Rôle de la forêt sur les crues

Forest effects on flood

Office National des Forêts, Délégation Nationale RTM

Abstract

Forest effect on flood flows, long matter of debate and affecting political decisions in many countries, seems obvious but is not as simple as it seems. Forest watersheds show a strong reduction of floods compared to bare lands, but this reduction is not so effective when compared to other vegetation types. For the study of complex processes involved in flood formation, looking at all aspects, one can think the forest soil as a reservoir; in the long term, forest increases the soil reservoir capacity ; but this capacity has its limits, with the effect being very small for extreme floods. Interest of tree populations, riparians or on the slopes, remains primarily in erosion and sediment transport control, which is an important factor in the area of torrential floods. Next, the impact of human activities is overviewed : expansion of forest areas or, more localy, their voluntary (clear cutting) or accidental (forest fire) reduction, silvicultural practices and their consequences limitating excess of sediment transport. More extreme in the case of small mountainous catchments, these results cannot be generalized to larger watersheds without caution. They provide materials to inform public opinion, and are a guideline to analyse impacts on flood genesis of soil occupation and its evolution.