Coastal NSW faces the possibility of a third bout of wild weather in less than a month as insurance advisers warn the risks from multiple storms are being underestimated.
Authorities are continuing to assess the damage from back-to-back big low-pressure systems that forced the evacuation from homes at Wamberal to Sydney’s north and stripped sand away from many beaches.
Central Coast Council alone estimates it has spent more than $100,000 on emergency repairs, and is rushing to import 600 storm-proof bags from Japan that are capable of containing four tonnes of material each.
But Risk Frontiers, a natural hazards research centre, said the risks from the storms on assets such as coastal roads and property are being underestimated because they are typically treated as single events. Since east coast lows come in clusters, the impacts are magnified.
The first of the two recent storms packed total wave energy hitting the coast with a ferocity expected about once every four years, or some 54.8 gigajoules per metre. The second was less extreme, but still packed a severity expected once each year at an estimated total energy of 11.8 GJ/metre, according to research by Thomas Mortlock, senior risk scientist.
“If treated as a single storm, the total energy was around 66.6 GJ/m, which is equivalent to the amount of erosive potential that could be expected of a single east coast low of a return period of approximately seven years,” Ryan Crompton, managing director of Risk Frontiers, said.
Data going back to the 1850s shows east coast lows, particularly the high-impact ones, tend to come in clusters. Since beaches typically don’t have time to recover lost sand in time for the following storm – or storms – the resulting cumulative risk needs greater attention by planners, Risk Frontiers said.
The recent combination “demonstrates how there can be an underestimation of coastal risk by assuming all [lows] drive independent erosion responses”, the consultancy said in a briefing note.
“If the cumulative erosion potential that exists with clustered … events is not incorporated into coastal hazard planning, then we may continue to under-appreciate the importance of event clustering,” it said.
Dr Crompton said that, while programs in NSW today were guided by the Coastal Management Manual that refers to storm clustering as an important facet of erosion risk, it was still “down to the individual plans as to the extent which this is considered in design criteria”.
“Addressing this problem is of particular importance given future sea level rise projections which will exacerbate the risk,” he said.
Mitchell Harley, a coastal researcher at the University of NSW said the clustering was relevant to understand the present erosion issues at Wamberal, Collaroy and Stockton, near Newcastle.
Dr Harley, though, said coastal planning has had to take into account the possibility of big storms in close succession. He noted the largest and fourth-biggest storms of the past 50 years hit within a fortnight of each other in 1974, and planners had to take into account the erosion risks of a repeat.
However, the so-called set backlines established in the wake of those tempests run through the middle of houses at Wamberal, implying that those threats remain serious if such events recurred.
For next week, though, weather models are uncertain about which way several possible lows might travel.
The Bureau of Meteorology is predicting one low is more likely to be further south than the two recent ones, and have more impact on the Victorian coast. A second low, forming late next week will most likely bring rain to inland regions rather than batter NSW beaches.
Ben Domensino, a senior forecaster with Weatherzone, said if the second of those two lows formed in the Tasman Sea, “you can expect coastal erosion impacts”.
With a large pool of cool air moving across from the west, there is the potential for “a dynamic set-up” once it reaches the warmer air over the Tasman Sea.
“We just won’t know for a few more days,” he said.
Eric Belzer, a Wamberal resident of 30 years, said residents were increasingly frustrated by delays on getting approval to build a seawall. While acknowledging the main obstacle remains determining who pays for the wall – that the council expects will cost $20,000 per metre – Mr Belzer said planning rules were inconsistent. The council approved an application to rebuild his 1950s cottage even though it assumed a wall was in place. They also rejected his plans to shift his new house several metres towards the back of his property, pressing him instead to shift it forward. The compromise approved will bring his house one metre closer to the beach.
The government on Friday announced it would set up a taskforce to settle on “a long-term solution to the coastal erosion issues” at Wamberal.
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Peter Hannam writes on environment issues for The Sydney Morning Herald and The Age.