Erosion of valley-floor bedrocks by sediment-laden flows

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(1) No quantitative studies on bedrock erosion due to sediment-laden flows have been conducted. In order to obtain a rudimentary knowledge on this subject a laboratory experiment on bedrock erosion by water flows without sediments was first performed using a recirculating flume. Artificial rocks made of a mixture of cement, sand, and water were used to simulate actual rocks in the field. In these simulated rocks having different strengths a slit was indented with various openings. To examine a flow in and around the slit, a visualization experiment was also conducted using neutral buoyant floats. When water flows into the interior of the slit, water particles hit the lower edge of the slit to act compressive force on the edge, so that this part is eroded more rapidly than the other part. The erosion is more vigorous as the opening of the slit becomes larger. The effect of opening tends to be more marked as the rock strength decreases. When erosion of the lower edge attains a certain critical value, water particles act no more compressive force on the edge. Difference in erosive force acting on the upper and lower edges becomes small, resulting in constancy of the relative height between the two edges.(2) An experiment using a recirculating flume was performed to investigate bedrock erosion due to sediment-laden flows with a constant speed. Rock samples used here were collected from the field and had different strengths. The samples were subjected to shear force of the flows. The result showed that (1) the erosion occurs linearly with time, i.e., no temporal variation in erosion rate is present, and (2) the relationship between average erosion rate, R , and compressive strength of rocks, S_c , can be expressed by R = 0.17(4.3 - 1n S_c ) where the units of R and S_c are mm/hr and kg/cm^2, respectively.