Author(s): Davide Cammarata; Mesfin Zenebe Gezahegn; Johannes Hubl; Georg Nagl; Roberto Ranzi
Linked Author(s): Roberto Ranzi
Keywords: Debris flow check dams residual risk dynamic load design standards
Abstract: In recent decades, debris flow phenomena in northern Italy and in the Alpine area in general appear to be more frequent, as a possible effects of climate change. Among the structural methods for controlling such events are check dams. The collapse of such structures can cause damage and loss of lives. To reduce the risk, such structures must be designed considering all possible stresses over the life of the structure. In the case of debris flows, dynamic stresses are of great importance. The design of such structures in the Italian Standards NTC2018 is not very strict in this sense, classifying them as 'retaining walls and similar' and limiting the consideration of this contribution to the discretion of the designer. The Austrian standard, which is at the forefront worldwide, dedicates a specific treatment to torrent control (ONR248XX series) and defines specific criteria for quantifying the dynamic stresses on a control structure due to a debris flow and the impact of a single boulder in the design and verification phase. We performed stability checks on structures designed in accordance with Italian standards, incorporating the specific Austrian criteria for dynamic stresses in the process and other criteria recently proposed in the literature. The result is that these structures, although compliant with the Italian standard, are not compliant with the more conservative Austrian standard. We therefore present two case studies in the Alpine region. Gravity check dams: On 29 October 2018, following a heavy rainfall event with return period ranging from 50 to 150 years for rainfall duration between 30 minutes to 72 hours in the Rio Rotiano catchment area (Trento, Italy), a series of three debris flow events occurred. Fourteen of the sixteen check dams collapsed almost completely. Analyses performed on some check dams indicate that the collapse occurred due to dynamic stresses caused by debris flows and boulders, which led to the loss of global and internal stability. Filter barrier: On 28 August 2020, following short but intense rainfall, with a return period estimated less that 20 years, in the Vallaro torrent catchment area, (Brescia, Italy), a violent debris flow was triggered, and the dynamic stresses, especially that of a boulder impact, caused the collapse of a filter dam just completed. Considering the impossibility of quickly adapting the large number of check dams present in Italian Alpine areas to the best knowledge of the effects of dynamic stresses, it is therefore necessary to warn that check dams located in catchment areas with strong debris flows, e. g. with thick and unstable glacial deposits, may not be safe enough and a residual risk remains, to be addressed with non structural measures, as warning systems or land and urban planning.
Year: 2025