RECOGNITION, CLASSIFICATION AND MECHANICAL DESCRIPTION OF DEBRIS FLOWS

Various types of flow or mass movement involving water and sediments o ccur on steep slopes in mountainous areas. Among them, debris flows ar e peculiar events during which a large volume of a highly concentrated viscous water-debris mixture flows through a stream channel. Througho ut the world these phenomena cause considerable damage but remain poor ly understood although a basic knowledge is already available concerni ng their recognition and propagation. Firstly, a synthesis of the usef ul practical criteria of recognition is proposed. Debris flows must be seen as intermediate phenomena between hyperconcentrated flows (inten se bed load transport) and landslides separated from them by sharp tra nsitions of some characteristics (celerity, deposit nature and flow ty pe). Two parameters, solid fraction and material type, thought to be a ppropriate for a sound and practical classification, are brought out, and the corresponding complete classification of flow and mass movemen ts in mountain areas is presented. Two extreme debris Row types are th us distinguished: muddy debris flows and granular debris flows. A crit ical review of recent advances in debris flow dynamics is then propose d. It is pointed out that adequate work must be carried out in the fie ld of non-Newtonian fluid mechanics. In particular, one fundamental rh eological property of debris flow materials is the yield stress, which explains thick deposits on steep slopes and can be inferred from fiel d measurements. Furthermore it can be used to estimate viscous dissipa tion within the bulk during flow. Relevant models predicting muddy deb ris flow dynamics are already available whereas further progress is ne eded concerning granular flows.