Surge Wall
The Shire of Carnarvon would like to thank Lotterywest for their assistance with the placing of seats, traffic bollards and interpretative signs on the South Carnarvon Surge wall. These will assist and encourage people who are wishing to walk along the surge wall and rest awhile to take in the abundance of wildlife and the wonders of the mangroves.
The surge wall was designed to reduce the threat to low lying areas of Carnarvon being inundated with flood waters and tidal surges from ocean flooding during severe storms and cyclones.
The actual surge wall from Carnarvon Road to Harbour Road is 2.7m AHD 1700m long and has a compacted crest. This section has been designed as an earth bund with rock facing on the seaward side, and seeded batters on the landward side. The limestone rock is to protect the seaward section from wave attack.
Background of the area
The low-lying coastal area of South Carnarvon has been subject to storm surge during a number of historic tropical cyclones. It is exposed to the ocean from the south across extensive mud flats and mangrove forest and to the west across the South Arm of the Gascoyne River.
There are more than 250 residential block in South Carnarvon with natural surface levels less than 2 metres above sea level. There is a risk to property from storm surge damage although to date, this risk has not been quantified. Construction of some form of physical storm surge protection for properties in South Carnarvon has been proposed for many years.
From Carnarvon Road around to Harbour Road the foreshore is exposed to the south across the mangrove flats. Under normal conditions there are rarely any waves breaking on the foreshore. However during cyclonic conditions, waves generated across the Gascoyne Flats may propagate through the submerged mangroves. Waves up to 0.5m with a 4 second period may break on the foreshore during a cyclone.
Historical records of storm surge in Carnarvon
The Bureau of Meteorology (BoM) records suggest tropical cylones effect the Carnarvon region every 5 years or so. Historically, a very significant cyclone impact occurs in the order of 20 years or so during recent history a severe cyclone has affected Carnarvon every 10 years of so. These include Tropical Cyclone (TC) Ingrid (1970), TC Beverly (1975) TC Hazel (1979) TC Herbie (1988) TC Vance (1999).
The most severe cyclone to effect the Carnarvon region was in March 1960, when gusts in the order of 178 km/hr (96 knots) were recorded at Carnarvon Airport. This was the largest recorded wind gust during a 90 year period.
What is a storm surge?
Storm surge is the elevation of still water level (SWL) above normal astronomic tide caused by meteorological effects. This is partly a barometric effect where low barometric pressure results in a rise in water level, and partly a wind set-up effect, where wind blowing onshore drives the surface water before it and against the coast. This results in elevated still water levels in coastal areas.
This is most pronounced for extensive shallow areas and is further exaggerated where water is pushed into an enclosed and narrowing area such as a deep bay similar effects can also result in a lowering of water level.
As a result of risk analysis studies, state and local Government departments' assistance, consultants, contractors and with the support of the community this wall is now in place to assist in the protection of the area of South Carnarvon and its residents.
Lotterywest
We thank again all participants for their support. If you would like some assistance with funding for worthwhile projects try this link to Lotterywest for information on available funding.
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