State-Frequency Analyses for Urban Flood Control Reservoirs

A methodology for determining reservoir stage-frequency relations is described that includes the following three component parts: (1) adjusting several-decade long historical sequences of daily streamflows for the effects of urbanization; (2) simulating reservoir system operations; and (3) performing a frequency analysis of the resulting series of maximum annual water surface elevations. Analyses are presented for the Addicks and Barker Reservoirs, which are operated by the U.S. Army Corps of Engineers to reduce flooding along Buffalo Bayou in Houston. Extensive urbanization occurring throughout the floodplain and watershed during the past 50 years includes development adjacent to reservoir lands upstream of the dams. Operating policies, including both allowable release rates and timing of gate operations, are demonstrated to significantly affect reservoir stagefrequency relations. The effects of different operating practices are shown to become more pronounced with increased urbanization.