Passive and active control of diversions to an off-line reservoir for flood stage reduction

Diversion of excess streamflow to an off-line reservoir is examined as a wave interference problem that can be controlled to reduce the cresting stage of a flood. Flood diversions create depression waves in the stream channel which, superimposed upon the flood, decrease flood stage. In the context of an O(102) km2 coastal watershed in northern California, numerical modeling was performed to compare the performance of three idealized diversion control strategies including passive control, weir control, and gate control. It was found that gate control, which creates a dam-break like flow into an off-line reservoir, can be optimized to accomplish 2–3 times the flood depth reduction of passive control. Capturing 2% of the runoff, for example, the cresting depth is reduced less than 1% with passive control but 2–4% by gate control. This is particularly important in areas with little off-line or over-bank storage, such as urban watersheds. Timing of the gate action is critical. Optimal control requires gate action slightly before peak stage arrives at the diversion structure, by a duration that scales with the capacity of the reservoir. The lead time of flood forecasts appears compatible with the lead time necessary to optimize diversions, and decision support systems should compensate for forecast uncertainty by early gate action. If by poor design or operator error the gate is opened too late, gate control becomes less effective than passive control.