Hyperconcentrated sand-water mixture flows over erodible bed

Field surveys and experiments in a tilting flume on hyperconcentrated sand-water mixture (SWM) flows were conducted. At slopes slightly smaller than the equilibrium slope, a rapid development of sand bars propagating upstream was observed. These bars consist of an almost horizontal terrace with subcritical flow, a steep lee-side slope with supercritical flow, and a hydraulic jump between subsequent bars. The erosion processes on the bar are limited by the high sand concentrations and flow velocities on the steep slopes. As a result, the erosion rate is more moderate than predicted by the pickup functions used in classical sediment-transport theory. A simple one-dimensional model is presented to study the behavior of triangular schematized bars. From an analysis of the equations it is concluded that an increase in erosion rate requires an increase in bed slope to maintain equilibrium flow conditions. Finally, simulations with the model are compared with observations in the field and in the tilting flume. The agreement appears to be reasonable.