Cross-shore morphodynamics of intertidal bars

Abstract

The beach recovery process determines the resilience of a sandy coast and is an important aspect of the coastal safety. Sediment stored underwater due to storms is transported onshore by the migration of subtidal and intertidal bars under mild wave conditions. The intertidal zone is an im-portant interface, connecting the marine and aeolian zone and facilitating the transition of hydro-dynamic to aeolian sediment transport. The aim of this study is to investigate the cross-shore mor-phodynamics of intertidal sandbars. The dominant cross-shore sediment transport processes can mainly be divided into surf zone and swash zone processes. The surf zone processes are primarily determined by the balance of wave nonlinearities, undertow and infra-gravity waves. In the swash zone, the cross-shore sediment transport is determined by the balance between the turbulent uprush and the gravity induced backwash. A recent terrestrial laser scanning (TLS) measuring campaign conducted at Kijkduin, the Nether-lands, provided new insight in the intertidal bar behavior. The results of one cross section are ana-lyzed. In a period of 6 weeks, two distinctive intertidal bars formed, grew and migrated onshore during mild wave conditions and eventually eroded again during a storm. Within 5 days, the upper intertidal bar migrated onshore over a distance of 25m and grew with a height of 0.3m, attributed to swash zone processes. Onshore sediment transport fluxes reached values of nearly 2 m3 per meter width in one tidal cycle.
The findings are compared with two XBeach models (surf beat model and hydrostatic swash model) which are used to reproduce the observed morphological behavior of the upper intertidal bar. Both models partly reproduce the onshore migration but show deviating results regarding the final growth of the intertidal bar. In contrast to the surf beat model, the morphological changes in the hydrostatic swash model are primarily induced by swash zone processes, which is comparable to the processes in the TLS measurements. Finally, a conceptual model is developed in which four intertidal bar regimes are classified based on the tidal water level. The distinction determines the dominant cross-shore processes for the for-mation, migration, growth and destruction of intertidal bars. The model shows that the swash zone processes are dominant for the onshore migration and growth of intertidal bars in the overwash regime, while the surf zone processes are dominant in the submersion regime. The findings presented in this study provide a better understanding of the intertidal bar behavior. Although the XBeach models did not reproduce the observed behavior completely, there are some pronounced similarities. Further research is required to increase the knowledge of intertidal bar behavior at a variety of sandy coasts and to improve the performance of rocesses-based models like XBeach.