Experimental investigation into the reduction of erosion of sand at high flow velocities

Significant reduction of the rate of erosion of a sand bed is obtained when sand is mixed with a small amount of bentonite. In previous experiments this behaviour has already been shown for relatively low flow velocities. In this case, the erosion process is dominated by grain-by-grain erosion, which is characterised by low ratios of the erosion velocity and permeability (v_e/k < 3). It is unknown whether these reductions in the erosion process also occur at relatively high flow velocities, where dilatancy-reduced erosion dominates (v_e/k > 3). Experiments were executed in a tilting flume to investigate the erosion rate of the sand–bentonite mixtures. In 13 different tests, the dry volume percentage of the bentonite additive, the diameter of the sand particles and the depth-averaged flow velocity were varied. The depth-averaged flow velocities ranged from 1 to 2 m/s and all erosion tests were performed under supercritical flow conditions. The experiments show that the bentonite additive did not influence the strength characteristics of the sand however the permeability did decrease significantly. This proves that the significant decrease of the erosion rate was caused by the decrease of the permeability of the sand and that the test conditions were in the dilatancy-reduced regime.