An abrupt change in long-term shoreline behaviour at Roches Beach, Tasmania: An early response to contemporary climate change-induced sea-level rise?

Abstract

This study analysed a 65-year air photo record for Roches Beach (Australia) which shows that after being mostly stable from at least 1946 until 1985, a major section of the shoreline abruptly switched to a net recession trend which persisted until 2011 when artificial sand replenishment began. Comparable changes have been identified on some other Tasmanian beaches but have only been rarely reported to date. At Roches Beach, consideration of potential causes has identified sea level rise (SLR) as the most plausible driver of the observed change, although increased wave height and vertical land movement (VLM) could also contribute. Climate change induced sea level rise is expected to eventually cause major recession of many shores, however most sandy beaches have not yet shown a clearly attributable response of this sort. We infer that the critical factor causing Roches Beach to undergo an early switch to shoreline recession in response to SLR is its mostly unidirectional littoral drift which transports sand into, through and out of its leaky embayment to a degree not seen in other beaches in the same coastal compartment of Frederick Henry Bay. This delivers a proportion of any eroded sand to the north end of the beach and beyond to a large sand sink at Seven Mile Beach. We deduce that the rising sea-level has increased the frequency and scale of upper beach erosion events, causing increasing net losses of eroded sand from the embayment until the formerly balanced sand budget changed to deficit. Storm records suggest that major erosion events abruptly tipped the beach into a recessional mode when its sand budget was close to deficit. Regional and local confounding processes that may prevent or overwhelm such responses to SLR are minor at this beach. These include swell wave directional variability and interannual sea level variability, both of which are minimal compared to many other Australian coasts. We contend that this allows sea level rise to be more persistently effective at changing the behaviour of Roches Beach than at many other sandy beaches where other causes of variability may be of sufficient magnitude as to mask or prevent persistent changes due to SLR. The factors identified as causing or allowing an early shoreline response to sea level rise will be widely applicable as indicators of other beaches likely to respond early to SLR. This underlines the importance of local studies to identify critical distinctions in susceptibility to SLR between what can be otherwise similar adjacent shores.