Energy Dissipation on a 4(H):1(V) Stepped Spillway

Approximately, 10% of the 11,000 flood control structures constructed with the financial and technical support of the Natural Resources Conservation Service are expected to have roller compacted concrete (RCC) stepped spillways to address inadequate spillway capacity due to changes in hazard classification. With the ability of placing stepped spillways over existing embankments, the NRCS will be able to increase the spillway capacity without necessarily raising the top of dam and increasing flood storage. This rehabilitation solution ultimately saves time and money, while still providing a safe method for the release of flood water downstream. Embankment overlaid stepped spillways typically have the same slope as the downstream face of the embankment. For NRCS structures, this means that the slopes are rather flat ranging from 2(H):1(V) and flatter. Yet, research and literature on flat sloped stepped spillways are quite limited. At the request of the NRCS, the USDA-Agricultural Research Service Hydraulic Engineering Research Unit in Stillwater, Oklahoma has been conducting a series of tests to investigate the effects various step heights have on the energy dissipation in the spillway chute having a 4(H):1(V) slope. A two-dimensional physical model of a stepped spillway was constructed in a 1.8 m (6 ft) wide concrete flume to evaluate the effects 3.8 cm (1.5 in) and 7.6 cm (3 in) steps have on the velocities and the energy dissipation. Flows ranging from 0.17 to 1.4 cubic meters per second (6 to 50 cfs) were tested. The likelihood of encountering low velocity, low air entrained flow near the crest to high velocity, high air entrained flow near the stilling basin dictated the use of an ADV, a pitot tube coupled with a pressure transducer, and a two-tip fiber optical probe for collecting centerline velocity profiles because each of these devices is used to capture unique data. The velocity profiles show the development of the turbulent boundary layer along the length of the chute. Energy losses along the length of the chute are also presented. Research conducted by others suggests insignificant differences in the energy loss as a result of changing the step height. This research is expected to assist engineers with the design of stepped spillways applied on relatively flat embankment dams. 16 pp., 2 tables, 12 figures, 24 references.