Subaqueous debris flow behaviour and its dependence on the sand/clay ratio: a laboratory study using particle tracking

Subaqueous debris flows were studied through a set of laboratory experiments examining the behaviour within the flow and the effects of varying the clay–sand mass fraction at a fixed water content. They confirm earlier experiments where mobility is enhanced by a lubricating layer and run-out is largely influenced by other mechanisms than the rheological properties of the debris flow itself for high clay content flows. Through particle tracking by high-speed video, a better understanding of the mobility of subaqueous debris flows has been established. For highly coherent flows, the heads of the flows were hydroplaning. In many cases, hydroplaning stretched, and eventually ruptured, the flowing mass. Fitting the observed velocity profiles to Bingham rheology and by comparing with standard rheological measurements of the pre-flow slurries demonstrates very substantial weakening and softening in the bottom shear layer, but not in the overlying plug layer. This effect is attributed to mixing with ambient water. Weakly coherent flows show an intensively fluidized front; the entire head breaks up into a turbulent flow. Behind the head, one finds a dense flow layer where sand grains drop out during the flow and form a depositional layer. The highest mobility is found for slurries with a low yield stress combined with sufficient competence to allow a moderate settlement of sand during flow. Similar mechanisms are likely to operate in natural subaqueous debris flows and may thus provide an additional explanation for the long run-out distances of those flows.