Field observations after the flash flood of 15-16 September 2022 in the Umbria and Marche regions (Central Italy)

Spatial and temporal scales of occurrence of flash floods, combined with the space and time scales of conventional measurement networks of rain and discharge, make these events particularly difficult to observe. The effective documentation of flash floods requires therefore post-flood survey strategies encompassing indirect reconstruction of peak discharges, observations of the geomorphic processes, and large wood entrainment and transport. An informal multi-disciplinary research group has been active on these themes for more than 15 years performing field surveys on flash floods and their geomorphic effects in Italy and other European countries. This contribution outlines methods and techniques for post-flood field data collection and describes their application to the flash flood that hit a sector of the Umbria and Marche regions on 15-16September 2022. The documentation of the flash flood reveals high peak flood discharges and a spatially complex flood response. Significant channel widening, mostly caused by bank erosion, has been observed in the hilly sectors of the channel. In the downstream channel reaches, the most relevant morphological effects of the flood consisted of the overbank deposition of coarse sediment, whereas the channels underwent only small topographic changes. Large wood transported in the Misa and Burano rivers was almost entirely eroded from the channel banks, while the supply of large wood from the slopes to the investigated channel reaches was negligible. The small width of most channels constrained the distance traveled by the longest logs, which was usually quite short. Among the lessons learned from the field study of this flash flood, we underline i) the need to properly represent the drought history to describe the actual antecedent soil moisture status, ii) the highly variable interaction of flood flows with floating large wood elements trapped at bridges, iii) the need of integrating the documentation of both hazard, impacts and vulnerability aspects to advance models for more effective flood risk management.