Structural susceptibility of the El Blanco bridge to flows involving large wood and sediment transport in the Blanco River, in the city of Chaitén (Patagonia, Chile)

Infrastructure in mountainous regions is particularly vulnerable when exposed to socio-natural hazards associated with extreme events, especially flood events involving the transport of large volumes of sediment and woody debris. In this context, understanding how such processes affect the structural stability of bridges is crucial for effective risk management and the planning of resilient infrastructure. This study examines the impacts of river floods, including large wood and sediment transport, on the "El Blanco Bridge" over the Blanco River in Chaitén, Chilean Patagonia, and the resulting susceptibility of the structure. The 2D Iber model, which solves the shallow water equations, was employed to simulate different flood scenarios as bi-phasic flows (i.e., water, inorganic and organic sediments, the latter are referred to as large wood, LW), evaluating the hydrodynamic loadings (i.e. pressure distributions and forces) on piers and their susceptibility to sliding, overturning and scouring. Critical flood scenarios that could pose a potential risk of infrastructure failure were identified by separately determining the associated peak discharge, sediment transport rates, LW loads and bed elevation changes. Compared to clear water flows, LW transport resulted in a reduction of the factor of safety against overturning and sliding, indicating higher hydrodynamic loads on the exposed structure. When sediment transport was considered, increasing flood flows slightly augmented maximum scour depth at the base of the piers. This study underscores the significance of hydrodynamic modeling of the Blanco River for natural risk management, and highlights the importance of considering LW transport when quantifying the safety of structures, especially in catchments where easily transportable LW sources may be found (e.g., in catchments following fires or volcanic eruptions).