f The Role of Sediment Composition in the Flow Behaviour of Submarine Landslides

Laboratory experiments on subaqueous mudflows at St. Anthony Falls Laboratory, the University of Oslo and NGI/ICG showed that the flow regime undergoes a remarkable transition as the slurry’s cohesion is varied. Flows of highly cohesive slurries exhibit plug flow in the head, hydroplaning, and rapid stretching, but create only a feeble turbidity current. Flows with low clay content generate a turbulent head and a pronounced turbidity current. Their body is fluidized, with the sand fraction settling out progressively. A simple model indicates that low-to-intermediate cohesion mudflows are a candidate for explaining extended and massive deposits of clean sand that have been found in abyssal plains far from their possible origin. The viability of this proposal depends crucially on the scaling properties of the system, which are discussed in some detail. Adapting the definition of the Reynolds number to flows of cohesive materials, the impact forces of debris flows on structures like pipelines can thus be described in a simpler and more universal way than hitherto, elegantly combining aspects of solid and fluid behaviour. Time permitting, the implications for numerical modelling of such flows and the reasons for the striking difference with sub-aerial debris flows will be briefly touched upon.