Debris floods in mountain streams: Insights from the Vaia Storm (October 2018) in the Tegnas catchment (Dolomites, Italy)

Abstract

Sediment transfer in mountain streams occurs via processes classified as debris flows, debris floods, and water flows. The analysis of the sediment-water flows occurring during high-magnitude floods is currently a scarcely explored issue. This knowledge gap is particularly striking for the debris floods, a water-driven flood flow with extremely high bedload transport that has been rarely investigated. This study investigates how the transport mechanisms activated in a mountain stream during a high-magnitude flood differ from those triggered during ordinary floods, focusing on the controlling factors and conditions required for debris flood occurrence. The study area is the Tegnas catchment (drainage area 51 km2), a mountain basin of the Dolomites (northeastern Italy) affected in October 2018 by a severe flood (Vaia Storm), whose recurrence interval has been estimated to approximately 200 years. We developed a post-flood survey protocol for distinguishing various flow types based on the features of the flood deposits. The transport processes typifying the stream network during ordinary floods and those occurred during the Vaia Storm were determined through the field survey of the deposits. We observed water flow as a response to ordinary events occurring along the entire Tegnas main stem. During the Vaia Storm, water flow still dominated, although debris floods were documented at several sub-reaches. The unit stream power induced by the Vaia Storm was calculated based on the peak discharge computed at the sub-reach scale using a rainfall-runoff model, and channel widening was determined measuring the pre- and post-Vaia channel width. The comparison between the high-magnitude and the ordinary flows allowed us to infer the existence of relationships between the transport mechanisms, the hydraulic forcing, and channel dynamics. The upheaval of the ordinary flow types did not occur along the entire stream network: water flows transitioned into debris floods only under unit stream powers exceeding the threshold of 5500 Wm–2 or downstream of sediment-injection sites. Debris floods occurrence appears to be facilitated by tributaries prone to debris flow and connected to the receiving stream, the injection of fine material into the flows (which can occur as consequence of channel-bank erosion or overbank floodwater re-entering the channel), and channels characterized by high slope and relative narrow section. The occurrence of debris floods caused higher channel widening than that induced by water flows. The complex relationships found between flow types and a range of controlling factors reveal that a 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 an accurate and reliable definition of flood hazard at the local (e.g., sub-reach) scale.