The Marlborough SLR assessment leverages the NIWA national inundation assessment (Paulik et al. 2023) that quantified extreme sea levels, including the effects of wave setup around Aotearoa New Zealand. The national assessment quantifies extreme sea level that includes astronomical tide, storm surge and an estimate of wave setup while considering various levels of future SLR.
The NIWA assessment for Marlborough spatially delineates coastal inundation including allowance for Relative Sea Level Rise (RSLR), that includes projected changes to Mean Sea Level (MSL) from climate change and Vertical Land Motion (VLM) over a 100-year planning timeframe, out to the year 2130.
The projections of absolute SLR incorporate the Intergovernmental Panel on Climate Change (IPCC) 2021 AR6 assessment. The assessment uses the projections within the report to demonstrate a range of potential socioeconomic futures, termed Shared Socioeconomic Pathways (SSP).
The SSP pathways range from SSP1-2.6, a very low greenhouse gas emission future to SSP5-8.5, a very high emissions future. SSP2-4.5 is an intermediate emission scenario that closely aligns with current global emissions’ reduction commitments via the Paris Agreement. Table 2-3 on page 13 of the NIWA assessment contains a translation of SSP scenario to absolute sea level rise ‘m’.
Recommended use of SLR scenarios is identified within the MfE interim guidance (August 2022, updated from 2017 guidance), which recommends Council use five selected SSP scenarios in combination with allowance for potential Vertical Land Motion (VLM) and a planning timeframe out to 2130. Including both Permanent Mean High-Water Springs (MHWS) and 1% AEP (Annual Exceedance Probability) storm-tide plus wave-setup events, plus projected relative sea-level rise over a 100-year planning timeframe.
For this assessment low-lying areas that are not directly connected to the coast are excluded from the commentary in the assessment. However, these areas are spatially captured to inform future assessment, the potential effects of this indirect connection are highlighted within the report.
The use of these scenarios forms an initial planning response in the wider context of developing dynamic adaptive plans for communities and infrastructure along the coast, these scenarios cover the period until a Dynamic Adaptive Pathways Planning (DAPP) strategy has been developed with the community, Iwi, and other stakeholders.
It’s important to note that these SLR scenarios and interim guidance are not intended to become de-facto recommendations as the community-driven DAPP process and resultant outcomes created with the Marlborough ‘community’ will replace these in time with more appropriate locally driven policy content and potential operational responses.
Council acknowledge that these scenarios, information, and projections may change over time, and that future work to keep this information current will likely be required., NIWA make several recommendations for further work, these recommendations will be taken to Environment and Planning Committee meeting for ratification (October 5 2023).
Spatial Data
The NIWA assessment quantifies Relative Sea Level Rise (RSLR) for the district by providing information specifically using the following scenarios.
- Two high-level inundation scenarios
- Permanent Mean High-Water Springs (MHWS)
- Intermittent Inundation of MHWS (1% Annual Exceedance Probability – 1% AEP) extreme sea level, equivalent to a 1 in 100-year event above the new MHWS.
- Three time periods
- Five future SSP scenarios
- SSP1-2.6,
- SSP2-4.5,
- SSP3-7.0,
- SSP5-8.5
- and SSP5-8.5 H+
The output of the assessment is a series of digital GIS features for each time period (3), inundation level (2), and sea level projection (5), a total of 30 layers.
For community information sharing and engagement these GIS files will be made available via Council’s ‘Smart Map’ service which the community can easily access; the map is structured around the scenarios identified above.
Go to the Sea level rise map
Inundation extent was further refined to be “direct inundation”, being inundation directly linked to the sea, or “indirect”, where the land area is lower than the inundation level but not directly connected to the sea. While not likely to the directly inundated, areas shown as indirect inundation are likely to be subject to secondary effects from climate change such as increasing groundwater levels or increased susceptibility to catchment-based flooding due to higher coastal water levels.
The assessment uses a “bathtub” model to produce inundation maps that show the spatial extent. The inundation data is generated by projecting an extreme sea level value across land, with any land that lies below the extreme sea level deemed to be inundated.
However, this simplified bathtub approach does come with caveats. Storm-tide peaks may typically last for only 1 to 3 hours around the time of high tide. This duration 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. Therefore, bathtub type models do not fully capture the dynamic and time-variant processes that occur during an inundation event, and usually result in an over estimation of coastal inundation.
For approximately 90% of the Marlborough assessment area (including the Marlborough Sounds and the East Coast south of White Bluffs / Te Parinui o Whiti) NIWA have high confidence that the report and spatial modelling is an appropriate representation of future SLR scenarios.
For the remaining area, specific to the Lower Wairau Plain (including Blenheim, Rārangi to Te Pokohiwi / Boulder Bank) NIWA recommends undertaking further refinement work via more robust hydro-dynamic modelling to improve the confidence in the modelling under future SLR scenarios.
This is because both the Wairau and Opaoa rivers and Vernon Lagoon heavily influence the tidal ebb and flow, hydro-dynamic modelling is expected to greatly improve the confidence in representing future SLR scenarios.
NIWA Report Recommendations
The following are NIWA’s recommendations to refine future inundation hazard and to improve the supporting information and confidence for the SLR assessment of the Lower Wairau Plain. Council business units agree with the direction of these recommendations, with a work programme to be developed to support them:
- A review of the managed waterway network, specifically for the location and operation of stop gates, be completed to ensure static mapping captures the current flood protection infrastructure.
- Review of localised stop bank breach locations highlighted in the mapping that result in inland flooding. These locations should be reviewed to ensure that the LIDAR resolves land levels in these areas adequately.
- Following refinement of the managed waterway infrastructure, refresh the inundation maps for the Lower Wairau area.
- Develop a dynamic 2D hydrodynamic model of the Lower Wairau to simulate permanent and extreme time varying coastal inundation. It is recommended that the SSP2-4.5, SSP5-8.5 and SSP5-8.5 H+ scenarios be adopted as a minimum for future inundation delineation. The 2D hydrodynamic modelling should be completed with a model such as X-Beach-GPU that resolves complex wave breaking processes in combination with simulation of tidal flow over complex bathymetries.
- The Lower Wairau is susceptible to catchment-based flooding which may be exacerbated by RSLR. It is recommended that a joint probability assessment is completed for the Wairau River discharge and coastal tidal levels. Inundation extent from both catchment-based inputs and coastal inundation should be quantified for the Wairau River catchment via 2D hydrodynamic modelling.
- The potential effects of RSLR on groundwater, particularly for the Wairau River area, should be assessed including salinisation to assist land use planning.
- As further VLM information becomes available it is recommended to update inundation assessments accordingly.
Council is currently scoping point (d) with NIWA. Once this work is completed Its results will be reported back to the Environment and Planning Committee, with the data from the subsequent hydro-dynamic modelling intending to replace the data from the initial bathtub assessment.
What is Happening Next?
This initial project focused on SLR and the initiation of Step 1 from the DAPP process (What is happening? preparation and context) and considers information collection needs for Step 2 (Hazard and Sea-level Rise Assessments) this comes from the MfE ten-step decision cycle guidance.
Council sees the report presenting the beginning of a longer-term process to implement an adaptation programme called Dynamic Adaptive Pathways Planning (DAPP). The DAPP process is a collaborative community engagement model that identifies values to be protected and prioritises areas, assets and infrastructure where the coastal environment is under threat of inundation from rising sea levels and storm surges. Using that process, implementation plan to avoid or mitigate the adverse effects of such outcomes on the community could be developed.
The 10-Step Decision Cycle / Dynamic Adaptive Pathways Planning (DAPP)
