Seasonal Shoreline Variability Induced by Subtidal Water Level Fluctuations at Reef‐Fringed Beaches

Abstract

AbstractThe comparative role of subtidal water level and wave height variations on seasonal shoreline changes was investigated at a reef‐fringed beach in southwestern Australia. The data set consisted of continuous sea level and wave records, monthly topographic beach surveys over a 2 year period, and bimonthly high‐resolution aerial images over a 7 year period. Shorelines were extracted from images and topographic surveys, and then an empirical orthogonal function (EOF) analysis was applied to both data sets. The temporal amplitudes of the first EOF mode of the image‐derived shoreline data set (∼60% of the variance) were most correlated with 30 day averages of the subtidal water level (variations up to ±0.2 m) driven by geostrophic adjustment of the Leeuwin Current. The geostrophic response of the Leeuwin Current was found to be further correlated with the phasing of the El Niño–Southern Oscillation, leading to higher water levels during La Niña and lower levels during El Niño, which as a consequence resulted in interannual variations in the shoreline behavior reflected in the first EOF mode. The temporal amplitudes of the second EOF mode (∼20% of the variance) were most correlated with 50 day averages in offshore wave height. Our results indicate that the seasonal beach response was primarily influenced by seasonal variations in offshore water level rather than by wave heights as has been generally observed in exposed beaches. A simple EOF‐based model is presented, which reproduces the alongshore‐variable shoreline response and its seasonal and interannual modulation due to El Niño–Southern Oscillation events over the study period.