Form and function of natural and engineered perched beaches

With predicted sea level rise and possible changes in storminess associated with climate change, it is not known how perched beaches, whether natural or engineered, will respond. Perched beach types range from beaches fronted by wall-like structures in the nearshore that may be connected to the dry beachface, to having underlying and outcropping geological formations. They may also be backed landward by hard structures such as cliffs or infrastructure instead of dunes. In this study, the global distribution of natural perched beaches from existing literature was mapped. 'Hot spots' include the West Indies, Central and South America, Pacific Island Atolls, Indian Ocean islands, the Red Sea and the Mediterranean region. Many of these areas are in the tropics and subtropics and hence are associated with beachrock with is common in areas with coral reefs. Mechanisms of beach control by coastal structures are still poorly quantified. Suggested mechanisms include: limited free-profile fluctuation, decreased sediment availability and increased erosion rates due to less water infiltration and raised beach groundwater tables. Field research in southwest Western Australia indicated that perched beach profiles may recover more slowly from the daily summer sea breeze erosion than exposed profiles. During a storm, perched profiles had less erosion, occurring lower on the beachface than exposed profiles. Perched profiles however did not recover as easily by accretion during low sea level phases of storm activity. Lower rates of recovery for perched profiles could be due to a scour step forming seaward of the rock formations inhibiting cross-shore sediment transport. Results indicate that natural perched beach behaviour has extreme spatial variation. Beach response to changing hydrodynamics is strongly dependent on the configuration and geometry of local coastal structures, whether natural and anthropogenic.