Counting the costs of major storm events before they hit our beaches

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As coastal councils brace for another hammering of their beaches over the weekend, new research methods are emerging that could help local communities assess the threats from beach erosion.

Rosh Ranasinghe, a former NSW coastal engineer, has studied the state's beaches for more than 15 years – including the frequently hit Collaroy-Narrabeen stretch – to develop new risk maps that can put a dollar-value per square metre of exposed properties.

Works at Collaroy beach earlier this month to shore up the collapsed foreshore after the recent east coast low. Photo: Peter Rae

The approach helps to identify more accurately the economically optimal setback line both now and in the future once climate change-related impacts such as rising sea-levels are taken into account.

"To avoid millions of dollars of damage in the coming years, I would strongly recommend that coastal risk and [optimal setback lines] be determined urgently for at least the 15 identified hotspots in NSW," Professor Ranasinghe, now with the UNESCO-IHE Institute for Water Education in the Dutch city of Delft, said. (See below for his estimate of the most at-risk sites in Collaroy-Narrabeen.)

The setback line marks the region beyond which the risk of damage faced by any property is too great to justify the economic gain the property might potentially provide, he said. By extension, the formula would Indicate which protective measures, such as a new seawall, would be justified if their expected return exceeded their cost.

Waverley Council Mayor Sally Betts with engineers examine the Bondi coastal walk on Friday ahead of another potential storm event. Photo: Peter Rae

Professor Ranasinghe said his modelling is being applied to places such as Kenya and Sri Lanka. Apart from Sydney's northern beaches, he recommends Stockton, The Entrance and Byron Bay be priority sites.

A spokeswoman for Planning Minister Rob Stokes said the government's new coastal management bill – including the upcoming State Environmental Planning Policy that would trigger the act – addresses issues raised by the new modelling.

"The coastal reform framework is consistent with the approach proposed," the spokeswoman said, noting the draft Coastal Management Manual specifically outlines requirements for councils to develop a probabilistic approaches to erosion risks.

"Determination of setback lines is a decision for councils to make as part of developing a local environmental plan or development control plan," she said.

Councils such as Waverley are continuing to assess the damage and remediation options after the first storm damage areas of Bronte Beach, Waverley Cemetery and the Coastal Walk.

"The storm event has been assessed by coastal engineers as a one-in-a-100 year event," Emily Scott, director of Waverley Renewal, said.

"There is potential the landslip area at [Waverley] cemetery could further collapse as a result of the storms this weekend, however, absolutely no graves are at risk of collapse – just the area of reclaimed land outside the historic boundary of the cemetery," Ms Scott said.