The Impact of a Flood Retarding Structure on Watershed Runoff Under Dry, Average, and Wet Climatic States

Flood damage to agricultural lands during the late 1930s and early 1940s prompted passage of the Federal Flood Control Act of 1944 (P.L. 78-534) and the Watershed Protection and Flood Prevention Act (P.L. 83-566) of 1953. As a result of these flood abatement programs, the USDA SCS constructed about 2,500 flood retarding structures (FRSs) in the State of Oklahoma to control runoff from about 22,000 km2. One of the pilot projects implemented in the flood abatement programs was the treatment of tributaries in the Washita River Basin in southwestern Oklahoma. The Little Washita River Experimental Watershed (LWREW) represents one such tributary that was instrumented during the 1960s to determine the downstream hydrologic impacts of the SCS flood retarding structures. Previous studies have investigated the impacts of FRSs on water yield, peak reduction, and flow frequency for tributaries in the Washita River Basin. Implicit in these investigations was the assumption that changes in measured runoff characteristics before and after construction of the FRSs were due only to the construction of these structures. Even though decade-long climate variations during the 1961-1990 period were recognized, they were not explicitly considered in previous analyses and resulted in difficult interpretation of results. Furthermore, flood frequency studies were limited by the short period of record before and after the installation of the FRSs. Computer simulation of watershed response provides the opportunity to determine the impacts of FRSs on runoff characteristics without the confusing effect of climate variations. The objective of this study is to determine the effect of a FRS on the flow regime and peak flows of subwatershed 442 within the Little Washita River Experimental Watershed in southwestern Oklahoma. Observed precipitation and streamflow data are complemented by computer simulations to overcome many of the shortcomings in previous analyses. Climate generation techniques are employed to assess of the magnitude of the impacts of the FRS relative to changes in watershed response due to decade-long climate variations. The results of this study provide a better estimation of the beneficial impacts of FRSs on downstream flow regime and the reduction of downstream flood risk under dry, average, and wet climatic conditions.