Large Wood Recruitment and Transport During a Severe Flash Flood in North-Western Italy

On October 25th 2011, the Magra river basin (North-western Italy) was hit by an extreme meteorological event, with hourly rainfall rates up to 130 mm h-1 and event rain accumulations up to 540 mm. Such large rainfall intensities originated flash floods in the main river channels and in some of the tributaries, causing severe damages and loss of lives. Numerous bridges were partly or fully clogged by large wood (LW) jams. A post-flood survey was carried out along the channels of two catchments that were severely and similarly affected by this event: Gravegnola (34.3 km2) and Pogliaschina (25.1 km2). The results obtained show an important change in the morphology of the channels, which width increased on average 10 and 3 times respectively in the two catchments. Large wood recruitment rates are very high, up to 1,270 m3/km, and most of it (70-80 %) is eroded from the floodplains, while the rest come from the slopes. Despite the variability observed, large wood dynamics seems to be related to the channel morphology. The lessons learned from this event may be useful for a management of mountain catchments which should explicitly take into account the crucial role of LW during extreme events, especially for its interaction with bridges.