Influence of the Initial Beach Profile on the Sediment Transport Processes During Post-Storm Onshore Bar Migration

Onshore bar migration is a characteristic bar behavior during post-storm beach recovery. The present large-scale experiments, feature bichromatic wave groups over an initially steep (1:15), fully-evolving beach. The same accretive wave condition is applied on two different post-storm beach profiles featuring outer and inner bars. They are characterized by a larger (smaller) shoreline erosion and a larger (smaller) outer breaker bar located farther away from (closer to) the shoreline depending on the larger (smaller) energy of the storm condition. After a considerable post-storm recovery time, similar equilibrium profiles are obtained, stressing the link between wave condition and equilibrium beach configuration. However, the evolution toward the equilibrium is different and depends on the initial morphological condition (post-storm beach profile). After the larger storm, the morphological evolution is termed accretive merging (AM) and characterized by merging of the two bars (outer bar dissipation). After the smaller storm, the morphological evolution denoted as accretive non-merging (AN) is characterized by onshore migration of the two bars with constant distance between them (bar maintenance). This study focuses on processes around the outer bar. During AN it features wave breaking, causing large suspended net offshore transport. AM, in contrast, mainly features bedload related to short wave asymmetries and low decomposed net transport rate magnitudes. High suspended net offshore transport occurs solely onshore of the outer bar trough. This causes filling of the bar trough and bar dissipation during migration. Additionally, processes around the outer bars are linked to accretion onshore of the bars and at the shoreline.