Shoreline instability under low - angle wave incidence

11 The growth of megacusps as shoreline instabilities is investigated by ex-12 amining the coupling between wave transformation in the shoaling zone, long-13 shore transport in the surf zone, cross-shore transport, and morphological 14 evolution. This coupling is known to drive a potential positive feedback in 15 case of very oblique wave incidence, leading to an unstable shoreline and the 16 consequent formation of shoreline sandwaves. Here, using a linear stability 17 model based on the one-line concept, we demonstrate that such instabilities 18 can also develop in case of low-angle or shore-normal incidence, under cer-19 tain conditions (small enough wave height and/or large enough beach slope). The wavelength and growth time scales are much smaller than those of high-21 angle wave instabilities and are nearly in the range of those of surf zone rhyth-22 mic bars, O(10 2 − 10 3 m) and O(1 − 10 days), respectively. The feedback 23 mechanism is based on: (1) wave refraction by a shoal (defined as a cross-24 shore extension of the shoreline perturbation) leading to wave convergence 25 shoreward of it, (2) longshore sediment flux convergence between the shoal 26 and the shoreline, resulting in megacusp formation, and (3) cross-shore sediment flux from the surf to the shoaling zone, feeding the shoal. Even though 28 the present model is based on a crude representation of nearshore dynam-29 ics, a comparison of model results with existing 2DH model output and lab-30 oratory experiments suggests that the instability mechanism is plausible. Ad-31 ditional work is required to fully assess whether and under which conditions 32 this mechanism exists in nature.