 |
| Titel |
Activity assesment of a corrected alpine torrential stream : from granulometry to historical records |
| VerfasserIn |
Benjamin Rudaz, Alexandre Loye, Michel Jaboyedoff |
| Konferenz |
EGU General Assembly 2010
|
| Medientyp |
Artikel
|
| Sprache |
Englisch
|
| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250039028
|
|
|
|
|
|
| Zusammenfassung |
| Torrential processes are amongst the most rapid and dangerous natural phenomenons
encountered in alpine regions. Their activity can be continuous and this process can be
described by a power law (intensity relative to frequency). However, some torrential
streams deal with a pulse process ; big events separeted by very calm periods.
The Saint-Barthélémy stream falls into that category. Situated in canton Valais
(Switzerland), it drains a catchment of 12.5 km2, composed of three geological
units (basements and flysch capped by the Morcles helvetic nappe) with quaternary
deposits. It created one of the biggest torrential cone of the Alps (~130*106 m3). The
frequency of these pulse events can be estimated at once every century. The last crisis
(1926 – 1930, >106 m3 transported in total) was followed by an intense campaign
of mitigation, consisting mainly of check dams, which are today all filled with
sediments.
The aim of this study is to understand the dynamics of the catchment and to use historic
activity to assess the current danger. Using high-resolution DEMs and the local sloping base
level method, the volume of sediments behind the check dams, and thus the sediment
productivity, can be calculated. The volumes obtained are then compared with those
calculated by comparing photogrametric-generated DEMs, which also helps localize the
active regions. The length profiles of the river network indicate the degree of activity in
relation to erosion and geology. Finally, the dominant transport process is identified using
field observations (forms, deposition patterns, local slope) and granulometric distribution of
the transported elements (ø < 20mm). By removing the artificially accumulated
volumes, the 1926 channel conditions can be reconstructed. This allows to calibrate
physically-based runout models on the 1926 debris flows, reconstructed using historic
sources. Once compared and affined, the obtained parameters are then applied on the
current channel, to test the effectiveness of filled dams as protection against debris
flows.
In conclusion, although the filled dams do not play their original rentention role anymore,
the results show that their protection against high intensity debris flow is strong.
This new function has to be taken into account in the future mitigation campaigns. |
|
|
|
|
|
|