Ocean Beach, Tasmania: A swell-dominated shoreline reaches climate-induced recessional tipping point?

Abstract

Global sea-level rise since the Nineteenth Century is expected to eventually cause recession of many shores, however most swell-exposed sandy beaches have not yet shown such response. This study analysed a 70-year air photo and beach profile record for swell-dominated Ocean Beach (western Tasmania) to show an abrupt change of long-term shoreline position variability circa 1980, from episodic erosion and accretion since at least 1947 to persistent recession with no recovery up to the present. Dating of back-dune peats exposed in the dune scarp showed that recent shoreline recession exceeds any in the last 1800 years. Investigation of potential causes identified recent-onset sea-level rise (SLR) on a tectonically-stable coast and increasing winds driving increased wave-setup as drivers with sufficient explanatory power to account for the observed changes, although data limitations and residual uncertainties mean additional contributing factors such as interdecadal wave direction changes cannot be ruled out. We hypothesise that Ocean Beach has experienced earlier recession in response to SLR and other climate change effects than many other beaches owing to exposure to a very high-energy storm-dominated wave climate, littoral drift efficiently delivering eroded sand to a large-capacity active sand sink, and low variability in swell-wave directions and inter-annual sea-levels. We hypothesise that sea-level rise with higher onshore wind speeds generating increased wave setup at Ocean Beach since before the 1980s has increased upper beach erosion event frequency until the formerly stable or gaining sand budget reversed to deficit. A major storm or storm cluster abruptly tipped the beach into its current recessional mode when its sand budget was close to deficit. Factors causing an early shoreline response to sea-level rise at this site are applicable more widely as potential indicators of beaches likely to respond earlier than others to climate-induced changes including not only SLR but also wind climate changes.