Coastal hazard maps - planning to withstand the test of time

New coastal hazard maps identifying land potentially at risk of flooding from storm surge and long-term erosion by the sea have been released by the Northland Regional Council.

The council’s chairman Bill Shepherd says the new maps come after the largest coastal hazards study ever undertaken in Northland.

“Their completion marks the end of a four-year project to update coastal hazard zones (some identified more than 10 years ago) and includes new areas identified following extensive peer-review and the use of modern topographic tools.”

Chairman Shepherd says the finalised maps will replace previous coastal hazard mapping and coincidentally, come as central government (via Ministry for the Environment) has just released its latest guidance on coastal hazards and climate change.

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

He says draft coastal hazard maps were released for public feedback from landowners in the mapped areas in 2016 and in mid-2017.  These resulted in about 55 submissions from the 8000 property owners contacted by the council which helped shape the final maps.

Generally, only minor adjustments have been made to the draft mapping.

“However, for a few sites – including Awanui, Ruawai and the Northern Wairoa –  a more refined modelling assessment has been undertaken, and final coastal flood hazard zones in these areas have been significantly reduced compared to the draft versions.”

In all, there are up to 13,500 potentially affected properties up and down the length of Northland’s east and west coasts.  These are documented in coastal erosion hazard maps for 31 sites and coastal flood hazard maps for 63 sites where survey information is available.

At some stage, after formal public consultation processes conducted by Northland’s three district councils, the new maps and any rules around them are likely to be incorporated into district plans.

The maps and rules would then be taken into account in district council decision- making on applications for subdivision, land use and building consents. Similar rules are already in place for the region’s existing coastal hazard zones.

“Identifying areas that could be at future risk of coastal hazards helps regional and district councils and landowners to manage risks associated with new development,” Chairman Shepherd says.

“It’s about future planning for our coast and creating strong and resilient coastal communities where new buildings can withstand the test of time.”

The maps show the estimated flood and erosion hazards in 50 years and 100 years’ time, based on conservatively predicted sea level rise scenarios of 0.4m during the next 50 years and 1.0m during the next 100 years.

“They also assume that coastal defences are not modified, and that coastal erosion continues unchecked during that time.”

Chairman Shepherd says present-day coastal flood hazard risk areas have also been identified for comparison with future scenarios.                                    

Councils throughout New Zealand are required to ‘give effect’ to national policy and help manage risks for land that’s susceptible to natural hazards, via regional and district policy and plans, as well as building and resource consents.

He says the regional council will continue to progressively investigate and map areas potentially affected by coastal hazards, and is currently implementing a region-wide aerial land elevation (LIDAR) survey due for completion by the end of 2018.

The survey will enable the council to further update the mapping for any coastal landform change, and extend the mapping around the entire Northland coastline.

From then on review and updates will be scheduled about once a decade or as significant new information becomes available.

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