THE IMPACT OF FLOOD RETARDING STRUCTURES AND VARYING CLIMATIC CONDITIONS ON STREAMFLOW AND SEDIMENT RESPONSE FOR A WATERSHED IN SOUTHWESTERN OKLAHOMA

Authors: Van Liew, Michael

Submitted to: Environmental and Water Resources Institute World Congress Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 5/12/2004
Publication Date: 6/27/2004
Citation: Van Liew, M.W. 2004. The Impact of flood retarding structures and varying climatic conditions on streamflow and sediment response for a watershed in southwestern Oklahoma. In: Proceedings of the Environmental and Water Resources Institute World Congress, June 27-July 1, 2004, Salt Lake City, Utah. 2004 CDROM.

Interpretive Summary: Small earthen dams referred to as flood retarding structures (FRSs) represent one of the most effective methods for reducing damages caused by flooding and sedimentation from agricultural land. The Little Washita River Experimental Watershed (LWREW) in southwestern Oklahoma provides an experimental setting to better understand the impacts of the FRSs on streamflow and associated soil losses. The impacts of the FRSs were studied on a 61 square mile subwatershed of the LWREW referred to as 526 that contained 13 FRSs and on a 208 square mile subwatershed referred to as 522 that contained 40 FRSs. Thirty years of rainfall data, along with computer simulations, were used to determine changes in the annual and daily streamflow and sediment characteristics at the outlet of the subwatersheds as a result of the FRSs for dry, average, and wet climatic conditions. Simulation results of this study show that with the impoundment structures in place, average annual streamflow decreased 14%, 3%, and 3% under dry, average, and wet climatic conditions on 526, and 10%, 4%, and 3% on 522. Results also show that the flood retarding structures reduced average annual sediment yield by 41%, 46%, and 43%, respectively, under dry, average, and wet climatic conditions on 526, and by 24%, 26%, and 25% for the respective climatic conditions on 522. Differences in the reduction of sediment yield between the two subwatersheds were due to such factors as the combined sediment storage capacities of the FRSs, their distance from the watershed outlet, and the percent of watershed area controlled by the structures. Results of this study highlight the importance of FRSs in reducing damages due to flooding and sedimentation and in helping to conserve soil and water resources on agricultural lands.

Technical Abstract: Flood retarding structures (FRSs) represent one of the most effective methods for reducing damages caused by flooding and sedimentation from agricultural land. The impacts of these structures on streamflow and sediment regime and their effectiveness in reducing watershed floods and associated soil losses under dry, average, and wet climatic conditions were investigated in this study. The Soil and Water Assessment Tool (SWAT) was used to determine differences in streamflow and sediment characteristics with and without flood retarding structures under varying climatic conditions on two subwatersheds of the Little Washita River Experimental Watershed (LWREW) in Southwestern Oklahoma. Model simulations were performed on subwatershed 526, which contained 13 FRSs controlling 63% of the 160 km2 drainage area, and on subwatershed 522 that consisted of 40 FRSs controlling 43% of the 538 km2 drainage area. Seven years of meteorological, streamflow, and sediment data were used for model calibration and validation. Thirty years of precipitation data, representing dry (1963 to 1972), average (1973 to 1982) and wet (1983 to 1992) climatic conditions, were then used to evaluate changes in streamflow and sediment characteristics due to the FRSs. Simulation results of this study show that with the impoundment structures in place, average annual streamflow decreased 14%, 3%, and 3% under dry, average, and wet climatic conditions on 526, and 10%, 4%, and 3% on 522. The flood retarding structures reduced average annual sediment yield by 41%, 46%, and 43%, respectively, under dry, average, and wet climatic conditions on 526, and by 24%, 26%, and 25% for the respective climatic conditions on 522. Differences in the reduction of sediment yield between the two subwatersheds were attributed to such factors as the combined sediment storage capacities of the FRSs, their distance from the watershed outlet, and the percent of watershed area controlled by the structures. Model simulations from this study confirm the importance of the flood abatement program in reducing flooding and soil losses from agricultural land, but also show that low flow conditions may be exacerbated by the FRSs.