Debris flow waves

We demonstrate that a numerical model based on mixture theory can capture the break-up of a flow into multiple waves formed of relatively dry granular fronts followed by more watery tails. In doing so it, it is shown that no variations in topography are necessary for the combined flow of solid and fluid phases down an inclined channel of constant gradient to break up into surge waves. The observed wave structure is consistent with field observations. The formation of small levees is also evident in our simulations. The production of levees is enhanced by the geometry of the channel. At the flow front, material is pushed towards the edges of the flow, onto the banks of the channel. The fluid phase drains down the banks back into the centre of the channel faster than the solid phase. As a result, a small amount of statically stable solid material is deposited at the edge of the flow, in the form of small levees.