Ground-Water Dams for Rural-Water Supplies in Developing Countries -;™;
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2 1 7 R G 6 8 Ground-Water Dams for Rural-Water Supplies in Developing Countries -;™;. ..,.,.... ... J L O •_* ' '-•»:•:•• . i^.-./-'i... !\'«-.- ::A:£\>J. FOR CCr»;iv:Ui\.:iTV WAT?./? SU by Gdran Hansonl\lilssone Ak d ban 3 "" ABSTRACT The use of ground-water dams to store water in regions with ari r tropicado l climate methoa s si d thas ha t received considerable attentio lasyearse w th fe t y n i B n. storing water behind subsurface dams in natural aquifers or e saninth d accumulate sann di d storage dams, the disadvan- tage f conventionaso l surface storage, suc higs ha h evapora- tion rates, pollution, siltation healtd an , h hazardse b y ma , avoided. The techniques are very old, but only recently have there been some attempts to make systematic studies an develoo dt p proper siting, design constructiod an , n methods. This paper present experience th s e gained from existing structures all over the world and describes the physical settin whicn gi techniquee hth appliede b y ma s . Fig . Genera1 . l principl subsurfaca f eo e dam. Design and construction alternatives are shown, and case studies from Indi Ethiopid aan presentede aar construce Th . - dammin f groungo d wate practices rwa ancieny db t tion of ground-water dams may be a feasible solution to civilizations in North Africa. More recently, various water-supply problems in many parts of the world if small-scale ground-water damming techniques have precede propey b d r plannin sitd egan surveys. been developed and applied in many parts of the world, notably in southern and East Africa, and in -r - BACKGROUND India. This paper summarizes the results of a state- use f surfaceo Th e reservoir scoro st e waten i r of-the-art study during which literature and data areas with dry climates has several serious disadvan- IT' from most worle partth f dso have been collected tages, suc pollutios ha n risks, reservoir siltation, (Nilsson, 1984). It also presents examples of and evaporation losses. Using ground water is one practical applications in two areas in Ethiopia and f overcomino y wa g these problems somn i t ebu , India (Hanson, 1984; and Nilsson, 1984). areas good aquifert availablno e thear r sy eo yma There are basically two different types of only yield sufficient quantities of water seasonally. ground-water dams, namely subsurface damd san Experience alsshows oha n that conventional sand storage dams. A subsurface dam is constructed development of ground water in developing below ground leve arrestd lan floe naturaa sth f w o l countries involves serious problems relateo dt aquifer, whereas a sand storage dam impounds operation and maintenance of drilling equipment, water in sediments caused to accumulate by the wells, and pumps. dam itself. During the last few years, considerable generae Th l principl subsurfacea f s i eo m da attention has been given to the use of ground-water shown in Figure 1. An aquifer consisting of fairly dams as a method of overcoming water shortage in permeable alluvial sediment smala n si l valley regions with tropicaarid dan l climates. Damming supplies water to a village by means of a shallow ground wate r conservatiorfo n purpose certainls i s y consumptioo t welle Du natura.e th d nan l ground- not a new concept. Ground-water dams were water flow aquifee th , draines i r d y durindr e gth constructed on Sardinia in Roman times and season consequentld an , wele yth l runs dryo T . prevent this trencacrosa ,g du valleye s hsi th , reaching dow bedroco nt r somko e other solid, aPh.D. Quaternary Geology, VIAK AB, P.O. Box 519, impervious layer. An impervious wall is constructed S-162 15 Vallingby, Sweden. in the trench, and when the dam is completed the ^'Research Engineer, Departmen f Lano t d Improve- trenc refilles i h d wit excavatee hth d materialA . men Drainaged an t , Royal Institut f Technologyeo , reservoir built in this way will not be drained and Stockholm, Sweden. may be used throughout the dry season, provided Received July 1984, revised July 1985, accepted August 1985. of course that the storage volume is sufficient to Discussion open until January 1, 1987. meet the water demand. \ 4 GEO'JN' . No Vol , 24 . P VVATER-July-August 1986 filled with sand. This artificial aquifer wile b l replenished each year during the rains and if the properls i m da y site constructedd dan , water wile b l reservoie seasonkepy durine th dr n us i te r gth .fo r Quite often a ground-water dam actually is a combination of the two types. When constructing a subsurface dam in a riverbed, the storage volume may be increased by letting the dam wall rise above ground level, thus creating an accumulation f sedimentso . Similarly, whe sanna d storagm eda is constructed, it is necessary to excavate a trench 'in the sand bed in order to reach bedrock or a Fig. 2. General principle of a sand storage dam. stable, impervious layer. Figure 3 shows a map on which all known ground-water dam sites are marked, and also where The general principle of a sand storage dam is it has been proposed, after preliminary investiga- shown by the example in Figure 2. The villagers tions implemeno t , techniquee th t schemee Th . n si collect their water fro smale mth l nonpcrennial Europ northwesterd ean n Afric mostle ar a y large- strea t mtimea s whe t carrieni s water r froo , m holes scale projects, wherea certain si n part f Ethiopiaso , dug in the shallow riverbed for a short period after East Africa, and Namibia, ground-water dams are the rains. The quantity of water stored is not suffi- used quite extensivel storags ya e reservoirr sfo cient, however,, to supply water to the village during small-scale rural-water supply. There is a long tradi- the entire dry period. By constructing a weir of tion of building ground-water dams in the arid suitable height across the streambed, coarse southwester ne Unite parth f o td State northerd san n particles carried by heavy flows during the rains are Mexico therd isolatee an , ear d example f subso - cause settleo dt eventualld an , reservoie yth r wile b l surface dams in Afghanistan, India, and Japan. General applicatio f sanno d dams. Sanconstructiom da d n site Planned/proposed sanconstructiom da d n General applicatio f sub-surfaco n e dams. Sub-surfac constructiom da e n site Planned / propose d sub-surfac constructiom da e n . 3. Ground-water dam sites and proposed construction areas. 498 PHYSICAL SETTING and quartzite, whereas areas underlain by rocks The rationale of damming ground water is the such as basalt and rhyolite tend to be less favorable. irregular availability of surface and ground water. Climat greaa s eha t influenc sedimenn eo t Ground-water damming techniques may thus be characteristic than si t governi t relationshie sth p applie arin semiarid di dan d areas where thera s ei between mechanical and chemical weathering. A need to conserve the scanty quantities of rainfall. lower rat f chemicaeo l weatherin arin gi d climates They are also highly suitable in areas with a resuly ma morn i t e coarse-grained sediments monsoon climate where there is a need to store (Sundborg, 1982). surplus water from rainy seasondurine us y r gdr sfo The stage following weathering in the periods. sediment cyclerosioe particlese th s th ei f no . topographicae Th l conditions govera o nt Topography and land use govern the extent of large exten technicae th t l possibilit f constructinyo g erosion, and an engineer interested in constructing the dams as well as achieving sufficiently large ; sand storage dams woul happe db fino yt d steep storage reservoirs with suitable recharge conditions .^slopes and low vegetation cover in the catchment and low seepage losses. The basin in which water is area, generally very contrary to the preferences of underlaie storee b b y o t dma bedrocy nb r ko agricultural and hydraulic engineers. unconsolidated formations with low permeability. The coarse sediments one wishes to trap by a generalls Ii t y preferabl sito et e ground-water dams sand generalle storagar m eda y transported be s da in well-define narrod an d w valley r riveso r courses. load therefors i t I . e necessar rainstorme th r yfo s This reduces construction costs and makes it producing the initial floods at the outbreak of the possible to assess storage volumes and to control rainy seaso sufficientle b o nt y othee heavyth n r O . possible seepage losses. This is obviously difficult hand, the flood intensity must not be so high that in areas with flat topography. On the other hand, stabilitthm eda jeopardizeds yi . Extremely high efforts mus made b t maximizo et e storage volumes flood intensity will also mean tha existence th t f eo without the construction of unnecessarily high natural riverbed limites si d (Jacob, 1983). dams. In mountainous areas with very high Subsurface dam mostle sar y commonly gradients, it may be difficult to find an acceptable constructe riverben di d aquifers consistin f sango d relation between storag eheight m volumda d .ean or gravel. Other types of aquifers that may be An optimum composition of riverbed material is damme weatheree dar d zones, alluvia r colluviao l l generally found in the transition zones between layers, or any type of overburden with sufficiently mountains and plains.