Scorpio V., Steger S., Cavalli M., Crema S., Marra F., Zaramella M., Borga M., Marchi L., Comiti F., 2021, Storm-induced geomorphic effects in a dolomitic catchment_ insights for the understanding of the hillslope-channel sediment coupling., 15 Convegno GIT - Geoscience and Information technology, Ripatransone (Ap), 20-21/12/2021,
URL: http://www.cnr.it/prodotto/i/462433

Several heavy precipitation events causing flash floods, debris flows, landslides, and morphological channel changes have occurred in Europe over the last years. In mountain environments, mass movements along the hillslopes are important sources of sediment supply to the rivers and may enhance the geomorphic effects of floods. The Stolla creek (catchment area_ 40 km2) is a confined/partly confined channel of the Dolomites (Easter Italian Alps), that was affected by an extreme flood in August 2017, and by a moderate flood in August 2020. The geomorphic effects caused by the two storms were investigated with the aims_ to compare the morphological changes induced by the two events in the channel and along the hillslopes; and to assess the impacts of the lateral sediment connectivity to the channel response. A multi-methodical approach was applied, including radar rainfall estimation, rainfall-runoff modeling, field surveys, remote sensing, geomorphological and statistical analysis. Hillslope and channel processes were mapped by comparing multitemporal orthophotos and changes in bed level elevation in the Stolla channel and sediment volume budget along the main channel and along the hillslope were accomplished using the difference of multi-temporal DTMs (DoD technique). Debris-flow connectivity to the main channel was derived by combining field evidence and geomorphometric analysis. The 2017 flood was caused by rainfall with a short duration (6 hrs) and a rain rate exceeding 45 mm h-1. More than 600 debris flows were triggered along the hillslopes. The Stolla channel experienced widening occurred through bank erosion, and overbank depositions and bed level aggradation up to 1.2 m or incision up to -2.2 m. Although 294,000 m3 of sediments eroded in the connected debris flows, limited volumes of sediments (< 1000 m3) were exported to the catchment outlet. The 2020 flood event was characterized by a lower rain rate (max 17 mm h-1) and a long duration (48 hrs) and did not trigger debris flows. The moderate magnitude of the flood peak did not lead to channel widening, but only bed incision (up to -1.4 m) in some reaches. The results show that different rainfall intensities and durations controlled the sediment transport effectiveness during the two floods and that the structural connectivity had an important role on the sediment cascade from the hillslopes to the main channel.

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