Converging RCC Stepped Spillways

The United States Department of Agriculture (USDA), Natural Resources Conservation Service (NRCS) estimates that nearly half of the small watershed dams constructed with their assistance will reach the end of their planned service life within the next 10 years. While many of these structures were originally built in rural environments, urban communities have encroached upon these structures over the years, making it difficult to address issues such as the maintenance of adequate spillway capacity to meet current dam safety standards. The urban infrastructure surrounding these structures limits the options available for dam rehabilitation. For instance, raising the dam or widening the existing auxiliary spillway to increase spillway capacity is often not a viable option due to land right constraints or changes in topography. Therefore, one of the more popular choices for increasing spillway capacity is the use of roller compacted concrete (RCC) stepped spillways. Two advantages of RCC stepped spillways are that they can be placed over the top of the dam and that they can provide considerable energy dissipation within the spillway chute. When placing an RCC spillway over an existing embankment dam, it is often desirable for the width of hydraulic control section to be greater than the width available to outlet the flow into the downstream channel. A generalized study utilizing a three-dimensional, 1:22 scale physical model was conducted to evaluate the influence converging training walls have on the flow characteristics on a stepped spillway. Vertical walls having convergence angles ranging from 0 degrees to 70 degrees were tested in conjunction with a spillway having a chute slope of 3(H):1(V) and a prototype step height of 0.3 m (1 ft). Water surface profiles in the central area of the chute and along the walls were collected and analyzed for each spillway configuration. Results of the study are presented in a format that will assist engineers in determining the required wall height for this type of a converging chute spillway.