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| Titel |
Recent human impacts and change in dynamics and morphology of ephemeral rivers |
| VerfasserIn |
J. A. Ortega, L. Razola, G. Garzón |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1561-8633
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| Digitales Dokument |
URL |
| Erschienen |
In: Natural Hazards and Earth System Sciences ; 14, no. 3 ; Nr. 14, no. 3 (2014-03-31), S.713-730 |
| Datensatznummer |
250118343
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| Publikation (Nr.) |
 copernicus.org/nhess-14-713-2014.pdf |
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| Zusammenfassung |
| Ephemeral streams induce flash-flood events, which cause dramatic
morphological changes and impacts on population, mainly because they are
intermittent and less predictable. Human pressures on the basin modify load
and discharge relationships, inducing dormant instability on the fluvial
system that will manifest abruptly during flood events. The flash-flood
response of two ephemeral streams affected by load supply modification due to
land use changes is discussed in a combination of geomorphic and hydraulic
approaches. During the Rivillas flash flood, intensive clearing on the basin
led to high rates of sediment flowing into an artificially straightened and
inefficient channel. The stream evolved from a sinuous single channel into a
shallow braiding occupying the entire width of the valley floor. Misfits and
unsteady channel conditions increased velocity, stream power and sediment
entrainment capacity and considerably magnified flood damage. Resulting
morphosedimentary features revealed a close relationship with the valley
floor post-flood hydraulic model, and pre-event awareness would have made it
possible to predict risk-sensitive areas. In the second case, the Azohía
stream, modelling of current pre-flood channel conditions make it possible to
determine channel narrowing and entrenchment in the lower alluvial fan
stretch. Abandonment of intensive agriculture, basin reforestation and
urbanization diminish load contribution and trigger channel incision. This
induces an increase in slope and velocity in the bankfull channel, producing
renewed erosive energy and thus activating upstream propagation of incision
and bank undermining. The absence of water-spreading dynamics on the alluvial
fan in favour of confinement in a single channel produces an unstable dynamic
in the system, also offering a false sense of stability, as long as no large
magnitude floods occur. When modelling flood-prone areas and analysing
hydraulic variables, it is important to detect possible anthropic
disturbances that may affect basin load budgets in order to anticipate
catastrophic consequences resulting from inappropriate fluvial management
before the occurrence of an extraordinary event. |
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