A multi-parameter index to support monitoring of Torrent Control Structures

Transversal torrent control structures (TTCSs) are fundamental components for managing mountain watersheds and mitigating geo-hydrological risks. With the growing frequency and intensity of extreme events driven by ongoing climate change, the status assessment of the structural condition of these structures has become an increasing challenge for watershed management strategies. To address this need, this study presents a Potential Fragility Index (PFI), a regional-scale screening tool specifically designed to identify structures potentially prone to damage. The PFI aims to classify TTCSs based on their potential fragility, identify potential hotspots, and support monitoring activities. The PFI methodology integrates intrinsic structural characteristics, such as construction material, height, and age, with extrinsic environmental factors, including substrate lithology and geomorphometric susceptibility to debris flows. Weights used to evaluate evaluating the relative importance of each factor in determining the potential fragility of the structures were assigned based on the evaluation of the answers to a structured questionnaire addressed to both professional practitioners and academic specialists. The index was implemented across the Fella, But, and Degano basins in the Friuli-Venezia Giulia region of northeastern Italy, covering an inventory of 3,594 structures. The conceptual coherence of the index was assessed through field observations of 282 structures across seven sub-catchments. The results demonstrate that structures located in headwater zones and along debris-flow propagation reach consistently exhibit higher PFI values, in agreement with deterioration patterns documented through field observations. In contrast, TTCSs located on alluvial fans and in low-slope channel reaches tend to exhibit lower fragility classes, reflecting the prevalence of depositional processes and reduced hydraulic stress. Notably, TTCSs sharing similar intrinsic and extrinsic characteristics may display contrasting current conditions depending on the dominant geomorphological processes within their respective sub-catchments, underscoring the role of local processes in controlling structural deterioration. The PFI proves to be both broadly applicable and transferable across a range of geomorphological contexts, as it is based exclusively on readily available regional datasets. It therefore represents a practical and scalable decision-support tool for guiding the prioritisation of maintenance actions and enhancing resource allocation in regional watershed management.