Flood protection ramps up with sea-level rise of 1.35m predicted in next 100 years

A policeman diverts traffic from a flooded Newlands Rd in July. (File photo)
KEVIN STENT/Stuff
A policeman diverts traffic from a flooded Newlands Rd in July. (File photo)

Stopbanks will be built higher and barrage gates stronger, as flood protection in the Wellington region ramps up in accordance with the latest climate science.

New flood protection infrastructure will need to plan for sea level rise of at least 1.35 metres, according to guidance proposed by the Greater Wellington Regional Council, as it plans to update its flood protection policy.

The Ministry for the Environment recommends all new infrastructure take into account the sea-level rise of approximately 1.35m by the year 2121 (a 100-year horizon), and a projected increase in rainfall of up to 30 per cent by 2120 in some catchments.

The regional council’s policy, presented to the climate committee on Tuesday, will trickle down into the lives of residents by way of local councils’ district plans, some of which are up for review in the next few months, and which set the regulations for what can be built, and where.

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MARTEN RABERTS
Big waves hit The Esplanade, between Ōwhiro Bay and Island Bay on Wellington's South Coast.

The current regional council policy was set in 2013; by the year 2100, the rainfall intensity was estimated to increase by 20 per cent – 10 per cent less than the new estimate – and sea-level to rise by 0.8m – an increase in the new estimate of 55 centimetres.

Regional councillor Thomas Nash said: “The data has become clearer, and it's got worse.”

Mitigation strategies will be undertaken on a catchment-by-catchment basis and use the Representative Concentration Pathway (RCP) – a greenhouse gas concentration trajectory adopted by the IPCC – of scenario 6 as a minimum.

People walk and cycle along the stop-bank at Avalon, Lower Hutt, but the infrastructure serves an important purpose when it comes to rising sea and river levels. (File photos)
John Nicholson
People walk and cycle along the stop-bank at Avalon, Lower Hutt, but the infrastructure serves an important purpose when it comes to rising sea and river levels. (File photos)

A RCP 8.5 scenario will be used for works that protect assets of “high community importance” – the location of a new hospital or protection of large populations.

The effects of climate change are expected to be increased rainfall, meaning greater flows within watercourses, severe storms becoming more frequent and less predictable (increasing erosion on infrastructure) and sea-level rise reducing the effectiveness of coastal flood defences.

Rising groundwater tables will reduce the amount of water the ground can absorb, leading to increased runoff, and higher river levels.

Before and after of Takarau Gorge Rd, which runs between Johnsonville and Makara in Wellington, after heavy rains flooded Ohariu Stream. (File photo)
Supplied
Before and after of Takarau Gorge Rd, which runs between Johnsonville and Makara in Wellington, after heavy rains flooded Ohariu Stream. (File photo)

Urban areas within the Hutt Valley, townships on the Kāpiti Coast, Masterton and Greytown in the Wairarapa, and rural areas throughout the region have been identified as particularly at risk.

The goal of this policy was “providing sound research to communities, in relation to their transport, infrastructure their homes”, Nash said.

“The best we can do is prepare people in the best possible way.”