Air water flow conditions on spillway channels downstream of Piano key weirs

Abstract

Piano Key Weirs (PKWs) are an improvement of labyrinth key weirs. This type of weir can be placed on the dams needing to be restored, which is an advantage given the number of ageing dams. PKWs are used as the crest of spillways in order to manage water discharge, mitigate flood risks, and maintain the structural integrity of the dams. However, few studies have been carried out on flow properties downstream of this type of weir.\\

The main aim of this research work is to compare different flow properties as the clear water depths, the velocities, the air concentration profiles or the dissipated energy for different spillway types for a steep slope. Three weirs are tested: a Creager weir and two PKWs with different geometries. For each weir, a smooth and a stepped spillway are placed, making a total of six configurations tested. The results are obtained with a double-tips conductivity probe.\\

Different conclusions were drawn from the results obtained in this work. Firstly, more air is entrained in the flow for stepped spillways than for smooth spillways. Moreover, in the case of smooth spillway, the air entrainment is more important for PKWs while, for the stepped spillways, the air entrainment can be considered to be the same for the different weirs. Then, a non-uniformity of heads across the width of the channel appears downstream the PKWs. The head variation is greater for smooth spillways than for stepped spillways. Finally, the stepped spillways dissipate more energy than smooth spillways. Moreover, the type of weir seems to have a small influence on the dissipated energy. Even if the difference is not very important, the dissipated energy is slightly greater in the case of a Creager weir than PKWs.