involved.
There is only one situation in which the economic benefits of human and water supplycan be quantified easily -i.e., where a need has been clearly identified and one or more alter-native sources of supply have already been considered and costed. The economic benefit isthen the minimum cost of any alternative solution. In other cases we can try to estimate thecost per cubic meter of water produced. This cost varies widely, however, according to thewater source (dug well, dam, tubewell), the pumping system (manual, thermal power, solarpower, and so on) and any distribution network. By example, we can suggest costs of the orderof CFAF 50-220/m3 for villages and small population centers.
(d) Fishing
Fishing is often an attractive income prospect, even without a fishery project and fish nursery.Outputs of 100 kg/ha/year are possible in permanent reservoirs, and annual incomes of theorder of CFAF 400,000 per fisherman have been recorded. It is, of course, possible to combinea fish-farming station with a water storage system. In that case, the road project costs have to beaugmented by the whole of the investment and input costs of the station.
SUMMARY OF CHAPTER 5
The economic choice of and justification for a road development project with or without water-retention features are based on the same principles as in any other project: discounted compari-son of construction, maintenance and operating costs with benefits to users (drivers and localpopulation).
The useful life of a project should be taken as 25 years; however, it is essential, to take accountof solid load.
Both traffic and beneficiary population will evolve over time, and this evolution must be as-sessed as precisely as possible. The benefits that accrue to road users from upgrading of a roadfrom "without water retention' to "with retention" are generally low As a rule it is the benefitsto the general population and farmers that justify the incremental cost of such a project.
44 | ECONOMIC CHOICE AND JUSTIFICATION
Chapter 5
* The agricultural benefits are assessed by estimating gross income from the crops concerned:prices of the main products and by-products minus the cost of inputs and other productionfactors (seed, equipment, tractor or team).
* The gross income figures are set against:
- the incremental cost of the road project;
- the value added of any crops already grown in the reservoir area;
* In the case of irrigation:
- the costs of infrastructure, pumping (if any) and water distribution within the perimeter;
- the value added of any crops already grown in the perimeter before the works were built.
* Among farming options, vegetable-growing yields the highest income and uses the project worksto the best advantage.
* Putting a value on the use of the water resources by people and livestock is more difficult. Thesometimes used figure of CFAF 50-200/m3 of water is no more than a rough average. A betterpicture of the situation can only be obtained through study of water reservoirs in the region andtheir usable annual production; however, there are generally few of these projects.
* Fishery resources must also be taken into account, where they can be quantified.
Chapter 6: Development andManagement of the Works
T his chapter focuses on water-retention works from two standpoints: (a) how road projectsincorporating water-retention facilities have come into being and fared in practice; and(b) the best ways to promote and improve such works (design, construction, mainte-
nance, management).
ORIGINS
Some water-retention works have been built without regard to socioeconomic benefits, solelybecause such a project seemed a more reliable way of crossing a wadi or a broad flood plain. Isthis motivation always valid? Between 1910 and 1930, it seems a number of submersible fordand water-retention works were built because the ability to determine realistic flood volumesdid not exist; in wide valleys this type of project allowed a much bigger safety margin for thedischarge of large flows. We now know how to calculate flood volumes with relatively goodaccuracy. Do cases nevertheless exist in which a road project equipped with water-retentionworks and spillway sill is recommended because of road technology?
Certain rivers in Niger, and perhaps in Burkina Faso, which tend to meander andhave, up to now, been crossed by means of conventional bridges could be equipped with araised sill and a reservoir. Unfortunately, the mission was unable to collect the complete his-tory of such a crossing or to assess the purely economic benefit, from the road standpoint only,of such a water-retention project. In the regions in question solid load could also totally nullifythe benefits of such a project. The most useful lesson emerging from the present study is that,in the past, some projects have been built purely for road technology reasons, and, in certaindifficult cases, designers should consider the possibility of a water-retention project.
Also common is a reservoir in a region with a pronounced dry season whose purpose isto supply water for the roadworks (wetting of fill and roadway materials, concrete mixing,equipment cleaning, and so on) and for personnel. It makes sense to combine the constructionof this (often temporary) worksite dam with that of the road in order to produce a final struc-ture able to serve the needs of nearby population groups. With respect to the economic calcu-lation of the project, the benefit of this water storage to the worksite is taken into account in theform of a reduction in earthmoving cost and worksite overhead compared with the amounts ofthese costs without water storage.
In the case of works that serve socio-agricultural purposes, the initiative to build wa-ter-retention works comes from one of the following four procedures:
(a) The Public Works Department embarks on a conventional roads project, and certainvillages ask that the opportunity be taken to create a water storage facility. This wish is
46 | DEVELOPMENT AND MANAGEMENT
Chapter 6
often expressed after the works have been started, the idea having been sown in theminds of the people living along the road by the power of the construction equipmentbeing used. The time available for the study is short, however, and there is no assur-ance that the project will be as satisfactory as it would have been if there had been time.Moreover, budget constraints can trim the project below its minimum economic scale.
(b) A similar procedure is initiated between the agricultural or water resources agency andthe Public Works department. One example is the Djeguenina Project in Mali, whichwas unfortunately abandoned because the project prepared by the water resources agencyarrived too late. The agricultural or water agency is, however, generally in a betterposition than the villages to know the progress of a road project and to study thevariant, or commission its study.
(c) On tertiary or rural roads the initiative often comes from the rural engineering orwater agency or even from an NGO. The agricultural or village water supply projectis then combined with a road crossing project.
(d) Finally, in the best scenario, the road agencies themselves ask the designer, in studyinga new road or reconstruction project, to consider the feasibility of incorporating waterstorage into certain crossing works. In practice, this is still rare. Perhaps the dissemi-nation of this report will promote its wider use.
DESIGN AND CONSTRUCTION
One needs to distinguish between development works on major roads and those on rural roadsand tracks. In the case of main roads, the process is the same in all the countries visited. Onrural roads, different methods are often used.
(a) Main network
The project owner or authority (maitre d'ouvrage) is the ministry responsible for roads. Theproject manager or executing agency (maitre d'oeuvre) is the roads agency, which usually del-egates its project study and works supervision responsibilities to engineering consultants.
In view of the sparse resources likely to be available to the project authority, interna-tional donors are always involved in project financing. Construction is carried out by a civilengineering contractor, usually a large firm. It is an advantage if the water-retention worksstudy/design can be done at the feasibility study stage. This requires the roads agency to in-clude the necessary clauses in the terms of reference for the road studies. It is not enough justto ask the engineering consultant to study this possibility for the various road projects. Withcurrent contracting methods - in which the cost of the study carries a great deal of weight inawarding the contract - there is a serious risk that the water-retention works study will beunsatisfactory because it is complex and calls for the involvement of several experts.
We recommend that the study be carried out in two stages: (i) identification of poten-tial sites and (ii) feasibility study of the sites selected.
Annex 2 sets forth the clauses that we suggest government agencies should include inthe terms of reference for road feasibility studies when they wish to consider water storagefacilities. A good indication of the project's likely benefits is if the project is initiated while theworks are in progress - at the express request of a village or a water resources agency. Theidentification phase can then be omitted. It is, nevertheless, advisable, prior to the final designstage, to carry out a feasibility study to determine the broad lines of the project: reservoir size,sill and dike levels, flood spillway design, and ancillary works (irrigation network, etc.). Thisfeasibility study can be done either by the roads agency or by its engineering consultant. It can,however, be entrusted to the national agency responsible for hillslope dams, which will oftenhave a better understanding of the problems and constraints associated with projects of thiskind than the road agency, and be better equipped to produce a good study quickly.
In all cases the final design project must be entrusted to the same designer as the rest ofthe road to ensure uniformity between the water-retention works and the roadworks under thecontract. A final important point: in none of the works encountered had the road agencycarried out or commissioned the ancillary work, such as clearing trees and brush from thereservoir area (see photo 15), and the downstream irrigation network. The responsibility forthis task was left to the agricultural agency or to private initiative.
(b) Rural roads
Some important rural road networks are built by private contractors under the same proce-dures as major roads; the only difference is that the road specifications are more modest inscale. The same comments and recommendations apply to such networks as to the main roadssystem except that if the ancillary works - notably the irrigation system - are built by a ruraldevelopment agency, they should be built at the same time as the roadworks. Cases also occur,however, in which water-retention works are built using much smaller resources and with fi-nancing by agencies seeking to develop HIMO works or by NGOs with strong grassrootsinvolvement in the construction. These projects are becoming more widespread and must notbe omitted, even if they involve certain risks.
In both the cases described above - a network built using modern, rapid constructionmethods and one calling for strong grassroots involvement right from the start - an institu-tional constraint is sometimes present: dear identification of the ministry or government agencyto which top-level responsibility for the proposed project belongs. The ambiguity on thispoint that prevails in some countries can generate harmful interagency rivalry or exhaust en-thusiasm. This problem is not, however, limited to water-storage roadworks but extends to theentire rural development field.
In the second case, with strong grassroots involvement in construction, the problem ofproject supervision must be resolved. A number of NGOs in Mali have entrusted constructionmanagement to a team of experts belonging to a local firm of engineering consultants. We
48 DEVELOPMENT AND MANAGEMENT
Chapter 6
strongly recommend this approach, since it is low cost and villagers and project authority caneasily locate the experts when necessary, a missing advantage when the worksite is managed bya foreign expert belonging to an NGO or an international donor.
MAINTENANCE
(a) Major roads
Where water-retention works are built on roads for which the road agency is responsible, it is,in principle, that agency which handles maintenance. In fact, such works are maintained nobetter and sometimes not as well as conventional roadworks -bridges and box culverts. Wa-ter-retention works include components, such as water intakes and stilling basins, that roadagencies know little if anything about and tend to neglect. Moreover, road agencies sometimesfeel that water management components -intake gates, flashboards and certainly irrigationnetworks - are not their concern. Management responsibility for water-retention works onmajor roads must be clarified: Where do the respective responsibilities of the road agency andthe management agency, if any, begin and end? Which agency, in the final analysis, is requiredto include the necessary appropriations in its maintenance budget? The recommended solu-tion is set forth in the next section (Works Management).
(b) Projects involving villages
In these kinds of projects the villages feel directly concerned, and small maintenance - thatwithin the village's technical and financial capability - is generally performed. For example,this is true of small flashboard works, in which the local people are kept aware year by year ofthe existence and benefits of the project through their water and crop management tasks.
The maintenance problems beyond their capacity are those that require large resources.The village or management agency must know exactly where to turn to obtain the necessaryfunds or have the work done.
WORKS MANAGEMENT
There are a number of fixed-sill works without water intake facilities that do not call for anyparticular management. They are mostly located on main roads. The road agency has neverconcerned itself with their management, and the village people make the best use of them theycan for water supply, cropfarming, vegetable growing and fishing. In contrast, projects incor-porating movable sills or water intakes need to be managed and monitored if they are to yieldall the hoped-for benefits. The mission's observations show that fixed-sill works, even thoseequipped with water-intakes, are rarely looked after by either their users or by the governmentagencies (water and rural engineering) that are in principle responsible for such developmentprojects.
Even where local residents have instigated a project they do not always manage it orperform the maintenance tasks within their capacity, such as changing a defective faucet, re-pairing cracks in facing and sowing grass.
If managed at all, movable-sill works are managed by either a national developmentcompany or an agricultural or village community (Mali, Mauritania).
Everything we saw points to the necessity, before any road/water retention project isstarted, for discussion among:
the agency building the projectthe project authoritythe beneficiary community (users).
The agenda should include a clear exposition of advantages and disadvantages of theproject and discussion of the terms of a contract with the future users under which they wouldperform clearly-defined tasks in exchange for the often large gift they will be receiving. If thepopulation refuses to participate, the proposed variant would of course be abandoned. Thecontract would define this grassroots involvement in the project: cleaning and clearing thefuture reservoir, setting up a management committee, collecting funds to pay for small mainte-nance equipment and materials (faucets, flashboards) and so on. The contract would clearlydefine the ownership of the project, the agency responsible for major repairs and the source oftechnical advisory assistance on maintenance, and the charges borne by the users. The projectauthority or authorities should monitor the project to ensure its efficient management. Thismonitoring could be of the kind applied in Burkina Faso (but on a larger scale): projectinspection tours, on-the-spot discussions with designated officials, and village meetings.
SUMMARY OF CHAPTER 6
The situation described above prompts a number of recommendations with respect to the mea-sures taken to promote road projects with water-retention features and to improve their operat-ing efficiency.
Such projects generally involve two government agencies:
- the agency responsible for building the road - i.e., the roads agency in the case of majorroads, and the ad hoc agency, which varies from country to country, for rural roads;
- the agency responsible for managing water resources, which also varies from country tocountry and sometimes even according to use: water supply, or agricultural development.
The first recommendation is a general one: these agencies must be identified clearly.
These projects also usually involve the participation of their direct beneficiaries (the users), whopossess only modest resources.
50 DEVELOPMENT AND MANAGEMENT
Chapter 6
* To ensure that the investment bears full fruit, we recommend that, in all cases, a contract beconcluded between the government agencies and the users that spells out:
- the ownership of the project, with all the attendant surveillance, maintenance and safetyimplications;
- the agency responsible for general management of the water resources, supervision of thereservoir (water storage, silting-up, etc.) and monitoring of day-to-day management;
- the agency responsible for day-to-day management of the water resources (users commu-nity); where appropriate, this management indudes small maintenance.
* We are not proposing standard contract formats here because the contracts can vary greadyaccording to national specifics and types of projects. We simply give, in Annex 2, a list of theessential points that should be explicated and addressed in the contracts.
* A project that involves irrigation or water distribution networks can have two owners:
- the owner of the works- the owner of the irrigation network, pumping station or other facilities.
* It is essential that each of the parties involved should be aware of his precise duties and respon-sibilities, which is by no means the case today.
_ 51
Chapter 7: Summary
T he case study in this report was financed by the Norwegian Trust Fund and headed andconducted by the World Bank as part of its Sahelian Operations Review. In providingexamples of and advice on the design of road projects with water-retention features in
the Sahel, the report aims to help authorities provide for water storage (temporary or perma-nent) in regions where water is a precious resources; and avoid over-dimensioning works onroads or tracks that carry very little traffic. The focus is on roadworks. Dam works, whosemain purpose is water storage, are not considered either in this report or in the manual.
The study area is the Sahel region proper, commonly defined as the area located be-tween isohyets 200 and 850 mm/year, plus a zone located further to the south which can reachisohyets of up to 1200 mm/year when, as is often the case, it also presents seasonal droughtproblems (outcropping rock and lack of water table).
A study mission traversed five Sahel countries in June-July 1992. This report summa-rizes the information collected and its resulting analysis.
DESCRIPTION AND PURPOSES OF THE WORKS
A great many kinds of roadworks allowing water management are found in the Sahel. They canbe dassified by whether they allow continuous road traffic (bridge, box culvert, conduit), orcreate traffic interruptions, however short (submersible ford); allow regular and flood waterlevels to pass without affecting the river regime, or permit permanent or temporary waterstorage; in the case of water-storage works, involve fixed components or movable gates.
For a description of the most common water management works, the reader is referred toChapter 2 of the report.
The most common works are:
(a) pond (storm basin)
(b) absorption well (drainage well or cesspool)
(c) stream-level paved ford
(d) raised paved ford, with or without conduits or box culverts, without sluicegates
(e) raised paved ford, with box culverts and sluicegates
52 | SUMMARY
Chapter 7
(f) combined road-dike structure with raised box culverts
(g) bridge equipped with fixed or movable spillway sill
(h) dam upstream of the road.
There are many other kinds of works, combining two or more of the types describedabove. There are also "balanced flow" works, which allow the water to circulate in eitherdirection and make it possible, over the entire length of a long dike-road in a flooded area andregardless of the area affected by a storm, to keep the water levels on each side of the road inbalance.
The study mission was struck by the differences among the countries it visited in termsof density and purposes of roadworks allowing good water management. While this differenceis due primarily to natural conditions, which vary from region to region, some aspects of it areapparently explained by administrative traditions.
Burkina Faso possesses by far the largest number of roadworks of the water-retentiontype. They are mostly fixed-sill works with a retention height of over 1.5 m. While thesecharacteristics arise from favorable natural conditions, they probably also reflect the keen aware-ness of the benefits of such works on the part of Burkina's road agencies, which since 1982 havesystematically asked designers to take them into consideration. The reservoirs are often perma-nent and serve mainly for human water supply and livestock watering. They are also often usedfor fishing.
In Niger, solid load and menader are major problems. Heavy solid-load volume hashindered the construction of many water-retention roadworks. The existing works are of thefixed-sill type and designed mainly to meet road needs (by creating water reserves for roadconstruction). While the road agencies are aware of the socioeconomic benefits of the reser-voirs, they have done little to promote them.
In Mali, the road agencies, while appreciating the benefits of water-retention works,cite their high construction cost and increased road destruction and closure risk. The waterresources agencies are behind most of the water-retention roadworks built or planned. Theworks are generally meant to improve agricultural production, and they include adjustable sills.
In Mauritania, water-retention works are essentially for farming. Human water needsare supplied by tapping the water table or deep resources. Where these do not exist, there areno people, and soil permeability and evaporation prevent the creation of permanent reservesusable for water supply or stock watering by means of road reservoirs. On the other hand,cropfarming is possible in Mauritania only in bottomlands, where water accumulates for sev-eral months and disappears through evaporation or infiltration. Controlled, movable-gate wa-ter reservoirs make it possible to create such moist zones artificially, permitting off-seasoncropfarming. For that reason, the works in Mauritania are of the movable-sill type and areintended for agriculture - no reservoir is permanent. Mauritania also has works located in the
main bed of the Senegal River that make it possible either to control the interior lakes orlengthen the time of submersion of the high areas of the main riverbed.
In Togo, the mission inspected only a road dam at Kabou, intended essentially forvillage water supply and livestock watering. It is probably representative of works that exist orcould be built in the dry zones, lacking a water table, located between isohyets 800 and 1200mm: northern Benin, northern Cote d'Ivoire and northern Nigeria.
FACTORS AND PARAMETERS
The various factors that determine the design or cost-benefit rating of a given roadworks projectare discussed below.
(a) Road traffic
Traffic governs, in particular, the choice between continuous and intermittent traffic designs.This choice calls for a specific calculation of the cost of alternative development works, theaverage annual number and duration of traffic holdups, and the length of the necessary diver-sion (where applicable) to avoid them. However, below 2 vehides/day a ford is almost alwayspreferable to a continuous-traffic project; with 30 vehicles/day or more, the continuous-trafficapproach is preferable; and between these extremes (2-30 vehides/day), an economic calcula-tion needs to be done.
(b) Topography
Topography is important from several points of view:
- Site selection. Generally, the best valley-crossing points are the same for a dike and abridge.
Longitudinal section of the valley.
- Cross-section of the valley. If the banks are steep, a bridge is usually the best solution.If, on the contrary, the valley is broad and flat, and the longitudinal section of theriver creates frequent and durable submersion of the valley (a common case in BurkinaFaso), a raised road (dike-road) is necessary. At the same time this allows a waterreservoir to be created upstream of the dike-road at no great cost.
- Water storage volume as a function of reservoir height. The greater the storage vol-ume for a given height, the greater the justification for water-retention facilities.
(c) Hydrodimatic data
Precipitation data are always fairly well known. Precipitation is generally concentrated in theperiod June-October, and the statistics give:
So | SUMMARY
Chapter 7
- average annual rainfall and year-to-year variations;
- instantaneous precipitation: maximum quantity of water falling in a given space of
time (ranging from a quarter of an hour to 48 hours) over a given period (5, 10 or
100 years). This parameter furnishes information on flood volumes to be expected in
small catchment basins. Instantaneous precipitation generally shows good correla-
tion with annual precipitation;
- monthly rainfall data: this information helps determine feasible crops and the corre-
sponding irrigation water needs.
Water flow data are much less well known. Streamflow ratio (volume of water carried
away, as a percentage of quantity of rainwater received by the catchment basin) varies substan-
daily, according to geology, plant cover, slope and rainfall itself. One example is a catchment
basin in Burkina Faso with an area of 182 km2 which posted annual average streamflow ratios
over three consecutive years of 12 percent, 6 percent and 1 percent.
Estimating annual average flow of a river into a given site thus involves a large risk of
error. Study of peak flows is essential in order to be able to dimension box culverts, bridges or
flood discharge works correctly (see the handbook). The design flood is the flow by reference
to which the project is dimensioned for total operating safety. The ten-year peak flow is gener-ally used in very small works projects, when submersion, or even destruction, does not entailserious consequences (loss of human life, substantial material damage). In the most common
works, a 100-year design flood is considered.
(d) Catchment basin
Despite the wide variation in streamflow ratio, catchment basin size is a vital basic parameterfor assessing water inflow volume.
Water-retention roadworks, which are always small reservoir projects (compared withlarge waterworks dams), never store large volumes of water. Generally speaking, cost per m3
stored rises rapidly with increasing catchment basin size, and the most cost-efficient projectsare those for catchment basins of moderate size with annual flows roughly equivalent to waterneeds.
The mission noted projects on catchment basins ranging in area from 4 to over 5,000km2. Once a catchment basin exceeds 500 kiM2, a project's water components, if designed tostore water, become very large. The incremental cost in relation to a conventional bridge isjustified only if the socioeconomic benefits that accrue from the reservoir are also very large.This means that the roadworks that are a priori most likely to allow economically efficientwater retention are those with catchment basins of 4-500 km2.
I 55
(e) Solid Loads
Solid loads caused by erosion of the catchment basin by meteoric water are often large in theSahel and need to be factored into the project calculations - they help determine the usefullife of the project and hence its economic justification. Solid load depends on many factors,particularly soil degradation and plant cover: volumes of 1,000 t/km2/year are not uncommonin the northern part of the region but can sometimes be close to zero in the south.
Large solid loads can spell doom for a water-retention project. Methods of combatingsolid loads include protecting the catchment basin, sowing grass, and reforestation. Thesemethods are complicated, expensive and slow to produce results. A project designer will factorin estimated solid load volume but never a potential reduction in it.
(f) Floating objects
Floating objects, such as branches and large clumps of grass, pose a serious danger to workswith low overhead clearance, especially low conduits and box culverts. Sahel region engineershave long known that it is always the same projects, year after year, that are obstructed in thisway. These projects are generally located in upstream catchment basins, with substantial popu-lations, where sedentary or itinerant farmers fell trees, practice slash-and-burn agriculture orfarm steep slopes along the river.
The most vulnerable types of works are conduits or box culverts laid at stream level.Submersible fords, on the other hand, easily allow floating objects to pass. A few floatingobjects sometimes lodge on raised fords without, however, endangering them. Raised fordswith box culverts or conduits have to be cleaned out to avoid prolonged submersion of theford.
To combat floating objects, the works need to be cleaned out after every storm; suchsystematic dearing is, however, very difficult to implement. Another solution is to install adevice that can stop floating objects 50-100 meters upstream of the project. The best methodis that of reinforced concrete posts (see photo # 12); it is effective and relatively cheap (seeManual).
(g) Evaporation
Evaporation has a direct impact on a reservoir's water balance. It depends on a number offactors, induding water or soil temperature, air temperature and humidity, wind and planttranspiration. In the Sahel, actual evaporation typically exceeds 1000 mm/year and can oftenreach or even exceed 2,000 mm. Although attempts have been conducted to combat evapora-tion, the results have been generally disappointing, and project design has to take natural evapo-ration into account.
56 | SUMMARY
Chapter 7
(h) Infiltration
Infiltration is never zero. In the case of a water-storage dam it has to be fairly low to allow thecreation of a water reserve. In all cases, it can jeopardize the existence of the structure throughleakage tunnelling.
Even if infiltration is high and, for example, prevents the reservoir from filling com-pletely, this does not mean that the benefit of the project is zero. It raises the water tablesupstream and downstream of the dam and facilitates water supply to human settlements. TheIfida Laba project, for example - a dike-road with raised lateral box culverts - has neverexperienced water entry into the culverts yet constitutes a permanent reservoir. The catchmentbasin area is 7.4 km2. The water disappears through infiltration, evaporation, and humanconsumption.
(i) Water quality
Warm water is subject to fermentation and rapidly becomes foul when it stagnates. This deg-radation accelerates as the volume of the pond shrinks under the combined effects of evapora-tion and infiltration. This situation can be aggravated by pollution caused by animals thatcome to drink there.
To retard water degradation as much as possible, certain construction measures needto be planned in advance and duly implemented to ensure proper use of the resource. Theyinclude: (a) clearing the flooded area of brush and trees and (b) sheer-finishing the reservoirbanks. For human water supply, the use of wells in conjunction with the reservoir and filteringof the water is advisable and indeed often done.
(j) Water balance -- Flood attenuation
This involves comparing (generally month by month): (a) reservoir water input (inflow - spill-way discharge) b) losses (infiltration, evaporation) (c) expected water consumption. Study ofthe water balance gives a better picture of optimum project size and in some cases can lead toincreasing the reservoir volume. In projects associated with farming perimeters, the waterbalance also helps the authorities develop precise water management rules.
An hour-by-hour water balance is also prepared in order to study the discharge of theproject design flood and to adapt all parameters of the spillway to the flows it will handle. Thishelps to assess the buffer effect of the reservoir (by accumulation of water between the spillwaysill and the upper level of the body of water discharged).
The difference between the flows entering and leaving the reservoir is generally verysmall for the (100-year) design flow of large catchment basins. Normally, in the case of a largecatchment basin, construction of a dam could not be justified by a substantial reduction inflood level. Only in the case of very small catchment basins (under 10 kmn2) can flood attenu-ation reduce the size of the water discharge works.
(k) Demographics and water needs
In a project designed to serve human needs, it follows that there must be a population voicingsuch needs in the vicinity. Water needs, other than for farming, are estimated on the basis oflocal human population size and livestock numbers. The local human population is made upof the people living within 5 km of the water resource - but population growth and popula-tion shifts must also be considered. A 20-year projection period is recommended. In calculat-ing forward, an average consumption of 20 liters/person/day should be assumed.
Livestock needs are as follows:
cattle: 30-40 liters/day (50 liters/day per adult animal under Sahelian conditions dur-ing the hottest times);
- small ruminants (sheep, goats): 3-4 liters/day.
(1) Crop suitability -- water needs
Determining a soil's crop suitability is a complex problem and a matter for agro-pedologistswith a good knowledge of the region. Agricultural research determines, for each crop, thequantity of water needed to ensure normal development of the plant during its growth cycleand the distribution of these needs over time. If rainfall cannot be counted on to supply thedesired quantity of water month by month, irrigation must be considered. Crop water needsare calculated by augmenting the plant needs (called evapotranspiration needs) by a given per-centage to take account of irrigation efficiency (main supply and distribution losses, unevendistribution within the parcel, deep percolation).
Below are some orders of magnitude of water needs under Sahel conditions. These needs haveto be augmented by a network efficiency ratio of at least 30 percent.
Maize or sorghum: 5-6,000 m3/ha.
Vegetable crops: 4,000 m3/ha.
Rice: at least 10,000 m3/ha for dry-season cycles. This thirsty crop will be generallyunsuitable for a perimeter located downstream of the reservoir.
Vegetable crops are often highly esteemed, especially around villages. They are highly labor-intensive and make it possible to meet the family consumption or marketing needs of thegreatest number of plot-holders.
58 SUMMARY
Chapter 7
CHOOSING TECHNICALLY AND ECONOMICALLY SOUND SOLUTIONS
(a) Introduction
The choice of good technical solutions depends on the parameters reviewed above and the unitprices of the different works Of course, the best project is the one that offers the best cost-
benefit ratio. From the technical standpoint, it is also important to adopt sound constructionalarrangements which inhibit deterioration of the works and avoid maintenance costs that nul-lify the expected savings of the investment.
(b) Basic technical choices
The main choices are:
permanent works or temporary works of the ford type
a water-retention project or a conventional bridge
in the case of a water-retention project:
- a fixed or a movable sill;
- a road laid on the dike or downstream of it.
With regard to the first of these problems, experiences with many projects are summarized insection "Road traffic" in Chapter 7.
Concerning the second problem, the basic factor is the existence of a water need andhence of nearby population. Many other factors must also be considered, including soil per-meability, availability of dike construction and slope-protection materials, and local topogra-phy. The favorable and unfavorable factors of the last two problems are listed in Table 4.3.
(c) Choosing construction specifications
The road specifications must be the same both within the project and outside it. The roadcross-section will be reduced (from two traffic lanes to one) only on very long works projects;an economic calculation is always necessary to justify this departure from the road standards.
If maximum water retention height is low (under 1.5 m) and the terrain is not toopermeable, the dike can be constructed as a simple road fill without any special arrangementsother than protection of the upstream slope. If, on the other hand, the water retention level ishigh and the terrain is permeable and prone to wash erosion, the dike must be designed as atrue dam with a tightness curtain or core and, where appropriate, an anchor core.
Slope gradients vary, with 1/2 the most common gradient. The slopes are subject to
erosion due to slap, floodwater and rainwater, and therefore must be protected. The protection
consists of a concrete curtain, mortared-stone facing, dry-stone facing set by hand or riprap.
Concrete protection and well-executed mortared stonework (with, in particular, drainage chan-
nels and proper footing foundations) are the most reliable methods.
In water-retention works, the flood spillway is the trickiest and often the most expen-
sive component. It comprises a sill (which may be fixed or movable) and various components
that channel the water from the sill to the old bed of the river downstream of the structure.
Spillway sills can be shaped, thin or thick. Shaped sills are made of concrete or stonework, and
thin sills are made of metal sheet piling or reinforced concrete. Thick sills generally consist of
an earthworks mass simply protected by a concrete or reinforced concrete curtain; the most
common cases are raised fords and raised box culverts. Movable sills consist of fixed vertical
rails incorporated into the structure to carry flashboard panels. The fixed rails are sometimes
replaced by simple rabbets made in the concrete. The flashboard panels we saw were long, low
structures made of wood or hollow metal panels (welded metal sheets), manufactured locally,
which can be handled manually.
The stilling basin provides the transition between rapid discharge of the water in the
guide channel (or just after it leaves the spillway sill) and its calm discharge into the river
downstream. It is an important device yet one with which road engineers are often unfamiliar,
and provides a pool within which the energy of the water is dissipated. It is an advantage if the
stilling basin is included in the construction plans. The manual shows how to calculate them
accurately. The lack of a stilling basin creates regressive erosion which culminates in destruc-
tion of the entire project.
(d) Economic choice and justification
The economic justification for a road project with water-retention or water pass-through fea-
tures is based on the same principles as any other project. The water-retention works are,
however, limited to a relatively short road segment, AB, within which the "with water reten-
tion" and "without water retention" approaches differ from each other - but above and below
which the roadworks remain unchanged.
The economic calculation is based on an evaluation of the following factors:
- vehicle operating cost on segment AB- construction cost- subsequent maintenance cost of the works- agricultural production- human water supply- livestock watering- other impacts of the project (such as sanitation benefits).
60 | SUMMARY
Chapter 7
Useful life poses a special - and unavoidable - problem in the case of water-reten-tion projects: the problem of solid load. If the reservoir silts up in less than 25 years, or evenlater, it is important to have a clear picture of the likely evolution of the reservoir and thedecline in water resource benefits from the starting level immediately following construction.
Construction cost has to be studied case by case. We have only assembled here theaverage unit costs observed in various Sahel countries in 1992, prior to devaluation of the CFAfranc (Table 4.1). Construction costs must, of course, be augmented by a construction supervi-sion factor (of about 7 percent).
Maintenance costs can vary fairly significantly, depending on whether the project has beendesigned generously or, on the contrary, economically. Annual maintenance costs can rangefrom 2 to 8 percent of construction costs. It is not easy to assess or evaluate the socioeconomicbenefits in monetary terms.
In the case of crops, the starting point is the farm-gate market value of the produce.From this we must, of course, deduct the cost of inputs (fertilizer, insecticides, etc.) and energy(pumping) and the value of any farming activities carried on before the reservoir was built.
Human and animal use of the water resource is even more difficult to evaluate. Thefigures sometimes used of CFAF 50-200/m3 are no more than rough estimates. The situationcould be clarified by studying a dam project in the region and its usable annual production;however, such projects generally do not exist. Any foreseeable fishery resources must be takeninto account.
MANAGING WATER-RETENTION WORKS
The term "management" is used here in its broad sense. It ranges from initiation and ensuingstudy of the project to management proper of the waterworks and resources.
(a) Origins of the project
In the past a number of submersible ford works were built because of a lack of hydrological dataand because in wide valleys this type of project allowed a much bigger safety margin for thedischarge of large flows. Today, it is rare for a road to spin off a water-retention project only tomake the necessary quantities of water available on site to meet the needs of the roadworks(wetting of fill and roadway materials, concrete mixing, equipment cleaning, and so on) and ofthe personnel. It makes sense to combine the construction of this temporary dam with that ofthe road to produce a final structure that can serve needs of nearby population groups.
Today, however, most water-retention roadworks projects are built with an eye to so-cioeconomic benefits. In some countries the initiative comes from the road agencies them-selves, which ask the designer, in studying a new road or reconstruction project, to consider thefeasibility of incorporating water storage into certain crossing works. In practice, this is still
61
rare - although we hope that the dissemination of this report will promote its wider use.Annex 2 presents suggested terms of reference to be included in a road study for that purpose.In other countries, the initiative comes from the water resources or rural engineering agency,assisted by the road agency or NGO.
(b) Agreement on and organization of construction studies
Road and water resources authorities-which are usually different-must be involved rightfrom the start. The Roads Directorate and its consultants do not normally concern themselveswith water management outside the road right-of-way. On the other hand, while the ruralengineering agency, the local population, and the assisting NGOs are directly affected by wa-ter-retention schemes, they do not normally participate in the preparation of a road project.This makes it necessary to adopt an attitude of openness to discussion and to include consen-sus-building mechanisms.
On main roads, the agency responsible for road studies must therefore consult withthe agricultural agencies and potential users concerning the benefits that would accrue fromincorporating water-retention works into road projects. While the feasibility study of theseworks can be delegated to the agricultural agencies or the NGOs, which are better placed thanthe road agencies to calculate the benefits of the potential reservoir, the design project must bedone by the road designer. In the case of tracks (unpaved roads), users and NGOs can assumebroader responsibility for the entire project.
(c) Beneficiary participation
Participation by beneficiaries in the financing and execution of the works can bring substantialadvantages. First of all, it ensures that there is a genuine demand for the works and that theusers' wishes have been considered in designing them. It also ensures that the project will beproperly operated and maintained, since the beneficiaries will have a sense of ownership. More-over, grassroots participation in the work helps develop local capacities which will be invalu-able in ensuring good maintenance.
(d) Maintenance organization and performance
Facilitating operation and maintenance must be a major factor in the design of the works.Their operation must be within the capability of the users or the local agencies or NGOs thatassist them in the field. Works that are easy to maintain are likely to last. These considerationsare reflected in the design of simple and robust projects and the use, at least on rural roads andtracks, of manual or highly labor-intensive construction methods. Consideration should alsobe given to preparing and disseminating operating and maintenance manuals or instructionbrochures. Finally, maintenance costs money, time and materials, and the designer shouldspecify which party is responsible in the event of an accident or rehabilitation, and who mustpay the maintenance costs. When feasible, the most durable arrangements are those underwhich the water user pays.
.- , - - - ---
1 63
Annex 1Bibliography
ECONOMIE ROUTIERE
1 L. ODIER1963Les avantages economiques des travaux routiers - Eyrolles, Paris
2 BCEOM - CEBTPJuin 1991Manuel des routes dans les zones tropicales et desertiquesTome 1: politique et economique routiere - Ministere de la Cooperation
3 B. COUKIS1983Utilisation des methodes manuelles dans les programmes de construction guidepratique pour organiser et conduire les travaux - BIRD Washington
HYDRAULIQUE
4 M. CARLIER1972Hydraulique generale et appliquee - Eyrolles, Paris
5 N. VAN TUU1981Hydraulique routiere - BCEOM - Ministere de la Coope'ration et du Developpement
PETITS BARRAGES
6 GRESILLON J.M. HERTER P, METRO T(EIER), LAHAYE J.P. (CIEH)1979Quelques aspects de l'hydraulique des barrages - Suggestions pour le dimensionnementdes petits barrages en Afrique sahelienne - Remarques relatives a l'etude des erosionshydrauliques sur sols coherents - Ministere de la Cooperation, Paris
7 Ministere de I'Agriculture, PARIS1977Technique des barrages en amenagement rural
64 BIBLIOGRAPHY
Annex 1
8 LANE E.W1935Security from underseepage - Transact ASCE vol 100p 1235
HYDROLOGIE
9 RODIER J., AUVRAY C.1965Estimation des debits de crue decennales pour les bassins versants de superficieinferieure a 200 km 2 en Afrique Occidentale, ORSTOM-CIEH, Paris (original de lamethode ORSTOM)
10 RODIERJ.1975Evaluation de l'coulement annuel dans le Sahel tropical africain - Collection Travauxet Documents - ORSTOM, Paris
11 J. RODIER - P RIBSTEINEstimation des caracteristiques de la crue decennale pour les petits bassins versantsdu SAHEL couvrant de 1 a 10 km2
12 ORSTOM1988Catalogue des etats de surface - Repertoire des aptitudes au ruissellement des solssaheliens
13 PUECH C., CHABI-GONNI D.1984Methode de calcul des debits de crue decennale pour les petits et moyens bassinsversants en Afrique de l'Ouest et Centrale (2eme edition) CIEH, Ouagadougou (originalde la methode CIEH)puis 1988Determination des crues decennales - CIEH
14 BERTON S.1988DOSSIER No 12: "La maltrise des crues dans les bas fonds - Petits et micro-barragesen Afrique de l'Ouest" - Collection "LE POINT SUR" - Cooperationfranfaise - GRET- AFVP - Agence de Cooperation Culturelle et Technique
HYDRAULIQUE AGRICOLE ET PASTORALE
15 SOGETHA1968Techniques rurales en Afrique ; les petits barrages en terre - Ministere de la CooperationCIEH
65
16 SOGETHA1971Utilisation agricole des eaux de crue en Afrique - Tome 1: Les dpandages de crue -CIEH, Ouagadougou
17 BCEOM, IEMVT1977Hydraulique pastorale -Techniques Rurales en Afrique - Secretariat d'Etat auxAffairesEtrang?res charg' de la Coope'ration, Paris
18 INSTITUT PANAFRICAIN DE DEVELOPPEMENT1977Decouvrir une agriculture vivri&re - Ed. Maisonneuve et Larose, Paris
CONSERVATION DES EAUX ET DES SOLS
19 ROOSE E.1992Introduction a la Gestion Conservatoire des Eaux et de la Fertilite des Sols (G.C.E.S.)- ORSTOM/FAO, Paris
20 C.T.ET.1979Conservation des sols au sud du Sahara - Collection Techniques Rurales en Afrique -Ministere de la Coope'ration, Paris
21 FAO1977Amenagement des bassins versants - Cahiers FAO, Rome
EAU ET SANTE
22 MONJOUR L., TOURNE F1981Problmes de sante en milieu sahdlien - Collection Techniques Vivantes - ACCT- CILF- PUFi Paris
23 INADES Formation1979L'eau et la sante - Livres 1 et 2 - INSP de Cote d'Ivoire, Abidjan
66 | BIBLIOGRAPHY
Annex I
MONOGRAPHIES
24 DIPAMA1992Sedimentation dans les barrages - Memoire de maitrise - Universite' de Ouagadougou
25 A. JOIGNEREZ - N. GUIGEN1992Evaluation des ressources en eau non perennes du Mali - ORSTOM
26 Dossier d'Appel d'Offres1985Etude de la route DORI - TERA - NIAMEY - Aic Progetti - Europrogetti - Autoritide Developpement integre de la region Liptako - Gourma
Etude complementaire pour l'etablissement du dossier d'execution de trois retenuesd'eau pour les besoins de chantier, utilisables d'une fa,on permanente par la popula-tion et le betail au Niger.
27 BCEOM - IRAM1987Schema Directeur de I'ADER DOUTCHI MAGGIA
28 LOUIS BERGER INT.Mai 1991Etude de mobilisation des eaux de revetement superficiel dans trois departements(Tahoua - Agadez - Zinder) - Rapport final - Direction du Genie Rural du Niger
1 67
Annex 2
Model Terms of Reference for Studies ofWater-Retention Works in Roads Projects
Where the economic appraisal study of a road construction or rehabilitation project includes thestudy of water-retention facilities, the latter study shall be divided into two phases. To that end, theterms of reference for the road study shall include the following clauses:
1. DEFINITION OF PHASE 1 OF THE STUDY
With respect to the crossing of any river with a catchment basin whose area is in the range 3km2 to 5,000 kM2, the Engineering Consultant shall study, case by case, whether it is feasible tobuild a water-retention facility upstream of the road and, if so, whether it would be advanta-geous to do so from the standpoint either of construction of the road, of water supply to thepeople living along it, or of a significant increase in agricultural production.
To that end, in the first phase the Engineering Consultant shall draw up and present tothe Agency a list of the crossings where it appears a priori that it would be advantageous to carryout such a study. This list shall be drawn up on the basis of existing maps and documents orvisual reconnaissance. It shall be accompanied by a brief report indicating:
- the reasons for choosing the crossings listed and for rejecting the others;
- the nature and volume of the surveying, geotechnical, hydrogeological and hydrologi-cal work the Engineering Consultant considers necessary for proper execution of phase2 of the study.
This report shall be produced within a maximum time limit of . months followingthe date of commencement of the studies.
The Agency shall notify its agreement or its requests for changes within a time limit of
2. DEFINITION OF PHASE 2 OF THE STUDY
The Engineering Consultant shall then perform a feasibility study of those works the study ofwhich has been definitively decided on by the Agency. This feasibility study shall include:
- a rapid agronomic and sociological reconnaissance;
- the results of the surveying work, in particular, the reservoir layout and water area/volume/height graphs;
68 | MODEL CLAUSES FOR TERMS OF REFERENCE
Annex 2
the results of the geotechnical or hydrogeological reconnaissance, core-samplings, andtesting work;
a hydrological study of the river, choice of the project design flood, and the hydraulicperformance of the works during passage of the design flood;
- 1:1000 or 1:500 plans of the project and 1:200 sections of the works (access dike andspillway), with all necessary detailed drawings for complete understanding of the project;
- calculation of infiltration rates within and beneath the works;
- recommended use of the stored water and month-by-month water balance, taking intoaccount inflow, infiltration and evaporation;
- a quantities estimate for the works, accurate to ± 20 percent;
- a comparative economic study of the project and its rate of return by reference to thealternative of a conventional road project without water retention;
- a report justifying the technical specifications adopted (stability of the works and ofthe protective devices adopted, means of combating leak tunneling, etc.) and the rec-ommended use of the water;
- proposals concerning the subsequent management and maintenance of the project:agencies, entities and user communities concerned; role and rights and responsibilitiesof each.
3. COMPOSITION OF THE TEAM
The Engineering Consultant shall assign the following to phases 1 and 2 of the study as definedabove:
- a hydrologist and a geologist (who may be the ones already assigned to the rest of theroad project);
- an agronomist and/or a water supply expert;
- a dam expert;
- an agricultural economist.
Each of these shall have ten years experience in the relevant profession. The hydrolo-gist, the agronomist, the water supply expert and the agricultural economist shall in additionhave a good knowledge of Sub-Saharan and preferably Sahelian Africa.
69
4. REMUNERATION OF THE ENGINEERING CONSULTANT
Depending on the method of remuneration prescribed for the road-only part of the economicappraisal (remuneration on a time-spent or a lump-sum basis), it is proposed to apply one ofthe following two formulas:
(a) Time-spent remuneration
The Engineering Consultant shall be remunerated on the basis of the worktime of the engi-neers and experts involved. Subcontracted surveying, geotechnical and hydrogeological worknecessary for study of the water-retention project shall be paid for separately, either on a lump-sum basis at the prices indicated in the report on phase 1 or by reimbursement of subcontrac-tors' invoices, the total of which shall not exceed the sum indicated in the phase 1 report.
(b) Lump-sum remuneration
For phase 1 of the study, the Engineering Consultant shall be remunerated by the global (orper-kilometer) lump sum of the study, which shall therefore include the services pertaining tothe establishment and justification of the list of projects that merit a water-retention study. Forphase 2 of the study, the Engineering Consultant shall be remunerated by lump-sum reim-bursement, at the prices indicated in the phase 1 report, of the surveying, geotechnical andhydrogeological work and by a lump sum of .......... per site, applicable to each of the worksprojects studied.
NOTE: These clauses have been draftedfor use in the study ofa very long road where the clientwishes to ascertain thefeasibility and merits of water-retention works. They can, ofcourse, alsobe helpfulfor the study of a single water-retention project orfor other projects, for example indefining the phase 2 feasibility study.
ii
i
I
iiI
i
i
71
Annex 3Terms of Reference for the Mission
1. OBJECTIVE OF THE STUDY
On a proposal from the World Bank, the Norwegian Trust Fund has agreed to fund a study onthe feasibility and suitability of including water retention schemes in road projects in the Sahelregion. Such schemes have important potential benefits:
i) better protection of the environment against water erosion and drought;ii) enhanced use of scarce water resources for agricultural purposes; andiii) stronger roads.
2. BACKGROUND
Water scarcity is an considerable constraint to agricultural activities in the Sahel, speciallyduring the dry season. It limits vegetable production and may also affect watering cattle, sheepand goats. Moreover, due to heavy rainfall during the wet season, large surfaces of land areexposed to flooding by temporary torrents (called "gullies" or "koris") which erode the top soil,and make some areas unsuitable for agriculture.
Roads are also affected by the seasonal differences of rain fall and drought. Koris canwipe out large sections of road, and so far, designers have given large dimensions to bridges tobe built over them, at a significant cost. But, sometimes koris change their beds during flood-ing, so these expensive bridges become useless while koris damage or ruin the road in otherplaces.
For the dual purpose of better protecting the road and retaining part of the rainwaterfor the dry season activities, some designers have replaced bridges over koris by low level dams.In some cases, this allows a significant decrease in the opened area required to evacuate the peakflood waters, because of the additional retention capacity. This new design concept mightbring significant advantages to local populations, and also reduce the cost of building andmaintaining roads. A study is needed to assess costs and benefits of this concept and to provideadvice to road designers.
3. SCOPE OF THE STUDY
The study will:
a) review past experiences, in selected countries of the Sahel, of road designs thatinclude water retention schemes and assess actual costs and benefits of suchschemes;
72 TERMS OF REFERENCE FOR THE STUDY
Annex 3
b) assess the suitability or limits of water retention schemes in road designs in the
region;
c) provide information and advice on how to design and implement such schemes,
supply sample drawings and sample technical specifications as appropriate to
be included in terms of reference and bidding documents; provide advice on
both operation and maintenance of these water retention schemes;
4. ORGANIZATION OF THE STUDY
The consultant, financed by the Norwegian Trust Fund and recruited by the World Bank, will
visit Burkina Faso, Niger and Chad, and discuss with professionals of the Ministries in charge
of Public Works, and of Hydraulics. He will visit selected sites of water retention schemes
included in road designs. He will analyze costs and benefits of these schemes with the relevant
authorities and collect all suitable information for the advisory part of his report. Then, he will
prepare a draft report (if possible in French) as detailed in the following paragraph. This report
will be reviewed and completed by AF5IN as a white cover draft to be sent for comments to the
African professionals visited by the consultants and to the Norwegian Trust Fund. After com-
ments have been received, the consultant and AF5IN will prepare the yellow cover report to be
sent to peer reviewers in the Bank. Then the consultant and AF5IN will issue the final report
to be circulated in the Sahelian countries, the Sahelian Department in the Bank, and to be sent
to the Norwegian Trust Fund. In addition, it is possible that a regional seminar will be orga-
nized on this topic for which the report would be used as a background document.
5. DETAILED OUTLINE OF THE REPORT
The report will contain the following:
a) policy chapter giving the broad conclusions of the case study, and highlight-
ing costs and benefits and limits of water retention schemes included in road
projects.
b) a case study report on the sites visited, including typical average costs for each
country;
c) an assessment of the feasibility and suitability of similar schemes in future
road construction projects, as well as for enhancing existing roads;
d) one or several sample design(s) of water retention schemes;
e) suitable clauses to be included in terms of reference for designers and in bid-
ding documents for road construction contractors; and
f) advice for operating and maintaining the water retention schemes.
Annex 4Photos of the Works
74 | PICTURES OF THE WORKS
Annex 4
M _ ,^.~~w~ _-_
Picture 1: BURKINA FASOStream-level ford. Notice the stilling basin created by the flood.
* .: .' .-> .M v:~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~. ..... . . ... ..... .... ..... ....
Picture 2: LATAPOA (Burkina Faso)Raised ford. End of dry season. There is still some water in the storage basin.
J , w i ! _ - --- T
_ _ a t.~~
Picture 3: LATAPOA (Burkina Faso)Watertake under the raised ford -- downsteam.
-7C :
- -if -- ------
Picture 4: NIAME (Mali)Raised ford under construction with culvert. On the right, a bridge which has been destroyedand rebuilt every year by the f lood and which will be replaced by the ford.
76 | PICTURES OF THE WORKS
Annex 4
-= X ----- ----
-re
,*~~~~~~~*
Picture 5: MAKANDIANA (Mali)Raised ford on a long dike -- Box culvert with sluicegate (under construction).
At~~~~~~~~~~t
Picture 6: GOHANGEN (Burkina Faso)Road embankment and raised ford view of the left bank upstream.
77
E~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Picture 7:TITA (Burkina Faso)Raised box culverts and channel.
. . . ... _...... .. _ .
Picture 8:TITA (Burkina Faso)Road embankment: the raised ford is out of the picture on the left.
78 | PICTURES OF THE WORKS
Annex 4
U.-~~~~~.
Picture 9: NATIABOUANI (Burkina Faso)Thick spiliway on the left of the bridge.The road embankment is protected against watererosion by masoning pavement.
'''' a4p . .. rmaawmnm~..
I -w p S > , -s __ r
s .. I I | | FS .............................. _ _ _ _ e ' vr-~~ .. .......................... . ............
.7 . .1 ]S f* -
Picture 10: QUEDOGO PETIT (Burkina Faso)The spiliway is made of steel plank upstream of the bridge.
I 79
Picture 1 TOUNGUENE (Mauritania)Works with cofferdam. In the foreground, the planks of the sluicegate.
-. 44-
A -- ~ s"
-~1 r{fF W W
3 .s. 4 $-~~~asr_ '-
Picture 12: BURKINA FASOBox culvert protected by plies across the stream.
80 PICTURES OF THE WORKS
Annex 4
Picture 13 Road FADA N'GOURMA a PAMA (Burkina Faso)Temporary dam and basin storing water for the road construction needs.
Picture 14: NATIABOUANI (Burkina Faso)Stilling basin downstream of the bridge view of the right bank.
I 81
-'-
.W]~~~~~~~~
Picture 15: Road FADA N'GOURMA A KANTCHARI (Burkina Faso)
zV .~ ~ ~ ~ I
Waterstorge -- Absece o tre cutting,and bsi_ cleaning
im . .1 , - -j
~~~ ~~~~ X ; H vFex>,,>t.-&* w * , ze k-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~'S,
.:-: -- -i --*
Picture 16: Road FADA N'GOURMA A KANTCHARI (Burkina Faso)Protecting the embankment downstream and drainage gutters.
82 | PICTURES OF THE WORKS
Annex 4
Radontea of te [email protected] of th spiliway.and th roa whc crse th chane by^'S A
"'7~~~~~~~~A
VAPicture 18 OAGADOuGOU ( a Fai so)_Conrteeav f ore dan Vie tto downste. t w
rase box culverts.
1 s _ - -=x _ 6~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~----- - ----
Picur _.__ OUGDUO Buknao
Cnrt pave fodadbrdedwntem
83
-- 64r | W iS}S ' sC=------
Picture 19: KABOU (Togo)Upstream watertake tower at the end of the dry season.
Picture 20:TABALAK (Niger)Water leveling works between north and south of the lake crossed by theTAHOUA-AR LIT.
i
I