Experimental study on self-accelerating turbidity currents

A self-accelerating current is a particle-driven gravity flow moving on a sloping bottom whose velocity increases in the downstream direction as a result of increasing suspended sediment concentration due to sediment entrainment from the bed. This implies that the net balance between deposition from the current onto the bed and erosion into the flow must be favorable to the latter; thus, a larger mass of particles is being picked up into suspension than is settling out. The self-accelerative stage cannot continue indefinitely. Either the downstream bed slope drops off to the point where self-acceleration cannot be maintained or an autosuspensive stage may be reached where the net balance between deposition and erosion is zero and the channel bed is partially or completely free of alluvium. Once such a state is reached on a constant bed slope, the current can persist indefinitely without any external supply of energy other than the potential energy offered by the slope itself. This paper documents experimental turbidity currents composed of lightweight plastic particles ranging from 20 to 200 mm with a specific density between 1.3 and 1.5. These particles were either noncohesive or slightly cohesive. The experiments were performed in a 15-m long flume with a bottom slope of 0.05. Self-acceleration of the head of the flow was achieved in some of the tests reported here. Measurements of velocity and suspended sediment taken at different stages of head evolution document this self-acceleration. In addition, these measurements are in agreement with previous empirical studies relating to head thickness, concentration, velocity, and water depth. Stratigraphic analysis of the deposit shows the key role bed material plays in determining whether a given turbidity current will or will not accelerate. This factor ties the dynamics of a self-accelerating current to the existence of deposits laid down by antecedent currents. The conditions of the present tests appear to fulfill previous autosuspension criteria relating to flow velocity, particle settling velocity, and bed slope. Densimetric Froude number similarity analysis is used to estimate equivalent parameters for field scale turbidity currents.