The impact of temperate reefs on 34 years of shoreline and vegetation line stability at Yanchep, southwestern Australia and implications for coastal setback

Abstract

Coastal setbacks are used to protect populations and infrastructure, and involve determining baselines and historic trends in shoreline movements. Assessing these movements can be difficult on coasts with especially complex geomorphology, such as due to rock and coral reefs. Reefs are often assumed to provide coastal protection and stability. However, little is known about the spatial variability of beach dynamics on beaches with reefs, over inter-annual and decadal time-scales. Spatial variability in inter-annual vegetation line and shoreline stability was assessed on three adjacent beaches at Yanchep, southwestern Australia, using 34years (1974–2008) of aerial photographs. Over 96% of the study area, beach width narrowed over the 34years. This trend was statistically significant on 83% of the middle beach, 43% of the northern beach, and only 29% of the southern beach. The maximum mean annual change in beach width was 1.7myr.−1 at the south end of the southern beach, the first mode of the Empirical Orthogonal Function (EOF) explained 45% of the variability alongshore, and correlated significantly (p<0.05) with the 90th percentile of annual wave height and annual mean sea level. Beach rotation and reef-controlled currents resulted in cellular beach morphodynamics, and vegetation line and shoreline movements varied greatly alongshore. On one occasion, the beach widened by more than 50m inter-annually while other areas eroded by 50m. The southern beach that was fronted by the most continuous and highest (supratidal) section of reef was the most temporally variable. Even the vegetation line here moved by up to 20m inter-annually, so cannot be assumed to be a ‘stable’ baseline for setback. The impact of reefs on inter-annual shoreline and vegetation line stability should be considered in future guidelines for determining coastal setback.