SubSurface Dams - Sustainable Sanitation and Water Management ...

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y adopting an appropriate dam structure.In this project, this problem was solved by selecting the dam site at a point near theconfluence of the Kolongo River into a larger river.(4) Salinization in reservoir areaThe subsurface dam is likely to cause accumulation of salt on the ground surface in thereservoir area due to the rise of reserved groundwater to the surface by evaporation.However, this phenomenon occurs only when the highest groundwater level is close to theground surface. It is thus possible to avoid this problem by setting the highest level ofreserved groundwater at a sufficient depth below the ground surface.In this project, the highest level of reserved groundwater (the depth of the crest of the dam)was thus set at 3 m below the ground surface.2-4 Requirements for a subsurface dam siteThe physical conditions (hydrogeological conditions) required for the site are as follows:(1) Presence of shallow groundwater with high fluidityThere must be groundwater at the dam site. This groundwater must have high fluidity aswell, because reserved water using a “cut-off wall” set up across stagnant water cannotincrease.In addition, it is desirable that this groundwater exists at a shallow depth because, if thegroundwater aquifer exists at a deeper depth, determining the hydrogeological characteristicsof the dam site would be more difficult, and the cost and technical difficulties of theconstruction of the subsurface dam would be much greater.(2) Presence of a porous layer (aquifer) for water storageThe higher the volume of pores (effective porosity) of the geological strata that form thereservoir layer, the more effective the water storage. This is because water is stored in thegeological strata. High effective porosity is necessary also for high water fluidity.(3) Presence of the surrounding basement rock with low permeabilityThe sides and the bed of the reservoir must consist of basement rock with low permeability.If there are big water bypaths, the dam cannot store water effectively.(4) Presence of a gorge of basement rock with low permeabilityTo construct a subsurface dam effectively, it is desirable to set up the dam at a bottleneckpoint, where basement rock with low permeability make a gorge with a vast aquifer upstream,as in the case of a surface dam.13
A gorge in the geological structure is suitable for thesubsurface dam site.GroundwaterflowThe strata for water storage musthave high effective porosity.The basement rock must be impermeable oralmost impermeable.Fig. 2.2: Requirements for a subsurface dam site14
Page 2 and 3: Photo: Construction of the subsurfa
Page 4 and 5: Subsurface DamSmall-Scale Surface D
Page 6 and 7: ContentsIntroduction1. General desc
Page 8 and 9: 1. General description of the Model
Page 10 and 11: Department of the Environmentand Fr
Page 12 and 13: According to the results of an anal
Page 14 and 15: A large amount of waterevaporates f
Page 18 and 19: 2-5 "Fossil valley", a suitable sit
Page 20 and 21: temperature of 30 to 34 degrees and
Page 22 and 23: 3-3 Distribution of fossil valleys
Page 24 and 25: 3-4 Surveys carried out in this pro
Page 26 and 27: Satellite image of the area around
Page 28 and 29: 3-4-2 Field explorationAt the 5 sit
Page 30 and 31: 3-4-3 Geophysical surveyAt the 5 si
Page 32 and 33: Groundwater level assumed from the
Page 34 and 35: 3-5 Detailed field surveys (test dr
Page 36 and 37: Groundwater level isalmost the same
Page 38 and 39: The village is on a peneplain with
Page 40 and 41: In this project at Nare, the "earth
Page 42 and 43: DownstreamFilled back soil3mCoarseg
Page 44 and 45: All the results of the quality test
Page 46 and 47: Photo 4.3: Construction of the subs
Page 48 and 49: 4) Basal stratum of the fossil vall
Page 50 and 51: 6. Observation of the water storage
Page 52 and 53: Table 6.1: Features of the faciliti
Page 54 and 55: PerchedwaterImpermeablelayer“Main
Page 56 and 57: Table 6.3: Annual rainfall in the K
Page 58 and 59: 3) The reserved water level during
Page 60 and 61: Ground surface level: 275.73mGround
Page 62 and 63: 6-4 Analysis of seasonal fluctuatio
Page 64 and 65: SandysiltSiltSand(white)Sand(moist)
Page 66 and 67:
(1) Relation between the rainfall w
Page 68 and 69:
4) Subsequently, the cycle in which
Page 70 and 71:
7. Other studiesIn addition to the
Page 72 and 73:
these facilities:- 3 mill stations,
Page 74 and 75:
Photo 7.3: Water-supply station for
Page 76 and 77:
8. Recommendations for future subsu
Page 78 and 79:
advisable to arrange rainfall stati
Page 80:
AcknowledgementsThe Model Project t
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