Experiments on turbidity currents over an erodible bed

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Abstract
Experiments were conducted to determine the behaviour of turbidity currents laden with non-cohesive silt (silica flour) moving down a slope the bed of which is covered with similar silt. Each current was sustained with constant inlet conditions for six to eight minutes. The motion of the head was not studied; measurements were concentrated on the continuous part of the current that was essentially constant in time but developing in space. Only supercritical currents were studied. The currents were free to erode sediment from, and deposit sediment on, the bed. Measurements of vertical profiles of downstream velocity and sediment concentration allowed for the development of approximate similarity relations. These relations can be used to estimate most of the shape factors that occur in the vertically-integrated equations of motion. The values do not deviate grossly from those found by using a "top-hat" function, i.e. a step function, such that velocity or concentration equals its average value in the layer and vanishes outside the layer. Coefficients of water entrainment from above, sediment entrainment from the bed, and bed resistance were determined. Where possible, the data were compared with other data and relations in the literature. Several modified relations were developed. Several researchers have suggested that a continuous supercritical turbidity current would ultimately either "ignite" (self-accelerate and entrain sediment in the net) or subside (decelerate and deposit sediment in the net) in the downstream direction. This ignition hypothesis has been offered as an explanation for the formation of submarine canyons. The experimental facility was too short to allow for a test of ignition. If the data obtained are used in conjunction with the equations of motion to extrapolate the experimentally measured currents farther downstream, however, ignition is predicted in several cases.