Combining field and aerial imagery monitoring for planning maintenance operations and strategies in an ungauged mountain catchment

Mountain catchments are highly sensitive to the impacts of global warming, which affects seasonal weather patterns, glacier retreat, permafrost thawing, and snow cover duration. These changes drive rapid transformations in their ecosystems and alter their hydrological and sedimentological regime, exacerbating their susceptibility to various hazards, including floods, shallow landslides, and debris flows, all closely tied to sediment dynamics. Consequently, sediment management plays a key role in developing watershed management strategies that lead to programme interventions for mitigating potential losses for the mountain communities.

In this context, an integrated approach combining field surveys and aerial imagery analysis is essential for evaluating the effectiveness of existing countermeasures (mainly torrent control structures) and for finding innovative solutions, especially in absence of sediment transport monitoring systems. This approach enables the collection of observations on lithology, geology, channel cross-section shape, longitudinal profiles, land use, active soil movements, and grain size distribution within sediment source areas and along the channel network. The field inspections of torrent control structures further provide a detailed assessment of their condition and functionality.

All these observations are essential for geomorphological approaches, statistical formulae, and hybrid methods to estimate potential debris flow volumes at both reach and catchment scales. Additionally, simplified rainfall-runoff modelling, such as the SCS-CN method, is employed to assess critical runoff thresholds that could trigger water-sediment flows. The outputs include the spatially distributed assessments of in-channel and hillslope sediment storage volumes, and the delineation of sediment source areas. Aerial imagery complements this process by verifying the spatial distribution and extent of sediment source areas and tracking land cover changes over time.

The proposed methodology was developed and applied to the Rovina Torrent basin, located in the Central Alps (Lombardy, North Italy). The basin is characterized by coniferous forest cover (37.2%) and extensive debris accumulations and lithoid outcrops (36.7%). In the study case, the results provide the typologies in terms of driving the triggering event mode within the catchment, the minimum critical rainfall for designing an early warning system, and the potential debris flow volume for adjusting the sediment trapping basin.

The outcome significantly enhances the accuracy of hazard mitigation strategies and supports the adaptation or redesign of the torrent control structures to better address evolving sediment dynamics. The combination of the scientific experience in similar mountain context and the additional field observations can quantitatively provide a robust diagnosis of the current scenario for planning maintenance operations, for proposing and designing alternative solutions to reduce the natural hazard and for effectively supporting the decision-making process including the strategic allocation of human and financial resources.