The design and operation of off-line storage basins for flood control is examined herein. The goal is to optimize the storage of riverside land such that peak flood stages are reduced. This is in contrast to using riverside land in the passive sense, allowing it to fill and empty with changes in river stage. Off-line storage basins offer the possibility of merging environmental and flood control objectives, in contrast to in-line storage that leads to unwanted environmental consequences.; Design and operational issues are examined using a flood model that simulates both flood dynamics and dynamics of flood wave-depression wave interaction. Depression waves form when water is suddenly diverted into off-line basins using a control action resembling a levee breach. San Francisquito Creek (SFC) in the California Bay Area is used as a site for the evaluation of off-line flood control. SFC runs through several environments, and little riverside land is available to participate in flood storage [CRMP 1998]. Results of the analysis show that active control of flood plain storage leads to the same reduction in flood stage as passive control using half as much flood plain area. However, active control may also provide less flood stage reduction resulting from poor operation of the off-line basin. That is, timing of the diversion and control of the diversion rate are critical operational parameters. Additionally, the degree of flood stage reduction scales with the available flood plain area, and in many cases complete mitigation of an event (i.e., no inundation) is possible with limited flood plain area.; Additional analysis was performed in a generalized channel system to evaluate the performance of off-line basins under a wider range of system conditions. Similar to observations in SFC, these studies show that active control allows for greater flood stage reduction than passive control. However, the diversion must be properly timed else the outcome can be worse. The generalized case also shows that control from both upstream and downstream is possible, and that downstream control becomes increasingly ineffective as the channel steepens. Analysis examining the relative effect of a number of system, design, and operational parameters is presented.
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