Observations of megacusp dynamics and their coupling with crescentic bars at an open, fetch-limited beach

Alongshore changes in shorelines along sandy beaches are a result of variations in wave conditions and nearshore morphology, which can cause straight shorelines to develop large-scale (100-1000 m wavelength) undulations, called megacusps, which cause local shoreline accretion and erosion. Megacusps are often morphologically coupled to alongshore variability in the nearshore sandbar (crescentic bars). Sandbar rhythmicity has been studied extensively, whilst megacusp dynamics and their coupling with crescentic bars have received less attention. This study uses a long-term dataset of hourly time-exposure video images and detailed propagated wave conditions to investigate megacusp dynamics and the corresponding sandbar-shoreline coupling at the low-energetic, tideless beach of Castelldefels (northwestern Mediterranean Sea, Spain). Megacusps were observed during 24% of the study period. Crescentic bars were present during 91% of the days with megacusps, whilst megacusps were not observed during 50% of the days with crescentic bars. Megacusp wavelengths (100-700 m) were comparable to those of crescentic bars, whilst cross-shore amplitudes (3-8 m) and alongshore migration speeds (0-15 m/day) were smaller than those of crescentic bars. No clear relation was observed between wave conditions and megacusp formation, migration and disappearance. However, megacusp presence was strongly linked to crescentic bar presence, as megacusps mostly developed some days after the formation of a crescentic bar. Coupling between shoreline and inner sandbar was significant during 74% of the time with simultaneous presence of megacusps and crescentic bars. No dominance of one particular coupling pattern was observed, although the pattern depended on the wave height and the wave angle. It is hypothesised that the wave height and angle determine the type of flow pattern over the inner crescentic bar (single or double rip cell circulations versus meandering currents) and thus control the type of coupling.