Report models coastal flooding risks resulting from climate change

A technical report that models future coastal flooding risk scenarios for six townships in Southland district has been made public.

The report, prepared by Great South for Southland District Council, is titled Southland Coast and Rakiura Stewart Island, Sea Level Rise & Extreme Sea Level Exposure Spatial Forecasting. It was presented to Council, together with an independent peer review by GNS Science, at a meeting today. This follows a public workshop on 1 November 2023 where preliminary modelling was shared.

Councillors requested a further report from staff about what the next steps will be.

Potential coastal flooding risks due to extreme sea level events plus sea level rise up to 280 years in the future are identified for six communities: Colac Bay Ōraka, Curio Bay, Fortrose, Riverton Aparima, Oban, and Waikawa.

The projections have been supplied as geographic files and integrated into map viewers, which are available on SDC’s website. The GNS Science peer review confirmed that the work is robust.

https://www.southlanddc.govt.nz/environment/climate-change/coastal-hazards/

“We will work with Environment Southland and Great South to integrate other risk information to create a fuller picture of climate change-related hazards for our townships,” SDC chief executive Cameron McIntosh said.

“This information will need to be operationalised so that it can be used by staff in a range of areas including asset management planning.”

Land information memoranda and project information memoranda for indicated properties will note potential coastal flooding risk, in order to fulfil Council’s legal obligations.

“The coastal adaptation planning process needs to commence as soon as possible, in partnership with mana whenua and residents, to ensure our communities are climate resilient,” Mr McIntosh said. “A review of the District Plan in light of the new coastal hazards information and within the wider policy context will also be needed.”

Great South’s report considers a range of factors that contribute to coastal flooding risks, drawing on the most current local data sources and utilising best practice methodologies. Mean high water spring (or high spring tide), storm tide, extreme sea level and sea level rise are considered, alongside vertical land movement.

Extreme sea level means the maximum reach of sea water when a high spring tide and a coastal storm coincide. Extreme sea level events have historically occurred once every 100 years, but are projected to become an annual event by 2050.

The sea level rise projections encompass four different scenarios, for years 2090, 2130 and 2300, in accordance with the Ministry of the Environment’s (MfE) 2022 Interim Guidance on the use of new sea-level rise projections. Due to the effects of sea level rise, in the future an extreme sea-level event would lead to more flooding of low-lying coastal areas.

All information is available on SDC’s website, and will be added to as local climate science knowledge is further refined.

Great South general manager strategic projects Stephen Canny said the area of climate change-induced sea level rise was a dynamic and changing area of science that was constantly being updated and refined as the rate of climate change is better understood and as new guidance is provided by the Intergovernmental Panel on Climate Change (IPCC) and the Ministry for the Environment.

Mr McIntosh said “our communities are already experiencing the effects of sea level rise and changing weather patterns. Council has commissioned this work to provide a basis for better decision-making into the future.

“Council has a responsibility to identify natural hazards, including those that will result from the effects of climate change in the future, and do our best to draw on this information in short, medium and long-term decision-making.”

SDC is a member of the Regional Climate Change Working Group. This inter-agency group is made up of tangata whenua representatives, elected representatives, and key staff from all councils in the region, working collaboratively to develop a Regional Climate Change Strategy and shape Southland Murihiku’s long-term response to climate change.

Southland District Council has set up an email address for those who have any questions: coastalhazards@southlanddc.govt.nz

The report, peer review, Story Maps and further information are available on SDC’s website, here: www.southlanddc.govt.nz/environment/climate-change/coastal-hazards/