Ground-Water Dams for Rural-Water Supplies in Developing Countries -;™;

Home , Amaravati River

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 evaporation 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. good aquifer characteristics. Infiltration conditions One of the basic conditions justifying the must be such that the reservoir is properly constructio subsurfaca depletioe f th n o s i m neda recharged during the rainy period. of ground-water storage through natural ground- storage Th e reservoir mus containee b t y db water flow. The gradient of the ground-water table imperviou r low-permeabilitso y layers that prevent and thu extene sth f ground-wateo t r flo geners wi - vertical and lateral seepage losses. In most cases ally a function of the topographic gradient. This bedrock will serve as the natural container, but fact indicates tha constructioe th t f subsurfacno e there are also examples of low-permeability over- dams is feasible only at a certain minimum topo- burden layers serving this purpose, suc buries ha d graphical gradient, which naturally varies according clay layers of alluvial origin, the upper clayey to local hydrogeological conditions. Most ground- portio weatheree th f no d profile lithomare th r o , g water dams in existence today have been in laterite profiles. constructed in areas with a 1-5% slope, but there The containing layer must be at such a depth are examples of dams constructed where gradients that it is technically possible to carry out the are 10-15%. excavation at reasonable cost. In general, the limit storage Th e capacit f sanyo d storage damd san t arouna seem e m5 b ,d 4- o sbutt , dependinn go subsurface dams constructed in riverbeds is a the scale of the scheme, it may be possible to function of the specific yield of river sediments. extend this considerably. If injected cutoffs are Dam therefore sar e preferably constructen di used instead of the conventional construction of geological environments wher weatherine eth g dams in trenches, there is no practical limit to the products contain a substantial amount of sand and depths possible. gravel. As a consequence, cases where dam The basic idea behind constructing a subsur- construction has met with success are mostly face dam for storage purposes is to arrest the found where the bedrock consists of granite, gneiss, natural flow of water; it is necessary to quantify

49 this flow so that the benefits of the scheme may be properly assessed before construction s startsi t i f I . • Con'crete . ' '. t economicallno y feasibl standare us o et d hydro- : • •-.'•'•* ' r jf - £> ^ • • • geological procedures for this purpose, a fairly •S7 • | » f. '. - Blocks or simple monitoring program indicating seasonal ' •*• ** ' .^ < ' •• — stone ground-water level fluctuations will give a good 'a '. i • .

Fig. 4. Clay dike. Fig. 6. Brick wall.

500 make it watertight is a fairly simple procedure Table 1. Heights of Some Subsurface Dam Types—Average (Ahnfors. 1980). The relatively high cost of bricks Values from Schemes Studied idrawbacksa , however therd alse an , ear o some Average height stabilitye doubtth o t s sa . Dam type in meters Using ferroconcrete means that steel rodr so Injected cutoff 10 wire mesh usede hav b t generall o et bu , y such Brick wall 6 materia availabls i l t reasonablea e cost methoe Th . d Concrete or stone masonry dam 6 f foro involve e m maius s work it e t nsth bu , Ferroconcrete dam 4 advantage is that very little material is needed to Clay dike 3 achiev verea y strong wall structure e b Th .o t s eha anchore solie th do dt reservoi r bottom. Using thin sheets of impermeable materials Beaumont (date unknown), Beaumon Kluged an t r suc tarres ha d fel r polyethyleno t inexpensivn a s ei e (1973), and Nissen-Petersen (1982). e cosalternativ f th material o ts a r fa cons s i sea - ':•; The upper limit of dam construction and thus cerned. The mounting of the sheets on wooden also the upper limit of storage volumes are set by frames and the erection process is rather complicat- the condition that the dam has to withstand the ed (Ahnfors, 1980) materiale Th . , especialle yth maximum peak flow allod discharga an r , wfo f eo polyethylene highls i , y sensitiv damago et e during peae th k flow belo bane wth k leve r througo l h erectio wels n a durins a l g refillin trenche th f go , spillways. and a minor rip will cause leakage losses. A small- A sand storage dam is generally constructed in scale scheme in south India where plastic sheets stages, as shown schematically in Figure 7. The were used has functioned very well for three years, basic idea is to limit the height of each stage in t remaini t seee b bu no st whethe materiae th r l will orde keeo rt psufficientla y high-water velocityo s , withstand high ground-water temperaturese th , that fine particle washee sar t fro reservoie dou m th r activities of microorganisms in the soils, root whil coarse eth e particles settle heighe Th . f eaco t h penetration othed an , r activities ove perioa r f do stage is determined by estimating the sedimentation time (Jayakumar, 1984). process in the reservoir (founded on experience Injected cutoffs have been used to arrest the from sedimentation in previous stages), from the flow in large or deep-seated aquifers in North exten f naturao t l sedimentatio streame th n ni r o , Africa and Japan (BCEOM, 1978-,Matsuo, 1977), calculations of water velocities in the reservoir. and to protect fresh water from pollution in This method of constructing the dam by Europe and the USA. For large projects and when addin stagw ne ega each seaso r eveno n less a deep aquifer is/dammed certainls i t i , feasiblya e frequently means that costs wil highee b l r than alternative. werm theda e ye constructe th woul f i e db fulo dt l The average heights of some of the dam types height directly methodo Tw . s have been trien di showe ar Tabln ni e mos1n I . t case crese a s th f o t order to solve this problem. One is to use a siphon, subsurfac keps i t somm a t eda e depth froe mth which discharges water over the dam and keeps the surface. Thi partls si o avoiyt d waterloggine th n gi flow velocity in the reservoir sufficiently high. This upstream area, and partly to avoid erosion damage method has not been found to be technically dame th commoA o . t n distancm e da fro e mth efficient and is very costly (Burger and Beaumont, surface cresth o about s t ei metere on t . date unknown). Another method is to leave a notch in the Sand Storage Dams whicm da h allow accumulatioe sth f sedimentno s When a subsurface dam is built, it is always possible to obtain at least some idea of the hydraulic characteristics of the existing aquifer material. When planning the construction of a sand storage dam, however, the material in which the water is supposed to be stored is still in the catchment area waiting to be transported to the dam site flooa y f unknowb o d n intensity propee Th . r Originol river btd design of a sand storage dam is therefore a more Layer f seno s d depositen i d e reservoith r complicated matter, involving more hydrological and hydraulic calculations. The subject has been Fig. 7. Construction principle of sand storage dam (from treate Wipplingey db r (195 1984)d 8an , Burged an r Burge Beaumontd an r , date unknown).

501 certaia o onlt p nyu height notce Th . thes hi n filled in befor e nexeth t rainy e reservoiseasonth d an ,s i r allowed to fill completely. This method has proved quite successful in Kenya (Werner and Haze, 1982). Figure 8 shows a concrete dam which fulfills Flushing valve Pipe for gravity extraction the basic requirements for a sand storage dam, i.e.

it is sufficiently massive to take up the pressure Grovel drain*^ "^Perforated pipe fro e sanwated mth dan r storereservoire th n di , Fig. 9. Extraction alternatives from a sand storage dam. and it is watertight. The dam also may be built of stonework r largeFo . r reservoirs taky e ema th t i , arcn fora hf m o dam . wele Th l through which water from subsurface alsy stony made ob b ma p eem u da e Th dam generalls si y extracte placee e b th y n di dma gabions or blocks sufficiently large to withstand reservoir or, for erosion protection purposes when the pressure. The gabion or block dam is covered dam buile s ar riverbeds n i t riverbanke th n i , . When on the upstream side by a thick layer of clay to aquifers with low permeability are dammed,, it may make it impermeable. It is also possible to place e necessarb construco yt seriea t f large-diameteso r the watertight clay layer as a core inside the dam. wells or collection chambers to create sufficient maie bod m heaa Th nda e f y b po alsn oca storage volume. stones covere concrety db e wall r stabilitsfo d yan If the community to be served by the scheme tightnessf thio sm typda eA . servesame th t esa is locatee d th downstrea sitd m ean da e th f mo timbridga s ea e ove smala r l strea Kenymn i a topographical condition favorablee sar possibls i t i , e (Werner and Haze, 1982). o extract t water fro reservoie mth gravityy b r y B . All sand storage dams must be very well using gravity extraction, problems with pump protected against erosion along the banks, and even installation, operation, and maintenance are more so at the dam toe, where the energy of water avoided .probleme Thesth e ear s whic todae har y during peak flo\vs will be extremely high. The best generally encountered in rural-water supply projects f avoidino y wa g erosiot a construco t s m ni da e th t in developing countries, even in projects where natural rock barst available no f suc I .e s har i t i , shallow well and simple hand pump technology is important to extend the dam wall several meters applied. Figur showe9 typicasa l sand storagm eda int wite rivee to oth h rproteco m t ban da d e kan th t where both extraction alternative illustratede sar . stone fillin r concretego . Ground-water dam spossibles shoulda r fa s a ,, anchoree b solin di d rock. This generally provides Common Aspects the best stability, and fracturing of the dam can be Ground-water dam generalle sar y combined avoided. Furthermore, it makes it possible in most with a drain along the upstream base of the dam. cases to control seepage below the dam. Anchoring functioe Th f thio n s drain, which generally consists achievee b y ma d usin gconcreta e foundation no of gravel or a slotted pipe surrounded by a gravel e rocth k surface roce th weathere s kf i I . s i t di filter, is to collect the water and transmit it to a important that the weathered material is fully well or through a gravity pipe to downstream areas. excavated before the dam foundation is made; If the permeability of the aquifer material is very otherwise, there will probably be a seepage below low, it also may be necessary to improve the flow the dam. Controlling seepage losses through fracture alon reservoie gth r botto usiny mb gsystea f mo zones is more difficult but may be achieved in collection, grave r slotteo l d pipe, drains some case pouriny sb g very thin mortar inte oth perpendicula dame th .o t r fracture system (Werne Hazed an r , 1982). A ground-water dam may also function as a means of recharging an already existing aquifer by Concrete latera r verticao l l flow. Figur show0 e1 exampln sa e from Namibia of how this can work (Sauermann, date unknown). The dike was utilized as an aquifer Erosion meany b wela f so l supplying wateairportn a o t r . protection t aquifecarre no Th yd enougdi r h wate meeo t r t the demand, so it was decided to construct a sand storag whicm eda h would increas rechargee eth . Sometime ground-watesa consideres i m da r da Fig . Concret8 . e sand storage dam. failure because it is drained out by a fracture

502 system t mightI . , however, the e possiblnb drilo et l annual ric aree o crope parth n cropi a tw f d o ts an , azone utilizd welth ean n i ground-watee l eth m da r a yea possiblee ar r . Ther neea r additionas ei dfo l as an artificial recharge structure (Hanson, 1984). wate expano t r aree dth a covere secona y db d If planned and executed properly, a scheme crop alsd irrigato an o,t thirea d crop. involving the damming of ground water will have factoe On r restricting the possibilitf yo direco n t negative impacsurroundine th n o t g constructing ground-water dam thin si s are thas ai t environment t shoulI . kepe db mindn i t , however, land holding generalle sar difficuls yi t smalli d tan , tha e techniquth t implementes ei vera n di y fragile to organize the cooperative effort needed when environment, where even a small change may have several farmer involvede ar s existino . Thustw e th ,g long-term physica wels a l socias a l l consequences. dam bote sar h situate quitn do e large farmse On . This calls for caution. Aspects which must be con- was constructed by a private person and the other possibilitsideree th e dar f upstreayo m waterlogging, governmenbuils a n wa o t t seede farth y mb effect downstrean o s m ground-water levels, salt Central Groundwater Boar f Indiado private Th . e accumulation pollutiod an , n problems causey db dam was constructed in Ottapalam in 1962-64. A the higher ground-water table. large-diameter well supplying fare wateth m o t r In some area t mighsi possible b t construco et t usually drie seasot y durin ddr ou therd e nan gth e a whole serie f interconnecteso d ground-water was a need to find an alternative solution. A 130 m dams. Potential has been identified along sandy long dike reaching dow bedroco nt buils kwa t riverbed Kenyn si a (Sdrlie, 1978 alond )an g narrow across a narrow valley, surrounded by outcropping and very long valleys in South India (Jacob, 1983). rock. "jessourse Th typa "( f silted-ueo p surface damf )o depte Th bedrocko ht , whic establishes hwa d Tunisi examplee aar f small-scalso e dams thay ma t prior to excavation by steel-rod sounding, is on the be placed at regular intervals along alluvial valleys reached averagan e m th maximu sa e5 t a m 9 f mo (El Amani, 1979). center of the well. The aquifer consists of residual soils which, at least in the upper layers, are sandy. CASE STUDIES The transition between the weathered layer and South India e underlyinth g fresh rocs distincts kwa i t i d an , Two subsurface dams have been constructed established during the excavation that the rock had in Kerala, South India e site situatee Th .s ar e th n di no major fracture zones. Palghat Gap, which is a graben dividing two hill The dike consists of a 4-inch plastered brick ranges parallel to the west coast. The altitude is wall (Figure 11). Wate extractes i r d froe mth about 50 m and rainfall averages 2400 mm, the aquifer through a gravel drain along the dike to a main par f whico t h occur June-Octobern si . series of storage wells connected to a pumping Agricultur intensivs ei e area watee th n Th .ei r well. The catchment area of the dam is about 10 available during the rainy season is enough for one wated haan , uses i r d mostl r complementaryfo y irrigation of 1.5 ha of paddy and, at the end of the dry season, 1 ha of coconut trees.

Pore-holy intc oqujftf Centrae builth m y da b t le GroundwateTh r Board was completed in 1979. In addition to supplying water for supplementary irrigation at the seed farm, it was also meant to serve as a pilot project for future dam construction. As a consequence, a scientific approach was used in the hydrogeological investigations and construction. alss owa Thiconstructem sda d acrossa narro catchmena ws valleha d yan t are f abouao t 20 ha. The bedrock consists of gneiss and granite which outcrops on the valley sides. The soils are sandy in the central parts of the valley and more fine-grained alon e sides gaveragth e Th . e specific yield, determined from tension-plat neutrond ean - probe measurements, is 7.5%. Other investigations carried out at the site were hammer sounding and Fig. 10. Ground-water dam for recharging purposes (from resistivity measurement establiso st depte hth o ht Sauermann, date unknown). bedrock, which at the deepest section is 4 m.

503 presented in a master plan for rural-water development in Hararghe Region (VIAK, 1977), which have forme basie r th dSwedis sfo h assistance th o et Ethiopian Government in the water sector. The first two ground-water dams in Ethiopia were constructed by the Ethiopian Water Works Construction Authority (EWWCA) in 1981. These two dams, located at Bombas and Gursum (Fugnambira) describee ar , d below positive Th . e experience gained from these pilot schemes has initiated a Research and Development Project with f furtheo m ai r e developinth methodologe gth d yan encouraging local application of ground-water dams in Ethiopia. The project is being carried out by EWWCA, with financial support from the Swedish International Development Authority (SJDA). The project is described in a Plan of Operation (VIAK, 1982).

Bombas A sand storage dam was constructed in 1981 t Bombasa villaga , f aboueo inhabitants0 50 t n o , the road between Harar and Jijiga in Hararghe Region. The dam and reservoir can be seen in Figure 12. Excavation was carried down to the bedrock at 3 m depth. The bedrock is composed of crystalline basement rocks. The excavation work s interruptewa unexpectey db d rains which once fig. 11. Brick wall and storage wells in the background, fille trenchee th d smads i wit f m heo da sand e Th . Ottapalam, India. solid concrete block abovraised m san e 8 eth d0. original sand surface. Downstrea walle th f m,o abous i e totam Th t da l 15lengte 0th mf ho , there is an erosion protection consisting of large and the crest was kept one meter below ground boulders which can be seen in Figure 12. On the leve orden i l o avoit r d waterloggin upstreae th n gi m photograph originae th , l masonry protectios ni area. The main part of the dam is made up by a shown, partly collapsed. Wate pipes i r gravity db y plastered brick wal t ther bu alsle ear o sections steea o downstreat lm tan0 30 kdame th f m,o consisting of tarred felt and plastic sheets. Two where the villagers collect water from a stand post. wells, connecte o eacdt h othe drily b r l holes The dam is full several months after the end throug unexpecten ha d rock bar, were constructed of the rainy seasons with a small discharge over- along the dike. walltoppinm da . e Thigth s indicates tha sane th t d The dam took three months to complete at a

total cos f U.So t . S7:500, including pumping equipment. One-third of this cost was for earthwork rese r equipmenth fo t d an constructiod an t n materials storage Th . e volum reservoie th f es o wa r estimate t 15,00da 0 in , which woul sufficiene db t 3 for supplementary irrigation of the second paddy crop and the raising of a third crop of black gram in a command area of 6'ha. The benefit/cost ratio at 8% interest rate was calculated to be 1.06.

Ethiopia e concepTh f ground-wateo t r dam bees sha n introduced recently into Kthiopia. The first proposal r ground-watesfo r dam Ethiopin i s a were Fig. San12 . d t storagBombasa m eda . 504 ::

g ~ffi&&-&$'££&.i'''2--'''': £*'•-..:'-%-'--v~*'"j;l''vV" - J*»&.--.'''. \~"v'-'-~**?^£ ifl5^-it^>*-.'^t^'^:-.'-^'.-^.-'.v; -,.<';'* ^;^^.C,-v '..'^"--'^;i-.'..-''»ixS£t« Fig . Subsurfac13 . t Gursuma m eda . recharges i m da grouny db d waterm . Wheda e nth is ful f watero l , ther dangea es i f pollutioro n from animals entering the reservoir area. The dam has functioned well and without any breakdown since inaugurateds wa t i projece cose th s f Th .to wa t U.S. $15,000. \ Gursum A subsurface dam was constructed at Gursum (Fugnambira), a town of 5,000 inhabitants, in 1981. The dam is shown in Figure 13. The structure is quite unique and is an example of a subsurface dam constructed in materials other than sand. Fig. Constructio14 . sana f ndo storagt Genda m eeda The parent materials at the dam site are silt and Balina. clay, with onl permeabilityw ylo overcomo T . e this, a large excavation was made. A dam wal walm madls i lda wa e f stonseo Th mad . e 15 masonre d an 4 y1 f stono e masonry plastered with cement. Because and shoul stageso raisee db tw firse n di Th . t stage of the low permeability of the parent materials, a was raised 1 m above the original sand structure. reservoir of 200 m3 was created and covered with This stag completes ewa Augusn di t e 1983th d ,an concrete semicircular rings restin pillarn go f so reservoir filled with san wated dan r durin gflooa d concrete blocks. in September 1983. However decrease th , e th f eo The flow of ground water in the soil is blocked water leve vers wa yl quickDecemben i d an , r 1983 by the dam. Additional water is piped by gravity the reservoir was empty. It has not yet been possible fro msprina g area upstrea dame th f m, o usin g to locate the leakage, whether under the dam or in perforate pipeC dPV s trenclaia n di h wit gradeha d filter. Wate pumpes i r d fro reservoie mwela th n i l r to an elevated reservoir for distribution to com- munal water point towne th watee n si .Th r supply has functioned well. The cost of the project was U.S. $150,000.

Research and Development Activities SIDA-sponsoree Th d researc developd han - ment project which is now being implemented involves experimenting with various local constructio nwall m materialvarioud da s an e th s n meansi s of extracting ground water. constructes wa firse m da Th t t Gendda e Balina neart Dirye t e Dawno s 198ha n ai t 3bu Fig. Gend15 . e Balina sand storag neam eda r completiof no come into operationshows i m Figuren ni da e .Th s Stage 1.

505 the reservoir. The foundation has now been storage dams for water conservation. Department of improve groutingy db . However t couli , e db Water Affairs, South-West Africa. 21 pp. possible that the leak is in the reservoir area, and Central Ground Water Board. 1980. SIDA-Assisted Ground- Water Project in Noyil, Ponnani, and Amaravati River e wateth drainins i r g into fault fractured an se th n i s Basins, Tamil Nadu and Kerala; Project Findings and bedrock. Should this be the case, ground water Recommendations. Government of India. 48 pp. could probabl extractee yb d through wells drilled El Amani, S. 1979. Traditional technologies and develop- int basemene oth woulm t da rockthi n di e sTh . ment of the African environments; utilization of case provide artificial recharg groune th f eo d water. runoff waters, the "meskats" and other techniques in Seven additional ground-water dams wile b l Tunisia. African Environment . Ilv , l 3-4. 11-12no , , pp. 107-120. constructed during the next few years by SIDA Hanson . 1984G , . Developmen f sub-surfaco t e e damth n si and the Swedish Red Cross. A subsurface dam is Hararghe region, Ethiopia. Interim Report, VIAK. under constructio t Ejna a Anan iwal m witf da o l ha Jacob . 1983C . V ,. Central Groundwater Board, India. solid block ferroconcreted san reinforcemene Th . t Personal communication. is made of wire mesh. There are also plans to con- Jayakumar, M. 1984. Central Soil and Water Conservation struct subsurface dams wallusinm da .ge clath n yi Research and Training Institute, Ootacamund, India. • Personal communication. Matsuo, S. 1977. Environmental control with underground CONCLUSIONS dams. Proceedings of the Specialty Session on Geo- Experience fro e schememth s discussed here, technical Engineering and Environmental Control. Ninth International Conferenc Soin eo l Mechanics as well as from other parts of the world show that and Foundation Engineering, Tokyo, July 1977. the construction of ground-water dams is a feasible pp. 169-182. solutio r improvinnfo supple gth f grounyo d water Nilsson, A. 1984. Ground-water dams for rural water areasy indr t shoulI . d not, however lookee b , d supply in developing countries. Report TR1TA-KUT upon as a universal method but as an alternative in 1034, Dept. of Land Improvement and Drainage, Royal Institut f Technologyeo , Stockholm. pp 6 5 . situations where water supply through conventional Nissen-Petersen, E. 1982. Rain Catchment and Water methods cannot be arranged or for some reason is Supply in Rural Africa; A Manual. Hodder & not suitable. Stoughton, Great Britain. 83 pp. The proper siting of ground-water dams Sauermann, H. B. Date unknown. Sand versus sun. South- necessitates a thorough knowledge of the hydro- West Africa. pp 3 . Sundborg, A. 1982. Erosion and sedimentation processes. geological conditions in the actual area. When a In: Sedimentation Problem Riven si r Basins. IHP tentative sitbees eha n selected mandators i t i , o yt Project 5.3, UNESCO, Paris. understan ground-watee dth r flow syste thao ms t Sorlie, J. E. 1978. Water conservation techniques proposed the dam can be correctly designed or, should the in North Turkana, Kenya. Hydrology in Developing site be unsuitable, unnecessary construction costs Countries. Nordi ReporP . cIH 2 , No Nationat l Com- avoidede b n r thesca Fo . e reason necessars i t si o yt mittees for the International Hydrological Programme in Denmark, Finland and Sweden, pp. 99-112. make general surveys as well as detailed studies on VIAK AB. 1977. Rural water development study for the selected sites. Due to socioeconomic conditions, Hararghe region . I-IIIv , . however, these investigations carrie e havb t o et dou VIA . 1982KAB . Three researc developmend han t projects at low cost. It is necessary to make generalizations in Hararghe region. Pla f operationno . and use simple geophysical methods. . HazeW Werner d . an 1982 . V , . Machakos Integrated Careful planning is also needed for the con- Development Programme, Kenya. Personal communication. struction phase schedulee b .o t Thi s sha d according Wipplinger . 1958O , storage Th . f wateeo sandn i r . South- to the seasons in order to keep costs down and to West Africa Administration, Water Affairs Branch, avoid damage caused by heavy storm flows. 1958 pp.7 10 . Wipplinger, O. 1984. Development of semi-arid regions. Unpublished memorandum. REFERENCES Ahnfors . 1980M , . Ground-water arresting sub-surface structures. Govt. of India, SIDA-Assisted Ground- C. Hanson received a B.Sc. in Geology, Chemistry, Water Project in Noyil, Ponnani and Amaravati River Hydrology,and Ph.D.a Quaternaryin and Geology from Basins, Tamil Nadu and Kerala. Report 1:16, 22 pp. the University of Stockholm, Sweden. Presently, he is . KlugerW . J Beaumont d . 1973an . D .. SedimentatioR , n ni responsible VIAK'sfor activities Ethiopia.in majorHis reservoir meana s sa f wateso r conservation. 1AHR activities are in the fields of rural and urban water supply, Congress, September 3-7, 1973, Istanbul. hydropower development, institutionand building (water Bureau Centra) d'Etudes pour ]es Equipements d'Outre-Mer technology). (BCEO.M). 1978 barrages Le . s souterrains. Ministere Ake Xilsson Researcha is Engineer with Depart-the Cooperationa J e d . pp 5 3 1 . ment Landof Improvement Drainage,and Royal Institute Burger, S. W. and R. D. Beaumont. Date unknown. Sand of Technology, Stockholm, Sweden. 506