Preliminary analysis of air-water flows on steep slope downstream of piano key weirs

owadays, piano key weirs represent an effective solution to build high discharge capacity weirs on the top of large dams. But lack of information exists on flow conditions downstream of this specific type of weir, for instance regarding the high velocity air-water flow taking place on steep slope spillway chutes. This information is yet of paramount importance, particularly for designing the energy dissipation system at the dam toe. This paper presents the results of a first systematic characterization of the air-water flow properties (flow depth, flow velocity, air concentration) in a smooth and a stepped spillway chutes downstream of two piano key weir geometries. For the sake of comparison and validation of the measurement technique, data have also been gained considering an ogee crest. The experimental facility was made of a 2.01 m high and 0.492 m wide chute with a slope of 52°. For the stepped configuration, 0.03 m high and 0.024 m long steps were considered. While air concentration and flow velocity profiles remain mainly unaffected by weir geometry, transverse water depth is not constant, and its distribution varies a lot depending on the piano key weir configuration. This can be explained by the numerous shock waves generated by the piano key weir complex geometry, which propagate and reflect along the chute. The study provides first insights into a better understanding on air water flow conditions on steep chutes downstream of piano key weirs, and paves the way to adapted design of chute sidewalls and downstream energy dissipation structures.