Strengthening the City's Walls

19 January 2010

Like King Canute, we at the City Council know we can't control the forces of nature - but we'll try hard to make sure they don't overwhelm us. Take the tides, and wave action. Both relentlessly chip away at the land around Wellington Harbour and our coastline - which is why we've budgeted up to $6 million a year for work to repair, maintain and enhance the city's sea walls and retaining walls.

Council officers plan repair work to battle erosion in Baleana Bay

Council officers plan repair work to battle erosion in Baleana Bay

There are about 270 sections of sea wall around the harbour and along the South Coast. Most are about 40-50 years old but some are considerably older. While most are in good condition, some need attention after years of battering.

Some sections of 'unprotected' foreshore - especially below Massey Road and Shelly Bay Road on the Miramar Peninsula - are also being eroded by wave action and need protection.

Council engineer Faiz Tawfeek says work is already underway on sections of sea wall in Kio and Balaena bays.

A contract has also been let for new sections of sea wall, and repairs, around the Miramar Peninsula. Most of the work, scheduled to start late this month, will take place between Shelly Bay and Karaka Bay and will include Pt Halswell and Kau, Mahanga, Shark and Scorching bays.

Faiz says a sea walls 'asset management plan' confirmed we have to do the upgrade and construction work. "We can't ignore these walls - we have to do the work. Otherwise the walls will eventually collapse - and then roads and private property will be at risk."

Sea wall design and construction methods will vary from site to site, but Faiz says the emphasis will be on building heavily-upgraded foundations. Most sea walls will be made of reinforced concrete - but in some cases the design will include rip rap - a layer of rocks designed to absorb and dissipate wave energy.

In most cases the rip rap will also help the sea wall to 'blend in' with the surrounding landscape. The installation of temporary coffer dams may be necessary in some cases to keep sea water out of the construction site.

City Council Infrastructure Director Stavros Michael says while erosion of the coastline is a constant issue, it is likely to be thrown into sharper focus if climate change does, as predicted, start to cause the sea level to rise.

"The potential effects of climate change are acknowledged in our asset-management planning - we disregard it at our peril."

Stavros says the Council is committed to upgrading and strengthening retaining walls all over the city. "This is a hilly town - we have a heck of a lot of retaining walls. Our aim is to bolster as many of them as possible so they can withstand heavy rains and earthquakes and thus keep our roads open and private property intact.