Erosion/accretion patterns and multiple beach cusp systems on a meso-tidal, steeply-sloping beach

This study examines daily variations in intertidal topography and beach cusps on a meso-tidal, steeply sloping beach during a five-month period. The observations are based on coastal imagery and frequent topographic surveys and are extracted from patterns of contours positioned 1 and 3 m above MSL (lower/higher beach-face, respectively). The intertidal topography was very sensitive to variations in the wave regime, triggering rapid transitions from erosive to accretive states and vice versa, as well as transformations of the cusp systems, often involving severe horn erosion and beach-face scarping. Distinct beach cusp systems were observed at different levels of the beach, with their evolution being controlled by: (i) wave forcing; (ii) tidal modulation of the wave power at different levels of the beach; and (iii) interactions between the existing beach cusp systems. The lower beach-face rhythmical patterns changed constantly and merged with the upper beach-face cuspate features, typically when the significant wave height exceeded Ho > 1.5 m. The upper beach-face pattern was persistent during the monitoring period, re-emerging with similar spacing and alignment, despite being eroded during energetic events; this persistence is also maintained by the interactions between swash and irregularities (e.g. embayments) in the dune/backshore morphology. The spacing of the observed beach cusps ranged from λc,min = 8 m to λc,max = 67 m; these values were underestimated by the edge wave and self-organization theories and were better predicted by the Sunamura (2004) formula. Beach cusp growth was linked to breaker angles of near-normal incidence and narrow-banded frequency wave conditions. Bay infilling was found to be the most dominant cusp reduction mechanism for larger features (λc > 30 m), while smaller features vanished during erosive conditions. The most intense cusp growth episodes were linked to accretive conditions, while larger features were shown to emerge during more energetic, erosion events. These findings highlight the need for more longer-term observations of diverse beach morphologies to formulate a global theory regarding beach cusp formation and evolution.

► Daily measurements of accretion/erosion and multiple beach cusps (BC). ► Higher beach-face BC were larger and persistent. ► Lower beach-face BC changed constantly; merged with higher ones when Ho > 1.5 m. ► Bay infilling observed for larger features, horn erosion for smaller features. ► Cusp growth during recovery; larger features emerged during energetic events.