Stabilized Formulation for Modeling the Erosion/Deposition Flux of Sediment in Circulation/CFD Models

In field-scale modeling, when the resuspension of sediment is modeled using a hydrodynamic model, a standard and common approach is to add a resuspension flux as the bottom boundary condition in the transport model. In this study, we show that the way of simply imposing an empirical bottom erosion formula as the flux is actually unrealistic. Its inability to stabilize the sediment concentration can cause excessive suspension fluxes in some extreme cases. Moreover, we present a modified erosion/deposition formula to model the resuspension of sediment. The formulation is based on volume conservation in the presence of erosion/deposition near the bottom. By taking into account the prescribed dry density of the bed material, the proposed formulation is able to produce realistic near-bed concentrations while ensuring model stability. The formulation is then tested in a one-dimensional vertical model and field modeling cases using a three-dimensional coastal circulation model. We show that the modified formulation is particularly important in modeling mud resuspension subject to the large bottom stress, which can be a result of waves or a strong river discharge.

NUMERICAL INVESTIGATION OF BORE HOLE FILLING VOLUME IN A COASTAL AREA

A numerical study is carried out to determine the parameters controlling the filling of a bore hole in Alderney Race. Our final goal is to identify the configuration minimizing the filling. We performed a one-way coupling between a coastal circulation model and a discrete element model for that purpose. Simulations used spherical particles and a monopile technology. We show that: as long as the distance between the hole and the residuals remains smaller than twenty meters, the flow direction plays a negligible role; interaction between moving particles and bottom roughness leads to a slight increase of the filling; the impact of ambient sediments strongly depends on seabed morphology and current effects.

Model Study of the Water Exchange in a Shallow Bay

Both the tidal prism and the water exchange duration can be used as the index of examining the residence time of the pollutants in a semi-enclosed bay, and it is significant to predict the long-term variation of water quality. A 3-dimensional coastal circulation model is used to reproduce the tidal regime in the Shenhu Bay. Simulated tidal currents are examined first, and the then the tidal prism is also compared to observations. Based on the validated simulations, the water exchange duration is further estimated by means of the numerical experiment. The simulations show that the half-exchange and 80%-exchange time are about 14 and 30 d, respectively.