Erosion warning with sea level rise

Properties within 90m of the current dune "vegetation limit" at Ocean Beach, Umina, are in danger of being eroded if sea levels rise by 90cm.

This is the implication of a report finding released last month by the Tasmanian Government, which looked at coastal vulnerability to climate change and sea level rise.

The report found that sandy shorelines backed by low-lying sandy plains, like the beach at Umina, were the most vulnerable to sea level rise and could expect shoreline recession.

The report's author, geoscientist Mr Chris Sharples, cited "the Bruun Rule, which states that horizontal shoreline recession due to sea level rise is most commonly in the range of 50 to 100 times the amount of sea level rise".

Mr Sharples said that the actual recession could be "much greater or much less than this".

A better prediction could only be made by "a detailed site-specific assessment and modelling exercise".

The rule gave "a basis to define a precautionary vulnerability envelope".

"This type of approach has been taken in the Western Australian State Coastal Planning Policy, which uses a multiplier of 100 times sea-level rise to define setback lines," he said.

"One possibility for applying such a precautionary vulnerability zone would be that proponents wishing to develop within such a defined zone would need to undertake sufficient site-specific assessments of recession vulnerability at their site as to demonstrate that their development is in fact not at risk, or that their development can be designed in such a way as to cope with any recession hazard."

A sea level rise of 90cm has been acknowledged in the draft Peninsula Urban Directions Strategy as the projected potential sea level rise which could apply to the Peninsula over the next 100 years.

The figure, which comes from the scientific Intergovernmental Panel on Climate Change, is also used in the Tasmanian report.

The draft Peninsula strategy suggests that minimum floor levels in low-lying areas could be raised by a metre, but it does not address the possibility of shoreline recession.

The report, Indicative Mapping of Tasmanian Coastal Vulnerability to Climate Change and Sea Level Rise, is available from the Tasmanian Government website www.dpiwe.tas.gov.au.

Mark Snell, June 20