The effect of sediment stratification on tidal dynamics and sediment transport patterns

Abstract

The western tip of southwest Korea is characterized by a tidally dominated, turbid, coastal environment. There are well-developed tidal flats along the coast and around islands, together with offshore sand ridges several tens of kilometers long in the west. The effects of bottom boundary layer (BBL) sediment stratification on sediment-transport and tidal dynamics in this environment were examined using a sediment transport model coupled with a three-dimensional tidal hydrodynamic model. Model experiments using two scenarios, with and without the effect of sediment-induced stratification, showed that BBL sediment-stratification influenced the spatial distribution and reduced the magnitude of net sedimentation. The presence of a sediment-stratified BBL also led to a reduction in suspended sediment fluxes and an increase in the vertical gradient of sediment concentrations in the water column. These variations occurred because sediment-induced BBL stratification leads to not only a reduction in bottom shear stress but also a decrease in buoyancy production of turbulent kinetic energy and an associated dampening of turbulence. The significant reductions in turbulence and bottom shear stress result in changes to the vertical-current structure of the M2 tide, including alteration of the tidal ellipse configuration and an increase in vertical shears of the tidal current amplitude and phase. These reductions also lead to a slight increase in tidal amplitudes due to the decreased tidal-energy dissipation. The model results indicate that feedback between the sediment-transport dynamics and hydrodynamics is an important factor in modeling sediment transport dynamics in tidally dominated, turbid, coastal environments. Such environments include the west coast of Korea and the Yellow Sea. A suitable approach to simulating the effects of this feedback may be the use of a modified bottom-drag coefficient as a stability function, together with the inclusion of the effect of sediment stratification on the hydrodynamics. Copyright 2005 by the American Geophysical Union.