Comparative analysis of the energy dissipation on a stepped spillway downstream of a Piano Key Weir

A systematic experimental study has been carried out to compare the residual energy at the toe of a stepped spillway of varied length considering a classical ogee-crested weir (OCW) or two different piano key weirs (PKW) at the top of the structure. Four spillway lengths have been considered on a wide range of discharge. An indirect method has been used to compute the residual flow energy at the spillway toe from water depth measurements in a horizontal channel downstream. The results indicate that uniform flow conditions, i.e. flow energy at the spillway toe independent of the spillway length, are reached on significantly shorter spillways downstream of a PKW than downstream of an OCW. However, the uniform flow energy at constant discharge on the spillway is found to vary depending on the weir type. In particular, it is smaller downstream of the OCW than downstream of the PKWs.