Numerical Modeling of the Undertow Structure and Sandbar Migration in the Surfzone

A process-based 3D numerical model for surfzone hydrodynamics and beach evolution was established. Comparisons between the experimental data and model results proved that the model could effectively describe the hydrodynamics, sediment transport feature and sandbar migration process in the surfzone with satisfactory precision. A series of numerical simulations on the wave breaking and shoaling up to a barred beach were carried out based on the model system. Analyzed from the model results, the wave-induced current system in the surfzone consists of two major processes, which are the phase-averaged undertow caused by wave breaking and the net drift caused by both of the nonlinear wave motion and surface roller effect. When storm waves come to the barred beach, the strong offshore undertow along the beach suppresses the onshore net drift, making the initial sandbar migrate to the seaside. Under the condition of calm wave environment, both the undertow and net drift flow to the shoreline at the offshore side of the sandbar, and then push the initial sandbar to the shoreline. The consideration of surface roller has significant impact on the modeling results of the sandbar migration. As the roller transfer rate increases, the sandbar moves onshore especially under the storm wave condition.

Numerical test of scale relations for modelling coastal sandbar migration and inspiration to physical model design

Physical model experiments on sandbar migration are widely conducted in the wave flume to reveal its hydro-sediment dynamic mechanisms.Selecting an appropriate scale relation is an important part of designing a laboratory experiment.However,evaluating the scale effect is complicated since it is impractical to design wave flume experiments based on strict geometry scales derived from the prototype beach or other laboratory beaches.Here,a process-based numerical model is used to test the Dean and Shields similitudes by comparing sandbar migration and sediment transport in different undistorted model scales using natural sediments.The numerical model provides excellent predictions for both offshore sandbar migration on a real-world beach and onshore sandbar migration in the wave flume.Consistent with previous studies,sandbar offshore migration and suspended sediment transport are similar when the Dean similitude is fulfilled.Sandbar onshore migration and bedload transport can be well reproduced only if the Shields similitude is achieved.Although previous laboratory experiments of sandbar migration generally used the Dean similitude,it is found that the Shields similitude is effective and indispensable for modelling sandbar onshore migration,which will be underestimated if the Dean similitude is only considered.Each scale relation cannot be used simultaneously for modelling both onshore and offshore sandbar migration in physical models.