An initial coastal inundation assessment for Marlborough looking out 100 years has been completed by the National Institute of Water and Atmospheric Research (NIWA), based on its national inundation assessment.

The assessment is a first step in a staged approach to delineate the future effects of sea level rise for the Marlborough district. Its primary objectives are to identify land potentially susceptible to sea level rise, and based on initial mapping, develop the future scope of work to refine the inundation extent.

The report was presented to the Council’s Environment and Planning Committee today.

“This report gives us a good understanding of expected sea level rise in Marlborough, through to 2130, although further research is needed to refine the inundation mapping for the Lower Wairau,” Committee Chair Gerald Hope said.

“Like many local authorities, Council has started planning for sea level rise. Some councils have completed their coastal hazard assessments - Marlborough is working through this process now.”

“Although we may not see the impacts of sea level rise for decades, there is a need to start preparing. We must be proactive and start the conversation with the community on how we respond to this challenge.”

“Unfortunately the effects of sea level rise are a real threat. As a community we’ll need to take proactive and deliberate action on how we accommodate rising sea levels and its effects on existing and future land and infrastructure."

“However, it’s important to note that this new study assessed a range of potential futures which also included a "worst case" which is used for land use planning.

The main areas of expected impact are low-lying parts of Picton, Havelock and the Lower Wairau River Plain. Elsewhere, the extent of inland inundation is limited due to Marlborough’s steep backshore. The report calls for further study to be undertaken on the Lower Wairau, including Blenheim and Rarangi to Te Pokohiwi/Boulder Bank.

Sea level rise is predicted to be approximately 20cm to 30cm by 2050; 38cm to 90cm by 2090 and 60cm to 1.67m by 2130. The NIWA assessment further considers the effects of ongoing land uplift and subsidence throughout the district to the year 2130.

The NIWA assessment uses a static inundation model (also called the bathtub model) to produce maps showing land that has the potential to be inundated. Inundation maps have been generated by adopting a static sea level across land and any land that lies below the level is deemed to be inundated.

The simplified static inundation model comes with caveats. Storm-tide peaks may typically last for only one to three hours around high tide, which may not provide sufficient time to inundate large land areas, particularly if seawater ingress rates are affected by narrow constrictions, such as drainage channels and culverts. As static inundation models do not capture these dynamic and time-varying processes the method usually results in an over estimation of coastal inundation, particularly for very low-lying and flat areas. However, static models resolve inundation well for steep backshore areas and for large increments of sea level rise.

The next step in the assessment process is to further refine the inundation extent where required via more complex methods and studies. The NIWA study has identified the Lower Wairau River Plain for priority assessment due to the complexity of the managed waterways which are likely to affect inundation extent and likely to be overestimated in this initial assessment. This work will begin straight away.

Members of the public can read the NIWA report, view the committee presentation and access the interactive mapping showing different sea level rise scenarios and three time periods (2050, 2090 and 2130) at www.marlborough.govt.nz/environment/climate-change

Councillor Hope said the Council had committed to implementing an adaptation programme - called Dynamic Adaptive Pathways Planning (DAPP).

“This is a collaborative community engagement model that identifies and prioritises areas, assets and infrastructure where the coastal environment is under threat of inundation. We have already written to iwi and will start planning a wider public engagement effort.”

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