Maps show Northland coastal erosion and flood threat to homes

Northland's coast has been newly charted showing where 13,500 properties potentially stand in the path of tide and time.

Climate change, storm surges, tsunamis, earthquakes and poorly placed human footprints could all increase the risk of flooding and coastal erosion at sites identified on Northland Regional Council's new coastal hazard maps.

Work to update existing maps showing coastal hazard zones, some charted more than 10 years ago, has taken four years and includes new at-risk areas.

Some of the information could give low-lying towns such as Dargaville, Ruawai and Awanui a sinking feeling.

On the positive side, flood, erosion or permanent inundation lines on some properties that were included in previous drafts have been reduced in size. Some of those adjustments follow landowners making submissions to the 2016 and mid-2017 drafts.

There are up to 13,500 potentially affected properties the length of Northland's east and west coasts — 3200km long and taking in several harbours including one of the world's largest, the Kaipara, and the Northern Wairoa River which is tidal for much of its 150km.

There are 31 sites on the coastal erosion hazard maps and 63 on the flood maps.

The maps show estimated flood and erosion hazards, projected over 50 years and 100 years' time, with conservatively predicted sea level rises of 0.4m over the next 50 years and 1m over the next 100.

"They also assume that coastal defences are not modified, and that coastal erosion continues unchecked during that time," NRC chairman Bill Shepherd said.

In terms of the mapping, ''hazard'' does not mean the destruction, breaking away or damage-potential of coastal structures, which may or may not exist in up to 100 year's time.

Modern topographic tools used to identify areas included hydro-dynamic modelling to gauge tidal zones and the ''old bath tub'' system, based on the level water might reach and stay at, NRC group manager of environmental services Bruce Howse said.

Mr Shepherd said the new maps were the result of the largest coastal hazards study ever undertaken in Northland.

They come just as central government, via Ministry for the Environment, released its own guidance on coastal hazards and climate change, he said.

"In that sense, Northland is now effectively ahead of the game in terms of identification of coastal flooding and erosion hazards."

After formal public consultation through Northland's three district councils, the new maps and applicable rules are likely to be put into the district plans.

They would then be taken into account in decision-making on subdivision, land use and building consents, as already occurs with the existing coastal hazard zones.

The coastal hazard maps and related reports can be viewed at www.nrc.govt.nz/coastalhazardmaps.