Medium term behavior and evolution of a beach cusps system in a low energy beach, Port Stephens, NSW, Australia.

Beach cusps are morphological features found on the beachface, consisting of steep-gradient, seaward-pointing cusp horns and gentle-gradient embayments. They are formed by swash action and usually develop under accretionary wave conditions and often display a marked alongshore rhythmicity, referred to as cusp spacing. There are two theories to explain beach cusp formation: standing edge waves and self-organization. This study investigates the medium term behavior of a cusp system over circa one year. Data collection was undertaken at Shoal Bay, Port Stephens, a ria-like drowned river valley located on SE Australia. The results showed the presence of two main cusp systems in this beach. Persistent cusps exist along the entire study area with spacing of 30 m at an elevation of 2 m above means sea level (MSL). Storm cusps are located on the upper part of the beach, around 3.5 m above MSL, with a bigger size and a longer spacing (50 m). We compared the cusp spacing with that predicted by the edge wave model and self organization model and the best results for both sets of beach cusps were found with the edge wave theory in the sub-harmonic version.