Monitoring sediment source areas in a debris-flow catchment using terrestrial laser scanning

Geomorphic changes caused by three debris-flow events in a small mountain catchment (Moscardo Torrent, Eastern Italian Alps) were investigated by means of multitemporal terrestrial laser scanning (TLS) surveys over three areas exposed to debris-flow dynamics. For these areas, DEM of Differences (DoDs) with a 0.2 m spatial resolution allowed the assessment of elevation uncertainty as spatially variable by applying the fuzzy inference system (FIS) proposed by Wheaton et al. (2010). Along with two commonly used parameters affecting Digital Elevation Model (DEM) accuracy, such as terrain slope and point density, we developed and employed in the FIS a new index, named Vegetation Noise, that takes into account the disturbance caused by the vegetation cover on the DEM accuracy. The results of volumetric budgets of the surveyed sediment source areas derived from the DoD analysis were compared with debris-flow volumes estimated from flow stage measurements at the instrumented channel reach. On the one hand, the discrepancies arising from this comparison underline the limitations of TLS surveys over morphologically complex areas such as debris-flow catchments. On the other hand, the TLS unveils the geomorphic changes at the scale of the single event, because a terrestrial survey is usually easier to carry out than an aerial one. The analysis of DEM quality and uncertainty indexes correlation may help to refine methods for spatializing elevation errors and improving the reliability of the models.