A field-based approach for flow typologies determination and analysis of their impacts on control works in a mountain catchment

A common approach to control sediment transport during floods involves the use of hydraulic structures installed within channel networks. During such events, sediment dynamics can vary depending on topographical, physical and hydraulic factors, leading to different geomorphological forms. While channel control works play a role in regulating the sediment transport, fluxes might affect the structures integrity as well. Transport mechanisms generate deposits with different morphological and sedimentological characteristics, which can be analyzed in the field using a post-event protocol to determine the dominant flow typologies along the channel network. The study focuses on the Vegliato Torrent (Italy), a 4.4 km2 mountain catchment with a 3 km long main channel divided into 50 sub-reaches and characterized by the presence of 22 structures. The basin was affected by a flash flood in 2021. The transport typologies considered in this study are debris flow, debris flood, hyperconcentrated and water flow. These flow typologies are associated with the characteristics of hydraulic structures, evaluated using two parameters: the Maintenance Priority index (MPi), which reflects the state and functionality of the control works, and the Sediment Continuity Ratio (SCR), which indicates the extent to which a structure promotes or disrupts sediment continuity during an event, also comparing to the other structures. The results show that 55% of the main channel is characterized by debris flow. Debris flood is also widely prevalent (43%), while only a few sub-reaches exhibit water flows (2%). The analysis shows that 67% of the damaged or low functioning structures that also promote sediment continuity are located within debris flow sub-reaches. Furthermore, around 50% of the structures that are efficient and promote discontinuity are located in debris flood sub-reaches. These results provide important guidance for authorities in developing sediment transport management plans and interventions, considering both transport typologies and hydraulic structures functionality.