Sediment-water flows in mountain catchments: Insights into transport mechanisms as responses to high-magnitude hydrological events

Sediment transfer in mountain streams occurs via processes classified as debris flows, hyperconcentrated flows, debris floods, and water flows under the control of the water energy and the amount of involved sediment. Despite the extensive documentation of the channel changes caused by high-magnitude hydrological events, the analysis of the sediment-water flows occurring during such floods is currently a minimally explored issue. This study investigated how the transport mechanisms activated in a mountain stream during a high-magnitude flood differ from those triggered during ordinary floods. It also evaluated the effectiveness of three morphometric approaches in predicting high-magnitude flows expectable in a channel sub-reach. The study area is the Tegnas catchment (Dolomites, Italy), a mountain basin affected in 2018 by a severe hydrological event (Vaia Storm), whose recurrence interval is approximately 200 years. We determined the transport processes typifying the stream network in the catchment during ordinary floods through field surveys and direct monitoring and compared these flows with the high-magnitude flow types that occurred during the Vaia Storm. Additionally, we examined the flows predicted using the morphometric approaches for high-magnitude events. We observed water flow as a response to ordinary events occurring along the entire Tegnas main stem, whereas debris flow ordinarily determines sediment transfer at steep tributaries. During the Vaia Storm, water flow still dominated along the Tegnas Torrent, although debris flows and debris floods were also documented at several sub-reaches of the main stem. The morphometric approaches satisfactorily predicted debris flows but often failed to recognize the debris floods occurred during the high-magnitude 2018 flooding. The analysis of different flow types enabled us to infer relationships among transport mechanisms, hydraulic forcing, and channel dynamics and to gain new insights on the poorly explored debris flood processes. Water flows transitioned into debris floods under unit stream powers exceeding the threshold of 5000 to 5500 W m or downstream of sediment-injection points. The occurrence of debris floods, which caused higher channel widening than that induced by water flows, appeared to be facilitated by the presence of tributaries prone to debris flow occurrence connected to a receiving stream, the injection of fine material into the flows, and channels characterized by high slope and narrow section. The morphometric approaches enabled adequate first-order discrimination of expectable high-magnitude flows, but a more detailed characterization that includes field observations is necessary to understand the transport mechanisms that can affect a specific channel site during high-magnitude hydrological events for a more accurate and reliable definition of flood hazard at the local scale.