From vol. 118, Summer 2018

By Lisa Dreier

EDITOR'S NOTE: In keeping with the topic of this issue, the intent of this article is to provide insight into technologies less commonly used in North America. In no way does the CAA endorse any particular technology or application mentioned. Lisa Dreier is a representative of Wyssen, which has commercial interest in some of the technologies discussed.

Figure 1: Overview of avalanche detection systems (radar, infrasound and geophones).

SNOW AVALANCHES pose a hazard for people and infrastructure during the winter season. Permanent measures (tunnels, steel structures, etc.) and/or active and passive temporary measures (e.g. road closures, evacuations, preventive avalanche release, avalanche forecasting, etc.) are used to mitigate this hazard. The preventive release of snow avalanches along traffic routes is often used where permanent measures are too expensive or not feasible to construct. Reliable feedback on the success of triggers makes preventive avalanche release more effective as knowledge of occurrence, frequency and size of avalanche events assists personnel responsible for avalanche control and forecasting.

A variety of detection systems are available and have been tested in operational use. Depending on the aim of the operation, the most suitable system should be selected (Table 1).

Table 1: Avalanche detection systems and their suitability for different operations.

RADAR SYSTEMS

Radars have been applied for the detection of avalanches for many years. In most cases Doppler radars are used, emitting electromagnetic waves at a certain frequency, which are then reflected and travel back to the radar (Gauer et al., 2007). Thus the radar requires line-of-sight of the avalanche paths in question. The radar can discriminate between moving and static targets and therefore measures the velocity of the avalanche front.

Experience with Radar
A long-range avalanche radar was installed in Ischgl, Austria in 2011, with the purpose of i) verifying the controlled release of avalanches and ii) gathering information about spontaneous avalanche activity. The radar is a standard operational tool of the safety staff (Steinkogler et al., 2018). The big advantage of the radar is the accurate detection of even small avalanche events. The shorter the distance to the radar antenna and the better the weather conditions (i.e. no rain, no snowfall), the smaller the detectable avalanches are (events of a few 100 m³ in a distance of 1.5 km were detected).

Since radar systems provide data in real-time, alarm thresholds can be defined which allow using the system for the automatic closure of traffic lines. Power can be provided byfuel cells or by permanent power supply if available.

Based on the success of the avalanche radar, the short distance avalanche radar with a 500 m range and less energy consumption was developed (Table 2). They are mounted directly on remote avalanche control systems (RACS) to get immediate information about the success of the avalanche release. This is a much-needed feature for verification of preventively released avalanches. Last winter a short-range radar was installed in Glacier National Park, Canada. The system detected 10 avalanche events triggered by the avalanche tower it was installed on, as well as by the adjacent tower. Other uses of this radar type, such as the detection of persons moving in the area endangered by avalanches, were also successfully tested
(Video: http://gpr.vn/PETRA).

Table 2: Summary and technical characteristics of radar, infrasound and seismic systems.

INFRASOUND
Infrasound waves are low frequency (<20 Hz) sound waves that are not perceived by the human ear. The infrasound technology is widely used for the detection of different natural (e.g. volcanic eruptions) and artificial phenomena (e.g. nuclear explosion). For avalanche monitoring, infrasound technology has significantly improved in recent years in terms of sensor design, noise reduction and processing algorithms (Ulivieri et al., 2011).

Typically, an infrasound detection system consists of a 4 to 5-element infrasound array, with a triangular geometry and an aperture (maximum distance between two elements) of approximately 150 m (Marchetti et al., 2015). During the winter season, the sensors are covered with snow, which helps to dampen ambient noise. This setup allows monitoring of the avalanche activity from all directions within a radius of 3 - 5 km (Table 2).

Experience with Infrasound
To gather information on avalanche activity of a larger area and to assist the local avalanche control team, an infrasound was first installed in 2012 in Ischgl, Austria. The goal was to gather information about avalanche activity from all avalanche paths in the area. Currently, nine systems are used operationally in Switzerland, Norway, Canada and USA (Figure 2). In Canada, an infrasound avalanche detection system has been operated in Glacier National Park for two winter seasons. Last winter the system detected 136 natural avalanches, 137 artillery explosions and 59 controlled avalanches. The detection system notified the forecasters of the onset of natural avalanche cycles and whether artificial avalanche control was successful. This information allowed the forecasters to plan and execute control sessions even more efficiently and thereby reduce closure times of the Trans-Canada Highway.

In Switzerland, Canada, and Norway extensive verification campaigns have been conducted over the last years (Steinkogler et al., 2016). The infrasound system was used to monitor certain avalanche paths which endanger local roads and to define the smallest avalanche size which can be detected. Although the system detected many of the smaller slides (size 1-2), they were not automatically visualized and identified as avalanches as they were below the defined thresholds. Mid-sized and large dry slab avalanches were correctly detected. Additionally, large dry avalanches could be detected up to 14 km away from the system.

Infrasound systems have been deployed in a variety of climatic conditions, ranging from a maritime climate in Norway, to lower elevations and high inner-alpine regions in Switzerland and Canada. At one of the locations, more than two metres of dense (250-300 kg/m3) snow with several ice layers covered the sensors which influenced the quality of the signals. Yet, a generally thick snow cover without ice layers has shown to filter out unwanted frequencies (e.g. traffic noise) and enhance the reliability of the system. Strong ambient noise, such as wind, has shown to complicate the identification of the avalanche signal.

The infrasound system proved to be a very valuable tool for gathering information about avalanche activity of multiple avalanche paths in a larger area. Since it is continuously monitoring it also provides data on spontaneous avalanche activity, which can be very useful information for the local avalanche control team (Figure 2, green arrows).

Figure 2: Example of infrasound detections in Glacier National Park, Canada. The system detects natural avalanches (green), controlled avalanches (red) and detonations of remote avalanche control systems (RACS) or artillery (yellow).

GEOPHONES
Geophones detect the ground vibrations induced by an avalanche in rather close distance to the sensor. So far, the installation of geophones was mainly done very close to the flowing path of the avalanche and the release areas. Avalanches can be reliably detected with approximately 50 m distance to the sensor (Table 2).

Experience with Geophones
Seismic sensors have been applied for operational and research purposes for many years (Perez-Guillen et al., 2016). Figure 3 shows an example where three geophones are deployed in the release area of a high alpine bowl. RACS allow for avalanche control to be performed during day or night and the geophones detect if an avalanche was released.

CONCLUSIONS
From an operational point of view, all systems have reached a technological level at which they work reliably both in terms of system stability and avalanche detection performance (Table 2). All three systems need a calibration period (a few avalanches of typical size for the avalanche path) to optimize the parameters and to be fine-tuned to the local conditions, minimizing false alarms. Generally, an intensive and well-prepared planning phase is essential to achieve the desired functionality and accuracy of the systems.

For authorities operating several avalanche release and detection systems, simplicity is one of the key demands. The integration of all relevant information from RACS and detection systems in one practitioner-friendly and easy to operate platform is crucial. A visualization of the results in a clear, simple way provides a good overview using a mobile phone or laptop (Figure 3).

Experiences with the short-range radar system and infrasound system installed in Glacier National Park, Canada, were recently presented at the CAA spring conference in Penticton by Jim Phillips (Parks Canada) and the author of this article. The presentation can shortly be viewed in the member section of the CAA website.

Figure 3: Integration of remote avalanche control systems (here “Sprengmast”) and geophone and radar (blue areas) detection systems in one user friendly web-platform.

REFERENCES
Gauer, P., Kern, M., Kristensen, K., Lied, K., Rammer, L.,Schreiber, H. 2007. On pulsed Doppler radar measurements of avalanches and their implication to avalanche dynamics, Cold Regions Science and Technology, 50, 55–71

Marchetti E., Ripepe M., Ulivieri G., Kogelnig A. 2015. Infrasound array criteria for automatic detection and front velocity estimation of snow avalanches: towards a real-time early-warning system. Natural Hazards and Earth System Sciences 3(4):2709-2737.

Pérez-Guillén, C., Sovilla, B., E. Suriñach, E., Tapia, M., and Köhler, A. 2016. Deducing avalanche size and flow regimes from seismic measurements, Cold Regions Science and Technology, 121, 25–41, 2016.

Steinkogler, W. , Meier, L., Langeland, S., Wyssen, S. 2016. Avalanche detection system: A state-of-the art overview on selected operational radar and infrasound systems, Interpraevent 2016, Lucerne, Switzerland.

Steinkogler, W., Langeland, S., Vera, C. 2018. Operational avalanche detection systems: Experiences, physical limitations and user needs, 7th Canadian Geohazards Conference, Canmore, Canada.

Ulivieri, G., Marchetti, E., Ripepe, M., Chiambretti, I., De Rosa, G. and Segor, V. 2011. Monitoring snow avalanches in Northwestern Italian Alps using an infrasound array, Cold Regions Science and Technology, Volume 69, Issues 2–3, December 2011, Pages 177–183P.