Catchment response to different severity storm floods: channel variations and hillslope sediment coupling dynamics

Abstract

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 of the two floods were investigated in the main channel and along the hillslopes with the aims: to compare the channel changes induced by the two events; 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 Digital Terrain Models. 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 in one hour. More than 600 debris flows were triggered along the hillslopes, among which 23 were connected to the Stolla. Important discontinuities for the sediment flux were represented by the floodplains. The Stolla channel experienced channel widening occurred through bank erosion, and overbank depositions. With ratio (ratio between the channel width after and before the flood) was between 1.3 and 4.9. Widening was accompanied by channel bed aggradation up to 1.2 m or incision up to -2.2 m. Widening through bank erosion was more common in narrower reaches, affected by higher flood power, and presenting higher connectivity with debris flows. Although 294,000 m3 of sediments were eroded in the connected debris flows and 12,380 m3 were transferred to channel from toe erosion processes, 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 the reaches where the 2017 event had caused channel-bed aggradation occurred.