Storm surge forces shoreline retreat

Those doubting the likely impacts of climate change might be convinced by a visit to Roches Beach in Tasmania, where residents saw the potential impact of rising sea levels on their doorstep on the weekend of July 9-10. Roches Beach, just 20 minutes east of Hobart on Frederick Henry Bay, is an area of special interest for ACE-CRC scientists who research sea level rise because its geomorphology and wave processes make it more than usually susceptible to erosion caused by storm surges. The leader of the ACE CRC Sea Level Rise Project, Dr John Hunter, UTAS geomorphologist Chris Sharples and surveyor Nick Bowden visited Roches Beach after the stormy weekend to observe the damage and survey the beach.  Over the weekend the Hobart temperature had hovered around 9 degrees Celsius. This deep low pressure system brought a 10-15 metre southerly swell which produced a storm surge lasting about 48 hours and eroding the beach’s shoreline by up to five metres. Houses built along the beachfront experienced flooding in their front yards and in some cases up to their doorsteps. Chris Sharples is the author of a technical report produced for ACE in 2010 into shoreline changes at Roches Beach. He said the erosion on this particular weekend was the worst single event he had seen at that location, and was more than double the erosion that had taken place during the past decade. Scientists have been monitoring Roches Beach for many years. The first set of profiles of the beach were recorded in 1988, but aerial photos from as far back as 1957 have allowed shoreline changes at Roches Beach to be traced over time. “Smaller storm events have been happening frequently here in the past 20 years, and because average sea-level is now higher than it was some decades ago, those smaller storms are now able to reach higher on the beach and erode more sand than they could previously,” Chris said. “The greater frequency of small erosion events means that the beach doesn’t have time to naturally build up again before the next storm, so the sand is being gradually lost.  Then the occasional big storm does even more damage because it is attacking a shore that is already depleted of sand by the frequent attrition of smaller events. ”  He said storm surges were not necessarily increasing in frequency, but with a higher sea level the damage they were doing was greater and as a result the shoreline was now located further landwards along most of the beach than it had been since at least 1957. Roches Beach is just one of 30 around Tasmania that are being monitored in the Tasmarc survey project, which is being run by Nick Bowden in conjunction with Dr Hunter.  Every month the high water mark and a profile at each beach are measured against a fixed survey marker. The aim of the project is to create a record of shoreline movement throughout Tasmania.

Authorised by the CEO of the Antarctic Climate and Ecosystems Cooperative Research Centre October 2019.

The ACE CRC was established and supported under the Australian Government’s Cooperative Research Centres Program.

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