Design Water Amount for Reservoir Management and Operation

Reservoirs for flood protection are normally designed by using the annual maximum design flood (AMDF). However, management and operation of a reservoir require regulation of intrayear runoff instead of interyear runoff. The intrayear runoff varies with time within a year; hence, reservoir operation and management should deal with different aspects of river runoff. Therefore, information on inflow in the near-future time interval that may be several days long is needed. The question that has long been under discussion, especially in developing countries, such as China, is whether the use of AMDF is reasonable for reservoir operation and management. This paper presents a new kind of design water amount for reservoir operation that considers the probabilistic characteristics of the intrayear runoff that the built reservoir must cope with. Such a design amount varies with time and inflow to the reservoir and reaches the traditional annual maximum design flood when the inflow reaches a high water level. This dynamic design water amount (DDWA) is important for addressing the issue of balancing the need for flood control with storage for other purposes, and the focus on keeping free capacity can lead to water losses if the flood levels are less than expected. Thus, the DDWA is an operational design criterion that should complement the traditional design flood computation. Hence, the use of such new design water amount may make the reservoir safer in a high-water period and a higher water level, leading to more benefit in the middle- and low-water periods. The analyses for a real example reservoir (Three Gorges Reservoir, China) demonstrate the use of the new design water amount. Applying the DDWA to the Three Gorges Project reservoir may ensure safety by flood protection and increase the ensuing benefit because it is a design water amount in the near-future time interval, which may or may not be in the flood period.