SubSurface Dams - Sustainable Sanitation and Water Management ...

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6. Observation of the water storage state by the subsurface damUnlike a surface dam, a subsurface dam does not allow direct observation of water storage.It can only be estimated from the groundwater level observed using facilities set up for thispurpose.This chapter describes the observation of groundwater level and other parameters carried outduring this project, as well as the water storage state estimated from the results of theobservations.6-1 Facilities for observation of water storage state (facilities for groundwaterobservation)Table 6.1 shows the features of the facilities for the observation of the water storage state(facilities for groundwater observation) set up in this project. The distribution of thesefacilities is shown in Fig. 6.1.Five of these facilities were equipped with hand pumps for water supply to the villagers (3pumps of which were still active in March 2003).(1) Observation wells of the all-strainer typeThis is an observation well formed using a plastic pipe with strainers (screens allowing theinflow of water) over all its length deeper than the 5.5 m below the ground surface, installedin a borehole (Fig.6.2-A).Before the construction of the subsurface dam, 5 wells of this type with automatic water levelrecorders were installed at the dam site and points on its extension to carry out continuousobservation of the groundwater level. After these wells were removed for the constructionof the dam, 9 wells of the same type were set up again, 4 of which were across the fossilvalley about 200 m upstream of the dam and the rest of which were set up at other places.Five of these wells were equipped with automatic water level recorders.(2) Large-diameter wellsThis is a dug large-diameter well (internal diameter is 1.8 m) whose structure is similar tothose prevailing as water-supply facilities in Burkina Faso. In this project, the height of therim of the well was set at 2 m above the ground surface, and the rim was surrounded byconcrete to protect the well from river floods.At about 100 m upstream and about 50 m downstream of the subsurface dam, “large-diameterwells” (OW-1, -2) were respectively installed for visual and comparative observation of thewater storage state by the subsurface dam. On the reservoir area of the “small-scale surfacedam with water gates” (see Section 7.(3)), 4 “large-diameter wells” (NP-1 to 4) were alsoinstalled for observation purposes. The water-pumping wells set up as a part of the“water-pumping station operated by solar energy” (see Section 7.(1)) were of the same type.(3) Sets of piezometers of different depthsWhen there is perched water above the "main" groundwater, the “observation wells of theall-strainer type” mentioned above do not show correctly the “main” groundwater levelbecause of the significant influence of the perched water. The presence of such perchedwater around the subsurface dam site of this project was suggested by observation during the47
excavation and the subsequent observation of groundwater using “wells of the all-strainertype”. To examine the presence of perched water and to observe the “main” groundwaterlevel, sets of piezometers, whose structure is shown in Fig. 6.2-B, were installed at 4 points(PA to PD).This is an installation consisting of 4 plastic pipes, each of which has a strainer only at itsbottom end, and is buried at different depths from each other. The water level in each pipereflects the level (and the pressure) of the groundwater at the depth of its bottom end.48
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 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 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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