El Chaparral dam model: rooster tail formation on high sloped spillway

A rooster tail characterizes the flow expansion downstream of rectangular dam spillway piers. In fact, especially for high Froude numbers, the jet trajectory shows a continuous oscillatory behaviour along the spillway. However, the jet impingement on the stilling basin can favourably increase the energy dissipation. Therefore, the correct assessment of the rooster tail characteristics is necessary to avoid some undesired effects and maximize the spillway discharge capacity. This paper aims to analyse the rooster tail characteristics downstream of chute expansion on a 50° slope using a scale model. The El Chaparral dam prototype is characterized by a maximum discharge of 6700 m3/s and the presence of an aerator in the spillway central section further contributes to increase the elevation of the rooster tail. The main characteristics of the flow propagation, jet height and width, shockwaves and wave reflection on the model side wall for different Froude numbers and gate openings have been analyzed and a photographic analysis of the flow propagation characteristics has been performed. Results have also been compared with an equation developed for flow expansion downstream of chute pier for supercritical zero gradient bottom.