Transient local scour by submerged three-dimensional wall jets: Effect of tailwater depth

The present study deals with scour caused by three-dimensional wall jets issuing from a square cross-section nozzle onto a non-cohesive sand bed. Experiments were conducted using two different nozzles and three tailwater depths. Velocity measurements were conducted using a one component laser Doppler anemometer. The densimetric Froude number was maintained below ten, while the jet expansion ratio was held greater than ten. The results indicate that the densimetric Froude number, tailwater depth and grain size to nozzle size ratio, all have an influence on the extent of scour caused by three-dimensional jets. However, each parameter has a dominant influence under different flow conditions. For example, at values of densimetric Froude numbers less than five, tailwater depth and grain size to nozzle width ratio have no effect on the maximum depth of scour. At higher values of densimetric Froude number, the effect of tailwater depth appears to be important for larger values of grain size to nozzle width ratio. Previous observations have indicated that the effect of the tailwater depth was significant for densimetric Froude number greater than ten. However, the present results indicate that the cut off value could be lower depending on the value of grain size to nozzle width ratio.; The present results show that the jet expansion ratio can have a significant influence, especially at very low tailwater depths, and the maximum depth of scour is not necessarily deeper at higher tailwater depths. A set of scaling parameters based on nozzle hydraulic radius, grain size and densimetric Froude number provides for a better scaling of the time variation of the scour parameters. It was also observed in the present study that the coarser particles are deposited towards the downstream end of the scour. In fact the coarsest particles are located on the downward slope of the ridge.