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Ground-Water Dams for Rural-Water Suppliesin Developing Countries

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by Gdran Hanson3 and Ake l\lilssonb

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ABSTRACTThe use of ground-water dams to store water in

regions with arid or tropical climates is a method that hasreceived considerable attention in the last few years. Bystoring water behind subsurface dams in natural aquifers orin the sand accumulated in sand storage dams, the disadvan-tages of conventional surface storage, such as high evapora-tion rates, pollution, siltation, and health hazards, may beavoided. The techniques are very old, but only recentlyhave there been some attempts to make systematic studiesand to develop proper siting, design, and constructionmethods. This paper presents the experience gained fromexisting structures all over the world and describes thephysical setting in which the techniques may be applied.Design and construction alternatives are shown, and casestudies from India and Ethiopia are presented. The construc-tion of ground-water dams may be a feasible solution towater-supply problems in many parts of the world ifpreceded by proper planning and site surveys.

BACKGROUNDThe use of surface reservoirs to score water in

areas with dry climates has several serious disadvan-tages, such as pollution risks, reservoir siltation,and evaporation losses. Using ground water is oneway of overcoming these problems, but in someareas good aquifers are not available or they mayonly yield sufficient quantities of water seasonally.Experience also has shown that conventionaldevelopment of ground water in developingcountries involves serious problems related tooperation and maintenance of drilling equipment,wells, and pumps.

During the last few years, considerableattention has been given to the use of ground-waterdams as a method of overcoming water shortage inregions with arid and tropical climates. Dammingground water for conservation purposes is certainlynot a new concept. Ground-water dams wereconstructed on Sardinia in Roman times and

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aPh.D. Quaternary Geology, VIAK AB, P.O. Box 519,S-162 15 Vallingby, Sweden.

^'Research Engineer, Department of Land Improve-ment and Drainage, Royal Institute of Technology,Stockholm, Sweden.

Received July 1984, revised July 1985, acceptedAugust 1985.

Discussion open until January 1, 1987.

Vol. 24, No. 4 GEO'JN'P VVATER-July-August 1986

Fig. 1. General principle of a subsurface dam.

damming of ground water was practiced by ancientcivilizations in North Africa. More recently, varioussmall-scale ground-water damming techniques havebeen developed and applied in many parts of theworld, notably in southern and East Africa, and inIndia. This paper summarizes the results of a state-of-the-art study during which literature and datafrom most parts of the world have been collected(Nilsson, 1984). It also presents examples ofpractical applications in two areas in Ethiopia andIndia (Hanson, 1984; and Nilsson, 1984).

There are basically two different types ofground-water dams, namely subsurface dams andsand storage dams. A subsurface dam is constructedbelow ground level and arrests the flow of a naturalaquifer, whereas a sand storage dam impoundswater in sediments caused to accumulate by thedam itself.

The general principle of a subsurface dam isshown in Figure 1. An aquifer consisting of fairlypermeable alluvial sediments in a small valleysupplies water to a village by means of a shallowwell. Due to consumption and the natural ground-water flow, the aquifer is drained during the dryseason, and consequently the well runs dry. Toprevent this, a trench is dug across the valley,reaching down to bedrock or some other solid,impervious layer. An impervious wall is constructedin the trench, and when the dam is completed thetrench is refilled with the excavated material. Areservoir built in this way will not be drained andmay be used throughout the dry season, providedof course that the storage volume is sufficient tomeet the water demand.

Fig. 2. General principle of a sand storage dam.

The general principle of a sand storage dam isshown by the example in Figure 2. The villagerscollect their water from the small nonpcrennialstream at times when it carries water, or from holesdug in the shallow riverbed for a short period afterthe rains. The quanti ty of water stored is not suffi-cient, however,, to supply water to the village duringthe entire dry period. By constructing a weir ofsuitable height across the streambed, coarseparticles carried by heavy flows during the rains arecaused to settle, and eventually the reservoir will be

filled with sand. This artificial aquifer will bereplenished each year during the rains and if thedam is properly sited and constructed, water will bekept in the reservoir for use during the dry season.

Quite often a ground-water dam actually is acombination of the two types. When constructing asubsurface dam in a riverbed, the storage volumemay be increased by letting the dam wall riseabove ground level, thus creating an accumulationof sediments. Similarly, when a sand storage damis constructed, it is necessary to excavate a trench

'in the sand bed in order to reach bedrock or astable, impervious layer.

Figure 3 shows a map on which all knownground-water dam sites are marked, and also whereit has been proposed, after preliminary investiga-tions, to implement the technique. The schemes inEurope and northwestern Africa are mostly large-scale projects, whereas in certain parts of Ethiopia,East Africa, and Namibia, ground-water dams areused quite extensively as storage reservoirs forsmall-scale rural-water supply. There is a long tradi-tion of building ground-water dams in the aridsouthwestern part of the United States and northernMexico, and there are isolated examples of sub-surface dams in Afghanistan, India, and Japan.

General application of sand dams.Sand dam construct ion sitePlanned/proposed sand dam constructionGeneral application of sub-surface dams.Sub-surface dam construction sitePlanned / proposed sub-surface dam construction

. 3. Ground-water dam sites and proposed construction areas.

498

PHYSICAL SETTINGThe rationale of damming ground water is the

irregular availability of surface and ground water.Ground-water damming techniques may thus beapplied in arid and semiarid areas where there is aneed to conserve the scanty quantities of rainfall.They are also highly suitable in areas with amonsoon climate where there is a need to storesurplus water from rainy seasons for use during dryperiods.

The topographical conditions govern to alarge extent the technical possibility of constructingthe dams as well as achieving sufficiently largestorage reservoirs with suitable recharge conditionsand low seepage losses. The basin in which water isto be stored may be underlain by bedrock orunconsolidated formations with low permeability.It is generally preferable to site ground-water damsin well-defined and narrow valleys or river courses.This reduces construction costs and makes itpossible to assess storage volumes and to controlpossible seepage losses. This is obviously difficultin areas with flat topography. On the other hand,efforts must be made to maximize storage volumeswithout the construction of unnecessarily highdams. In mountainous areas with very highgradients, it may be difficult to find an acceptablerelation between storage volume and dam height.An optimum composition of riverbed material isgenerally found in the transition zones betweenmountains and plains.

One of the basic conditions justifying theconstruction of a subsurface dam is the depletionof ground-water storage through natural ground-water flow. The gradient of the ground-water tableand thus the extent of ground-water flow is gener-ally a function of the topographic gradient. Thisfact indicates that the construction of subsurfacedams is feasible only at a certain minimum topo-graphical gradient, which naturally varies accordingto local hydrogeological conditions. Most ground-water dams in existence today have beenconstructed in areas with a 1-5% slope, but thereare examples of dams constructed where gradientsare 10-15%.

The storage capacity of sand storage dams andsubsurface dams constructed in riverbeds is afunction of the specific yield of river sediments.Dams are therefore preferably constructed ingeological environments where the weatheringproducts contain a substantial amount of sand andgravel. As a consequence, cases where damconstruction has met with success are mostlyfound where the bedrock consists of granite, gneiss,

and quartzite, whereas areas underlain by rockssuch as basalt and rhyolite tend to be less favorable.

Climate has a great influence on sedimentcharacteristics in that it governs the relationshipbetween mechanical and chemical weathering. Alower rate of chemical weathering in arid climatesmay result in more coarse-grained sediments(Sundborg, 1982).

The stage following weathering in thesediment cycle is the erosion of the particles.Topography and land use govern the extent oferosion, and an engineer interested in constructing

; sand storage dams would be happy to find steep.^slopes and low vegetation cover in the catchment

area, generally very contrary to the preferences ofagricultural and hydraulic engineers.

The coarse sediments one wishes to trap by asand storage dam are generally transported as bedload. It is therefore necessary for the rainstormsproducing the initial floods at the outbreak of therainy season to be sufficiently heavy. On the otherhand, the flood intensity must not be so high thatthe dam stability is jeopardized. Extremely highflood intensity will also mean that the existence ofnatural riverbeds is limited (Jacob, 1983).

Subsurface dams are mostly commonlyconstructed in riverbed aquifers consisting of sandor gravel. Other types of aquifers that may bedammed are weathered zones, alluvial or colluviallayers, or any type of overburden with sufficientlygood aquifer characteristics. Infiltration conditionsmust be such that the reservoir is properlyrecharged during the rainy period.

The storage reservoir must be contained byimpervious or low-permeability layers that preventvertical and lateral seepage losses. In most casesbedrock will serve as the natural container, butthere are also examples of low-permeability over-burden layers serving this purpose, such as buriedclay layers of alluvial origin, the upper clayeyportion of the weathered profile, or the lithomargin laterite profiles.

The containing layer must be at such a depththat it is technically possible to carry out theexcavation at reasonable cost. In general, the limitseems to be at around 4-5 m, but, depending onthe scale of the scheme, it may be possible toextend this considerably. If injected cutoffs areused instead of the conventional construction ofdams in trenches, there is no practical limit to thedepths possible.

The basic idea behind constructing a subsur-face dam for storage purposes is to arrest thenatural flow of water; it is necessary to quantify

49

this flow so that the benefits of the scheme may beproperly assessed before construction starts. If it isnot economically feasible to use standard hydro-geological procedures for this purpose, a fairlysimple monitoring program indicating seasonalground-water level fluctuations will give a goodestimate.

One prerequisite for using ground-water damsfor rural-water supply in developing countries isthat it must be possible to do so at low cost. Alarge part of the total project cost is generally theconstruction material; it is therefore imperativethat suitable material is available locally and atreasonable cost. The various alternative materialsthat have been or may be used are described below.The identification of such materials should be donein the initial stages of planning the construction ofa ground-water dam.

DESIGN AND CONSTRUCTIONThe principal designs of subsurface dams and

sand storage dams were shown in Figures 1 and 2.Since the two types are basically different, thepresentation of design and construction alterna-tives has been divided into two parts, followed by apart covering certain aspects common to bothtypes.

Subsurface DamsThe actual storage volumes of subsurface

dams in existence today range from a few hundredto several million cubic meters and the designs arein consequence quite different. The followingpresentation concerns mainly small-scale schemes.

The most common way of constructing asubsurface dam is to build a dam in a trench exca-vated across a valley or riverbed. The earthworkinvolved may be carried out by manual labor sincethe excavation depths are generally not more than3-6 meters. Subsurface dams are generallyconstructed at the end of the dry season, whenthere is little water in the aquifer. There is usuallysome flow, however, and this must be pumped out

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during the construction. After the dam has beenconstructed and drains installed, the trench isrefilled with the excavated material. It is importantthat the refill is properly compacted by mechanicalmeans and watering.

The clay dike in Figure 4 is an alternative thatis very suitable for small schemes in highly perme-able aquifers of limited depth, such as sandy river-beds. Clayey topsoils are generally available closeto any construction site which means they can beexcavated and transported to the site at low cost.The use of clay is a labor-intensive alternative butrequires no skilled labor. Possible drawbacks arethe large excavations generally required, the needfor proper compaction and the risk of erosiondamage to the clay surface due to the flow ofground water. The latter can be avoided byprotecting the dike with plastic sheets.

A concrete dam covered on both sides withblocks or stonework, as shown in Figure 5, is analternative involving slightly more advancedengineering for which skilled labor is needed. Itnecessitates the use of form work and theavailability of cement and gravel. One advantage isthat it is possible to raise it above the level of ariverbed and use it for further sand accumulation.This type of dam may also be constructed entirelyof stonework.

Bricks are generally available or may bemanufactured from local clay. Building a brickwall, as shown in Figure 6, and plastering it to

Fig. 4. Clay dike.

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Fig. 6. Brick wall.

make it watertight is a fairly simple procedure(Ahnfors. 1980). The relatively high cost of bricksis a drawback, however, and there are also somedoubts as to the stability.

Using ferroconcrete means that steel rods orwire mesh have to be used, but generally suchmaterial is available at reasonable cost. The methodinvolves the use of form work, but its mainadvantage is that very little material is needed toachieve a very strong wall. The structure has to beanchored to the solid reservoir bottom.

Using thin sheets of impermeable materialssuch as tarred felt or polyethylene is an inexpensivealternative as far as the cost of materials is con-cerned. The mounting of the sheets on woodenframes and the erection process is rather complicat-ed (Ahnfors, 1980). The material, especially thepolyethylene, is highly sensitive to damage duringerection as well as during refilling of the trench,and a minor rip will cause leakage losses. A small-scale scheme in south India where plastic sheetswere used has functioned very well for three years,but it remains to be seen whether the material willwithstand high ground-water temperatures, theactivities of microorganisms in the soils, rootpenetration, and other activities over a period oftime (Jayakumar, 1984).

Injected cutoffs have been used to arrest theflow in large or deep-seated aquifers in NorthAfrica and Japan (BCEOM, 1978-,Matsuo, 1977),and to protect fresh water from pollution inEurope and the USA. For large projects and whena deep aquifer is/dammed, it is certainly a feasiblealternative.

The average heights of some of the dam typesare shown in Table 1. In most cases the crest of asubsurface dam is kept at some depth from thesurface. This is partly to avoid waterlogging in theupstream area, and partly to avoid erosion damageto the dam. A common distance from the damcrest to the surface is about one meter.

Sand Storage DamsWhen a subsurface dam is built, it is always

possible to obtain at least some idea of thehydraulic characteristics of the existing aquifermaterial. When planning the construction of a sandstorage dam, however, the material in which thewater is supposed to be stored is still in the catch-ment area waiting to be transported to the dam siteby a flood of unknown intensity. The properdesign of a sand storage dam is therefore a morecomplicated matter, involving more hydrologicaland hydraulic calculations. The subject has beentreated by Wipplinger (1958 and 1984), Burger and

Table 1. Heights of Some Subsurface Dam Types—AverageValues from Schemes Studied

Average heightDam type in meters

Injected cutoffBrick wallConcrete or stone masonry damFerroconcrete damClay dike

106643

Beaumont (date unknown), Beaumont and Kluger(1973), and Nissen-Petersen (1982).

':•; The upper limit of dam construction and thusalso the upper limit of storage volumes are set bythe condition that the dam has to withstand themaximum peak flow, and allow for a discharge ofthe peak flow below the bank level or throughspillways.

A sand storage dam is generally constructed instages, as shown schematically in Figure 7. Thebasic idea is to limit the height of each stage inorder to keep a sufficiently high-water velocity, sothat fine particles are washed out from the reservoirwhile the coarse particles settle. The height of eachstage is determined by estimating the sedimentationprocess in the reservoir (founded on experiencefrom sedimentation in previous stages), from theextent of natural sedimentation in the stream, orcalculations of water velocities in the reservoir.

This method of constructing the dam byadding a new stage each season or even lessfrequently means that costs will be higher thanthey would be if the dam were constructed to fullheight directly. Two methods have been tried inorder to solve this problem. One is to use a siphon,which discharges water over the dam and keeps theflow velocity in the reservoir sufficiently high. Thismethod has not been found to be technicallyefficient and is very costly (Burger and Beaumont,date unknown).

Another method is to leave a notch in thedam which allows the accumulation of sediments

Originol river btd

Layers of send deposited inthe reservoir

Fig. 7. Construction principle of sand storage dam (fromBurger and Beaumont, date unknown).

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only up to a certain height. The notch is then filledin before the next rainy season, and the reservoir isallowed to fill completely. This method has provedquite successful in Kenya (Werner and Haze, 1982).

Figure 8 shows a concrete dam which fulf i l lsthe basic requirements for a sand storage dam, i.e.it is sufficiently massive to take up the pressurefrom the sand and water stored in the reservoir,and it is watertight. The dam also may be built ofstonework. For larger reservoirs, it may take theform of an arch dam.

The dam may also be made up by stonegabions or blocks sufficiently large to withstandthe pressure. The gabion or block dam is coveredon the upstream side by a thick layer of clay tomake it impermeable. It is also possible to placethe watertight clay layer as a core inside the dam.

The main dam body also can be a heap ofstones covered by concrete walls for stability andtightness. A dam of this type serves at the sametime as a bridge over a small stream in Kenya(Werner and Haze, 1982).

All sand storage dams must be very wellprotected against erosion along the banks, and evenmore so at the dam toe, where the energy of waterduring peak flo\vs will be extremely high. The bestway of avoiding erosion is to construct the dam atnatural rock bars. If such are not available, it isimportant to extend the dam wall several metersinto the river bank and to protect the dam toe withstone filling or concrete.

Common AspectsGround-water dams are generally combined

with a drain along the upstream base of the dam.The function of this drain, which generally consistsof gravel or a slotted pipe surrounded by a gravelfilter, is to collect the water and transmit it to awell or through a gravity pipe to downstream areas.If the permeability of the aquifer material is verylow, it also may be necessary to improve the flowalong the reservoir bottom by using a system ofcollection, gravel or slotted pipe, drainsperpendicular to the dam.

Concrete

Erosionprotection

Fig. 8. Concrete sand storage dam.

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Flushing valvePipe for gravity extraction

Grovel drain*^ "^Perforated pipe

Fig. 9. Extraction alternatives from a sand storage dam.

The well through which water from subsurfacedams is generally extracted may be placed in thereservoir or, for erosion protection purposes whendams are built in riverbeds, in the riverbank. Whenaquifers with low permeability are dammed,, it maybe necessary to construct a series of large-diameterwells or collection chambers to create sufficientstorage volume.

If the community to be served by the schemeis located downstream of the dam site and thetopographical conditions are favorable, it is possibleto extract water from the reservoir by gravity. Byusing gravity extraction, problems with pumpinstallation, operation, and maintenance areavoided. These are the problems which are todaygenerally encountered in rural-water supply projectsin developing countries, even in projects whereshallow well and simple hand pump technology isapplied. Figure 9 shows a typical sand storage damwhere both extraction alternatives are illustrated.

Ground-water dams should, as far as possible,be anchored in solid rock. This generally providesthe best stability, and fracturing of the dam can beavoided. Furthermore, it makes it possible in mostcases to control seepage below the dam. Anchoringmay be achieved using a concrete foundation onthe rock surface. If the rock is weathered it isimportant that the weathered material is fullyexcavated before the dam foundation is made;otherwise, there will probably be a seepage belowthe dam. Controlling seepage losses through fracturezones is more difficult but may be achieved insome cases by pouring very thin mortar into thefracture system (Werner and Haze, 1982).

A ground-water dam may also function as ameans of recharging an already existing aquifer bylateral or vertical flow. Figure 10 shows an examplefrom Namibia of how this can work (Sauermann,date unknown). The dike was utilized as an aquiferby means of a well supplying water to an airport.The aquifer did not carry enough water to meetthe demand, so it was decided to construct a sandstorage dam which would increase the recharge.Sometimes a ground-water dam is considered afailure because it is drained out by a fracture

system. It might, however, then be possible to drilla well in the zone and utilize the ground-water damas an artificial recharge structure (Hanson, 1984).

If planned and executed properly, a schemeinvolving the damming of ground water will haveno direct negative impact on the surroundingenvironment. It should be kept in mind, however,that the technique is implemented in a very fragileenvironment, where even a small change may havelong-term physical as well as social consequences.This calls for caution. Aspects which must be con-sidered are the possibility of upstream waterlogging,effects on downstream ground-water levels, saltaccumulation, and pollution problems caused bythe higher ground-water table.

In some areas it might be possible to constructa whole series of interconnected ground-waterdams. Potential has been identified along sandyriverbeds in Kenya (Sdrlie, 1978) and along narrowand very long valleys in South India (Jacob, 1983).The "jessours" (a type of silted-up surface dam) ofTunisia are examples of small-scale dams that maybe placed at regular intervals along alluvial valleys(El Amani, 1979).

CASE STUDIESSouth India

Two subsurface dams have been constructedin Kerala, South India. The sites are situated in thePalghat Gap, which is a graben dividing two hillranges parallel to the west coast. The altitude isabout 50 m and rainfall averages 2400 mm, themain part of which occurs in June-October.

Agriculture is intensive in the area. The wateravailable during the rainy season is enough for one

Pore-holy intc oqujftf

Fig. 10. Ground-water dam for recharging purposes (fromSauermann, date unknown).

annual rice crop, and in part of the area two cropsa year are possible. There is a need for additionalwater to expand the area covered by a secondcrop, and also to irrigate a third crop.

One factor restricting the possibility ofconstructing ground-water dams in this area is thatland holdings are generally small, and it is difficultto organize the cooperative effort needed whenseveral farmers are involved. Thus, the two existingdams are both situated on quite large farms. Onewas constructed by a private person and the otherwas built on a government seed farm by theCentral Groundwater Board of India. The privatedam was constructed in Ottapalam in 1962-64. Alarge-diameter well supplying water to the farmusually dried out during the dry season and therewas a need to find an alternative solution. A 130 mlong dike reaching down to bedrock was builtacross a narrow valley, surrounded by outcroppingrock.

The depth to bedrock, which was establishedprior to excavation by steel-rod sounding, is on theaverage 5 m and reaches a maximum of 9 m at thecenter of the well. The aquifer consists of residualsoils which, at least in the upper layers, are sandy.The transition between the weathered layer andthe underlying fresh rock is distinct, and it wasestablished during the excavation that the rock hadno major fracture zones.

The dike consists of a 4-inch plastered brickwall (Figure 11). Water is extracted from theaquifer through a gravel drain along the dike to aseries of storage wells connected to a pumpingwell. The catchment area of the dam is about 10ha, and water is used mostly for complementaryirrigation of 1.5 ha of paddy and, at the end of thedry season, 1 ha of coconut trees.

The dam built by the Central GroundwaterBoard was completed in 1979. In addition tosupplying water for supplementary irrigation at theseed farm, it was also meant to serve as a pilotproject for future dam construction. As a conse-quence, a scientific approach was used in thehydrogeological investigations and construction.

This dam was also constructed across anarrow valley and has a catchment area of about20 ha. The bedrock consists of gneiss and granitewhich outcrops on the valley sides. The soils aresandy in the central parts of the valley and morefine-grained along the sides. The average specificyield, determined from tension-plate and neutron-probe measurements, is 7.5%. Other investigationscarried out at the site were hammer sounding andresistivity measurements to establish the depth tobedrock, which at the deepest section is 4 m.

503

fig. 11. Brick wall and storage wells in the background,Ottapalam, India.

The total length of the dam is about 150 m,and the crest was kept one meter below groundlevel in order to avoid waterlogging in the upstreamarea. The main part of the dam is made up by aplastered brick wall but there are also sectionsconsisting of tarred felt and plastic sheets. Twowells, connected to each other by dr i l l holesthrough an unexpected rock bar, were constructedalong the dike.

The dam took three months to complete at atotal cost of U.S. S7:500, inc luding pumpingequipment. One-third of this cost was for earthworkand the rest for equipment and construct ionmaterials. The storage volume of the reservoir wasestimated at 15,000 in3 , which would be suff ic ientfor supplementary irrigation of the second paddycrop and the raising of a th i rd crop of black gramin a command area of 6 'ha. The benefit/cost ratioat 8% interest rate was ca lcula ted to be 1.06.

EthiopiaThe concept of ground-water dams has been

introduced recently into Kth iop ia . The firstproposals for ground-water dams in E th iop ia were

504

presented in a master plan for rural-water develop-ment in Hararghe Region ( V I A K , 1977), whichhave formed the basis for Swedish assistance to theEthiopian Government in the water sector.

The first two ground-water dams in Ethiopiawere constructed by the Ethiopian Water WorksConstruction Authority (EWWCA) in 1981. Thesetwo dams, located at Bombas and Gursum(Fugnambira), are described below. The positiveexperience gained from these pilot schemes hasinitiated a Research and Development Project withthe aim of further developing the methodology andencouraging local application of ground-waterdams in Ethiopia. The project is being carried outby EWWCA, with financial support from theSwedish International Development Authority(SJDA). The project is described in a Plan ofOperation (VIAK, 1982).

BombasA sand storage dam was constructed in 1981

at Bombas, a village of about 500 inhabitants, onthe road between Harar and Jijiga in HarargheRegion. The dam and reservoir can be seen inFigure 12. Excavation was carried down to thebedrock at 3 m depth. The bedrock is composed ofcrystalline basement rocks. The excavation workwas interrupted by unexpected rains which oncefilled the trenches with sand. The dam is made ofsolid concrete blocks and raised 0.8 m above theoriginal sand surface. Downstream of the wall,there is an erosion protection consisting of largeboulders which can be seen in Figure 12. On thephotograph, the original masonry protection isshown, partly collapsed. Water is piped by gravityto a steel tank 300 m downstream of the dam,where the villagers collect water from a stand post.

The dam is full several months after the endof the rainy seasons with a small discharge over-topping the dam wall. This indicates that the sand

Fig. 12. Sand storage dam at Bombas.

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Fig. 13. Subsurface dam at Gursum.

dam is recharged by ground water. When the damis full of water, there is a danger of pollution fromanimals entering the reservoir area. The dam hasfunctioned well and without any breakdown sinceit was inaugurated. The cost of the project wasU.S. $15,000.

\Gursum

A subsurface dam was constructed at Gursum(Fugnambira), a town of 5,000 inhabitants, in1981. The dam is shown in Figure 13. The struc-ture is quite unique and is an example of a subsur-face dam constructed in materials other than sand.The parent materials at the dam site are silt andclay, with only low permeability. To overcome this,a large excavation was made. A dam wall was madeof stone masonry plastered with cement. Becauseof the low permeability of the parent materials, areservoir of 200 m3 was created and covered withconcrete semicircular rings resting on pillars ofconcrete blocks.

The flow of ground water in the soil is blockedby the dam. Additional water is piped by gravityfrom a spring area upstream of the dam, usingperforated PVC pipes laid in a trench with a gradedfilter. Water is pumped from a well in the reservoirto an elevated reservoir for distribution to com-munal water points in the town. The water supplyhas functioned well. The cost of the project wasU.S. $150,000.

Research and Development ActivitiesThe SIDA-sponsored research and develop-

ment project which is now being implementedinvolves experimenting with various local construc-tion materials in the dam walls and various meansof extracting ground water.

The first dam was constructed at GendeBalina near Dire Dawa in 1983 but has not yetcome into operation. The dam is shown in Figures

Fig. 14. Construction of a sand storage dam at GendeBalina.

14 and 15. The dam wall is made of stone masonryand should be raised in two stages. The first stagewas raised 1 m above the original sand structure.This stage was completed in August 1983, and thereservoir filled with sand and water during a floodin September 1983. However, the decrease of thewater level was very quick, and in December 1983the reservoir was empty. It has not yet been possibleto locate the leakage, whether under the dam or in

Fig. 15. Gende Balina sand storage dam near completion ofStage 1.

505

the reservoir. The foundation has now beenimproved by grouting. However, it could bepossible that the leak is in the reservoir area, andthe water is draining into faults and fractures in thebedrock. Should this be the case, ground watercould probably be extracted through wells drilledinto the basement rock. The dam would in thiscase provide artificial recharge of the ground water.

Seven additional ground-water dams will beconstructed during the next few years by SIDAand the Swedish Red Cross. A subsurface dam isunder construction at Eja Anani with a dam wall ofsolid blocks and ferroconcrete. The reinforcementis made of wire mesh. There are also plans to con-struct subsurface dams using clay in the dam wall.

CONCLUSIONSExperience from the schemes discussed here,

as well as from other parts of the world show thatthe construction of ground-water dams is a feasiblesolution for improving the supply of ground waterin dry areas. It should not, however, be lookedupon as a universal method but as an alternative insituations where water supply through conventionalmethods cannot be arranged or for some reason isnot suitable.

The proper siting of ground-water damsnecessitates a thorough knowledge of the hydro-geological conditions in the actual area. When atentative site has been selected, it is mandatory tounderstand the ground-water flow system so thatthe dam can be correctly designed or, should thesite be unsuitable, unnecessary construction costscan be avoided. For these reasons it is necessary tomake general surveys as well as detailed studies onthe selected sites. Due to socioeconomic conditions,however, these investigations have to be carried outat low cost. It is necessary to make generalizationsand use simple geophysical methods.

Careful planning is also needed for the con-struction phase. This has to be scheduled accordingto the seasons in order to keep costs down and toavoid damage caused by heavy storm flows.

REFERENCESAhnfors, M. 1980. Ground-water arresting sub-surface

structures. Govt. of India, SIDA-Assisted Ground-Water Project in Noyil, Ponnani and Amaravati RiverBasins, Tamil Nadu and Kerala. Report 1:16, 22 pp.

Beaumont, R. D. and J. W. Kluger. 1973. Sedimentation inreservoirs as a means of water conservation. 1AHRCongress, September 3-7, 1973, Istanbul.

Bureau Centra) d'Etudes pour ]es Equipements d'Outre-Mer(BCEO.M). 1978. Les barrages souterrains. Ministerede Ja Cooperation. 1 35 pp.

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storage dams for water conservation. Department ofWater Affairs , South-West Africa. 21 pp.

Central Ground Water Board. 1980. SIDA-Assisted Ground-Water Project in Noyil, Ponnani, and Amaravati RiverBasins, Tamil Nadu and Kerala; Project Findings andRecommendations. Government of India. 48 pp.

El Amani, S. 1979. Traditional technologies and develop-ment of the African environments; util ization ofrunoff waters, the "meskats" and other techniques inTunisia. African Environment, v. I l l 3-4, no. 11-12,pp. 107-120.

Hanson, G. 1984. Development of sub-surface dams in theHararghe region, Ethiopia. Interim Report, VIAK.

Jacob, V. C. 1983. Central Groundwater Board, India.Personal communication.

Jayakumar, M. 1984. Central Soil and Water ConservationResearch and Training Institute, Ootacamund, India.

• Personal communication.Matsuo, S. 1977. Environmental control with underground

dams. Proceedings of the Specialty Session on Geo-technical Engineering and Environmental Control.Ninth International Conference on Soil Mechanicsand Foundation Engineering, Tokyo, July 1977.pp. 169-182.

Nilsson, A. 1984. Ground-water dams for rural watersupply in developing countries. Report TR1TA-KUT1034, Dept. of Land Improvement and Drainage,Royal Insti tute of Technology, Stockholm. 56 pp.

Nissen-Petersen, E. 1982. Rain Catchment and WaterSupply in Rural Africa; A Manual. Hodder &Stoughton, Great Britain. 83 pp.

Sauermann, H. B. Date unknown. Sand versus sun. South-West Africa. 3 pp.

Sundborg, A. 1982. Erosion and sedimentation processes.In: Sedimentation Problems in River Basins. IHPProject 5.3, UNESCO, Paris.

Sorlie, J. E. 1978. Water conservation techniques proposedin North Turkana, Kenya. Hydrology in DevelopingCountries. Nordic IHP Report No. 2, National Com-mittees for the International Hydrological Programmein Denmark, Finland and Sweden, pp. 99-112.

VIAK AB. 1977. Rural water development study forHararghe region, v. I-III.

VIAK AB. 1982. Three research and development projectsin Hararghe region. Plan of operation.

Werner, V. and W. Haze. 1982. Machakos IntegratedDevelopment Programme, Kenya. Personal com-munication.

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C. Hanson received a B.Sc. in Geology, Chemistry,and Hydrology, and a Ph.D. in Quaternary Geology fromthe University of Stockholm, Sweden. Presently, he isresponsible for VIAK's activities in Ethiopia. His majoractivities are in the fields of rural and urban water supply,hydropower development, and institution building (watertechnology).

Ake Xilsson is a Research Engineer with the Depart-ment of Land Improvement and Drainage, Royal Instituteof Technology, Stockholm, Sweden.

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