Catchment-based perspective for debris-flow hazard assessment in alluvial fans: test site application in four alpine catchments

In many mountainous contexts, the increasing urbanization and the limited availability of bottom of the valley areas have favored building expansion in zones potentially prone to hydraulic phenomena typical of alpine environments. This makes a rigorous and systematic risk assessment, aimed at supporting appropriate urban planning tools. Defining risk requires characterizing the hazard, and therefore this study aims to develop a workflow for its determination. The workflow is divided into two phases: 1) identification of the mountain hydraulic process (catchment–fan data sheet) and the definition of volumes available for homogeneous segments of the hydrographic network, and 2) physically based modeling. The first phase integrates field surveys, photo-interpretation analyses, and geomorphometric analyses with the goal of estimating the sediment volumes available and mobilizable in the catchment during hydraulic events typical of mountainous environments. These volumes are then used as inputs in hydraulic simulations for hazard delineation. The study area is located in Lombardy, more precisely in Camonica Valley, and includes four catchments situated in different parts of the valley (lower, middle, and upper valley), whose different geological and geomorphological conditions influence the mountain hydraulic phenomenon characterizing each catchment. The study presented aims to illustrate the first part of the workflow and show the preliminary results related to the Figna stream catchment, located in the central portion of Camonica Valley. These results were obtained through the compilation and analysis of the information contained in the catchment–fan data sheet, which allows identification of the dominant mountain hydraulic process. The information in the catchment–fan data sheet includes: morphometric analysis of the catchment and fan, analysis of historical events and description of the fan, with particular attention to geomorphological evidence (e.g., flow lobes, inverse grading of deposits, presence of large boulders, and vegetation scars). These activities can be supplemented, if necessary, by focused and brief field inspections in accessible areas considered significant for the study.