Flood hazard in mountain streams: the key role of geomorphic processes during high magnitude events

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.