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| Titel |
Climatic and geomorphic controls on flash flood response in Europe |
| VerfasserIn |
Lorenzo Marchi, Marco Borga, Emanuele Preciso, Eric Gaume |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250039835
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| Zusammenfassung |
| High-resolution data enabling identification and analysis of the hydrometeorological
causative processes of flash floods have been collected and analysed for 25 extreme flash
floods (60 drainage basins) across Europe. Criteria for flood selection were high intensity of
triggering rainfall and flood response and availability of reliable high-resolution data.
Hydrometeorological data collected for each event were checked by using a hydrological
model. The derivation and analysis of summarising variables has made it possible to outline
some characteristics of flash floods in various morphoclimatic regions of Europe. Peak
discharge data for more than 50% of the studied watersheds derive from post-flood surveys in
ungauged streams. This stresses both the significance of post-flood surveys in building and
extending flash flood databases, and the need to develop new methods for flash-flood hazard
assessment able to take into account data from post-event analysis. Catchments do not
need to be particularly steep to favour flash flooding. However, relief is important
since it may affect flash flood occurrence in specific catchments by combination of
two main mechanisms: orographic effects augmenting precipitation and anchoring
convection, and topographic relief promoting rapid concentration of streamflow.
Examination of data shows a peculiar seasonality effect on flash flood occurrence, with
events in the Mediterranean and Alpine-Mediterranean regions mostly occurring
in autumn, whereas events in the inland Continental region commonly occur in
summer, revealing different climatic forcing. Consistently with this seasonality effect,
spatial extent and duration of the events is generally smaller for the Continental
events with respect to those occurring in the Mediterranean region. Furthermore, the
flash flood regime is usually more intense in the Mediterranean Region than in the
Continental areas. The runoff coefficients of the studied flash floods are usually rather
low (mean value: 0.35). Moderate differences in runoff coefficient are observed
between the studied climatic regions, with higher values in the Mediterranean region.
Antecedent saturation conditions have a significant impact on event runoff coefficients,
showing the influence of initial soil moisture status even on extreme flash flood
events and stressing the importance of accounting soil moisture for operational
flash flood forecasting. The runoff response displays short lag times (mostly < 6
hours). The identified relations between watershed area, stream length and response
time enable determination of a characteristic velocity of the flash flood process
(at basin scales less than 350 km2), defined as the ratio of characteristic length
(mean river length) and time (response time or lag time), equal to 3 m s-1. This is
related to the celerity with which the flood wave moves through the catchment. |
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