Assessing geomorphic changes induced by a debris-flow: a case study in the Dolomites

Minute F., Cavalli M., Comiti F., 2018, Assessing geomorphic changes induced by a debris-flow: a case study in the Dolomites, GIT 2018, Sarzana (SP), 11-13/06/2018,

The study of landscapes and their dynamics has become fundamental to understand natural hazard mechanisms and the better planning of mitigation measures to prevent damages and loss. Mountain regions present higher risk due to complex geology and morphology. South Tyrol is particularly prone to natural hazards, 256 recorded events in 2016, 62 of which classified as debris-flow. This process is characterized by high mobility, speed and energy, prerequisites that can sometimes lead to catastrophes (e.g. Sarno, 1998). A major event occurred on the August 5th, 2017 in the Braies/Prags area (Dolomites). Heavy rainfall triggered a large debris-flow in the Schade catchment, which hit the village of Ferrara/Schmieden. The event caused damages to infrastructures and buildings but no casualties. The aim of this study is to analyze the debris-flow event by using a Digital Elevation Models (DEMs) differencing approach to estimate the mobilized materials and identify the spatial patterns of erosion and deposition in the catchment. The estimation of the eroded and deposited volume was performed with the software Arc-GIS by computing a DoD (DEM of difference) which allows assessing elevation changes through time. In the study case, pre- and post-event DEMs were available (dated 2010 and 2017, respectively). The process required a sophisticated elaboration of the data. An accurate difference between two elevation surfaces is possible only if the two DEMs are completely overlapped, satisfying the principles of concurrency and orthogonality. The co-registration of the two point clouds has been carried out through the Iterative Closest Point (ICP) algorithm of the free software Cloud Compare. The values of the matrix of transformation have been derived by analyzing stable areas. The DoD was at first calculated for the entire catchment (around 4 km2) but, even if a robust approach encompassing DEMs error estimate and errors propagation into the DoD, results were not satisfying due to large differences in terms of representativeness of the two DEMs in input. The solution was the selection of a mask of the areas affected by erosion or deposition and effectively connected to the main channel network. A first net volume estimation was computed by estimating the DoD minimum level of detection considering a constant error of 20 cm for the 2017-DEM and 35 cm for the 2010-DEM. A more accurate elaboration was obtained by using a fuzzy approach, which estimates the error and propagates uncertainty into the DoD with a spatially variable approach. The resulted net balance was -115.719 ± 46.043 m3 and this value was taken as a reference by personnel in charge for watershed management for building a 100.000 m3 sediment retention basin at the catchment outlet. The results obtained from this study can be used for landscape planning including mitigation solutions and developing more robust hazard maps.

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