Air Concentration and Velocity Measurements on Self-Aerated Flow Down Stepped Chutes

The U.S. Bureau of Reclamation (Reclamation) developed instrumentation to measure air concentration and water velocity in large flumes as well as in smaller indoor models, namely the large outdoor flume facility located at Colorado State University (CSU) in Fort Collins, Colorado, and the stepped spillway model chute assembled at the National Laboratory of Civil Engineering (LNEC), Lisbon. The basic components of both the air concentration probe and the back-flushing Pitot tube are briefly reviewed in this paper. Typical model and near prototype air concentration and velocity profiles are shown, illustrating the highly-aerated, highly-turbulent feature of skimming flow over stepped spillways. The experimental results suggest that self-aerated skimming flow over stepped spillways behave similarly to self-aerated flow on smooth chutes. The shape of the velocity distribution seem however affected by the large flow resistance.