| dc.description.abstract | Recent observations have shown that the onshore return of bars is not always an alongshore-uniform process, but that horns of crescentic subtidal bars can detach and become an isolated feature in the morphology of the coast. Subsequently, it has been observed that these features can propagate shoreward as a spatially coherent structure. These fragments have been termed Shoreward Propagating Accretionary Waves (SPAWs; Wijnberg & Holman, 2007) and when they merge with the intertidal beach can act as a local natural nourishment, causing an alongshore variation in sand supply. The exact role of the onshore migration of SPAWs and the importance within the bar-beach- dune system remains unknown. Therefore, it is the aim of this paper to identify which processes control the migration of a SPAW and how this could affect the nearshore morphology, based on the numerical model 2DBeach. The results show that the migration of the SPAW is predominantly due the wave-non-linearity and that the observed circular currents provide sediment fluxes but did not contribute significantly to the onshore migration of the SPAW. A higher angle of wave incidence is not favourable for the onshore migration of a SPAW, due to the alongshore current and coinciding oblique migration path. The dimensions of the SPAW did not influence the processes regarding the onshore migration of the SPAW. However, a larger SPAW did need a longer period of time to migrate onshore. During low energetic conditions (wave height of 0.5m) the SPAW remained at its location as the waves were not high enough to break over the SPAW. When the SPAW did migrate onshore new insights were formed with respect to a trail of sand at the area the SPAW had migrated across as this has not been observed prior to this study. This implies that the SPAW loses sand over time, while migrating onshore. Therefore, a further offshore cross-shore position acts negatively to the migration success of a SPAW. On this the hypothesis is made that even though the wave conditions are favourable for the migration of a SPAW if the cross-shore distance is to far or the through is to deep the SPAW is unable to reach the inner bar or shoreline as too much sediment is lost during the migration. Due to the fact that the SPAW acts as a perturbation, the coast is more dynamically active, resulting in a faster increase in the alongshore variability than a no SPAW case. However, this does not result in a higher alongshore variability at the end of the simulation. If the SPAW does reach the inner bar or shoreline the amount of volume that the SPAW has at that moment defines the way the SPAW interacts with the existing bar-rip system. A more voluminous SPAW creates an energetic rip current at both sides of the SPAW simultaneously and creates a new bar- rip system. While, a small SPAW, on the other hand, that has lost most of its volume will solely be a nourishment for the existing bar-rip system. Summarizing, the perfect conditions for a SPAW to bring sand onshore are shore normal or slightly oblique incoming waves, which have the height to break over the SPAW. | |
