Internal Structure of a Self-Accelerating Turbidity Current

Abstract

A self-accelerating current is a particle-driven gravity flow moving on a sloping bed whose velocity surges in the downstream direction as a result of increasing suspended sediment concentration entrained from the bed. This concept although introduced decades ago has proved elusive to be measured, but it is critical to explain the increasing evidence that turbidity currents are arguably the leading process for conveying sediments down the continental slope and beyond. It implies that both flow velocity and suspended concentration increase due to a feedback loop up to the point where some external condition restrains the process. The higher the velocity is, the larger the grain size and sediment concentration that can be kept in suspension; conversely, the denser the flow is, the faster it moves downslope. This work documents laboratory experiments of plunging (hyperpycnal) turbidity currents composed of noncohesive particles ranging from 20 to 200 μm with a specific density of 1.5. The experiments were performed in a 15-m-long flume with a bottom slope of 5%. Self-acceleration of the front and body of the flow was achieved during the experiments and documented using both video recordings and ultrasonic velocity profilers (UVP). The UVP backscatter signal was calibrated with physical samples and used for estimating suspended sediment concentration profiles. The plunging mechanism played a lead role during the experiments by purging the flow from suspended material beyond its capacity and from coarse grains beyond its competence. Self-acceleration was observed in some of the performed tests. Acceleration of the current front was clearly recorded with the video cameras, while the UVP recordings captured acceleration within the current body along the flume. The measured depth-averaged velocity in the current body was higher than the velocity of the current front, thus indicating an internal circulation pattern that shapes the current front and head. Our experiments indicate that a supercritical flow regime is a necessary but not a sufficient condition for the occurrence of a self-accelerating turbidity current. Whether the current self-accelerates or not on a certain sloping bottom seems to be determined by flow discharge, concentration, size of suspended material, and availability of entrainable sediment at the bed. Deposits formed during the experiments showed that turbidites generated by self-accelerating currents presented both less pronounced thinning and fining downstream rates than those generated by decelerating or depositional currents. This signature may be used in the sedimentary record to discern the turbidite forming flow.