Effects of check dams on sediment dynamics in a debris-flow catchment

The Moscardo Torrent (catchment area 4.1 km2, range in elevation between 890 and 2043 m), a small stream in the Eastern Italian Alps, displays a high frequency of debris flows (commonly one event per year) that deliver large amounts of sediment to the receiving stream, and cause concerns for infrastructures located on the alluvial fan and near the confluence. Over the last decades, hydraulic control works were implemented in the main channel to limit bed erosion and to stabilize channel banks. Although the objectives of training works have been only partly achieved, check dams and hillslope stabilization works have affected the sediment transfer from hillslopes to the channels and along the main channel. The effects of hydraulic control works were investigated by means of multitemporal Structure from Motion (SfM) surveys over an area exposed to debris-flow dynamics in which two new check dams were built. Ground-based SfM surveys were taken before and after three debris-flow events (occurred between June and July 2016), allowing the generation of four high-resolution Digital Elevation Models (DEMs). Geomorphic changes caused by the debris-flow events have been assessed in order to produce the DEM of differences (DoDs with a 0.2 m spatial resolution) that allowed estimating erosion and deposition volumes in the study area. Furthermore a debris-flow monitoring system has been in operation in the Moscardo Torrent the analysis of the videos and of the hydrographs recorded by ultrasonic sensors permitted to assess the debris-flow volumes. These estimates were used to characterize the magnitude of events in support of the topographic analysis. By examining the changing pattern of erosion and deposition over time it was possible to understand the check dams’ effects on sediment dynamics. Preliminary results show that the new check dams effectively stored sediment transported by the three debris flows. However, once the check dams will be completely filled, they will lose their functionality, letting sediment flow downstream along preferential paths defined by the torrent control works and by the morphology of debris-flow deposits. Moreover, debris-flow lobes deposited upstream of the check dams could act as sediment sources further increasing downstream debris-flow magnitude.