Clay shrink-swell

Clay shrink-swell

Some clay soils swell when wet and shrink when dry. Over successive dry seasons, that movement cracks foundations and walls. The damage builds slowly rather than in one event. Clay shrink-swell prices both Property damage (% of asset value) and Business interruption (% of annual revenue).

What the map shows

• Today: two rasters. First the global clay shrink-swell risk class (1 low, 2 medium, 3 high) (sheet). Then the CHELSA precipitation of the driest month, 1981-2010, the climate that triggers the movement (sheet).
• Projected: the map does not move; it shows the reference climatology. The engine reads projected driest-month rainfall at 2035, 2050 and 2080 for its trigger, so the loss can change with the horizon.

How the loss is computed

The engine reads the susceptibility class at the site's point. A class-to-loss table, scaled by the ground vulnerability of the site type and by a dryness trigger from the driest month's rainfall, gives the damage per activation; an activation rate turns it into an annual loss. Business interruption is a loss-of-use relation on the ground loss. The table is measured in shape on 43,417 French CatNat drought-subsidence recognitions and in scale on the CCR claim cost. The hazard stays tagged Proposal. See Damage functions: The geotechnical trio and Climate risks.

Clay shrink-swell is measured as a slow drift: the Exclude slow-drift baselines switch sets its baseline to zero so that only the change counts.

Sources

• National hazard and soil maps: BRGM (France), IGME (Spain), BGS (UK), BRO (Netherlands), NGU (Norway), UC Davis (USA), IBGE (Brazil), AAFC (Canada). BRGM: Licence Ouverte Etalab 2.0. IGME: IGME licence, commercial use allowed, credit line below. BGS GeoClimate UKCP18 Open: Open Government Licence v3. BRO: CC0 1.0. NGU: NLOD 2.0. UC Davis: Licence not confirmed (CC BY 4.0 reported). IBGE: Licence not confirmed. AAFC: Open Government Licence – Canada.
• Source: © NC Geological Survey of Spain (IGME), map of expansive clays at 1:1,000,000 (IGME_Arcillas_1M).
• ISRIC SoilGrids (CC BY 4.0), for the proxy used where no national map exists.
• CHELSA-CMIP6 v2.1 (Karger et al.), precipitation of the driest month. CC0 1.0.
• Calibration: French CatNat drought-subsidence register (1988-2023) and the CCR claim cost.

Limits

• A screening layer, not a siting tool. Outside the national maps, the class comes from a soil-based proxy.
• The dwelling value behind the loss scale and the activation frequency are posed, not measured.
• The calibration is French detached housing, extrapolated to every other asset and country.
• The intermediate-emissions trigger is the midpoint of the low and high emissions bands, because CHELSA publishes no SSP2-4.5.