Ruawai residents question new Northland coastal hazard maps for the area

NRC climate resilience coordinator Matt de Boer outlines coastal hazard mapping to farmers and others at a recent Kaipara District Council drainage committee meeting in Ruawai. Photo / Susan Botting

Some Ruawai residents are pushing back against Northland's new draft coastal hazard mapping.

Northland Regional Council (NRC) has released coastal flooding hazard maps for the entire Kaipara Harbour - as part of new region-wide coverage showing risk areas.

They show a bigger area of coastal flooding risk across Ruawai in a climate change-influenced future. Some parts of Ruawai flats are slightly above current sea levels, others half a metre below.

Raupo drainage committee chair Ian Beattie said the mapping used the worst-case scenario and didn't take into account that Ruawai was protected by an engineered system of at least three metre-high stopbanks.

NRC climate resilience co-ordinator Matt de Boer outlined the new mapping to local farmers, businesspeople and others at a recent quarterly Kaipara District Council (KDC) drainage committee meeting in Ruawai. It was the mapping's first public community meeting appearance since being released by NRC on April 14.

Beattie said the mapping had not taken the stopbanks into account adequately when calculating how sea level rises may influence local coastal flooding. Ruawai's $14 million Raupo flood protection and drainage network covers 8700 hectares, managing flooding risk via 70 kilometres of stopbanks, 140 kilometres of canals and drains, 52 floodgates and a flood pump.

Beattie said some exposed stopbanks had been topped up to four metres.

But while de Boer said the mapping had taken the stopbanks into account, he accepted there could be further consideration given. NRC would work with the committee to look further at the maps.

He said the government required councils to take a 1.2-metre sea level rise into account, with emphasis on a potential bigger 1.5-metre increase. Ruawai Promotions and Development Group chairman Bruce Crompton said his local district already had great flood protection infrastructure.

Ruawai resident opposed to new coastal hazard mapping say the town's stop banks system is adequate to mitigate any sea level rises.

It should be this, rather than forcing planning rules on to new development, that managed local sea level rise risk.

Crompton said new Ruawai housing already had to be built much higher than previously, due to new planning rules. The meeting hadn't allayed his concerns about this.

The new mapping means full coverage for Kaipara Harbour's northern coastline for the first time, its land parcels among 20,000 around Northland's coast - a 60 per cent increase on the 12,500 previously.

Coastal erosion mapping has also expanded. There are now 41 Northland sites mapped, up from 30 previously. Roughly 2500 land parcels are mapped, 500 more than previously.
Northland's district councils must take the NRC's new coastal flood and erosion hazard mapping into account.

Northland could be facing a sea level rise of up to 1.5 metres, new coastal hazard mapping assumes.

The NRC's new coastal erosion and flooding hazard mapping uses this figure as its "extreme" scenario to calculate risk for climate change adaptation planning.

The new mapping is built around updated 50 and 100-year government projections and new aerial survey data (at www.nrc.govt.nz/coastalmaps) and is now open for public feedback before being finalised in July.

Sea level rise, storm surge and wave run-up is applied to local elevations, land coverage and tidal information in the new coastal erosion and flooding maps - to be used for decision making to manage coastal hazard risks.

One of the three-metre stopbanks Ruawai residents say will deal with any sea level rises.

The four scenarios are:

■ Current day - shows areas susceptible to coastal flooding in a 1-in-100 year storm now and into the future, with no allowance for sea level rise.

■ 50-year projection – shows areas susceptible to coastal flooding in a 1-in-50-year storm, with a projected sea level rise of .6 metre by 2080.

■ 100-year projection – shows areas susceptible to coastal flooding in a 1-in-100 year storm, with a projected sea level rise of 1.2 metres by 2130.

■ 100-year "extreme" projection – shows areas susceptible to coastal flooding in a 1-in-100-year storm event, with a projected sea level rise of 1.5 metres by 2130.