Influence of tailwater depth, sediment size and densimetric Froude number on scour by submerged square wall jets

Abstract

In this paper, an effort is made to better understand the scour caused by submerged square jets in a cohesionless sand bed. The variables of interest are the densimetric Froude number, the tailwater depth and sediment grain size. Experiments were carried out at three different values of densimetric Froude number, a range of tailwater conditions varying from low to very high submergence, and two different sand bed grain sizes. Velocity measurements were conducted using a laser Doppler anemometer. At a given densimetric Froude number, progressing from the start of the test towards asymptotic conditions, the geometric parameters used to describe scour are found to be sensitive to tailwater conditions and the ratio of the nozzle size-to-grain size. The present results indicate that the effect of nozzle size-to-grain size ratio can be important and needs to be incorporated in the interpretation of scour. The effect is reduced as asymptotic conditions are reached. At a given tailwater depth, the scour geometry tended to increase with increasing densimetric Froude number and the effect was prominent at all tailwater depths. Turbulent bursts have been noted to have an important role in the scour process, and are more distinguishable with the finer bed material. At low values of the relative tailwater depth, the flow and the corresponding scour pattern tends to be nonsymmetrical. Moreover, at low values of tailwater depth and higher densimetric Froude number, the scour pattern is quite different from the other test conditions. Empirical equations describing the scour geometry are proposed for the asymptotic state. These relations are found to be valid for a wide range of test conditions.

Keywords:

• Asymptotic state
• cohesionless soil
• jet scour
• migrating dune
• nozzle size-to-grain size ratio
• tailwater ratio
• turbulent bursts