Monitoring of torrent control structures through a multi-parameter index

Transversal torrent control structures (TTCSs) are one of the most emblematic measures for erosion control and sediment retention in mountain streams. TTCSs are prone to decay that may undermine their stability and effectiveness: their monitoring is essential to plan appropriate conservation strategies. This work aims to develop an index applicable at the regional scale as a screening tool to identify the structures potentially more prone to damage. For this purpose, a Potential Fragility Index (PFI) is developed. This index is applied to TTCSs (check dams and bed sills) and is based on intrinsic and extrinsic characteristics of the structures. Specifically, intrinsic factors include structural height, year of construction, and construction material, while extrinsic factors include the location along the watercourse and lithology. The index has been developed and tested in three catchments located in the easternmost sector of the Italian Alps (Friuli-Venezia Giulia Region): Fella, But, and Degano catchments with areas of 700 km2, 325 km2 and 320 km2, respectively. The PFI is intended to: (i) classify TTCSs according to their potential fragility; (ii) identify potential hotspots; and (iii) support monitoring activities during an initial screening phase. The results are qualitatively assessed for 282 structures within selected sub-catchments through field surveys to evaluate the conceptual coherence of the index.