HYDROPLANING OF SUBAQUEOUS DEBRIS FLOWS

We report laboratory experiments that demonstrate that the fronts of s ubaqueous debris flows can hydroplane on thin layers of water. The hyd roplaning dramatically reduces the bed drag, thus increasing head velo city. These high velocities promote sediment suspension and turbidity- current formation. Hydroplaning causes the fronts of debris flows to a ccelerate away from their bodies to the point of completely detaching from the bodies, producing surging. Instigation of hydroplaning is con trolled by the balance of gravity and inertia forces at the debris fro nt and is suitably characterized by the densimetric Froude number. The laboratory flows constrain hydroplaning to cases where the calculated densimetric Froude number is greater than 0.4. The presence of a basa l lubricating layer of water underneath hydroplaning debris flows and slides offers a possible explanation for the long run-out distances of many subaqueous flows and slides on very low slopes.