Research Highlight: Mohamad Rouhnia
Mohamad Rouhnia is a fourth year PhD student working in the area of environmental hydraulics and sediment transport. He received his BS and MS degrees from the University of Tehran in Iran, and he is currently working on an experimental study examining influence of interface shear instabilities in the removal of sediment from buoyant plumes.
Current Research Summary: Dispersal of terrestrial sediment in an estuary is highly dependent on the vertical removal of sediment from the river plume. Traditionally, sediment removal rates from buoyant river discharges have been scaled using settling velocity of sediment particles. However, for buoyant mud plumes lofting over saltwater, removal of sediment is potentially promoted by processes such as flocculation of cohesive particles, convective sedimentation triggered by diffusion or settling of particles, and interfacial mixing caused by the shear. In the presence of shear, interfacial turbulence mixing is likely to enhance sediment removal from bottom of the plume. The goal of this research is to improve modeling of vertical downward sediment flux from buoyant river plumes under the effects of shear instabilities through physical experiments. We specifically seek the answer to the question: Is the effect of shear instabilities significant enough to compete with flocculation enhanced individual particle settling when the two are simultaneously occurring? We hypothesize that shear instabilities increase the deposition rates from river plumes due to the turbulent mixing at the interface. To test our hypothesis, a set of experiments are designed with an initially stagnant lower layer but flowing upper layer, carried out in a stratification flume that allows for shear instabilities to be involved in addition to floc settling. Comparison of floc settling velocities and settling velocities extracted from the experiments, under the influence of shear instabilities, is the tool to test the hypothesis. Floc measurements are made using a separate floc camera system setup and physical floc samples. Floc sizes are then used to calculate floc settling velocities. Calculated velocities are compared to the actual removal rates obtained from the experimental measurements of concentrations.