Large wood recruitment processes and transported volumes in Swiss mountain streams during the extreme flood of August 2005

Published Date
Geomorphology
Available online 11 October 2016, doi:10.1016/j.geomorph.2016.10.011
In Press, Corrected Proof — Note to users

Author 

Nicolas Steeb ^,

Dieter Rickenmann

Alexandre Badoux

Christian Rickli

Peter Waldner

Swiss Federal Research Institute WSL, Zürcherstrasse 111, 8903 Birmensdorf, Switzerland

Received 8 January 2016. Revised 5 October 2016. Accepted 7 October 2016. Available online 11 October 2016.

Highlights

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  Wood budgets and transport diagrams established for four study catchments
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  Landslides and debris flows were dominant recruitment processes in headwater streams.
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  Bank erosion predominated in the lower part of mountain rivers
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  Estimated deadwood contribution of 12%

Abstract 

The extreme flood event that occurred in August 2005 was the most costly (documented) natural hazard event in the history of Switzerland. The flood was accompanied by the mobilization of > 69,000 m³ of large wood (LW) throughout the affected area. As recognized afterward, wood played an important role in exacerbating the damages, mainly because of log jams at bridges and weirs. The present study aimed at assessing the risk posed by wood in various catchments by investigating the amount and spatial variability of recruited and transported LW. Data regarding LW quantities were obtained by field surveys, remote sensing techniques (LiDAR), and GIS analysis and was subsequently translated into a conceptual model of wood transport mass balance. Detailed wood budgets and transport diagrams were established for four study catchments of Swiss mountain streams, showing the spatial variability of LW recruitment and deposition. Despite some uncertainties with regard to parameter assumptions, the sum of reconstructed wood input and observed deposition volumes agree reasonably well. Mass wasting such as landslides and debris flows were the dominant recruitment processes in headwater streams. In contrast, LW recruitment from lateral bank erosion became significant in the lower part of mountain streams where the catchment reached a size of about 100 km². According to our analysis, ~ 88% of the reconstructed total wood input was fresh, i.e., coming from living trees that were recruited from adjacent areas during the event. This implies an average deadwood contribution of 12%, most of which was estimated to have been in-channel deadwood entrained during the flood event.

Keywords

Large wood

Mountain streams

Wood transport

Mass balance

Large flood

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  Corresponding author.

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