Chute Spillway for Big Sioux River, Sioux Falls, South Dakota, Hydraulic Model Investigation

Abstract

Abstract : The Big Sioux River flows through Sioux Falls, located in Minnehaha County, South Dakota. A Reconnaissance Study completed in 1988 identified a Federal interest in upgrading the existing flood control project at Sioux Falls by raising the levees. A diversion channel bypasses the 25.7-km (16-mile) loop of the Big Sioux River, avoiding the populated downstream area and a 24-m (80-ft) drop at the natural falls downstream of the downtown area. The city of Sioux Falls requested U.S. Army Corps of Engineers assistance in 1969 because of a heavy snow pack and other conditions conducive to a major flood event. The levee across the channel from the diversion spillway chute stilling basin suffered erosion damage and was in danger of being breached. A 1993 Feasibility Report by the Omaha District examined deepening the stilling basin, raising the stilling basin walls, and raising the levees. The design of the chute stilling basin was based on sound engineering procedures; however, the need for a model study was necessary to optimize chute and stilling basin features to obtain the most economical design. The model verified that modifications to the stilling basin walls were necessary. While modifications to the stilling basin were not necessary, it was determined that channel realignment would increase energy dissipation. The following information was obtained from the model: flow characteristics and stilling basin performance, chute wall heights to prevent overtopping, downstream levee heights to prevent overtopping, riprap requirements for protection downstream of the structure, and wave heights of existing flow for design of riprap protection of the levees. The model study was used to determine the type of modifications to the basin. Serious consideration to lowering the basin was made.