Wave-supported gravity currents in continental shelves
Hooshmand Salemian, Abbas
MetadataShow full item record
Wave-supported gravity currents (WSGC) are one of the most important processes causing cross shelf sediment transport. This dissertation studies the physics behind WSGC in continental shelves using experimental observations. An analytical model for predicting sediment transport due to these events on the shelf is proposed and validated. First, the presence of the sediment bed and its effects on the flow structure is investigated in detail. The presence of sediment on the bed significantly alters the structure of the wave boundary layer relative to that observed in the absence of sed- iment, increasing the turbulent kinetic energy (TKE) by more than a factor of three at low wave orbital velocities and suppressing it at the highest velocities. In the low velocity regime, the flow is significantly influenced by the formation of ripples, which enhances the TKE and Reynolds stress and increases the wave boundary layer thick- ness. In the high velocity regime, the ripples are significantly smaller, the near-bed suspended sediment concentrations are significantly higher and density stratification due to suspended sediment concentration becomes important. In this regime, the TKE and Reynolds stress are lower in the sediment bed runs than in comparable runs with no sediment. The transition between regimes appears to result from washout of the ripples and increased concentrations of fine sand suspended in the boundary layer, which increases the settling flux and stratification near the bed. Second, experimental results for bulk, gradient and flux Richardson numbers are shown and proper scaling is proposed. It is shown that a bulk Richardson number using the maximum buoyancy frequency has a critical value of 1/4 , which is the value theoretically predicted. However, a bulk Richardson number using the average buoy- ancy frequency and a prescribed sediment concentration profile has a critical value of 0.03. This value is nearly one order of magnitude smaller than the assumed critical number of 1/4 from steady tidally driven currents. Third, WSGCs are modeled using an analytical solution to the Navier-Stokes equations, and validated using experimental results. The velocity field is solved for zero slope condition and a down-slope velocity due to gravity is added for slope conditions. The final velocity profiles agrees very well with experimental down-slope velocities. The solution of the model is provided for laminar and turbulent conditions, and for exponential and a constant suspended sediment concentration profiles in the mud layer. Additionally, the criteria for laminar or turbulent regimes are proposed based on the Reynolds number of surface waves. Finally, experimental results for suspended sediment concentration profiles, ve- locity, turbulence and wave boundary layer are compared to field observation and numerical simulations of WSGCs. The predicted down-slope velocity of a layer sus- pended and moved by surfaces waves is 1-2 orders of magnitude smaller than that observed in the field correlated with similar wave conditions.
- Civil engineering