Water from Small Dams

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The crest (top of dam wall) must always be at least one metre higher at the middle thanat the ends to prevent a wash-out of the middle section by a heavy thunderstorm.In addition, height must be increased by 10% if the soil is compacted by a tractor. Thismust be increased by 20% if compacted by oxen and by 30% if the dam wall is notcompacted at all. These increased heights are called the settlement allowance, becausewhen the reservoir of a newly built dam gets filled with water, the soil in the dam wallwill settle and lower the middle of the dam wall, which endangers the safety of the dam.SpillwaysThe two ends of the curved wall of hillside dams function as spillways to allow surpluswater from the reservoir to flow safely out of the dam. Heavy rain-showers on largecatchments produce huge volumes of run-off water that must pass over the spillwayswithout eroding the ends of the dam wall otherwise water might destroy the whole damwall.Spillways should therefore be reinforced by placing large stones against the ends of thedam walls. Long-rooted grass with runners should be planted between the stones toprevent overflowing water from eroding the stones. The floor of the spillways should alsobe covered with stones interplanted with grass to prevent erosion. If the floor of thespillways is steep a concreted stone-masonry structure may be needed.Enlarging a catchmentShould the volume of run-off water not be sufficient to fill a pond, then a catchment canbe enlarged by diverting run-off water from another catchment into the pond by means ofsoil bunds as explained in the former chapter.Enlarging a water reservoirDams having catchments with sufficient run-off can be enlarged to hold much morewater by deepening the reservoir and using the excavated soil to increase the height of thedam wall. Labour for enlargement of dams might be obtained by:• Allowing neighbours to collect water free of charge, if they will excavate andtransport one wheelbarrow of soil for every one jerrycan of water they fetch.• Charging people a fixed amount for every jerrycan of water they fetch and using themoney raised to hire people for further excavation work.28
A hillside dam built by a tractorploughing in circles against the damwall in Zambia. It took the farmertwo weeks to complete theconstruction works.A hillside dam built manually at thefoot of a rock outcrop in Kitui,Kenya, some 40 years ago. The damis still providing clean water fordomestic use from a hand-dug wellsunk into a seepage line downstreamof the dam wall.An example of a hillside dam harvesting rainwater run off the Machakos-Kitui road.29
Page 1 and 2: Water from Small DamsA handbook for
Page 3 and 4: ContentsAcknowledgement ………
Page 5 and 6: AcknowledgmentsMuch gratitude is du
Page 7 and 8: TECHNICAL VOCABULARYASAL = Arid and
Page 9 and 10: Chapter 1.Introduction to small dam
Page 11 and 12: Borrow pits, also called murram pit
Page 13 and 14: Rock catchment dams can either bebu
Page 15 and 16: Chapter 2.Considerations before con
Page 17: It is difficult to estimate the wat
Page 21 and 22: If the answers to the questions lis
Page 23 and 24: 3.3 OwnershipValley dams are normal
Page 25 and 26: Chapter 4.Charco damsFarmers and ca
Page 27 and 28: Since there are no documented guide
Page 29 and 30: 4.4 ConstructionThe construction si
Page 31 and 32: Simple equipment for measuring leve
Page 33 and 34: Another very positive feature of hi
Page 35: For manual excavation, divide the a
Page 39 and 40: The rainwater runoff from the catch
Page 41 and 42: 6.4 Measurements to be taken of a d
Page 43 and 44: The sketch shows that a freeboard 1
Page 45: Homogeneous dam walls can be built
Page 48 and 49: 6.8 Bill of Quantities (BQ) and cos
Page 50 and 51: The table below shows all the tools
Page 52 and 53: 6.9 An example of a complete design
Page 54 and 55: It is a good idea to have an experi
Page 56 and 57: Water drawn directly from a dam res
Page 58 and 59: Usually, soil for building the side
Page 60 and 61: The usual types of pit latrines wit
Page 62 and 63: Silt traps made of vegetation plant
Page 64 and 65: 9.2 Washed-out dam wallsThere are s
Page 66: ReferencesBrown, L.N. 1965. Small e

Water from Small Dams

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