Depositing and Eroding Sediment-Driven Flows: Turbidity Currents

Abstract

Laboratory experiments were conducted to observe the behavior of turbidity currents in the vicinity of the slope break between a submarine canyon and its associated depositional fan. Sediment-laden and saline hydraulic jumps were produced. The vertical structure of the currents was found to depend on flow regime. Velocity and sediment concentration profiles showed a good degree of similarity as the current evolved in the downslope direction. The grain size of the suspended sediment did not affect the vertical velocity distribution in weakly depositional flows. Grain size showed a clear tendency to decrease vertically. Smaller grains were more uniformly distributed vertically. The saline and turbid hydraulic jumps showed similar characteristics. The amount of water entrained by the flows while going through a jump was small. The change in flow regime caused a marked reduction of the bed shear stress downstream of the jump. The thickness of the deposit left immediately downstream of the jump increased as the ratio of the bed shear velocity downstream of the jump to the particle fall velocity decreased. A clear correlation between turbidite thickness and grain size was observed. The thickness of the turbidites was seen to decrease roughly exponentially with distance. Any increase in the thickness of the deposits near a canyon-fan transition will not be due to the break in slope itself but rather due to the hydraulic jump induced by the break. It was observed that a fine-grained turbidity current of sufficient strength can entrain substantial amounts of bed sediment into suspension. The rate of sediment entrainment showed a clear tendency to increase with current velocity. Prominent bedforms developed during the experiments. The bedforms were found to have an important effect on boundary shear stress. The effect of the bedforms was removed successfully by modifying the technique of Nelson and Smith (1989) for open channel flows. Two formulations to evaluate sediment entrainment rates were obtained using data from equilibrium open channel suspensions, one for well-sorted sediment and the other for poorly-sorted sediment. An integral model for steady, spatially developing turbidity currents driven by poorly.sorted sediment was developed and used as a basis for the analysis of the experimental results.