Climate Acute RisksFlood depth — 10-year return, present climate (GIRI)

Flood depth — 10-year return, present climate (GIRI)

This layer estimates how deep floodwater would stand at a given location during a flood of the kind expected roughly once a decade, under present-day climate.

Source: GIRI (UNEP-GRID / CDRI) Year: 1979-2016 Category: Physical risks · Mitigating services · Climate Acute Risks Coverage: Global Format: Raster grid (~90 m) Used in risk analysis: No — display layer since 2026-09-11; the Flood mitigation services flag reads GIRI flood depth RP100 (baseline, 90 m) at ≥ 1.55 m

Changed on 2026-09-11. The riverine flag reads GIRI flood depth RP100 (baseline, 90 m), the raster the Direct Physical Loss engine prices the riverine arm from, so the screening view and the priced loss explain a riverine site with the same picture. The two were kept apart while the engine read a ~2 km resample against this layer's ~90 m; the engine now reads GIRI at ~90 m, so that reason is gone. The threshold could not carry over — the swap changes both the return period and the unit — so it was measured: the extent this layer flags at ≥ 1 m is reproduced by the 100-year layer at ≥ 1.55 m; the target layer's page carries the calibration. This layer stays in the catalogue as a display layer: the once-a-decade flood reads differently from the once-a-century one and is worth seeing on its own.

What it shows

This layer estimates how deep floodwater would stand at a given location during a flood of the kind expected roughly once a decade, under present-day climate. By giving a site-level water depth rather than a broad category, it can tell apart floodable and non-floodable ground within river corridors, deltas and urban areas, making it well suited to screening flood exposure precisely.

How it is built

The layer is produced by UNEP-GRID for the Global Infrastructure Resilience Index (GIRI), developed with the Coalition for Disaster Resilient Infrastructure. It is based on a distributed hydrological model (Silvestro et al., 2013, 2015) driven by a global meteorological dataset for the present-climate run, coupled with a hydraulic model that converts simulated river discharge into flood depth. The output is a global grid of inundation depth at roughly 90 m (3 arc-second), for a 10-year return period and a present-climate baseline (1979–2016). It represents both river and rainfall-driven flooding; coastal storm surge is not included and is covered by a separate coastal-flood layer. Forward-looking climate-scenario versions are available as separate layers.

How to read it

Higher values mean deeper projected floodwater at that location for a once-in-a-decade flood, indicating greater exposure. Locations with little or no depth are effectively outside the modelled flood footprint at this return period.

Class thresholds

The underlying layer models a physical flux: flood inundation depth, in centimetres (cm) — the modelled depth of standing floodwater at each ~90 m cell for a 10-year return period under the present-climate baseline (1979–2016). The raster stores depth in centimetres; the five risk classes map to the following thresholds (shown in metres, as the legend labels them):

ClassFlood depth (10-year return)
Very low< 0.5 m (< 50 cm)
Low0.5 – 1 m (50 – 100 cm)
Moderate1 – 2 m (100 – 200 cm)
High2 – 4 m (200 – 400 cm)
Very high≥ 4 m (≥ 400 cm)

This layer no longer raises a flag. Until 2026-09-11 a site was flagged on the Mitigating services dimension when it exceeded 100 cm (1 m) here — the Moderate class or above; the flag now reads GIRI flood depth RP100 (baseline, 90 m) at 1.55 m, the value that flags the same extent.

How these thresholds were set. This is a Darwin in-house classification: round metre-scale cutoffs (0.5, 1, 2, 4 m) applied to the continuous GIRI flood-depth surface to bin it into five interpretable hazard classes. The breaks are not taken from GIRI or any external published standard — GIRI publishes a continuous depth raster, and the class boundaries are Darwin's own choice of round-number depth thresholds.

Two deliberate threshold choices, noted for transparency.

  • The bands describe submersion severity, not event rarity. This layer and the coastal depth layer — Deltares coastal flood depth RP100 (2018 baseline), which replaced the combined Deltares + WRI layer archived on 2026-09-14 — read depth on the same 0.5 / 1 / 2 / 4 m bands even though they represent different return periods (GIRI here is a 10-year event; the coastal layer is a 100-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. A 1 m depth at a 10-year return is a more frequent hazard than 1 m at a 100-year return, so the return period in each layer's title should be read alongside the class.
  • The same layer is binned two ways for two purposes. The site-level map uses the absolute physical bands above; the country/region risk profiles re-bin the same GIRI surface by 20/40/60/80 quantiles ([188, 261, 360, 450] cm). 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

GIRI — Global Infrastructure Resilience Index (UNEP-GRID / CDRI, 2023). Licensed under CC BY 3.0 IGO. https://giri.unepgrid.ch/.

Comparison with the WWF Risk Filter Suite

This layer maps 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:

WWF sub-indicatorSourceAggregationClass thresholds
Flood Occurrence (B3_1)Dartmouth Flood Observatory — count of large flood events 1985–2021Sum per basin0 · 1–2 · 3–10 · 10–30 · >30 events
Flood Hazard (B3_2)JRC Global River Flood Hazard Maps — RP100 max depth per pixel (0 outside extent)Median per basin, then even quantiles≤0.0514 · ≤0.0530 · ≤0.0567 · ≤0.0718 · >0.0718 m

Why our thresholds differ from WWF's

At first glance WWF's flood-hazard class breaks (≈5–7 cm) look wildly lower than Darwin's (0.5–4 m). This is not a real disagreement — the two layers measure different things and bin them on different principles:

  • Unit of analysis. WWF takes the median depth over an entire HydroBASINS L7 basin, where most pixels lie outside the 100-year flood extent and are recorded as 0 m. That median is therefore diluted to a few centimetres and only carries meaning relative to other basins. Darwin reads the modelled depth at the individual ~90 m pixel under the site, so the number is the actual water depth at that location.
  • Relative vs. absolute classification. WWF classifies by quantiles (each class is one-fifth of all basins), so its breaks are a ranking, not a physical depth. Darwin uses absolute physical depth bands (knee-deep ≈ 0.5 m, etc.), which is what site-level screening needs.
  • Different hazard and return period. WWF's hazard component is JRC fluvial RP100; Darwin's GIRI layer is fluvial + pluvial at RP10.

In short, the difference is justified by design: Darwin's purpose is to discriminate floodable from non-floodable ground within a basin, which a basin-median quantile cannot do. Reusing WWF's centimetre-scale breaks would be meaningless at pixel level.

Where Darwin does aggregate this layer to a coarser unit — the country/region risk profiles — it switches to a WWF-style 20/40/60/80 quantile classification (thresholds [188, 261, 360, 450] cm over ~450 geographies), i.e. methodologically aligned with WWF's relative approach, differing only in the aggregation unit (country vs. basin) and source (GIRI vs. JRC/Dartmouth).

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.

DimensionENCORE service / pressureProximity trigger (this layer)Activity trigger (entity)
Climate hazardsFlood mitigation services— (read from GIRI flood depth RP100 (baseline, 90 m), above 1.55 m; this layer used to trigger above 100 cm)“Flood mitigation services” pressure ≥ 4

Legend

Symbolised field: Flood depth (10-year return)

Value (Flood depth (10-year return))LabelColour
1Very low (<0.5 m)#fee08b
50Low (0.5–1 m)#fdae61
100Moderate (1–2 m)#f46d43
200High (2–4 m)#d73027
400Very high (>4 m)#7b3294

Generated from darwin/layers/layer-flood-depth-giri-10yr-present.toml and risk_indicator_pairs.toml (develop).

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