Experimental Observations on the Initiation of Sand-Bed Erosion by an Upward Water Jet

Erosion of a saturated sand bed by a vertically upward water jet was studied experimentally. Direct observations on the behavior of sand-bed erosion were obtained by increasing the flow rate at a small increment until fluidization developed to the surface of the sand layer. With the increase in flow rate, the sand bed underwent different stages: no cavity, a stable cavity above the jet, an unstable cavity propagating upward, and full fluidization. The mechanism for the cavity evolution was investigated, and an analytical model was developed to predict the critical flow rate for full fluidization. The model predictions compared well with the measurements in the experiments with different sand-layer thicknesses.