Simulation of the Impacts of Flood Retarding Structures on Streamflow for A Watershed in Southwestern Oklahoma under Dry, Average, and Wet Climatic Conditions

ABSTRACT: Following the devastating floods of the 1940s, thousands of flood retarding structures were constructed in the Great Plains. The impacts of these structures on streamflow characteristics and their effectiveness in reducing floods under dry, average, and wet climatic conditions were investigated in this study. The setting for the study was a 160 km2 (61.9 mi2) experimental watershed in southwestern Oklahoma that contained 13 flood retarding structures, which controlled 65% of the drainage area. Thirty-three years of precipitation and eight years of streamflow data, in conjunction with computer simulations, were used to evaluate changes due to the flood retarding structures in annual, monthly, and daily streamflow characteristics at the outlet of the watershed. Simulation results indicate that installation of the flood retarding structures leads to a decrease in average annual streamflow of about 3%, which was attributed to an increase in average annual evaporation of this same amount due to the free water surface of the reservoirs created by the flood retarding structures. On a monthly time scale, the simulation results showed that under dry climatic conditions, the flood retarding structures caused a reduction in mean streamflow for all months of the year. On the other hand, during average and wet climatic conditions, changes in mean streamflow varied by month with May and October showing the greatest decreases, respectively. As expected, the greatest impact of the flood retaiding structures was on daily flow characteristics and maximum daily flows. Noticeable differences include the reduction of maximum daily discharges on the day of a storm event and the increase in streamflow from principal spillway releases in the days immediately following storm events. Annual maximum daily discharges were reduced by about 33%, with the reduction of the 5 and 10 year maximum daily flows being 23.8 to 14.6 cms (840 to 515 cfs) (39%) and from 36.0 to 20.6 cms (1,270 to 727 cfs) (43%), respectively. This study demonstrates the effectiveness of the flood retarding structures in reducing annual peak runoff events, thereby reducing flooding and related property damage. However, the flood retarding structures also reduce low streamflow values by trapping baseflow runoff from drainage areas above the impoundment structures. From a water quality and stream habitat preservation point of view, maintenance of a minimum baseflow can be critical.