Numerical modeling of cross-shore sediment transport and sandbar migration

Nearshore processes on barred beaches are studied with a process-based numerical model. The two major goals of the study are to expand the body of knowledge about nearshore processes on barred beaches gaining a better understanding of the physical mechanisms affecting bar migration events and to enhance the numerical model in order to accomplish realistic simulations of bar migration events. The numerical model is used to study the effect of physical processes on the hydrodynamics and morphodynamics in the nearshore environment.The hydrodynamics on barred beaches and mechanics of sediment transport related to bar migration are studied on storm time scales. The numerical model system consists of a linear spectral refraction-diffraction model, REF/DIF S, a quasi-3D nearshore circulation model, SHORECIRC, energetics-based sediment transport models, and a morphological evolution model. A laboratory experiment conducted in the Delta Flume, Netherlands which had an erosive test stage with offshore bar migration followed by an accretive test stage with onshore bar migration is used for modeling purposes and verifications.The sediment transport is driven by the near-bed wave orbital velocities and the undertow current at the bottom. For that reason, accurate predictions of nearshore hydrodynamics are important for predictions of morphodynamics and successful simulations of bar migration events. A number of enhancements are made to the wave and circulation modules of the numerical model system specifically for simulations on barred beaches. The model modifications and enhancements are: (1) a combined breaking wave parameter with a spatial variation in the wave model (2) a method accounting for breaking wave persistence in the wave model (3) a method accounting for the new breaker roller lag in the wave model (4) the dynamic pressure component in the radiation stress forcing (5) a roller contribution with different depth variation options for the short wave forcing in the circulation model (6) wave height instead of water depth as the turbulent length scale in the eddy viscosity calculations in the circulation model (7) a slope term for the default sediment transport formula.The depth-integrated cross-shore momentum balance in the surf zone shows the strong dependence of the mean water level on radiation stresses. The effect of surface shape parameter and the roller face angle on radiation stress and mean water level predictions are investigated. In reality, the organized wave energy is transferred to roller development over a transition distance and the roller does not immediately contribute to the radiation stresses therefore, showing the importance of the roller lag mechanism for mean water level predictions.The cross-shore variation of the vertical momentum balance is studied to observe the variation of forcing agents of the undertow current. The cross-shore pressure gradient is the most dominant forcing term affecting the depth structure of the undertow current. The effect of different depth variations of the roller contribution to the short wave forcing on the undertow current is investigated. The mechanism accounting for breaking wave persistence and the mechanism accounting for the roller lag are shown to be important for predictions of the undertow currents on barred beaches.The performance of the model in predicting offshore bar migration and onshore bar migration events on storm time-scales is investigated. The skewed wave orbital velocities are introduced to the linear wave model by an empirical parametrization method and are found to contribute strongly to the onshore bar migration. The enhancements made to the wave dissipation and roller are found to significantly affect the predicted migration of the bar as well as the maintenance of the trough.