Assessing sediment dynamics and check dams efficiency in a debris-flow catchment using multi-temporal topographic surveys

Torrent control works have always been a fundamental tool for preventing torrential hazard in mountain catchments, where the sediment transport phenomena as debris flows are one of the most dangerous geomorphic processes affecting small steep basins. The linkages between sediment source areas on the hillslopes and channel network, along with the temporal and spatial distributions of channel storage, are key controls of debris-flow occurrence and magnitude. Consequently, the prevention of natural hazards related to debris-flows requires a better understanding of sediment dynamic. Among the hydraulic engineering structures, grade control dams and sediment retention dams are the most effective and common technique to manage debris flows and debris floods hazard. These structures could have important effects on sediment dynamic. Therefore, an integrated approach that analyses the debris-flow dynamic and its interactions with torrent control works, is needed to assess the efficiency of the realized structures and to improve the long-term hazard management at catchment scale. In spite of the widespread presence of such hydraulic structures in steep mountain streams worldwide, very little researches considered the role of check dams on sediment dynamics in debris-flow environments over time to enhance the planning of the torrent control works. The monitoring of debris-flow events, the estimation of debris-flow magnitude and frequency, and the analysis of spatial patterns in terms of eroded and deposited volumes, are fundamental to improve the sediment dynamic understanding. In the last two decades, High-Resolution Topography (HRT) has provided new opportunities to characterize debris-flow activity at different scales. Between these, the application of Structure from Motion (SfM) photogrammetry paired with Multi-View Stereo (MVS) algorithms has become a low-cost method to collect HRT at multiple temporal and spatial scales, also in rugged or inaccessible environments like those observed in debris-flow catchments. SfM allows carrying out HRT with high frequency; nevertheless, the SfM technique is limited at broad spatial scales. Therefore, other technologies as LiDAR surveys could be used to assess the sediment dynamic also at catchment scale. However, the use of HRT required the design of appropriate workflows for data post-processing and uncertainty assessment to compare multi-temporal surveys, especially in a topographically complex environment. In this research, the effects of torrent control works on debris-flow dynamics were investigated by means of multi-temporal SfM and LiDAR surveys in the Moscardo torrent (eastern Italian Alps) where several check dams have been built over time. Methodological workflows enabled the realization of multi-temporal Digital Elevation Models (DEMs) which were compared (i.e., DoD) to quantify the debris mobilized and the time evolution of erosion and deposition patterns in debris-flow channels equipped with check dams. The DoDs data were integrated with a sediment connectivity analysis to have a whole assessment of debris-flow dynamic. The results show that the check dams considerably modified debris-flow dynamics in the studied channel but their performance cannot be considered satisfactory. They temporary stored volumes of debris just after their construction, but soon when the structures were filled the check dams acted as sediment sources that increased debris-flow magnitude. Moreover, the sediment paths flowed around some check dams. These processes triggered the slope foot erosion and activated shallow landslides, further sediment source areas for debris-flow process. The analysis proposed in this work could help to improve design approaches and to obtain more realistic cost-benefit ratios of the adopted strategies and, in this way, select the best solutions.