Wellington's eastern suburbs are vulnerable to sea level rise. Photo / Mark Mitchell
Wellington's eastern suburbs are vulnerable to sea level rise. Photo / Mark Mitchell
The threat of sea level rise played a key role in the decision to move Wellington's light rail route away from the airport and to the southern suburbs instead, a ministerial briefing reveals.
The document, released to the Herald under the Official Information Act, assessed resilience issues in the capital's$7.4 billion transport plan, called Let's Get Wellington Moving (LGWM).
It also said the existing Mt Victoria tunnel was very high risk in a major earthquake because landslides could damage or block the entrances.
LGWM was born out of an agreement in 2014 to resolve congestion between Ngauranga and the airport in the east.
But by 2021 officials were mulling an alternative route to the south.
The ministerial briefing from June this year said known resilience risks in the east, including earthquakes, liquefaction, tsunamis, flooding and sea level rise, were a "key factor" in the route change.
An artist impression of light rail outside the main Wellington train station. Image / Supplied
Parts of suburbs like Kilbirnie, Rongotai and Miramar were only two metres above sea level, it said.
"In combination, these risks severely limit the potential for intensified urban development in these suburbs, which is necessary to support the business case for MRT investment."
The route to the south was preferable because areas targeted for housing in the likes of Mount Cook, Newtown, Berhampore and Island Bay were "largely unaffected" by sea level rise or tsunami risk.
Wellington Airport chief executive Matt Clarke said he was comfortable with the change in route.
"As long as there's increased capacity to get people to and from the airport. The proposed mix of road improvements, rapid transit buses and a new tunnel should deliver that," Clarke said.
"The status quo clearly isn't good enough. At peak times it can take longer to get to the CBD than the flight from Auckland or Christchurch. That's no good for Wellingtonians, for visitors to our city, or the environment."
The Government's preferred option for LGWM includes a second Mt Victoria tunnel and bus priority to the east.
Wellington's new airport bus has been a success. Photo / GWRC
Clarke said the service was working well and could be quickly scaled up to meet demand.
But despite the move away from the east, the briefing noted the new preferred light rail route still travelled through two areas at risk of sea level rise.
One was the waterfront section between the railway station and Taranaki St, which was a risk borne by all infrastructure in the city centre anyway.
The other problem was the southernmost section of the route at Island Bay, including the terminal, which is four metres above sea level.
Design measures to mitigate the risk would be considered during the detailed business case phase, the briefing said.
But the report still found Option 2, not the Government's preferred Option 1, was the most resilient.
Option 2 has bus rapid transit running to the south rather than light rail.
LGWM programme director Sarah Gardner said Option 2 scored higher because bus rapid transit didn't travel on tracks and could adapt to move around damage after an earthquake, whereas a light rail vehicle ran on a fixed route and could not be diverted from that route to avoid damage.
"It's important to note that resilience is just one of the many considerations in selecting the preferred option," she said.
Option 1 rated higher for new housing opportunities and passenger capacity.
Mass rapid transit is the centrepiece of LGWM, while a second Mt Victoria tunnel is the other big project in the mix.
The Government wants to turn the existing tunnel into one dedicated for walking and cycling, and build a new tunnel with two lanes for cars and two lanes for public transport.
The existing tunnel was identified as high risk due to potential landslides and its importance as a key link between the city, eastern suburbs, and the airport.
Gardner said like many parts of Wellington, the hill slopes of Mt Victoria were vulnerable to landslides which could damage or block the entrance to the tunnel.
"A new tunnel would provide opportunities to design the entrance and exit to better withstand landslides. For example, the entrance and exit could extend further out from the slope, offering better protection from any landslide that occurs," Gardner said.
