Basin-scale analysis of the geomorphic effectiveness of flash floods: A study in the northern Apennines (Italy)

• Published in: The Science of the total environment Vol. 640-641; pp. 337 - 351
• Main Authors: Scorpio, V., Crema, S., Marra, F., Righini, M., Ciccarese, G., Borga, M., Cavalli, M., Corsini, A., Marchi, L., Surian, N., Comiti, F.
• Format: Journal Article
• Language: English
• Published: Netherlands Elsevier B.V 01.11.2018
• Subjects: basins; Bed level changes; Channel confinement; Channel widening; Flash flood; Flood geomorphic hazard; floodplains; floods; islands; Italy; landslides; rain; regression analysis; riparian vegetation; rivers; sediments; Stream power; streams; watersheds; Italy; Flood geomorphic hazard; Flash flood; Channel widening; Stream power; Channel confinement; Bed level changes
• ISSN: 0048-9697; 1879-1026; 1879-1026
• DOI: 10.1016/j.scitotenv.2018.05.252

Large floods may produce remarkable channel changes, which determine damages and casualties in inhabited areas. However, our knowledge of such processes remains poor, as is our capability to predict them. This study analyses the geomorphic response of the Nure River (northern Italy) and nine tributaries to a high-magnitude flood that occurred in September 2015. The adopted multi-disciplinary approach encompassed: (i) hydrological and hydraulic analysis; (ii) analysis of sediment delivery to the stream network by means of landslides mapping; (iii) assessment of morphological modifications of the channels, including both channel width and bed elevation changes. The spatial distribution of rainfall showed that the largest rainfall amounts occur in the upper portions of the catchment, with cumulative rainfall reaching 300 mm in 12 h, and recurrence intervals exceeding 100–150 years. The unit peak discharge ranged between 5.2 and 25 m3 s−1 km−2. Channel widening was the most evident effect. In the tributaries, the ratio between post-flood and pre-flood channel width averaged 3.3, with a maximum approaching 20. Widening was associated with channel aggradation up to 1.5 m and removal of riparian vegetation. New islands formed due to the fragmentation of the former floodplain. In the Nure River, the average width ratio was 1.7, and here widening occurred mainly at the expenses of islands. Bed level dynamics in the Nure were varied, including aggradation, incision, and overall stability. The flood geomorphic effectiveness was more pronounced in the middle-higher portions of the basin. Planimetric and elevation changes were well correlated. Regression analysis of the relationship between widening and morphological/ hydraulic controlling factors indicated that unit stream power and confinement index were the most relevant variables. The study provides useful insights for river management, especially with regard to the proportion of the valley floor subject to erosion and/or deposition during large events. [Display omitted] •Basin-scale analysis of geomorphic effectiveness of a large flood•Integrated geomorphological and hydrological-hydraulic approach•Valley floor of confined channels almost completely reworked by the flood•Magnitude of channel widening linked to bed elevation changes•Varied response of floodplains and islands depending on local flood intensity