Synchronised patterns of erosion and deposition observed at two beaches

Abstract

A similarity in the observed variability through time of two or more individual beaches (here quantified by synchronous changes in shoreline position and subaerial sand volume) can have significant implications to the increased focus worldwide on the need for expanded coastal monitoring. If it can be established that one or a select few beach sites exhibit ‘regionally representative’ behaviour in response to varying and/or changing wave and other climatic forcing, then this will assist to guide the extent and scope of the ongoing monitoring effort required. In this study, two beaches located on the southeast Australian coastline of similar size and orientation but separated by 35km, were simultaneously monitored and compared for a period spanning 3years. Intensive sub-aerial surveys were conducted each month at both sites using ATV-mounted RTK-GNSS. The purpose was to establish whether detailed, embayment wide patterns of erosion and deposition quantified at either site were indicative of the behaviour observed at the other.During the data collection period, storm wave events caused substantial fluctuations in monthly sub-aerial sand volumes (in the range of 60–70% of mean volume), including month-to-month changes at the berm and episodic foredune erosion and recovery. Principal component analysis of the two beaches revealed that both were dominated by oscillation (corresponding north–south behaviour) rather than rotation (contrasting north–south behaviour) over this three-year period. Comparison of alongshore-averaged volume time-series between the two sites indicated a similar total range (68m3/m and 78m3/m) and the average difference between the two sites (4.5m3/m) was found to be of the order of 50% of the average monthly change (9.7m3/m and 10.2m3/m). Corresponding spatial patterns in the occurrence of erosion/accretion along the two sites were also compared and found to closely correspond. Five synchronised ‘snap-shot’ observations from historical aerial photogrammetry and surveys at both sites spanning the period 1974 to 2011 (one per decade) suggest coherence between the sites extends beyond the three-year intensive monitoring period reported here.