Clay shrink-swell risk (global)

Source: Multi-source composite (national hazard maps + ISRIC SoilGrids) Year: 2020 Category: Physical risks · Mitigating services · Geological Coverage: Global Format: Raster grid (~1 km) Used in risk analysis: Yes — gates Mitigating services

What it shows

This layer maps the global risk of clay shrink-swell, the behaviour of expansive soils that swell when wet and shrink when dry. These ground movements can damage buildings, roads and other infrastructure. The layer offers a worldwide screening view of where expansive-soil hazard is most likely, helping flag sites where ground stability deserves closer attention.

How it is built

Where precise national hazard or soil maps exist, they are used directly — including France, Spain, the United Kingdom, the Netherlands, Norway, the United States, Brazil and Canada. Elsewhere, a calibrated proxy fills the gaps, built from a decision rule on global soil properties (clay content, cation-exchange capacity and the probability of expansive Vertisol soils). The proxy was trained on the French, Spanish and UK national hazard maps (matching the exact class in 58% of cases and within one class in 94%) and validated against global Vertisol references in the tropics. National areas appear fine-grained while proxy areas are coarser at roughly a 1 km grid. This is a screening layer, not a siting tool.

How to read it

Values fall into three classes: 1 Low, 2 Medium and 3 High. Higher classes indicate greater susceptibility to expansive-soil ground movement. National-map areas are more detailed than proxy areas; treat the proxy regions as indicative.

Class thresholds

This is a categorical hazard-susceptibility layer, not a continuous physical flux: it expresses the susceptibility of expansive (shrink-swell) clay soils as a three-level qualitative class rather than a measured quantity in physical units. There is therefore no single numeric break in one unit — the class is assembled from heterogeneous national hazard maps where they exist (France, Spain, UK, Netherlands, Norway, USA, Brazil, Canada) and, elsewhere, from a decision-tree proxy on SoilGrids clay content, cation-exchange capacity and ISRIC Vertisol probability.

A site is flagged on the Mitigating services dimension when the layer reaches the High class (3) (template: "High clay shrink-swell susceptibility zone (class 3)").

How these thresholds were set. This is a Darwin in-house classification: the three-class susceptibility scale harmonises the various national hazard-map classes onto a common 1–3 Low/Medium/High scheme, and the proxy decision tree was trained against the French, Spanish and UK national hazard maps (matching the exact class in 58% of cases, within one class in 94%) and validated against global Vertisol references. The underlying class boundaries come from those national mapping agencies' own methodologies and the calibrated proxy rule, not from a single published external standard.

Source

BRGM, IGME, BGS, BRO, NGU, UC Davis, IBGE, AAFC and ISRIC SoilGrids (CC-BY).

Comparison with the WWF Risk Filter Suite

There is no equivalent indicator in the WWF Risk Filter Suite. Clay shrink-swell is a geophysical ground hazard rather than a biodiversity or water risk, so it falls outside the WWF methodology and is a Darwin extension.

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: Shrink-swell risk

Generated from darwin/layers/layer-clay-shrink-swell-global.toml and risk_indicator_pairs.toml (develop).