SubSurface Dams - Sustainable Sanitation and Water Management ...

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(1) Relation between the rainfall within the drainage basin and the rate of streamflowThe statistical analysis of the daily rate of streamflow at the crossing point of the KolongoRiver with the old main road in Nare Village and of the daily rainfall (*) within the KolongoRiver basin provided the relationship shown in Fig. 6.8.(*The rainfall within the drainage basin was estimated from the rainfall data measured inNare/Koulikare, Ouanobian, Noka and Kaya.)(2) Relation between the rate of streamflow and the recharge of groundwaterFrom the rate of streamflow measured in 2000 and 2001 and the estimated recharge ofgroundwater, the following relationship was presumed:Recharge of groundwater = about 10 to 15% of the amount of streamflow (*) from July toOctober(*Rate of streamflow at Nare Village)(3) Relation between the rainfall within the drainage basin and the recharge of groundwaterThe relationships given above in (1) and (2) make it possible to estimate the recharge ofgroundwater from the rainfall measured within the Kolongo River basin.(4) Dimensions of the reservoir area of subsurface damThe survey of a longitudinal profile of the reservoir area showed that the slope of the groundsurface was 0.65/1000. The digging of the observation wells also revealed that the thicknessof the fossil valley sediment (water storage layer of subsurface dam) in the reservoir area wasnot very different from that at the dam site. Therefore, assuming that the slope of the bottomof the reservoir layer is equal to that of the ground surface, the dimensions of the reservoir areestimated as follows:- Width of the reservoir area: About 150 m on average (lowest estimate)- Maximum extent of the reservoir area: 13.4 km upstream of the dam- Maximum groundwater level: -3 m below ground surface- Volume of the reservoir layer: About 9,000,000 m 3- Maximum water storage capacity: 1,800,000 m 3 (assuming that the effective porosityof the reservoir layer is about 20%)(5) Results of water balance analysis in the reservoir areaFrom the results of water balance analysis on recharge of groundwater, water storage andwater leakage, the water storage state in the reservoir area of the subsurface dam wasestimated as shown in Table 6.5.63
Table 6.5: Change in water storage state by the subsurface dam(1) Recharge ofgroundwater(2) Water leakage Increase inreserved water(1) - (2)(m 3 )Total amount of reservedwater at the end of the dryseasonIn the rainy 1,200,000 (1,200,000) (1,200,000)season of 1998At the end of the990,000 210,000 210,000dry season of1999In the rainy 1,200,000 (1,200,000) (1,410,000)season of 1999At the end of the990,000 210,000 420,000dry season of2000In the rainy750,000 (750,000) (1,170,000)season of 2000At the end of the990,000 -240,000 180,000dry season of2001In the rainy 1,200,000 (1,200,000) (1,380,000)season of 2001At the end of the990,000 210,000 390,000dry season of2002Total 4,350,000 3,960,000 390,000 390,000Note: Water storage by the subsurface dam actually started in the rainy se(6) Forecast of water storage in the futureAs Table 6.5 shows, the water leakage from the reservoir area of the subsurface dam isestimated to be about 990,000 m 3 per year. Therefore, when an extraordinary drought occursas in 2000, reserved water at the end of the dry season of the following year will decreasecompared with the previous year.However, assuming that these droughts are extremely rare and the annual recharge ofgroundwater on average is about 90% that observed in 2001 (about 1,200,000 m 3 ), i.e.1,100,000 m 3 , the water storage in the future will change in the following way:1) With the recharge of groundwater (the increase in reserved water) during the rainyseason, the reservoir layer of the subsurface dam will be "full" in the rainy season of 2005.The reserved water will then be about 1,800,000 m 3 .2) However, due to water leakage from the reservoir layer, the reserved water willdecrease to about 800,000 m 3 at the end of the dry season of 2006 (until the beginning ofthe following rainy season).3) With the recharge of groundwater of 1,100,000 m 3 during the rainy season of 2006 asassumed, the reserved water will reach the maximum capacity of about 1,800,000 m 3 , andthe excess water of about 100,000 m 3 will overflow the crest of the subsurface dam.64
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Photo: Construction of the subsurfa
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Subsurface DamSmall-Scale Surface D
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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 16 and 17: y adopting an appropriate dam struc
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 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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