How inundation is mapped

Data

The main dataset is the 30-meter resolution Shuttle Radar Topography Mission (SRTM) Digital Elevation Model (DEM), downloaded via Google Earth Engine. It was preprocessed in ArcGIS Pro V3.5 to correct voids, fill sinks and remove anomalies in flat terrain.

Terrain below 20 meters elevation was extracted for focused analysis, as this zone is the most vulnerable to near-term sea-level rise.

The result is a grid layer with one field for each sea-level rise scenario, published to ArcGIS Server as dynamic map and feature services, each with its own API endpoint, and drawn in the ROC Datahub with Esri Leaflet.

The map supports layer toggling, scenario switching, zoom and pan, and pop-up information windows.

Scenarios

Sea-level rise was simulated with a modified bathtub method: projected increments, based on global climate projections including the IPCC Sixth Assessment Report (AR6), are added to a baseline tidal level approximated from regional mean higher high water (MHHW). Each field in the layer holds one scenario.

SLR_1m

+1.0 m

SLR_2m

+2.0 m

SLR_3m

+3.0 m

How a cell is classed

Inundation was mapped by subtracting the DEM from each water surface: where the water level exceeds the ground, the cell is marked as potentially inundated. A cell is then classed by the first scenario that floods it.

Only hydrologically connected zones, those continuously linked to the ocean or tidal systems, are counted as inundated. This was checked with the Region Group function in ArcGIS Pro.

Limitations

Low-lying depressions larger than one acre that are not connected to the ocean were kept but symbolized separately, to avoid overestimating risk. Inundation depths were also mapped for each scenario, and a land-water mask limits the visual output to terrestrial areas.

A bathtub model ignores waves, storm surge, tides beyond the baseline and coastal defences, so it shows exposure to sea-level rise rather than a forecast of flooding.