Geomorphic effectiveness of check dams in a debris-flow catchment using multi-temporal topographic surveys

Torrent control works, such as grade control dams and sediment retention dams, are structural measures for controlling debris flows and debris floods. In spite of the widespread presence of such hydraulic structures in steep mountain streams worldwide, there are still few studies monitoring the effects of check dams on sediment dynamics. The use of repeated topographic surveys allows us to characterize debris-flow activity at multiple temporal and spatial scales and its interaction with torrent control works. Structure from Motion (SfM) technique paired with Multi-View Stereo (MVS) algorithms represents a low-cost opportunity to conduct such multi-temporal surveys. This enables us to better study the effects of individual debris flows, track geomorphic changes and evaluate the effectiveness of torrent control works (e.g. check dams). The effect of check dams on sediment dynamics is investigated by means of multi-temporal topographic surveys before and after debris-flow events in the Moscardo torrent (eastern Italian Alps) where two check dams have recently been built. Multi-temporal SfM based on images taken from the ground in combination with imagery taken by an Unmanned Aerial Vehicle (UAV) is used to obtain DEMs and to study topographic changes through the comparison of repeat DEMs (DEM of Difference, i.e. DoD). The results show that the new check dams considerably modified sediment dynamics in the studied channel but their performance cannot be considered satisfactory. The sediment flowed around the upstream check dam on the right wing, while deep erosion observed downstream of the check dam threatened to undermine the foundation's stability. Moreover, debris-flow lobes deposited upstream of the check dams could act as sources of sediment further increasing downstream debris-flow volume. The analysis proposed in this work could help improve design strategies and permitting the identification of a proper site for check dam building.