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Debris-flow observation station

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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 of radar and ultrasonic sensors to measure the flow depth, geophones on check dams to determine flow speed, and a video camera 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).


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.