Adapting to sea level rise in Lyttelton Harbour costly

Wednesday, 18 October 2023

An aerial view of Governors Bay-Teddington Rd around Allandale Domain. The Christchurch City Council is consulting on some potential options for six Lyttelton Harbour communities to deal with rising sea levels.

Christchurch could face a bill into the hundreds of millions of dollars to move roads and infrastructure away from the rising sea and storms in Whakaraupō Lyttelton Harbour.

For example, moving Teddington Rd to higher ground between Governors Bay and Gebbies Pass Rd could cost as much as $170 million. An old landfill at sea level in Allandale might cost $65m to $100m to dig up and remove.

The city council is seeking public views on potential “pathways” for five low-lying harbour locations - Rāpaki, Allandale, Teddington, Charteris Bay and Purau - plus Koukourarata in Port Levy.

Gallipoli Wharf at Rapaki could be replaced with a more resilient wharf for $3.2m to $4.8m, or up to $1.1m could be spent raise it so it can last longer.

Locally, sea levels have risen by about 10cm since 2005, and the council anticipates a further 14cm to 23cm of sea level rise by 2050, and 38cm to 1 metre by 2100.

Initially, the risk to infrastructure is not from high tides but from “large storms”, which cause deeper floods over larger areas and coastal erosion.

Potential options include managed retreat of assets like roads, the council says.

The city council could adopt a “hold the line” approach, where public assets were defended by options such as flood-proofing, but this approach may only work for so long, council team leader coastal hazards adaptation planning Jane Morgan said.

“Alternatively, we could work with nature and retreat assets further inland, which costs more at the outset, but reduces the overall risk,” she said.

The sea level in the harbour has risen 10cm in the last 15 years, the council says.

Gallipoli Wharf at Rapaki, built in 1916 to honour Māori who served at Gallipoli, had about 10 to 20 years left before maintenance was no longer worth the increasing cost or disruption, the council said. It could be replaced with a more resilient wharf slightly further inland for $3.2m to $4.8m. Alternatively, up to $1.1m could be spent raise the wharf so it could last longer.

At Purau, Purau Drive and Camp Bay Rd are vulnerable to sea level rise. Moving the roads would likely cost between $37m and $55m, while protection would likely cost $15m to $22m.

There was flooding in Teddington after tsunami waves from Chile came up the harbour in 2010.

The public boat ramp at Charteris Bay would cost about $660,000 to $980,000 to flood-proof and protect. It would likely cost a few hundred thousand dollars to simply remove it and not rebuild, the council estimated.

Roads near Teddington and elsewhere already flood during storms. Tsunamis originating in Chile flooded the head of the harbour in 1960 and 2010. Rather than spend $170 million moving the main road, a cheaper shorter-term option was to raise the current road between 40cm and 100cm for an estimated $15m to $50m. However, this would probably further encroach on Canterbury’s largest and most diverse salt marsh.

The council noted it was responsible for public infrastructure and property owners may face their own costs. If a road was moved, for instance, new driveways may be needed.

The 1.4-hectare Allandale landfill was closed in 1993 and covered with soil and plants. To prevent erosion and the release of contaminated material into the harbour, rock “armouring” was added in 2010, which the council expected to be effective for 10 to 20 more years.

It could add more rock in the future, at an estimated cost of $1.3m to $1.9m, instead of $100m for removal and remediation.

The wharf at Koukourarata/Port Levy is mainly used for recreation, but it regularly floods in king tides. Upgrades will be needed in two to 10 years. A new wharf could cost $6.5m, while raising and protecting it will likely cost as much as $2m.

“We don’t yet have all the information about what the different options might look like … it’s important to get people’s thoughts on them now, before we invest time and money drawing up plans that might not align with communities’ views,” Morgan said.

The feedback will help the council come up with “preferred pathways” next year that will be presented to city councillors for a decision to either accept, amend or reject.