Analysis of past and future droughts causing clay shrinkage in France

Abstract

Clay-shrinkage-induced subsidence can cause permanent damage to buildings if the drying extends below the foundations. Clay soils and damage are widespread in France. The causes of clay shrinkage are understood at the micro-scale, but the same reasoning cannot be applied at the large spatial scales that are critical for land management because the phenomenon depends on very local parameters. In this study, clay shrinkage occurrence factors are characterized and the global number of insurance claims is statistically quantified, without considering the risk of damage for each house. A drought index specifically designed for clay shrinkage is used to analyze past and future soil moisture droughts that may cause subsidence by calculating annual drought magnitudes for France. The index is based on Interactions between Soil, Biosphere, and Atmosphere (ISBA) land surface model simulations. It is calculated for several vegetation types. A comparison of the annual values of this index with the number of insurance claims for subsidence shows that the presence of trees near individual houses must be taken into account. Historical and projected simulations are performed, with the main difference being the meteorological forcing provided to ISBA. The historical simulation covers the years 2000–2022 and uses the SAFRAN atmospheric reanalysis. The projected simulation covers the years 2006–2065 and uses an ensemble of climate models under Representative Concentration Pathways (RCPs) 4.5 and 8.5. The historical simulation shows particularly widespread droughts in France in 2003, 2018, 2019, 2020, and 2022. In 2022, particularly high drought index values are observed throughout the country. Projections show that drought conditions are expected to worsen in the future, especially under RCP 8.5 compared to RCP 4.5. The projections diverge significantly after 2046, and both the north and south of the country are equally affected.