The present study is the continuation of a research program conducted at Colorado State University in cooperation with the United States Bureau of Reclamation (Bureau) concerning overtopping spillway flows. Tests were conducted at near prototype conditions on an existing outdoor flume located at Colorado State University's Engineering Research Center. Horizontal steps fabricated from lumber and plywood were placed in the flume providing a simulated stepped spillway with step heights of h = 1.0 ft and h = 2.0 ft. Additional testing of the smooth surface chute with the steps removed was conducted for comparison purposes.

Roller Compacted Concrete (RCC) is becoming an increasingly popular method of constructing and protecting dam embankments. RCC naturally lends itself to a stepped configuration by the construction technique of roller compacting successive horizontal concrete lifts. To date, there have been numerous RCC stepped spillways constructed worldwide, yet there is the lack of a general design that quantifies the hydraulics characteristics of the overtopping flow for a given step height, dam height, and slope.

Specialized instrumentation, provided by Colorado State University and the Bureau, was used to collect air concentration and velocity data within the stepped spillway flow. The data were analyzed to quantify flow depth, energy dissipation, and an estimate of flow resistance in the form of the Darcy friction factor f. Results of the analysis were then used to propose a design procedure for estimating the hydraulic characteristics of stepped spillway flow using typical required information.

Overflow conditions occur on spillways during regular periods of release or during emergency flood events. During extreme events, overtopping of the entire crest o f the dam may occur and the downstream face is exposed to erosive flow conditions compromising the integrity of the dam or leading to catastrophic failure. In recent years, there has been an increased interest in the use of stepped overlays on dams and spillways to dissipate the erosive energy from overtopping flows.