Debris-flow magnitude-frequency relationships for mountainous regions of central and northwest Europe

Debris-flow activity within a given area can be defined in terms of magnitude and frequency. When for such an area ranges of event magnitudes can be related to the corresponding frequencies, the regional characteristics of debris-flow activity will be obtained. From the viewpoint of hazard assessment this magnitude-frequency characteristic is an essential element of the debris-flow process. For larger areas, (supra)regional comparison may also be undertaken on this basis. However, the data available often specify magnitude insufficiently. For the present study, published data specifying both magnitude and frequency (a very limited amount) were used for the Alps, the Tatra Mountains, Scotland, Norway, and Swedish Lapland in addition to material for the French Alps from the author's research. Activity levels within Northwest European mountains were found to be lower than in the Alps or the Tatras. Possible explanations are discussed, in which precipitation patterns and debris availability play a prominent part. However, the general conclusion has to be that firm statements about the causes of observed differences in magnitude-frequency patterns cannot yet be given.