L’érosion hydrique des sols
L’érosion déplace les matériaux constituant les sols essentiellement sous l’influence des pluies, souvent renforcées par les activités humaines. Ce phénomène naturel majeur de la dégradation des sols cause une perte annuelle estimée à 25 milliards de tonnes à l’échelle mondiale. Quant à l’Europe, la Commission européenne estime que 17 % de sa surface, soit 26 millions d’hectares, est affectée par l’érosion hydrique.
Erosion displaces soil materials essentially under rainfall influence, often intensified by human activities. This major natural soil degradation phenomenon causes an estimated annual loss of 25 billion tonnes globally. As for Europe, the European Commission estimates that 17% of its surface, or 26 million hectares, is affected by water erosion.
3 chapters
- Soil erosion hazardGeographic distribution• In metropolitan France, moderate to very high soil erosion hazard (probability of erosion occurring at a given intensity) affects approximately one-fifth of the territory. In northern France, arable lands are particularly vulnerable due to sparse vegetation cover for part of the year. Conversely, in the north of the Paris Basin and the southwest, erosion is linked to soil sensitivity to surface crust formation (crusting) that triggers runoff. Finally, high erosion hazard in the Alpine arc and Corsica is explained by intense rainfall combined with steep slopes, or by agricultural zone heterogeneity in eastern Brittany.Erosion-limiting factors• Multiple factors influence erosion: soil properties, relief, vegetation cover, and climate. On silty soils particularly prone to crusting (surface crust formation blocking water infiltration and promoting runoff), runoff can occur on gentle slopes (under 5%) during moderate rainfall. While erosion can damage plants or seedlings on agricultural lands, certain practices can limit such damage: crops perpendicular to slope, hedgerow conservation, no-till cultivation techniques.
- Soil lossesExtent and affected areas• Soil losses from water erosion are estimated at 1.5 t/ha/year on average, with high variability (up to 20% of territory affected by very high rates). Vineyards, and to a lesser extent cultivated lands and orchards, are most affected. Silty zones in the North and cultivated plains of the Garonne valley are also heavily exposed to soil losses.Most exposed regions and irreversibility• In approximately half of regions, over 20% of the surface is affected by erosion exceeding 2 t/ha/year: intensive cereal-growing regions (Centre, eastern Normandy, Île-de-France, Hauts-de-France) and eastern Occitanie, as well as intensive livestock regions in the West (western Normandy, Brittany, Pays de la Loire). However, soil loss exceeding 1 t/ha/year can be considered irreversible over 50 to 100 years.
- Muddy water flowsCatastrophic phenomenonErosion can nonetheless take catastrophic forms by causing muddy water flows. Their frequency is a good indicator of chronic soil erosion.Claims and statistics• Damage caused by these flows triggers claims under natural disaster provisions. 8,800 orders were thus recorded in France between 1982 and 2016. Over this 30-year period, 18% of natural disaster status recognitions related to mudslides were recorded in the first two years, likely due to the implementation of this administrative procedure. The evolution of the number of orders appears partly correlated with rainfall trends.Prevention toolsAuthorities have tools to limit flow occurrence: risk prevention plans (PPR) and urban planning documents (municipal maps, PLU, SCOT, DTA).
