Our sandy beaches and shorelines are subject to natural processes of coastal erosion (sediment loss) and coastal accretion (sediment gain). Beaches or shorelines will often experience a cycle of erosion followed by accretion, with the duration of the erosion-accretion phase ranging from weeks (the period of a storm event and post-storm recovery) to longer periods over a year(s), a decade or several decades.
In some areas of the district, coastal erosion is already affecting roads, reserves and private property. Mitigation measures (such as walls, rock revetments, sand push-ups) are already undertaken on parts of the district’s coastline. Climate change will result in sea level rise and an increase in storm severity and frequency. This will increase the exposure of our coastal margins to coastal erosion and accretion, creating new erosion hazards in areas that have not previously been exposed. Both Golden and Tasman Bays have typically low energy wave climates. As such recovery times following big storms generally take longer than other parts of New Zealand's coast line. Given this, any increase in the frequency of storms could result in the exceleration of erosion along the district’s coastline.
The Ministry for the Environment has developed a series of factsheets which provide further information on the different elements of coastal processes. View the factsheets on the Ministry for the Environment website(external link).
Over the years a range of formal and informal structures have been erected around the districts coastline. These structures include rock armouring, rock revetments, walls, earth bunds or stopbanks, and causeways. In many cases the effectiveness of these structures, their condition and ongoing maintenance, along with their legal status, is uncertain. As sea levels rise the continued presence and functioning of these structures cannot always be guaranteed.
Under the Tasman Resource Management Plan structures in the coastal environment may require resource consent.
Coastal erosion at Moturoa/Rabbit Island from ex-tropical Cyclone Fehi (Feb 2018)
We have started work on a natural hazards plan change(external link) to the Tasman Resource Management Plan (TRMP). As part of the process, we are improving our knowledge about local hazards and sharing that information with affected landowners and communities.
Recent work undertaken by Auckland University (Aotearoa’s Coastal Change Dataset(external link)) has mapped shoreline change in Tasman and Golden Bays. It uses both aerial and satellite imagery, dating from the earliest available aerial imagery through until 2021. This data was then added to by Council staff to include earlier shoreline information from survey plans as well as more recent aerial imagery. Using this combined data, shoreline change rates per year were calculated for the district’s sandy coastlines.
Generally, erosion and accretion rates vary significantly and over varying time scales and can be significantly different in shorter periods compared to a trend averaged over a longer period. Analysis of the data suggests that there has been an acceleration of erosion rates within the district since the early 2000’s.
We’ve also started a project to bring together our natural hazards spatial data into a single, online, easy to use, natural hazards map viewer. The mapped shoreline changes will be included as a layer in the natural hazards map viewer, available early 2025.
How is the Council using this coastal erosion information?
Consideration of coastal erosion has been embedded in the Council’s work programmes for a number of years. We use this information in:
Next Steps
The next steps in reviewing the TRMP natural hazards plan change(external link) will be to engage with the community on the ‘issues and options’ for managing our natural hazards, including coastal erosion. Engagement is proposed to begin early to mid 2025 and will seek the community’s views on the options for updating the TRMP provisions that relate to natural hazards.
Landowners, iwi, stakeholders and the wider community will have opportunities to input into the natural hazards TRMP plan change as we progress through the plan drafting stages.
Where do I find information about what to do following a natural hazard event?
Information, advice on getting prepared and what to do during a natural hazard event can be found on the Nelson Tasman Emergency Management Group website.(external link)
The Natural Hazards Commission Toka Tū Ake(external link) also has information and advice for home owners, tenants and home buyers to ensure your home is safer for natural hazards.
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