Dans les régions de grande culture du Nord-Ouest de l’Europe, certaines situations morphologiques et pédologiques bien identifiées peuvent générer des inondations catastrophiques par émission et concentration de ruissellement. L’état de surface du territoire agricole influe-t-il sur la manifestation de ces phénomènes exceptionnels de la même manière qu’il le fait sur ceux qui sont plus fréquents et moins graves ? C’est la question abordée dans ce texte. L’étude est conduite à partir de l’analyse d’une chronique des catastrophes sur 30 ans dans le pays de Caux (Seine Maritime). Deux types de catastrophes sont distingués : celles qui succèdent à de longues périodes de pluies et surviennent en hiver; celles qui sont dues à des orages et se produisent au printemps. L’état de surface du territoire agricole a un rôle déterminant sur le déclenchement des inondations; on le constate si l’on examine la répartition des catastrophes au cours de l’année, ainsi que leur chronologie au cours des 30 ans. La maîtrise des inondations doit donc être conçue à partir du territoire agricole situé en amont.

In the arable regions of north-western Europe, some morphological sites can generate runoff and thus cause catastrophic flooding. The paper addresses the following questions: does soil surface state influence these exceptional events as it does for more frequent events? In the Pays de Caux (near the mouth of the Seine), a particularly sensitive region, an inventory of catastrophic flooding, based on events recorded in a local weekly, has been made over a 30-yr period. These events are presented according to their occurrence throughout the year (fig 2), their chronology over the 30-yr period (fig 3) and their spatial repartition (fig 2). Two kinds of catastrophe can be distinguished: 1), winter floods which result from long rainy periods (table III) and need large watersheds (table I); those which cause the most damage are floods due to an exceptionally long and rainy period during wheat sowing; after that time, they are late enough to allow the soil surface to become crusted (table VI); 2), the flash flood caused by spring storms do not need large watersheds (table I). They are correlated both with storm frequency in spring and the amount of winter rain (table IV). However, the correlation of these catastrophes with spring storms is not perfect in that the highest frequency of spring floods appears in June but storms are equally likely to occur in May and in June (fig 4); this is interpreted as an effect of soil surface state. Large amounts of winter rain have a tendency to create rills which in turn increase spring catastrophes by reducing the runoff concentration time. The chronology of flash flood over the 30 yr (fig 3) can be explained by climatic conditions but also by agricultural land use. With agricultural inventories (table II), it is possible to induce soil surface states in January and May (table V) and thus represent, for a long period, the evolution of soil surface state in these 2 months (fig 5). It would seem that the floodings in the last 10 yr were caused by the increase in streaming surfaces. So the distribution of catastrophes throughout the year as well as over a 30-yr period show the effect of soil surface state of agricultural land on flooding. Thus flood control should be planned taking into consideration the soil surface state of the agricultural area.