Estimation of flood flows on the Big Sioux River between Akron, Iowa, and North Sioux City, South Dakota

This report presents estimated flood flows for specified frequencies at selected locations on the Big Sioux River between the Akron, Iowa, streamflow-gaging station and North Sioux City, South Dakota. The selected locations include: at the Akron gaging station, downstream from the Richland-Westfield Creek Basins, downstream from the Brule Creek Basin, downstream from the Upper West Boundary Big Ditch and Rock Creek Basins, downstream from Broken Kettle Basin, and downstream from North Sioux City. The flood flows for the 10-, 50-, 100-, and 500-year recurrence intervals will be used to support a Federal Emergency Management Agency Flood Insurance Study. Four methods were used to estimate the flood flows. The first method involved the use of drainage-area ratios raised to specified exponents to transfer the flood-frequency relation from the Akron gage to the selected downstream locations. The second method was a flood-frequency analysis based on a summation of the Akron gaging-station peak flows and concurrent tributary daily flows from within the various study reaches. The third method was an independence/dependence analysis of the Akron gaging-station flows and the tributary flows from the various study reaches. The fourth method was a flood-frequency analysis assuming complete dependence of the Akron peak flows and the tributary peak flows from the various study reaches. Based on the various analyses that were done, the drainage-area-ratio method best estimated the flood flows for the Akron to North Sioux City reach of the Big Sioux River. The best estimates of 10-, 50-, 100-, and 500-year flood flows at the location downstream from North Sioux City are 35,300, 70,400, 89,100, and 142,000 cubic feet per second, respectively.