Stronger seawalls make coast more resilient

24 June 2016

New and strengthened seawalls at Breaker Bay and Island Bay are the latest in an ongoing programme of infrastructural improvements designed to help protect vulnerable parts of Wellington from severe southerly storms.

This photo shows a seawall that has been built at Karaka Bay since a big storm and high seas caused erosion in the area in  2013. The colours of the stones used in the wall are similar to natural rocks in the area, so the wall blends in with its  surroundings.

Karaka Bay seawall

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It is 3 years ago this week since a major storm and high seas battered the coastline, damaging roads, footpaths and properties around the south coast.

Since then, Wellington City Council has carried out about $4 million worth of major protection work at Shelly, Princess, Karaka, Island and Breaker bays, plus numerous smaller road and seawall repairs around the harbour and south coast. The work has been partly funded by the NZ Transport Agency and insurance.

More work is planned at Lyall Bay, Karaka Bay and west of Island Bay near Victoria University’s Coastal Ecology in the coming financial year, and there is more to come over the next few years as temporary solutions are replaced with longer lasting ones.

Work at Lyall Bay to protect the road and footpath and reinstate the sand dunes will include rock protection work by Dorrie Leslie Park, and filling the gaps in the existing concrete seawall behind the dune.

Mayor Celia Wade-Brown says the intensity and number of storms affecting Wellington’s south coast have risen in recent years and the reality is that these parts of the city will be under increasing pressure.

 “Improved seawalls are an important part of the mix. The Council is also working on a number of other fronts to help make the city more resilient to climate change, bigger storms and sea-level rise.

“There is much coastal planting to be done and sometimes, softer environmental solutions are better than hard engineering. Prioritisation of future funding will be determined by the resilience strategy now under development,” she says.

Seawall design varies from site to site but all the new walls are being built with much stronger foundations so they can potentially be extended upwards as the sea-level rises.

Some – like the high new seawall above Princess Bay – are made from reinforced concrete.

Others – like one of the new walls at Shelly Bay and the new Breaker Bay one – are built using layers of very large rocks that absorb and dissipate wave energy and blend in with the surrounding landscape.

Councillor Iona Pannett, Chair of the Council’s Environment Committee, says the new rock seawall at Breaker Bay was built using nearly 3000 tonnes of rock. It will help protect a low-lying stretch of coast where high seas sometimes sent waves and debris crashing over the road and into properties.

“This was a risk to residents as well as people passing through this area on foot, bikes or in cars.”

The Princess Bay seawall was constructed to protect the road after storms in February and June 2013 caused serious erosion that undermined some 80 metres of footpath.

“It was built upright to preserve as much of the beach as possible and has been designed to last 100 years,” Cr Pannett says. 

In Island Bay, the entire wall has been strengthened – not just the storm-damaged section. This involved realigning sections that were on a lean and using some 290 steel rods and concrete to secure the top of the wall to its foundations.

A new concrete support beam below the ground will also help make the wall stronger, safer and more resilient to high seas and storm surges.

Sections of the original wall have been repaired and reused to help retain the heritage character of the 1930s wall.