The artificial lakes which are created by constructed dams across river care called reservoirs. It may fail either due to excessive leakage of water or a result as a rapid sedimentation.
There are the main geological problems connected which are as under follows:-
The amount of leakage of water from the reservoir is controlled by the depth of water table. If the water table is so near the ground surface that the water table level in the reservoir does not rise above it, no serious loss by leakage will occur. On the other hand of water table lies deep below the ground surface, the water level in the reservoir will stand above it. As a result, leakage will occur and its amount will depend on the permeability of rocks.
During the geological investigation it is necessary to locate the highly permeable rocks that are present in the reservoir area. The rocks which are highly fissured, intensely jointed, faulted or have solution channels are likely to cause serious leakage form the reservoirs.
Generally the leakage of water from the strata that have downstream dip, will be more that those which have upstream dip, if the permeable rock bed outcrops on the valley slopes in the reservoir, it may not only cause leakage but may also cause land slide, such a land slide may produce an opening in the reservoir rim through which the stored water may escape.
Silting of Reservoir
The reservoir built on river which carry large amount of sediment, may silt up very soon and its water storage capacity may be reduced considerably. The amount of silt produced and supplied to the rivers depends mainly upon the lithological character and topography of the catchments area. The river flowing over the soft rocks and high gradient areas, carry greater amount of silt. On such rivers silt traps may be constructed upstream in order to check the rate of silting in it.