Post-flood surveys for assessing the consequences of a flood event: the study case of Pioverna torrent

Flooding represents one of the most significant natural disasters, responsible for fatalities and huge economic damages. Such events cause markable changes on the fluvial-geomorphic landforms and on the riparian vegetation, modifying remarkably the fluvial environment from different points of view. Although there is an improvement of public awareness of dangers related to such phenomena, the lack of systematic and comprehensive investigations associated with environmenmntal controls and consequent impacts remains evident. In this regards, post-event surveys represent a necessity to gain experience and knowledge for adapting methodologies for forecasting and technical countermeasures design. A complete documentation of a flood event should include a number of field- and desk-based activities that, however, are demaning in terms of time-consuming and human resources. Moreover, geomorphological surveys must be carried out immediately after the flood event to detect clear references that can be generally cleaned up by restoration activities and/or further flooding events. This study shows a field suvey procedure achieved in analysing the flash flood occurred on June 12th 2019 along a 2 km stretch of Pioverna torrent in Valsassina (Lombardy, Italy). The post-event surveys consist in collecting meteorological data, digital photogrammetric observations, field evidences of the flood, delimitation of flooded areas, reconstruction of peak discharge and interviews with eyewitnesses. Few days after the event, rainfall data have been downloaded by the neighbouring meteorological stations. Few weeks after, detailed topographic survey using the Unmanned Aerial Vehicle (UAV) photogrammetry and permanent Ground Control Points as check points has been carried out to build the Difference of the Digital elevation models. Necessarely, a pre-event survey carried out with the same methodology has to be available for the same stretch. The results show that, first, the adopted methodology is a good balance between accuracy, speed, effort and acquired information. UAV photogrammetric survey allows to efficiently detect both the geomorphological changes and the flood-damaged areas. Secondly, the change in vegetation patterns confirms the strong control of fluvial geomorphological processes; in particular, differences in vegetation response are evident in eroded streambank and in sediment deposition areas. Finally, this study underlines how the post-flood surveys have a strategic importance for a wide spectrum of multidisciplinary aspects, from ecology to hydraulics, and offers an example of how different tools and activities can be efficiently combined to obtain a faithful reconstruction of the flood event and of its conseuquences and damages. A standardisation of such a procedure should be drafted into specific guidelines and included into the plan of post-emergency management.