Alongshore variability in crescentic sandbar patterns at a strongly curved coast

Sandbars, submerged ridges of sand parallel to the shoreline, tend to develop crescentic patterns while migrating onshore. At straight coasts, these patterns form preferably under near-normal waves through the generation of circulation cells in the flow field, whereas they decay under energetic oblique waves with associated intense alongshore currents. Recently, observations at a man-made convex curved coast showed an alongshore variability in patterning that seems related to a spatiotemporal variability of the local wave angle (Sand Engine). Here, we aim to systematically explore how coastline curvature contributes to alongshore variability in crescentic pattern formation, by introducing local differences in wave angle and the resulting flow field. A nonlinear morphodynamic model was used to simulate the patterns in an initially alongshore uniform sandbar that migrates onshore along the imposed curved coast. The model was forced by a time-invariant and time-varying offshore wave angle. Simulations show that the presence of patterns and their growth rate relate to the local breaker angle, depending on the schematization of the offshore angle and the local coastline orientation. Growth rates decrease with increasing obliquity as both refraction-induced reductions of the wave height as well as alongshore currents increase. Furthermore, simulations of variations in coastline curvature show that patterns may develop faster at strongly curved coasts if this curvature leads to an increase in near-normal angles. This implies that beaches where the coastline orientation changes substantially, for example, due to km-scale nourishments, become potentially more dangerous to swimmers due to strong currents that develop with pronounced bar patterns.