The Coastal Storm Inundation dataset maps the extent of sea water coverage expected around the Auckland Region during sustained coastal storm-tide flooding events. Elevated storm-tide sea levels are predicted based on the joint probability of a high tide, a storm surge (which is the rise in sea-level caused by wind action and low barometric pressure related to storm events), and wave setup. (Wave setup is the elevation of the mean sea level at the shoreline due to breaking waves. Larger dynamic wave processes like wave runup and overtopping are not included in this mapping so should be considered on top of these water levels. Rainfall and freshwater flooding are also not included in this data, but are available separately in other geomaps layers e.g. ‘Flood Plains’, or on the Auckland Flood Viewer).

Coastal inundation (or coastal flooding) is mapped for a range of Annual Exceedance Probability (AEP) events, where the AEP is the % probability of an event occurring each year. Extreme events can also often be referred to by their return period, or Average Recurrence Interval (ARI). The table below gives the conversion between the two terminologies:

The extreme sea level events were calculated between 2013 and 2019, as compiled in Carpenter, N., R Roberts and P Klinac (2020), Auckland’s exposure to coastal inundation by storm-tides and waves, Auckland Council technical report, TR2020/24, Auckland’s exposure to coastal inundation by storm-tides and waves (knowledgeauckland.org.nz)

Increments of sea level rise have been applied on top of storm-tide sea level events in order to assess the increasing coastal flooding hazard into the future. Sea-level rise values applied currently align with the projections by the Intergovernmental Panel on Climate Change sixth assessment report (2021), and the Ministry for the Environment (2022) Interim guidance on the use of new sea-level rise projections, which updates the Ministry for the Environment Coastal Hazards and Climate Change Guidance for Local Government (2017).

In MfE’s (2022) interim guidance, (excluding vertical land movement) one metre sea-level rise is projected to occur between 2095 - >2200, depending on the emission scenario used. Two metre sea-level rise is projected to occur in the longer term (beyond 2150).

MfE’s (2022) Interim guidance recommends the inclusion of vertical land movement (VLM) in relative sea level rise considerations. These are not included in the above sea level rise predictions due to the high VLM variability across the region. Vertical land movement is generally predicted to increase the rates of relative sea level rise for the Auckland region so should also be incorporated in planning and design.

Refer to Interim guidance on the use of new sea-level rise projections | Ministry for the Environmentand NZ Sea Risefor more information on MfE’s interim guidance on sea level rise and vertical land movement.

The inundation mapping is based on the Digital Elevation Model ground levels surveyed by aerial LiDAR between 2016-2018.

Coastal inundation mapping is not available for some scenarios for the Parakai-Helensville area as this area’s complex dynamics requires hydrodynamic modelling to accurately assess the coastal storm inundation extent. Hydrodynamic modelling has not yet been carried out for all scenarios (e.g. 1.5m sea level rise) but is available for most 2% & 1% AEP scenarios and all tidal (MHWS) scenarios. Please see technical report 2020/024for information on the hydrodynamic modelling.

Extreme sea levels for the Auckland region were derived by NIWA in 2013 (Part 1 of Technical Report 2020/24). From 2016-2019, additional extreme sea level data was gathered for:

• The east coast estuaries (NIWA, 2016; Part 2 of Technical Report 2020/24)

• Parakai/Helensville Harbour (DHI, 2019; Part 3 of Technical Report 2020/24)

• Great Barrier Island (NIWA, 2019; Part 4 of Technical Report 2020/24)

In 2020, these levels were projected onto the land topography (derived from the 2016-2018 LiDAR survey) by Stantec to establish the extent of coastal flooding. Additional coastal flooding scenarios (annual exceedance probability events + sea level rise) were mapped in 2023 by Watershed Engineering and mean high water spring tides + sea level rise were mapped by NIWA.

(Note: The studies informing this mapping of coastal inundation generally used Auckland Vertical Datum 1946. Auckland Council is now transitioning to using New Zealand Vertical Datum 2016.)

Update Cycle:

Adhoc when improved data becomes available.

This data is available to the public on the Geomaps viewer and on Auckland Council Open Data portaland some coastal flood mapping is copied into LIM reports.