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| Titel |
Sediment transfer dynamics in the Illgraben |
| VerfasserIn |
G. L. Bennett, P. Molnar, B. W. McArdell, F. Schlunegger, P. Burlando |
| Konferenz |
EGU General Assembly 2012
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 14 (2012) |
| Datensatznummer |
250068198
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| Zusammenfassung |
| Quantification of the volumes of sediment removed by rock-slope failure and debris flows
and identification of their coupling and controls are pertinent to understanding mountain
basin sediment yield and landscape evolution. We analyzed photogrammetrically-derived
datasets of hillslope and channel erosion and deposition along with hydroclimatic variables
from the Illgraben, an active debris flow catchment in the Swiss Alps, spanning 1963 – 2010.
Two events in the recent history of the catchment make it particularly interesting and
challenging to study: a large rock avalanche in 1961, which filled the channel with sediment,
and the construction of check dams along the channel in the late 1960s and 1970s. We aimed
to (1) identify the nature of hillslope-channel coupling, (2) identify the dominant controls of
hillslope sediment production, channel sediment transfer and total sediment yield, (3)
observe the response of the channel system to the 1961 rock avalanche and check dam
construction, and (4) develop a conceptual model with which to investigate sediment
transfer dynamics in various scenarios, including the absence of check dams along the
channel.
The study captures a multi-decadal period of channel erosion in response to the 1961 rock
avalanche, punctuated by shorter cut-and-fill cycles that occur in response to changes in
hillslope sediment supply and changes in transport capacity. Hillslopes eroded rapidly at an
average rate of 0.34 myr¯ 1, feeding the channel head with sediment. A near doubling of
hillslope erosion in the 1980s coincided with a significant increase of air temperature and
reduction in snow cover duration and depth, whilst precipitation variables did not change
significantly. We find that the main influence of check-dam construction on channel sediment
transfer was an initial reduction in sediment transport and a drop in debris flow activity
between 1963 and 1986. After 1986 sediment storages in the channel were filled and
debris flow activity resumed. During this time hillslope erosion exceeded channel
erosion by 0.14myr¯ 1 indicating that hillslopes eroded independently of channel
incision. Channel sediment transfer was transport-limited at the scale of the study as
suggested by the aggradation of the channel in periods of very high hillslope flux and its
apparent relation to variables connected to runoff generation such as precipitation and
snowmelt.
We have developed a conceptual model of sediment transfer based on our data set with
which to investigate sediment transfer dynamics in a probabilistic sense. A stochastic
sediment input from the hillslopes is generated from our magnitude-frequency model of
landslides. Sediment is fed into a hillslope storage component, where a fraction is
redeposited in long-term storage. Sediment in short-term storage is subsequently fed
into the channel system and is only removed given a transport event (rainfall or
snowmelt) of sufficient magnitude and sufficient available sediment. Transport
events are generated stochastically using a weather generator. We calibrate the model
with available data. Despite its simplicity the model reproduces the storage and
discharge behavior of the channel system observed over the study period and enables us
to test the sensitivity of the system to different parameters and system structures. |
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