Abstract
Several heavy precipitation events caused flash floods and landslides in Europe over the last years. In mountain environments, mass movements are important sources of sediment supply to the rivers.
The Stolla creek is a 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 of the two floods were investigated with the aims to compare the channel changes induced by the two events and to assess the rule of the sediment connectivity. A multi-methodical approach was applied, including radar rainfall estimation, rainfall-runoff modeling, field surveys and comparison of multitemporal orthophotos and DTMs.
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 and bed level aggradation or incision. 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 in some reaches.
The main results of this study suggest that:
* different rainfall intensities and durations controlled channel changes and sediment transport effectiveness;
* sediment supply from hillslopes played a relevant impact in channel adjustments;
* the structural connectivity strongly influenced the sediment cascade.