Stability analysis of turbulent water jets onto both rigid and mobile beds

A laboratory-theoretical investigation on the stability of jets developing downstream of a drop structure is presented. Two series of laboratory experiments were performed with both a mobile and a flat rigid bed downstream of the drop. The discharge and the downstream water depth were varied and two kinds of jet flow were observed developing downstream of the drop: a surface wave jet and an oscillating jet. The os-cillating jet is an unstable flow configuration. Both velocity and free surface flow measurements were per-formed. Starting from laboratory measurements, a mathematical model, which investigates the stability of the surface wave jet both over a flat and a scoured bed, was developed. The modified De Saint-Venant equations, i.e. with correction terms accounting for the curvature of the streamlines, were obtained from the theoretical velocity and pressure profiles. The stability of the surface wave jet is affected by the presence of the scoured bed, which weakens its stability properties.