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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.

Availability of high-resolution morphometric survey datasets is not an issue any longer, especially after the wide adoption of laser scanning technology to collect data. Nevertheless, this abundance of data is not always matched by the techniques and the procedures used to process it and to turn it into information useful for decision makers. The work we present here was aimed at exploiting high-resolution morphometric data collected using two different technologies, i.e. Aerial Laser Scanner (ALS) and Terrestrial Laser Scanner (TLS). The two datasets were combined to generate a series of Digital Elevation Models (DTMs) that were in turn used to assess the effects of the construction of a new check dam on sediment transport modelling in a small Italian mountain catchment (Rio Moscardo). The fusion of the two datasets allowed the generation of three DTMs. The first DTM (scenario 1, from ALS data only) describes the morphology before the check dam construction. The second DTM (scenario 2, from ALS and TLS data) describes the morphology right after the check dam construction, therefore without upstream debris deposition. The third DTM (scenario 3, from ALS and TLS, modified) describes the morphology a few years after the dam construction, with consistent upstream debris deposition. The three DTMs were used as a basis for the calculation of a spatial sediment Connectivity Index (IC), to assess how different portions of the catchment are connected to the catchment outlet (target) in terms of debris flow delivery. The modelling results clearly show that in scenario 2 the check dam acts as an effective sink for the debris, almost disconnecting the catchment portion upstream to the dam (15% of the catchment area) from the catchment outlet. In scenario 3, where due to debris deposition the slope of the area just upslope to the dam has strongly decreased, the IC values of the catchment portion upstream to the dam revert to the ones shown in scenario 1, where strong connection to the outlet was detected.