The paper presents a methodology developed to determine the spatial "debris-flow initiation hazard". Through the field study of twelve catchments prone to debris flows in the French Alps and the laboratory analyses of many samples of debris-flow deposits and source-area materials, data on the environmental setting are provided. The potential debris-flow source-areas are localized, and their characteristics compared. Relying on this analysis, the author has selected five predisposition factors that are relevant to debris-flow initiation, which are related to slope angle, geology, mass movements, volumes and granulometric properties of superficial deposits. The data related to these factors were quantified and processed using an algorithm, and maps of the "debris-flow initiation" hazard were produced. The methodology provides a special emphasize on geotechnical data and quaternary geology. They illustrate the spatial distribution of potential events and allow identifying potential source areas and planning works in order to mitigate the risk.
Debris flows have been reported for hundred of years in mountainous areas, and have induced important economic and human losses around the world. Nowadays, the economic development and urbanization of mountainous valleys lead to an increase of the risk downstream as the potential impacts of debris flows are higher. In the Maurienne valley (Savoie district, France), the construction of a motorway raised focus on the risk related to mass movements. Indeed, the layout is crossing several streams that have initiated major debris flows in the past few years. Through field investigations of the environmental context of several catchments, a comprehensive method for the identification of debris-flow hazard assessment has been developed.
As the costs associated to loss of life and property from natural hazards is continually increasing, research on hazard assessment has evolved with development of multiple approaches.