Reservoir planning

A reservoir stores water when it is plentiful to release it when it is scarce. Planning one means deciding how much storage is needed for a target supply, where to locate the dam, how to divide the storage among uses, and how long the reservoir will last before sediment fills it.

Storage is divided into zones. Dead storage, below the lowest outlet, holds sediment and cannot be released by gravity. Live storage, between the minimum drawdown level and the full reservoir level, supplies irrigation, drinking water and power. Above full reservoir level, the flood cushion up to the maximum water level absorbs part of the design flood, and freeboard above that protects the dam from waves.

The live storage needed for a given demand can be found from a mass curve, the cumulative inflow plotted against time, by drawing demand lines tangent to its peaks and measuring the largest gap below them. The sequent peak algorithm does the same in a table: it carries forward the cumulative deficit, resetting to zero whenever inflow refills the reservoir, and the largest value is the required storage. Longer records, including droughts, give more reliable answers, and evaporation and seepage losses are added.

Sedimentation reduces capacity every year. The trap efficiency of a reservoir, which depends on the ratio of capacity to annual inflow, estimates how much incoming sediment stays. Catchment treatment, sediment flushing and sluicing extend reservoir life.