NSW Coastal Overwash - 2025

Brief description

This dataset provides a regional-scale assessment of coastal overwash and related inundation hazards along sandy beaches of the New South Wales (NSW) coastline. The dataset comprises spatial layers containing shore-normal transects spaced at 100 m intervals across more than 800 km of open sandy coastline.

For each transect, the dataset includes:

* Average beach slope and associated standard deviation derived from airborne LiDAR surveys.
* Elevation of the overwash threshold for the local backbeach feature (e.g., dune crest, seawall).
* Total Water Level (TWL) estimates for baseline (2020) and future decadal intervals out to 2150, under various sea level rise scenarios associated with the Intergovernmental Panel on Climate Change’s shared socio-economic pathways (i.e. SSP1-2.6, SSP2-4.5, SSP3-7.0, SSP5-8.5 medium confidence and SSP5-8.5 low confidence). For each sea level rise scenario, five annual exceedance probabilities were assessed (AEPs: 100%, 20%, 5%, 2%, and 1%) and key percentiles (17th, 50th, 83rd, 95th, and 99th) of the associated TWLs.
* Corresponding overwash likelihood classifications: Likely, Potential, and Unlikely.

The methods used for this assessment involved modelling total water levels (TWL), i.e. the combined effects of tides, storm surge, probabilistic wave runup and future sea level rise. Still water levels (from astronomical tides and storm surge) were derived from tide gauge data, while runup was calculated probabilistically using ensemble simulations incorporating beach slope distributions. A validated empirical runup formula (Atkinson et al., 2017: https://doi.org/10.1016/j.coastaleng.2016.10.001) was selected through model evaluation against observed storm debris lines.

Future total water levels were estimated by adding sea level rise randomly sampled from future sea level rise distributions fitted to the Intergovernmental Panel on Climate Change (IPCC) sea level projections at tide gauge sites along the NSW coast for several Shared Socioeconomic Pathways (SSPs). Predicted total water levels were then matched against back beach elevation thresholds (e.g., dune crest elevation, seawall crest elevation) to examine both current and potential future coastal overwash and associated inundation likelihoods (e.g., Likely Inundation). Coastal overwash related inundation likelihoods were determined by comparing TWL estimates against local backbeach thresholds (e.g., dune crest, seawall crest) and applying a classification system based on indicative percentiles of the TWL exceedance probability distributions. If the backbeach feature elevation was less than the 83rd percentile of the 1% AEP TWL distribution, then coastal overwash was classed likely. If the backbeach feature elevation was between the 83rd and 99th percentile of the 1% AEP TWL distribution, then the coastal overwash was classed potential. If the backbeach feature elevation was greater than the 99th percentile of the 1% AEP TWL distribution, then coastal overwash and associated inundation was classed unlikely.

Detailed information on file naming conventions, directory structure, shapefile attributes, and layer descriptions is provided in the accompanying ReadMe documentation. Readers are also encouraged to refer to the accompanying technical report (NSW Coastal Erosion and Inundation Hazards and Exposure Assessment – Technical Report, 2025) for detailed information on the methodology, assumptions, and modelling procedures.