Rapid adjustment of shoreline behavior to changing seasonality of storms: Observations and modelling at an open-coast beach

Abstract

An 8-year time series of weekly shoreline data collected at the Gold Coast, Australia, is used to examine the temporal evolution of a beach, focusing on the frequency response of the shoreline to time-varying wave height and period. Intriguingly, during 2005 the movement of the shoreline at this site changed from a seasonally-dominated mode (annual cycle) to a storm-dominated (~monthly) mode. This unexpected observation provides the opportunity to explore the drivers of the observed shoreline response. Utilizing the calibration of an equilibrium shoreline model to explore the time-scales of underlying beach behavior, the best-fit frequency response (days-1) is shown to be an order of magnitude higher post-2004, suggesting that a relatively subtle change in wave forcing can drive a significant change in shoreline response. Analysis of available wave data reveals a statistically significant change in the seasonality of storms, from predominantly occurring at the start of the year pre-2005 to being relatively consistent throughout the year after this time. The observed change from one mode of shoreline variability to another suggests that beaches can adapt relatively quickly to subtle changes in the intra-annual distribution of wave energy.