Modeling the equilibrium of tide-dominated ebb-tidal deltas

Abstract

This study focuses on identifying physical mechanisms that lead to symmetric, tidedominated ebb-tidal deltas. An idealized morphodynamic model is developed and analyzed to demonstrate that these deltas can be modeled as morphodynamic equilibria (no evolving bathymetry). It is assumed that the large-scale alongshore tidal currents are small compared to the cross-shore tidal currents, that waves have shore-normal incidence, that the tidal velocity profile over the inlet is symmetric with respect to the midaxis, and that the Coriolis force can be ignored. The modeled tidal hydrodynamics are characterized by an ebb jet during the ebb phase of the tide and a radial inflow pattern during flood. Two residual eddies are formed. The mechanism behind these current patterns is explained with vorticity concepts. The modeled bottom patterns are similar to those of observed symmetric tide-dominated ebb-tidal deltas. In the center of the tidal inlet an ebb-dominated channel is observed that branches further offshore into two flooddominated channels. At the end of the ebb-dominated channel a shoal is present. Varying the tidal prism, the width of the tidal inlet, the wave height, and the bed slope coefficient in the sediment transport formulation within the range of observed values leaves these patterns qualitatively unchanged. However, the exact extent and shape of the modeled deltas are affected by these parameters. Compared to observations, the modeled ebb-tidal delta is smaller and the ebb-dominated channel is shorter. The observed exponent in the power law relation between sand volume of the delta and the tidal prism is recovered and explained with the model.