Impact of Converging Chute Walls for RCC Stepped Spillways: Fid-Bau Portal

Impact of Converging Chute Walls for RCC Stepped Spillways

Hunt, S. L. / Kadavy, K. C. / Abt, S. R. / Temple, D. M.

Roller compacted concrete (RCC) stepped spillways are growing in popularity as a method for providing overtopping protection for many aging watershed dams with inadequate spillway capacity. Land rights are often not obtainable for widening existing earthen spillways, and in other cases, topographic features and land use changes caused by urbanization limit the ability to modify the dimensions of the embankment or spillways. An advantage of stepped spillways is that they can be placed over the top of an existing embankment dam without increasing the height of the dam or without widening the existing auxiliary spillway. Furthermore, stepped spillways provide considerable energy dissipation in the chute, potentially reducing stilling basin size. The U. S. Department of Agriculture (USDA) Natural Resources Conservation Service (NRCS) has proposed to design a converging RCC stepped spillway for Big Haynes Creek watershed dam site 3 in Georgia. The design calls for a 100 m (330 ft) ogee crested weir section, with a 3(H):1(V) spillway chute converging 52° to the stilling basin located at the toe of the structure. The peak runoff from a probable maximum precipitation (PMP) event is expected to generate a spillway discharge of 763 m³/s (26900 cfs). To assist with the design of this spillway and with future designs based on similar design parameters (i.e. chute slope, step height, etc.), a study utilizing a three-dimensional, 1:22 scale, physical model was conducted to evaluate the flow characteristics in the spillway. This study is the first known attempt at developing generalized design criteria for vertical training walls for converging stepped spillways. The results of the study will be discussed herein.