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| Titel |
From source to sink: analysis of debris flow events assessed by Dendro-geomorphology. |
| VerfasserIn |
Sara Savi, Michelle Bollschweiler, Markus Stoffel, Fritz Schlunegger |
| Konferenz |
EGU General Assembly 2011
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250046688
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| Zusammenfassung |
| We quantify the frequency distribution of the ca. 2.5 km2-large Schimbrig catchment that is
located in the central Swiss Alps. We assess this distribution using patterns of tree
ring records that register growth perturbations caused by external forcing such as
landsliding and debris flows. In particular, tree-ring analyses represent a very helpful
tool for the dating of natural hazards and can provide a better understanding of the
modality of sediment transfer and of the connectivity between sediment sources and
sinks.
The study catchment can be divided into two distinct tributaries: the eastern area,
occupied by an active earth slide (Schimbrig landslide) that underwent high slip rates several
centimetres to meters per day between September 1994 and May 1995 translating a total of
350,000 m3 of material, and the western segment, characterize by a deeply incised network of
mixed debris flow and alluvial channels (50 m maximum incision) bordered by hillslopes that
host shallow and deep-seated landslides less than 15,000 m2 large. The entire catchment is
mainly underlain by sandstone-mudstone alternations of the Eocene Subalpine Flysch that
have been reworked by glaciers during the LGM. The unconsolidated sediments
left by ice retreat are one of the main sediment sources causing instability in this
region. The climate in the Schimbrig area is dominated by a very high precipitation
rate.
Intense rainfall events may trigger landslides and debris flows from the steepest slopes of
the channel network. Therefore we focused our study to this area of the catchment with the
aim to reconstruct frequency and magnitude of debris-flow events occurring in the channel
using dendro-geomorphic techniques. We collected a total of 500 tree cores on the fan, along
the main channel and in the catchment area. Results from the fan area indicate that more
than 20 events occurred during the past 170 years with an average return interval
of 7 years. The most relevant event, affecting more than one-fourth of the trees,
occurred in 1994-1995 and seems to be related to the highest activity of the Schimbrig
landslide. Others important events, involving between 15 and 20% of the trees,
occurred in 1997, 1966, 1956, 1951; 1883 and 1859. Spatial analyses of affected trees
indicate different flow tracks for the different events. Interestingly, the 1997 event
affected a large portion of the fan (ca. 40%) but not its entire length, indicating a
source area somewhere in the central part of the fan (i.e. sediment sourced from a
tributary stream or from channel erosion farther upstream and subsequent sediment
remobilization on the fan. ). The 1994-1995 event created damage to trees on the entire fan
and is linked to the highest activity of the Schimbrig landslide that most likely
represents the sediment source for this particular event. The 1966event was more
probably related to a lateral landslide occurring on the slope of the left river bank
where the trees registered most relevant damages. Further analyses will focus on
the assessing in more detail the sediment sources in the catchment’s headwaters. |
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