Sandbar straightening under wind-sea and swell forcing

• Sandbar straightens under mild wave forcing.
• Wave incidence angle controls the alongshore uniformity of a sandbar.
• Nondimensional sediment fall velocity poorly predicts beach state transitions.

Sandbar straightening on natural beaches is known to occur following storm events. However, on a typical south-west Western Australia beach, sandbar straightening is observed under low amplitude wind-sea forcing in summer and remains alongshore-variable following high wave events associated with winter storms. During summer, diurnal variations in wave height and direction are associated with a strong sea breeze cycle. At the peak of the sea breeze the local wind-sea approaches the beach obliquely, driving strong alongshore currents. The occurrence of the seasonal, obliquely-incident wind-sea provides the opportunity for testing the role of the wave angle on sandbar straightening. The location of the bar crest is observed using time exposure video images and the alongshore variability is quantified using the standard deviation of the sandbar position. The role of wave incidence angle on alongshore-variability is highlighted, as the sandbar straightens in the presence of oblique wave forcing associated with summer sea breezes, and remains alongshore-variable with normally incident wave forcing. The transition from alongshore-variable to alongshore-uniform also depends on the antecedent bathymetry, in particular the distance between the shore and the bar crest. When the sandbar is further offshore, alongshore variability persists in the presence of strong sea breezes.