Abstract
Abstract. Hydraulic hazards in alluvial fan areas are mainly related to torrential floods and debris flows. These processes are characterized by their fast time evolution and relevant sediment load. Rational approaches for the estimation of hazard levels in flood-prone areas make use of the maps of depth and velocity, which are provided by numerical simulations of the event. This paper focuses on national regulations regarding quantitative debris-flow hazard mapping and compares them to a simple conceptual model for the quantification of the hazard levels on the basis of human stability in a flood. In particular, the proposed method takes into account, in a conceptual fashion, both the local slope and the density of the fluid, that are crucial aspects affecting stability for processes in mountain environments. Physically-based hazard criteria provide more comprehensible and objective maps, increasing awareness among stakeholders and providing more acceptable constraints for land planning.
Highlights
Debris flows are rapid mass movements, characterized by strong granulometric heterogeneity, typical of steep mountain basins
The deposition areas, usually characterized by separation of the fluid and the solid phase, are located on alluvial fans, which are themselves clear evidence of such geomorphic events
The definition of the return period of the debris flow event refers to the liquid discharge only, disregarding the effect of sediment availability on the overall volume and concentration
Summary
Debris flows are rapid mass movements, characterized by strong granulometric heterogeneity, typical of steep mountain basins. The hazard maps were obtained from the combination of the maximum velocity and depth envelopes with the Austrian, Japanese and Swiss thresholds These maps were compared to those obtained by applying a simple conceptual model describing human stability in a flow. The hazard assessment based on the value of the flow specific energy, which would correspond to the water depth at a stagnation point, does not allow separate accounting of the effects of velocity (e.g. the drag on an object) and water depth It does not consider relevant features such as the density of the fluid and the local slope. The yellow zones identify the debris-flow prone areas, whilst in the red ones Fd overcomes the building resistance This model accounts for the variability of the flow force due to the fluid density
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