Abstract
Flash floods can cause great geomorphological changes in ephemeral fluvial systems and result in particularly severe damages for the unprepared population exposed to it. The flash flood in the Storelva river in Utvik (western Norway) on 24 July 2017 was witnessed and documented. This study assessed the causes and effects of the 2017 flood and provides valuable information for the calibration and validation of future modelling studies. The flooded area at peak discharge, maximum wetted and dry areas during the entire event, critical points and main flow paths were reconstructed using on-site and post-event (i) visual documentation, such as photographs and videos, and (ii) aerial surveying, such as orthophotographs and laser scanning, of the lowermost reach. The steep longitudinal slope together with the loose material forming the valley and riverbed contributed to a large amount of sediment transport during this extreme event. Steep rivers such as the Storelva river have very short response times to extreme hydrologic conditions, which calls for exhaustive monitoring and data collection in case of future events, as well as modelling tools that can emulate the hydro-morphodynamics observed during events such as the 2017 flash flood.
Highlights
Europe is currently experiencing one of the most intense flood-rich periods in the past five centuries and flood seasonality is accentuated, with increasing flood events occurring in all regions in their flood-rich season [1]
More than half of the 1500 flood events reported in Europe in the last 150 years were flash floods and information on human or monetary losses was available in most cases, whereas the total flooded area was available for only 10% of all the events [2]
The extreme event presented affected several rivers; this study focuses on the Storelva river, located in Utvik, western Norway (Figure 1a)
Summary
Europe is currently experiencing one of the most intense flood-rich periods in the past five centuries and flood seasonality is accentuated, with increasing flood events occurring in all regions in their flood-rich season [1]. The flood-rich period we are currently experiencing is anomaly separated temporally from previous flood-rich periods–there are circa (ca.) 100 years of historical flood registry with very few events–the most recent flood-rich period is significantly warmer than its interflood period–unlike all the previous flood-rich periods–and floods occur 15% more frequently than during the flood seasons of previous flood-rich periods. These factors explain the unpreparedness of civilians and stakeholders to unexpectedly frequent, severe and spatially extended floods. This paper portraits its story with the aim to provide insights into its causes and effects and to facilitate the use of such a flash flood as a case
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