EGU2020-9682
https://doi.org/10.5194/egusphere-egu2020-9682
EGU General Assembly 2020
© Author(s) 2022. This work is distributed under
the Creative Commons Attribution 4.0 License.
Flood hazard in mountain streams: the key role of geomorphic
processes during high magnitude events
Nicola Surian1, Andrea Brenna1, Marco Borga2, Marco Cavalli3, Francesco Comiti4, Lorenzo
Marchi3, and Mattia Zaramella2
1
Department of Geosciences, University of Padova, Padova, Italy (nicola.surian@unipd.it)
2
Department of Land, Environment, Agriculture and Forestry, University of Padova, Padova, Italy
3
Research Institute for Geo-hydrological Protection, National Research Council (CNR IRPI), Padova, Italy
4
Faculty of Science and Technology, Free University of Bozen-Bolzano, Bolzano, Italy
Although channel dynamics (i.e. channel lateral mobility, intense sediment and wood transport)
are commonly dominant processes in mountain streams during high-magnitude floods, hazard
assessment still mostly focuses on water flooding only. Therefore, there is a need to include river
geomorphological hazard to produce reliable flood hazard mapping and define effective
mitigation measures. This work deals with the “Vaia” storm that occurred in the Eastern Alps (Italy)
on 27-30 October 2018. Our aims are (i) to improve the understanding of geomorphic processes in
response to large floods and (ii) to improve the prediction capability of the reaches more prone to
undergo intense channel dynamics (e.g. channel widening, in-channel sedimentation) during such
events.
An integrated approach was deployed to study the flood event in the Cordevole river catchment
(876 km2). The approach includes (i) analysis of geomorphological processes, by comparing remote
sensing data acquired before and after floods and field survey (e.g. recognition of different flow
types); (ii) hydrological and hydraulic analysis (collection of rainfall and streamflow data,
estimation of peak discharges at multiple sites in ungauged streams, and model-based
consistency check of rainfall and discharge data); (iii) landslide mapping and analysis of sediment
delivery to the channel network.
Intense sediment and wood transport took place. A wide range of transport processes (i.e. debris,
hyperconcentrated and water flows) was recognized in the channel network and notable channel
aggradation occurred at specific location (e.g. in channelized reaches). Channel widening was the
most relevant geomorphic response along the fluvial network. Width ratio (i.e. channel width after
/ channel width before the flood) reached up to 2.1 and 4.4, respectively in the Cordevole and in its
tributaries. Locally, the valley slopes were eroded (e.g. slope retreat up to 14 m). This means that
the lateral channel dynamics affected not only large portions of the valley floor (e.g. forested
floodplain) but also the valley slopes, especially if made of Quaternary deposits or soft bedrock.
These results have several implications in terms of flood hazard assessment in mountain streams.
Since channel widening is a major process (streams may take up the whole floodplain and, locally,
erode the valley slopes), so-called “river morphodynamic corridors” need to be defined and
integrated into flood hazard maps. During high-magnitude floods the sediment mobilization may
take place through mechanisms (e.g. hyperconcentrated flows) that can be different from those
expected for ordinary water floods. Since major channel changes commonly occur during large
floods, their prediction is needed and should accompany flood hydraulic modelling to obtain
reliable flood event scenarios.
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