EXPERIMENTS ON RAPID DEPOSITION OF SAND FROM HIGH-VELOCITY FLOWS

Observations of sand transport and rapid deposition by strong sand-lad en surges in an open channel, by means of highspeed motion pictures, r evealed that the dominant effect is a process by which a laminar shear ed layer, with sand concentrations approaching the threshold for immob ilization by grain interlocking, develops as suspended sand becomes co ncentrated near the base of the flow. The laminar sheared layer climbs vertically with time as sediment is progressively immobilized at the base of the layer and added at the top from the overlying turbulent fl ow The mobility of the laminar sheared layer is probably enhanced by c ontinuous upward flow of interstitial water that is trapped within the layer as the layer accumulates. As the flow weakens and the concentra tion of suspended sand in the flow decreases, the laminar sheared laye r thins and/or becomes immobilized, giving way to the familiar weak tr action transport on a well-defined immobile sand bed. Particles reside in the laminar sheared layer only briefly, and move only a short dist ance before immobilization; the laminar sheared layers seem not to sha re essential features with traction carpets. Much of the coarser, stru ctureless lowermost parts of thick turbidites might have been emplaced by processes not unlike those described here. Our results suggest tha t the entire turbidity current need not be of extremely high density f or an initial deposit to be formed rapidly from extremely high near-be d sediment concentrations.