Coastal flood depth (Deltares + WRI Aqueduct, combined)

Source: Deltares + WRI Aqueduct Year: 2018 Category: Physical risks · Mitigating services · Climate Acute Risks Coverage: Global (coastal) Format: Raster grid (~90 m) Used in risk analysis: Yes — gates Mitigating services

What it shows

This layer maps how deep coastal flooding could be at a given location for a severe storm-surge event, expressed as inundation depth in metres. It represents a one-in-a-hundred-year coastal flood under present-day conditions. By giving site-level depth rather than a broad category, it helps distinguish exposure within long, low-lying coastal stretches such as deltas and gulf coasts.

How it is built

The layer averages two open global flood-model products on a fine (~90 m) coastal grid: the Deltares Global Flood Maps (using a vegetation- and building-corrected elevation model, 2018, with surge driven by modern reanalysis) and the WRI Aqueduct coastal flood hazard product (historical baseline, no subsidence or flood defences, resampled to the finer grid). Both share the same underlying surge-model lineage but differ in their elevation models and in the vintage of the surge forcing; averaging them yields a middle-ground depth estimate whose extent follows the broader of the two footprints. Wave setup and coastal protection structures such as dykes and levees are not included.

How to read it

Values are inundation depths in metres for a one-in-a-hundred-year coastal flood: higher values mean deeper potential flooding and greater exposure. Because defences and wave setup are excluded, depths are best read as a hazard screening signal rather than a guaranteed local outcome.

Class thresholds

The underlying layer models a physical quantity: coastal flood inundation depth, in metres (m) — the modelled water depth at a given pixel for a one-in-a-hundred-year (RP100) coastal flood under present-day conditions. The five risk classes map to the following thresholds on that depth:

A site is flagged on the Mitigating services dimension when the modelled depth exceeds 1 m (RP100) — i.e. it reaches the Moderate-risk class or above (proximity template: "Coastal flood inundation depth > 1 m (RP100)").

How these thresholds were set. This is a Darwin in-house classification: round-number depth cutoffs (0.5 / 1 / 2 / 4 m) applied to the averaged Deltares + WRI Aqueduct depth raster to give an interpretable five-class submersion scale. The breaks are Darwin's own banding of the continuous depth field, not a published external standard.

Two deliberate threshold choices, noted for transparency.

• The bands describe submersion severity, not event rarity. This layer and the GIRI flood-depth layer use the same 0.5 / 1 / 2 / 4 m bands even though they represent different return periods (this coastal layer is a 100-year event; GIRI is a 10-year event). The bands answer "how deep would the water be?", which is comparable across return periods; they do not encode how often the flood occurs. Read the return period in each layer's title alongside the class.
• The same layer is binned two ways for two purposes. The site-level map and proximity flag use the absolute physical bands above; the country/region risk profiles re-bin the same depth surface by 20/40/60/80 quantiles ([0.39, 0.52, 0.66, 0.82] m). This is intentional — absolute depth is what a site needs, while a relative ranking is what comparing whole countries needs (and it mirrors WWF's quantile approach, see below).

Source

Deltares Global Flood Maps (via Microsoft Planetary Computer) and WRI Aqueduct Floods Hazard v2.

Comparison with the WWF Risk Filter Suite

This layer relates to the WWF Water Risk Filter (WRF) "Flooding" risk category (and the Biodiversity Risk Filter S3_7 Flooding theme). In WWF that category is a composite of two basin-level sub-indicators, both aggregated to HydroBASINS Level 7 catchments:

Why our thresholds differ from WWF's

WWF's flood-hazard class breaks (≈5–7 cm) sit far below Darwin's coastal-depth breaks (0.5–4 m), but the two are not comparable as physical depths:

• Unit of analysis. WWF takes the median depth over an entire HydroBASINS L7 basin, where most pixels are outside the flood extent (0 m), so the basin median collapses to a few centimetres and is meaningful only relative to other basins. Darwin reads the modelled inundation depth at the ~90 m pixel under the site.
• Relative vs. absolute classification. WWF uses quantiles (each class = one-fifth of basins); Darwin uses absolute physical depth bands suited to site-level submersion screening.
• Different hazard. WWF's hazard component is JRC river flooding (RP100); this Darwin layer is coastal storm-surge inundation (Deltares + WRI Aqueduct, RP100), which WWF's flood category does not isolate.

The difference is therefore justified by design: a basin-median quantile cannot tell apart submerged from dry ground along a long, low-lying coast (Camargue, Mekong, US Gulf, Bangladesh delta), which is precisely what this layer exists to do.

Where Darwin aggregates this layer to a coarser unit — the country/region risk profiles — it adopts a WWF-style 20/40/60/80 quantile classification (thresholds [0.39, 0.52, 0.66, 0.82] m over ~410 geographies), i.e. methodologically aligned with WWF's relative approach, differing only in the aggregation unit (country vs. basin) and source.

Risk analysis

A site is flagged on a dimension by combining a proximity trigger (this layer) with an activity trigger (the entity's ENCORE pressure/service). Proximity only → Potentially material; proximity and the matching ENCORE pressure/service is material → Very material; neither → Not material.

Legend

Symbolised field: Coastal submersion risk

Generated from darwin/layers/layer-coastal-flood-depth-deltares-wri-combined.toml and risk_indicator_pairs.toml (develop).