Showing papers by "H.E. de Swart published in 2000"

Multiple morphodynamic equilibria in tidal embayments

Abstract: The possible morphodynamic equilibria of tidal embayments are investigated within the framework of a one-dimensional model The equilibria are defined by a steady profile of the erodible bottom. The extension with respect to earlier studies is that the embayments have arbitrary lengths L with respect to the tidal wavelength. This implies a much richer dynamics due to the possibility of tidal resonance and new sediment transport contributions that are caused by internally generated overtides and residual currents. If the system is only forced by an externally prescribed M2 tide at the seaward boundary, a unique morphodynamic equilibrium is obtained for all embayment lengths smaller than the frictional length scale of the tide. Bottom friction causes tidal resonance to occur for a shorter length than a quarter of the frictionless tidal wavelength. This shift is smaller than would occur in the case of a fixed bed profile since the equilibrium condition induces larger water depths. If an externally prescribed overtide is added to the forcing, more than one type of morphodynamic equilibria can be found. For L values smaller than the M4 resonance length scale the bottom profiles are strongly concave, with locally large water depths, and the water motion resembles a standing tidal wave. For longer embayments another type of equilibria, characterized by a weakly concave bottom profile and a traveling tidal wave, appears. For sufficiently strong amplitudes of the externally prescribed M4 tide, multiple morphodynamic equilibria are found. The maximum L, beyond which morphodynamic equilibria cease to exist, decreases with increasing influence of external overtide and bottom friction. These model results show an overall good agreement with field observations.

Tidal and morphologic properties of embayments: Effect of sediment deposition processes and length variation

Abstract: An idealised morphodynamic model is analysed to study the properties of both the vertical and horizontal tide in a rectangular embayment with an erodible bottom. Forcing is due to a prescribed tidal constituent at the entrance. The model is based on the shallow water equations, sediment is transported as suspended load and the divergence of the tidally averaged sediment flux determines the bottom evolution. It is further assumed that the embayment is in morphodynamic equilibrium, i.e., the bottom profile is steady. The main results are that the tidal and morphologic properties strongly depend on the sediment deposition process and on the embayment length. A depth-dependent deposition term causes bed profiles to become convex. With increasing embayment lengths the profiles become concave and tidal resonance is found at a length which is slightly smaller than a quarter of the frictionless tidal wave-length. This shift is less than in case of a fixed bed because water depths increase with increasing embayment lengths. The morphodynamic equilibria do not exist for lengths larger than the frictional length scale of the tide.