Dams

General conditions to be considered when construct

There must be a lack of seismic and volcanic activity
Valley sides need to be stable so mass movement is unlikely. If beds dip in towards the resevoir then landslides may occur
The river catchment needs to have sufficient rainfall and be underlain by impermeable rocks to promote surface runoff for collection in the resevoir.
If it is a drinking water source it is important there are no exposed mineral viens containing toxic elements such as lead or arsenic

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Strong and competent with a high load bearing stenght to support the combined weight of the dam and water.
Ideally it should have a crystalline igneous or metamorphic rock or cemented sedimentary rock. - clay, mudstone and shale are weak, incompotent rocks so are generally unsuitable
Impermeable to prevent leakage of water from the reservoir
Rock type also needs to be uniform. If the dam is situated on two rock types, differential subsidence may occur making the dam unstable

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Beds that are horizontal or dip upstream provide stable foundations
Beds dipping downstream are unstable. There is the potential for leakage and slippage along the bedding planes that may lead to the collapse of the dam

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Faults - zones of permeability and weakness and may have diffferent rock types on either side. The weight of the dam and the water can reactivate old faults leading to an increase in seismic activity
Joints are zones of permeability that can cause leakage. Joints are often more closely spaced than faults
Anticlines make unstable foundations because slippage can occur alog bedding planes on the fold limbs. Tension joints on the crest of the anticline may allow leakage of water from the reservoir
Synclines are stable but water can bypass the dam through permeable beds

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Embankment dams - a clay core is held in place by piles of rock or earth. These are built in broad, shallow valleys. The weight of the dam is spead over a wide area so the foundations do not be very strong but they do require very large quantities of fill material
Masonry and concrete dams - can be arch, butress or gravity dams. They need a supple of cement and aggregate to make the concrete

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Grouting - holes are drilled into the rock and liquid cement pumped in. The cement fills pore spaces, joints and fissures, reducing permeability and increasing rock strength
Clay or plastic lining - prior to filling, the reservoir is lined with an impermeable material such as clay or plastic to prevent leakage of water into the underlying rock. Clay is a good choice because if there is a local supply it will be cheap
Cut off curtain - an impermeable barrier, usually made of concrete, constructed as an extension below the dam. Cut off curtains are particualy effective at prevent leakage from dams situated on synclines. They aslo have a further benefit of strenghtening the foundations and prevent slippage of beds dipping downstream

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Flooding of land for the reservoir - may result in loss of agricultural land or villages being drowned and people have to relocate elsewhere. Where forsets are flooded, the decaying vegetation releases large quantities of carbon dioxide and methane
Damage to aquatic ecosystems - may be caused by a change in water depth, may prevent fish such as salmon swimming upstream to their breeding grounds, the dam may even cause ecological problems beyond the river mouth by reducing amount of nutrients entering the sea
Silting of the reservoir, water released from dam spillways contains very little sediment as it is trapped in the reservoir (deltas dont form)
Risk of downstream flooding

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