Abstract
Abstract. Understanding and modelling the dynamics of large wood (LW) in rivers during flood events has spurred a great deal of research in recent years. However, few studies have documented the effect of high-magnitude flash floods on LW recruitment, transport and deposition. On 25 October 2011, the Magra river basin (north-western Italy) was hit by an intense rainstorm, with hourly rainfall rates up to 130 mm h−1 and event rain accumulations up to 540 mm in 8 h. Such large rainfall intensities originated flash floods in the main river channels and in several tributaries, causing severe damages and loss of lives. Numerous bridges were partly or fully clogged by LW jams. A post-flood survey was carried out along the channels of two catchments that were severely and similarly affected by this event, the Gravegnola (34.3 km2) and Pogliaschina (25.1 km2). The analysis highlighted a very relevant channel widening in many channel reaches, which was more marked in the Gravegnola basin due to highly erodible material forming the slopes adjacent to the fluvial corridor. Large wood recruitment rates were very high, up to 1270 m3 km−1, and most of it (70–80 %) was eroded from the floodplains as a consequence of channel-widening processes, while the rest came from hillslopes processes. Overall, drainage area and channel slope are the most relevant controlling variables in explaining the reach-scale variability of LW recruitment, whereas LW deposition appears to be more complex, as correlation analysis did not evidence any statistically significant relationship with the tested controlling variables. Indeed, in-channel LW displacement during the flood has been mostly limited by the presence of bridges, given the relatively large width attained by channels after the event.
Highlights
Floods are the natural hazard which affect the largest number of people on a global scale (Jonkman, 2005), and within these events, flash floods are the cause of the highest mortality rates (Doocy, 2013)
The magnitude of large wood (LW) recruitment and deposition which occurred during the October 2011 flash flood in the study basins was characterized by a very high variability at different spatial scales, i.e. catchment, sub-catchment and reach scale
Most LW recruitment originated from the fluvial corridors, with a much smaller contribution from the hillslopes, similar to what was observed during the 2005 flood events in Switzerland (Waldner et al, 2007)
Summary
Floods are the natural hazard which affect the largest number of people on a global scale (Jonkman, 2005), and within these events, flash floods are the cause of the highest mortality rates (Doocy, 2013). Flash floods are defined as sudden events with high peak discharges, produced by severe thunderstorms that are generally of limited areal extent (IAHSUNESCO-WMO, 1974). Despite their relevance, these phenomena are poorly understood, mainly because they cannot be extensively monitored (Borga et al, 2014). The intense precipitations originating the rapid and large increase in discharge in the channel network frequently trigger slope instabilities, such as landslides and debris flows, during the same event (Borga et al, 2014). These hillslope processes, coupled to the fluvial dynamics, supply large volumes of both sediments and large wood (LW) to the channels in forested catchments (Comiti et al, 2008)
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