A massive $6.7 million line of defence is being re-installed at a cyclone-vulnerable section of a Gold Coast beach.
While its abundance of surf waves has made the Gold Coast a popular playground for surfers, these high-energy waves also cause a great deal of erosion. An average of 500,000 cubic metres of sand is subjected to longshore drift along the area’s 42km of beaches per year. During cyclonic conditions, waves of over 11m high have been recorded in some areas.
Built in 1923, the original Narrowneck seawall was one of the first constructed on the Gold Coast. The Department of Main Roads (DMR) constructed a highway along the Narrowneck section of the Gold Coast in 1922. When erosion threatened the highway the following year, DMR constructed the original seawall from timber piles and log sleepers.
A council spokeswoman says seawalls have a “long and proven” record on the Gold Coast. “They are the preferred and endorsed option as a hard coastal protection measure,” she says. Seawalls are flexible structures that are designed to move under wave attack; they may also be topped up in situ.
To further protect the highway from the dynamic wave forces, the seaward face of the timber wall was later armoured with large rock boulder in the early 1950s. The original timber ramps that were used to slide the large rock boulders into place are almost 100 years old and still in place today.
Following site investigations in 2014, which included localised excavation along the existing seawall, the city concluded that 900m of the Narrowneck seawall needed to be reconstructed. As a result, the Narrowneck seawall was scheduled for design and construction as part of the 2015/2016 seawall program. Work is scheduled for completion by October 2016 and will comprise three stages.
Sheet piling will be installed at specific sections to address localised slumping and seawater intrusion. In addition to locally sourced rock, the city will also re-use rock from the existing seawall. Rock sizes range from two to four tonne for the primary armour and up to 360kg for the secondary armour rock layer. A geotextile filter layer suited to marine applications is being used to prevent the leaching of sand through the seawall.
Just 30m separates the Narrowneck seawall from the Nerang river, meaning the area could be cut in two during a cyclone.
The spokeswoman says the seawall program is integral to ensure infrastructure is protected from coastal hazards. “The implementation of a continuous seawall is necessary to provide protection to property, open space and infrastructure,” she says. “Incomplete sections of the seawall may allow erosion to occur between and behind the walls during storm events. The city’s defences against storm damage are being boosted.”