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| Titel |
Effects of large floods on channel width: recent insights from Italian rivers |
| VerfasserIn |
Vittoria Scorpio, Margherita Righini, William Amponsah, Stefano Crema, Giuseppe Ciccarese, Laura Nardi, Davide Zoccatelli, Marco Borga, Marco Cavalli, Francesco Comiti, Alessandro Corsini, Lorenzo Marchi, Massimo Rinaldi, Nicola Surian |
| Konferenz |
EGU General Assembly 2017
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 19 (2017) |
| Datensatznummer |
250145260
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| Publikation (Nr.) |
 EGU/EGU2017-9183.pdf |
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| Zusammenfassung |
| Variations of channel morphology occurring during large flood events (recurrence
interval > 50-100 years.) are very often the cause of damages to buildings and
infrastructures, as well as of casualties. However, our knowledge of such processes
remains poor, as is our capability to predict them. Post-event campaigns documenting
channel changes and linking them to hydrological and morphological factors thus
bear an enormous value for both the scientific community and river management
agencies.
We present the results of an analysis on the geomorphic response associated to 4 large
floods that occurred between October 2011 and September 2015, affecting several catchments
in Northern Italy (Magra-Vara, Trebbia, Nure rivers) and Sardinia (Posada and Mannu di Bitti
rivers), characterized by different climatic, lithological and geomorphological settings. The
analysis considered more than 400 channel reaches characterized by a drainage area ranging
from 39 to 1,100 km2 and featuring a wide range of lateral confinement, mostly within the
partly- and unconfined conditions.
The approach to flood analysis encompassed: (i) hydrological and hydraulic analysis; (ii)
analysis of sediment delivery by landslides to the channel network; (iii) GIS-based and field
assessment of morphological channel modifications. For the Nure River flood event
(September 2015) a quantitative assessment on average bed level variations was
also carried out. Return period for maximum hourly rainfall intensities and peak
water discharges exceeded in all basins 100 yr, in some cases even 300 yr. Very
high unit peak discharges were estimated, reaching 8.8 m3 s−1km−2 in the Nure
River (205 km2) and up to 30 m3 s−1km−2in few Magra River tributaries (5-10
km2).
Notable channel widening (post-flood width / pre-flood width > 1.1) occurred in 83% of
studied reaches, and it was found more relevant in the channels with narrower initial width,
i.e. along the relatively steep tributaries. For these tributaries, the ratio between post-flood and
pre-flood width presents an average value of 4.2, with a maximum approaching 20. In the
main river channels, due to the presence of wider sections and lower slope, the ratio resulted
< 5, on average 1.3.
The analysis of width ratio vs. flood peak unit stream power shows that the minimum unit
stream power required to cause relevant widening was about 500 Wm−2. Nonetheless, some
reaches affected by unit stream power as high as 4,000 Wm−2 exhibited limited or no
widening at all. Indeed, a statistical analysis on the relationship between widening and both
morphological and hydraulic controlling factors indicates that unit stream power and
confinement index were the most relevant variables, whereas sediment input from mass
wasting processes seems to have a localized influence. Remarkably, the analysis
of subset referring to Trebbia and Nure basins showed that channel widening is
strongly associated to bed aggradation, and that steeper tributaries underwent higher
aggradation despite their larger sediment transport capacity. These results points
out that geomorphic changes due to large floods are controlled by several factors
that induce a highly variable pattern of change even within the same river basin. |
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