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| Titel |
Erosion rates, sediment transport and characteristic discharge in a transient landscape in the Entle catchment (northern border of the Central Alps, Switzerland) |
| VerfasserIn |
Fabien van den Berg, Fritz Schlunegger, Kevin Norton |
| 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 |
250032610
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| Zusammenfassung |
| The 65 km2-large Entle catchment is located at the northern border of the Central Alps of
Switzerland and is underlain by various lithologies including Flysch, carbonate sequences,
Molasse deposits and glacial till. It has been subjected to headward knickpoint migration
since the termination of the LGM (16 ± 3 ka), due to a base level fall upon glacial
retreat.
The incised portions of the catchment were delineated within a GIS environment in an
effort to calculate volumetric differences between the glacial surface and the modern
topography. The sediment budget estimates yield an average erosion rate of 1.93 ± 0.36
mm.yr-1 in the incised reaches, and a maximum local erosion rate of 11.47 ± 2.15 mm.yr-1.
Assuming that there has been no erosion elsewhere, the basin-wide averaged erosion rate is
estimated at 0.31 ± 0.06 mm.yr-1. This is consistent with 10Be-based denudation rates
measured in adjacent catchments.
Although constant erosion rates are generally assumed for studies involving 10Be
analysis, field evidence indicate that headward knickzone migration through bedrock and
unconsolidated glacial till has destabilized the surrounding hillslopes, resulting in supply of
large volumes of sediment to the trunk channel by landsliding and/or debris flows
downstream the knickzone. This additional influx of sediments may raise the local base level
within the incised reach, thus perturbing the migration of the knickzone for a limited time
interval. This time span critically depends on the relative importance between the probability
density function (PDF) of the sediment particle size supplied by mass failure processes
and debris flows, and the characteristic water discharge magnitude to remove that
material.
Measurements of the PDFs of the sediment particles along the incised Entle reach
together with the application a simple long profile stream-power model for the entrainment
and transport of sediment allow the identification of characteristic bed-forming discharge
magnitudes. In particular, the model illustrates that (i) the stream power increases in the
knickzones (as expected), and that (ii) highest stream power values are calculated where
tributary debris flow fans and landslides are being cut, illustrating the importance of
hillslope-derived perturbations. Most important, the long-term averaged grain size threshold
seems to correspond to that of the 5-year flood. Furthermore, the critical mobilized grain size
for the 1-year flood exceeds the observed grain size at all sites along incised reach, suggesting
that the majority of the sediments are readily carried away by even small floods.
However, the large blocks derived by landslides (up to 11m in diameter, with origin in
the glacial till) are very likely to remain in place, not being affected by any flood. |
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