Reliability Concepts in Reservoir Design

A reservoir's function is to regulate natural inflows, which vary irregularly, to provide outflows at a more regular rate that is determined by water demand for one or more uses (water supply, irrigation, hydropower), temporarily storing the surplus, when inflows exceed outflows. Reservoir reliability is defined as the probability that the reservoir will perform the required function, i.e. provide the outflow required to satisfy the water demand, at a specified period of time under stated conditions. The traditional reservoir design procedures are more commonly based on empirical approaches. It is shown, however, that the reliability concept is a more rational basis, and provides easy and accurate computational procedures, for reservoir design and operation. Under some simplified assumptions, a simple explicit expression relating reservoir size, yield and reliability is extracted. This expression can be used for preliminary stages of a reservoir design. For more detailed and accurate studies, a generalized solution procedure based on stochastic simulation of inputs is outlined. Keywords: hydrologic persistence; hurst phenomenon; mass curve; range; reliability; reservoir dead storage; reservoir design; reservoir flood storage; reservoir size; reservoir yield; storage; simulation; stochastic models; water demand