Debris floods and channel widening in mountain rivers: Examples from the Vaia Storm (October 2018) in the Cordevole River catchment (Dolomites, Italy)

Mountain rivers experience channel widening as a response to high-magnitude hydrological events. Several studies indicate that the unit stream power (ω_pk) and the lateral confinement (CI) are among the most important constraints to explain channel modifications induced by a flood. That said, with the same controlling factors, a relatively broad spectrum of width ratios (WR = channel width after/before the flood) is commonly observed in real case studies. Sediment transport in mountain streams occurs via processes classified as debris flows (DFW), debris floods (DFD) and water flows (WF). This study aims to test if different flow-types are one of the drivers of channel response to floods, specifically investigating if there is a relationship between DFD (i.e. a transport condition characterized by extremely high bedload) and intense channel widening.

The case study is the Cordevole catchment (Dolomites, Italy; drainage area of 857 km²), which in October 2018 was affected by a severe hydrological event (Vaia storm). Besides the main stem of the Cordevole River, we considered four of its tributaries (Tegnas, Pettorina, Liera and Corpassa torrents). WR was determined at the sub-reach scale through aerial photographs analysis and ω_pkwas calculated considering the discharge at the flood peak provided by hydrological modelling. A post-flood survey allowed us to determine the flow-types that occurred at each sub-reach of the Tegnas Torrent during the event. The possible upheaval from WF to DFD along the other streams was determined considering the presence of conditions required for local occurrence of DFD (i.e. ω_pk exceeding 5000 Wm^-2 and/or DFW tributaries delivering large amount of sediment into a receiving stream).

DFD sub-reaches of the Tegnas Torrent experienced widenings that, at the same ω_pk, were 2-3 times larger than WR of WF sites. These processes-specific relationships were used to recognize sub-reaches of the other streams were an “anomalous widening” occurred during the Vaia event, i.e. sites where WR was significantly larger-than-expected for a specific ω_pkunder conditions of WF. Among 117 sub-reaches, anomalous widening was recognized at 13 and 6 sub-reaches of the Liera (WR up to 16) and Pettorina (WR up to 10) torrents, respectively. All these sub-reaches were characterized by the presence of conditions required for DFD occurrence during a high-magnitude flood, allowing us to infer that the process responsible for sediment transport during the Vaia event was likely DFD. Contrariwise, no sub-reaches of the Cordevole and Corpassa streams experienced anomalous widening, likely because WF occurred along their whole courses due to their morphological characteristics (e.g. wide channel before the flood) and/or lower magnitude of the flooding locally induced by the storm.  

These results suggest that an extraordinary-widening characterizes DFD channel sites, which, during a severe flood, can be affected by channel changes remarkably more intense than those occurring in response to WF. For this reason, in addition to hydraulic and morphological constraints, the different sediment–water flows possibly occurring at a sub-reach should be considered as a further controlling factor for channel modifications and, consequently, for prediction of geomorphic hazard at local scale.