Energy Dissipation on Flat-Sloped Stepped Spillways: Part 1. Upstream of the Inception Point

In recent years, hazard classifications for many existing embankment dams have changed because hydrologic conditions have been altered. Consequently, many of these dams no longer provide adequate spillway capacity according to state and federal dam safety regulations. Stepped spillways are a popular choice for providing increased spillway capacities to existing embankment dams. Stepped spillways in these applications are typically placed over the existing embankment or auxiliary spillway; thereby, the chute slope is the same as the downstream embankment face or auxiliary spillway slope. Design guidelines and literature in general for these stepped spillways are very limited, so further research on these stepped spillways is warranted. A two-dimensional, physical model was constructed to evaluate the inception point, velocities, and energy dissipation in a 4(H):1(V) spillway chute having 38 mm (1.5 in.) high steps. Model unit discharges ranging from 0.11 m3 s-1 m-1 (1.2 cfs ft-1) to 0.82 m3 s-1 m-1 (8.8 cfs ft-1) were tested. Water surfaces, bed surfaces, and velocities were collected during the tests. An inception point relationship provided by H. Chanson may be used to determine the inception point for slopes as flat as 4(H):1(V) when F* ranges between 10 and 100. The velocity profiles transition from uniform at the crest to approaching a one-sixth power law distribution at the inception point for all tested flows. Energy losses increase in a linear fashion from near zero at the crest to approximately 30% near the inception point for all tested flows. This research will assist engineers with the design of stepped spillways applied on relatively flat embankment dams.