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Title: SIMULATION OF THE IMPACTS OF FLOOD RETARDING STRUCTURES ON STREAMFLOW FOR A WATERSHED IN SOUTHWESTERN OKLAHOMA

Author
item Van Liew, Michael
item Garbrecht, Jurgen

Submitted to: Journal of Soil and Water Conservation Society
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/1/2003
Publication Date: 7/1/2003
Citation: Van Liew, M.W., Garbrecht, J.D. 2003. Simulation of the impacts of flood retarding structures on streamflow for a watershed in southwestern Oklahoma. Journal of Soil and Water Conservation Society. 58(6):340-348.

Interpretive Summary: Thousands of small dams referred to as flood retarding structures (FRSs) were constructed throughout the Great Plains of the United States following severe flooding during the 1940s. These structures consisted of an earthen dam and an inlet for controlling the release of water, which is referred to as a principal spillway. These FRSs reduced the threat of damage caused by flooding and stream bank erosion. The Little Washita River Experimental Watershed (LWREW) in Southwestern Oklahoma provides an experimental setting to better understand the impacts of the FRSs on streamflow characteristics. A 61 square mile subwatershed within the LWREW contained 13 FRSs and was investigated in this study. Forty years of rainfall and sixteen years of streamflow data, along with computer simulations, were used to determine changes in the annual, monthly, and daily streamflow characteristics at the outlet of the subwatershed as a result of the FRSs. Results of the study show that with the exception of a slight increase in annual evaporation, no relevant change in annual streamflow could be attributed to the FRSs. On a monthly basis, the FRSs slightly reduced streamflow during the two wettest months (May and October) of the year, and also decreased streamflow somewhat during the summer months. As expected, the greatest impact of the FRSs was on the daily streamflow characteristics. Noticeable differences in daily flows included the reduction of peak discharges on the day of a storm, and the increase in streamflow due to principal spillway releases in the days immediately following storm events. Test results show that with the FRSs in place, annual flood peaks on the subwatershed were reduced by 37%. Results of this study highlight the importance of FRSs in reducing damages due to flooding and in helping to conserve soil and water resources on agricultural lands.

Technical Abstract: Following the devastating floods of the 1940s, thousands of flood retarding structures (FRSs) were constructed in the Great Plains, as a result of flood abatement programs that were authorized by the federal government. The impacts of these structures on the streamflow regime and their effectiveness in reducing watersheds floods under current and changing conditions were investigated in this study. The setting for the study was a 160 km2 experimental watershed in southwestern Oklahoma that contained 13 FRSs which controlled 65% of the drainage area. Forty years of precipitation and sixteen years of streamflow data, complemented by computer simulations, were used to evaluate changes in annual, monthly, and daily streamflow characteristics at the outlet of the watershed as a result of the FRSs. With the exception of a slight increase in annual evaporation, no relevant change in annual streamflow could be attributed to the FRSs. On a monthly basis, the FRSs slightly reduced streamflow during the two wettest months (May and October) of the year, and also decreased streamflow somewhat during the summer months. As expected, the greatest impact of the FRSs was on the daily flow regime and daily peak flows. Noticeable differences in daily discharges as a response of the FRSs to rainstorms include the reduction of peak discharges on the day of an event and the increase in streamflow due to principal spillway releases in the days immediately following storm events. Annual floods were reduced on average 37 percent, and 5 and 10-year floods were reduced from 32.7 and 49.6 cms to 20.4 and 30.3 cms, respectively. Results of this study accentuate the importance of the flood abatement program in reducing damages due to flooding and in helping to conserve soil and water resources on agricultural lands.