Scour process due to symmetric dam spillways crossing jets

Abstract

The presence of a dam in a river deeply modifies the natural dynamic process, including sediment transport, debris flow and flow regime occurring both in the upstream and in the downstream branches. Thus a dam cannot be considered as an isolated element in the context of a river, and the study of all the cited phenomena has to be conducted taking its presence into account. In particular, it affects both the global and the local river erosive mechanisms. Moreover, risk management policy considers, among other variables, that in all likelihood toe erosion is the one affecting dam failures. In this perspective, even the local scour phenomenon due to plunging jets assumes a particular importance for all river engineers and has to be carefully taken into consideration in order to prevent structural risks and to correctly design protection measurements downstream of hydraulic structures. Previous studies extensively analysed both scour geometry and flow features due to one single jet. The present paper aims to study both the flow features and the scour hole lengths in the case in which two symmetric crossing jets are used. In particular, experiments were conducted by varying hydraulic and geometric parameters: jet discharge, tailwater depth, jet vertical angle and the angle between the crossing jets. Moreover, it was experimentally proven that the vertical distance of the jet crossing point from the water surface deeply influences the scour hole length. A comparison of scour hole morphology due to an equivalent single jet geometric condition is also proposed, in order to understand under what conditions crossing jets reduce the scour depth.

Keywords:

• Plunging jets
• crossing jets
• scour morphology
• hydraulic features