Recent numerical investigations reveal the existence of four distinct regimes of wave-induced fine sediment transport ranging from well-mixed transport, to the formation of a lutocline, and eventually a complete flow laminarization over a range of sediment availabilities and settling velocities. The numerical model is based on an Eulerian-Eulerian two-phase formulation simplified for fine sediment (small particle response time) while resolving all the scales of turbulence-sediment interactions. This project will further investigate four critical science issues related to these regimes via numerical simulations. Firstly, a complete phase map will be constructed of flow regimes as a function of wave Reynolds number, bulk Richardson number (sediment availability) and nondimensional settling velocity with a series of carefully designed simulations. The results will highlight the major differences between the tidal and wave boundary layers in response to sediments. Secondly, with a better understanding of the onset of laminarization, the model will be enhanced to support non-Newtonian rheology in order to study the interplay between rheological stress and turbulence modulation in determining the transition of flow regimes and hydrodynamic dissipation. Thirdly, mean current will be added to the simulations. Wave-current interaction may enhance the mud layer thickness and transport, as observed in a recent field study at the shelf of Waiapu River (New Zealand). However, if the current is too strong, sediments can be re-entrained and become well-mixed and hence the formation of gravity flow is prevented. Finally, the model will be expanded for transport of coarser grains. Concurrent transport of clay and silt due to decreasing wave Reynolds number will be first studied. Next step will be to simulate a complete polydispersed system using a direct quadrature method of moments approach. Polydispersed simulation efforts allow insights into the processes causing the observed microstratigraphy in mud-dominant environments.
Several prior field observations on continental shelves reveal a variety of seabed states due to wave-current driven sediment transport. The occurrences of these seabed states have several critical implications. For example, the formation of a lutocline indicates trapping of fine sediments near the bed and the resulting large density anomaly may yield significant offshore sediment transport on the shelf through wave-supported gravity flows. When surface waves propagate over a muddy seabed, high wave dissipation rate is often observed during the waning stage of a storm as the fluid mud layer becomes laminarized. A recent microstratigraphy study of mud deposits suggests a three-part sedimentary microfabric that can be associated by processes occur during wave-supported gravity flow events. The main challenges of modeling wave-induced fluid mud transport are the coupling between sediment and turbulence, the transitional nature of turbulent flow, rheology and the polydispersed nature of transport. This research addresses these challenges and the results will be valuable in further interpreting critical processes observed in the mud-dominant coastal environment.
The project will improve our understanding of the resuspension and delivery of fine sediment across the continental margin, which is a critical element of the sediment source to sink study. This study will also improve the ability to predict the surface layer properties of the seabed which is critical to underwater exploration and wave prediction. Using wave tanks already available, a hands-on laboratory experiment to visualize the existence of wave boundary layer and the intermittent nature of the mixing process near the bed will be developed by undergraduate students. This newly-designed experiment will be used in Engineering outreach activities taking place annually during the summer session of each institution. In Year 3, this experiment will be added to the curriculum in the undergraduate fluid mechanics laboratory at U. Delaware.
Transport mode of fine sediment near the seabed: Study to advance understanding of carbon cycling, coastal/marine water clarity and ecosystem dynamics Delivered by the rivers, fine sediments are the main vehicle to carry terrestrial organic carbon, nutrients and contaminants to the ocean. On the decadal timescale (or longer), the amount of these fine sediments delivered to the ocean also reflects the anthropogenic impact on the watershed. Due to many counteracting oceanographic factors, fine sediments delivered directly by the rivers cannot be too distant from the land and their initial deposition is located in estuaries, bays and inner shelves. Their subsequent journey to the deeper ocean is then driven by sediment resuspension process due to waves and currents. During this journey, the transport of these fine sediments further influences water clarity, ecosystem dynamics, carbon cycling and coastal geomorphology. The primary goal of this study is to quantify the occurrence of different modes of fine sediment transport during resuspension in the coastal environment using theories and computer simulations based on fluid mechanics principles. As discussed next, the main outcomes of this study advance our understanding and predictive capability in carbon cycling, water clarity and ecosystem dynamics in the coastal environments. A series of computer simulation of fine sediment transport in the bottom boundary layer near the seabed driven by waves reveal the existence of three distinct transport modes associated with sediment availability and wave intensity. The amount of resuspended sediment in the water column, quantified by sediment availability, is directly associated with critical shear stress of erosion and sediment settling velocity. For more rigid and consolidated sediment bed, critical shear stress of erosion is large and dilute transport mode I is obtained (see Figure). Sediments are well-mixed in the water column due to high turbulence driven by waves and currents. In this mode, water clarity can be lower and the direction of sediment transport is determined by the oceanic currents, which is mainly parallel to the coast. For newly deposited fine sediment without experiencing sufficient consolidation, critical shear stress of erosion is lower, more sediments are suspended from the bed and the two-layer transport mode II is obtained (see Figure). In this case, suspended sediments are sufficient to attenuate flow turbulence in the boundary layer through sediment-induced stable density stratification. Such mechanism gives the well-known two-layer flow structure in stratified flow with a sharp negative sediment concentration gradient, called lutocline, located at about 5~10 cm above the bed. Lutocline effectively separates the lower turbulent transport layer from the upper layer where mixing is suppressed. When transport mode II occurs, exchange of nutrients and gas between the seabed and the upper water column is suppressed. Moreover, large amount of sediments are accumulated within the bottom O(10) cm, which drives downslope gravity flow and triggers offshore delivery of fine sediments. For very soft, poorly consolidated mud bed with very low critical shear stress of erosion, very large amount of suspended sediments can significantly suppress turbulence and trigger the laminarized transport mode III (see Figure). Sediments start to settle onto the bed due to diminishing turbulence. However, depending on the structure of aggregates of cohesive sediment, hindered settling may become dominant and results in the formation of large amount of less mobile, unconsolidated sediment suspension, called fluid mud. The occurrence and dynamics of these transport modes need to be parameterized in large-scale coastal modeling systems in order to better predict their impacts on carbon cycling, ecosystem dynamics and coastal geomorphology.
