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Debris Flow Observation Station at the Illgraben near Leuk (VS)

Murgang 28.6.2000 im Illgraben

28. Juni 2000 (image: C. Graf, WSL)

debris-flow front 1 July 2008 at Illgraben, VS, Switzerland

1. Juli 2008 (image: WSL automatic camera)

Since 2000 the WSL has been observing naturally-occurring debris flows at the Illgraben, an exceptionally active catchment located near the village of Susten (Leuk), in Canton Valais. The observation station is equipped with both standard and unique instrumentation, intended to provide information to help improve our understanding of the debris-flow process. Collaborators from many universities both within Switzerland and abroad use data from the Illgraben along with new observations to gain a better understanding of debris flows and their formation. A multi-level warning system for the community was installed in 2007 to provide alarms to evacuate people from the active channel and to alert the local security commission.

Frequent Debris-flow activity at the Illgraben

The Illgraben catchment ranges from the summit of the Illhorn mountain to the confluence with the Rhone River. The steep slopes in the upper catchment are underlain by geological rock formations and structures which allow rapid mass wasting by landslides and rockfall and the production of abundant sediment deposits in steep gullies. The geometry of the catchment effectively focuses water runoff on the sediment deposits where debris flows form either by direct entrainment of sediment deposits by runoff or small landslides which grow in volume by entraining sediment from the bed of the torrent channels. Debris flows occur typically following heavy rainfall produced by thunderstorms or weather fronts. On average 3 to 5 debris flows, and additional debris floods, are observed every year.

Research at the Illgraben

The initial research station at the Illgraben, consisting radar and ultrasonic sensors to measure the flow depth, geophones on check dams to determine flow speed, and a video were described by Hürlimann et al., 2003. A large force plate and additional sensors were installed in subsequent years (McArdell et al., 2007). Special measurement campaigns, often in projects originating from external partners, have also been performed to better understand certain aspects of the debris flow process, such as channel-bed erosion and deposition (Berger et al., 2011a, Schürch et al., 2011), the use of infrasound to detect debris flows (Kogelnig et al., 2014), and advanced seismic methods to explore the formation of debris flows and their propagation along the channel (Burtin et al., 2014). The debris-flow entrainment (bulking) module of the RAMMS debris-flow runout model (Frank et al., 2015) is based on careful repeated laser scans of a reach at the Illgraben channel collected by researchers at Durham University (Schürch et al., 2011). In collaboration with Geobrugg AG, flexible ring-net barriers were installed in the channel bed at the Illgraben. The results of that project (Wendeler et al., 2006, 2008; Wendeler, 2016) were used to improve the design of flexible debris-flow barriers.

To better understand the formation of sediment deposits and the mobilisation of debris flows, several research projects were launched, including investigations of hillslope-channel sediment coupling (Schlunegger et al., 2009) observations of bedrock landslides (Caduff et al., 2014) and the formation of sediment deposits in gullies (Berger et al, 2012, Bennett et al., 2012, 2013) which are then mobilized to become debris flows. The long-term chronology of landslides and an analysis of the torrent-catchment system (Bennett et al., 2013) in collaboration with Prof. Peter Molnar at the ETH in Zürich could also be used to develop a relatively simple model to explain many aspects of the formation of debris flows at the Illgraben (Bennett et al., 2014). more...

Multi-level alarm system at the Illgraben for warning

In the Spring of 2007 a debris-flow warning system was installed at the Illgraben to provide alarms for the community of Leuk (Canton Valais). The design of the system benefited from the research experience gained by the research projects which were performed at the Illgraben (Badoux et al., 2009). The overall concept includes the dissemination of information to residents and tourists, repeated observations of potential problem areas in the channel and catchment, and the actual alert system consisting of sirens and flashing lights. When a debris flow is triggered, sensors within the catchment automatically detect the process and issue both alarms at various points along the channel as well as information for local hazard managers. Depending on the type of event (debris flow, debris flood, flood), the time between detection in the catchment and the arrival of debris flows at populated areas on the fan can be up to 15 minutes. In the first two years of observation, the system generated 28 alarms, of which 6 were caused by debris flows, 21 by floods or debris floods, and only one false alarm (Badoux et al., 2009). Recent research (Abancó et al., 2012) helps to improve the interpretation of the data to further increase the quality of information used by hazard managers and ultimately aims to further increase the reliability and accuracy of warning systems. more...


Coordination of scientific activities
Warning system and site management


Abancó, C.; Hürlimann, M.; Fritschi, B.; Graf, C.; Moya, J., 2012:
Transformation of ground vibration signal for debris-flow monitoring and detection in alarm systems. Sensors 12: 4870-4891 

Badoux, A.; Graf, C.; Rhyner, J.; Kuntner, R.; McArdell, B.W., 2009:
A debris-flow alarm system for the Alpine Illgraben catchment: design and performance. Nat. Hazards 49: 517-539.

Bennett, G.L.; Molnar, P. Eisenbeiss, H.; McArdell, B.W., 2012:
Erosional power in the Swiss Alps: charcterization of slope failure in the Illgraben. Earth Surf. Process. Landf. 37: 1627-1640.

Bennett, G.L.; Molnar, P.; McArdell, B.W.; Schlunegger, F.; Burlando, P., 2013:
Patterns and controls of sediment production, transfer and yield in the Illgraben. Geomorphology 188: 68-82.

Bennett, G.L.; Molnar, P.; McArdell, B.W.; Burlando, P., 2014:
A probabilistic sediment cascade model of sediment transfer in the Illgraben. Water Resour. Res. 50: 1225-1244.

Berger, C.; McArdell, B.W.; Schlunegger, F., 2011a:
Direct measurement of channel erosion by debris flows, Illgraben, Switzerland. J. Geophys. Res. 116, F01002, doi: 10.1029/2010JF001722: 18 p.

Berger, C.; McArdell, B.W.; Schlunegger, F., 2011b:
Sediment transfer patterns at the Illgraben catchment, Switzerland: Implications for the time scales of debris flow activities. Geomorphology 125: 421-432.

Burtin, A.; Hovius, N.; McArdell, B.W.; Turowski, J.M.; Vergne, J., 2014: Seismic constraints on dynamics links between geomorphic processes and routing of sediment in a steep mountain catchment. Earth Surf. Dyn. 2: 21-33.

Caduff, R.; Kos, A.; Schlunegger, F.; McArdell, B.W.; Wiesmann, A., 2014: Terrestrial radar interferometric measurement of hillslope deformation and atmospheric disturbances in the Illgraben debris-flow catchment, Switzerland. IEEE Geosci. Remote Sens. Lett. 11, 2: 434-438.

Frank, F.; McArdell, B.W.; Huggel, C.; Vieli, A., 2015:
The importance of entrainment and bulking on debris flow runout modeling: Examples from the Swiss Alps. Nat. Hazards Earth Syst. Sci. 15, 11: 2569-2583.

Hürlimann, M.; Rickenmann, D.; Graf, C., 2003:
Field and monitoring data of debris-flow events in the Swiss Alps. Can. Geotech. J. 40: 161-175.

Kogelnig, A.; Hübl, J.; Suriñach, E.; Vilajosana, I.; McArdell, B.W., 2014: Infrasound produced by debris flow: propagation and frequency content evolution. Nat. Hazards 70, 3: 1713-1733.

McArdell, B.W.; Bartelt, P.; Kowalski, J., 2007:
Field observations of basal forces and fluid pore pressure in a debris flow. Geophys. Res. Lett. 34, L07406, doi:10.1029/2006GL029183: 4 pp.

Schlunegger, F.; Badoux, A.; McArdell, B.W.; Gwerder, C.; Schnydrig, D.; Rieke-Zapp, D.; Molnar, P., 2009:
Limits of sediment transfer in an alpine debris-flow catchment, Illgraben, Switzerland. Quat. Sci. Rev. 28: 1097-1105.

Schürch, P.; Densmore, A.L.; Rosser, N.J.; McArdell, B.W., 2011:
Dynamic controls on erosion and deposition on debris-flow fans. Geology 39: 827-830.

Wendeler, C.; McArdell, B.; Rickenmann, D.; Volkwein, A.; Roth, A.; Denk, M., 2006:
Field testing and numerical modeling of flexible debris flow barriers. In: Ng, C.W.W.; Zhang, L.M.; Wang, Y.H. (eds) Physical Modelling in Geotechnics - 6th ICPMG '06. Vol. 2. London, Taylor & Francis. 1573-1578.

Wendeler, C.; McArdell, B.; Volkwein, A.; Denk, M.; Gröner, E., 2008:
Debris flow mitigation with flexible ring net barriers - field tests and case studies. WIT Trans. Eng. Sci. 60: 23-31.

Wendeler, C., 2016:
Debris-Flow Protection Systems for Mountain Torrents - Basic Principles for Planning and Calculation of Flexible Barriers. WSL Report. 44: 297 pp. (PDF)