Such a system is now in operation in Gildeskål outside Bodø in Northern Norway and another in Vinje in Telemark. County Road 472 in Nordland has a stretch of road of about 1 km which is struck by avalanches mostly every year. In Sandneslia, just above this stretch, NGI has installed an automatic system for avalanche alert. The system is based on geophones that registers ground vibrations generated by the moving snow masses.
The network of geophones and accompanying transmittors is linked to traffic lights that automatically stops traffic when an avalanche is registered. In the winter of 2017-18 the system was put into permanent operation by the Norwegian Public Roads Administration, after successful testing since 2015.
- So far we have not had avalanches that has reached the road, only smaller ones which have stopped further up the hillside. The monitoring and warning system, however, proved to work according to specifications and has the features we wanted, says Gunne Håland, senior engineer and engineering geologist in the Public Roads Administration, and responsible for this Project.
There are a total of six avalanche paths along the exposed road stretch. The avalanche warning system is set up to monitor three of these paths, but results from the test period show that the measurement system also detects vibrations from the avalanches in the other paths.
The geophones detects the vibrations in the rock masses. An avalanche on its way down one or more of the avalanche paths will create a continuous stream of vibrations in the ground as long as the snow is in motion. Other potential sources of vibration, such as a running deer, heavy gust winds or hammering rainfall may also create vibration, but these will have different characteristics.
-If the monitoring system registers vibrations over a period longer than about 3 seconds, it will indicate an avalanche. Then the system will send an avalanche alert to the traffic lights on the exposed road below, and at the same time the Road Traffic Centre in Mosjøen will get the alert, says Yme Asgeir Kvistedal, senior engineer in the department of Instrumentation and Monitoring at NGI.
The red light at each end of the exposed stretch of road effectively prevents cars to enter the avalanche-prone zone. The time it takes to register an avalanche in the hillside happens very quickly, since it is a wireless sensor network. Cars that already are located inside the avalanche-prone stretch of road, between traffic lights, will therefore have ample time to get out into the safe zone, before the avalanche, if big enough, reaches the road.
On the opposite side of the narrow fjord the Public Roads Administration has installed a radar system that monitors the movements on the mountainside in Sandneslia. This system can confirm avalanches registered by the geophone system.
- The reason we now have two systems, is that we already had planned and ordered a system for avalanche detection by radars, at the time when we decided to test the monitoring system based on geophones, says Gunne Håland. –The geophone system is a pilot project that was custom-made by NGI. Since we were not sure how well this system would work, as it was a pilot project, we decided to supplement and compare it with the radar monitoring. My present assessment is that both monitoring systems do their job successfully, and confirm one another.
On assignment from the Public Roads Administration an avalanche monitoring and warning system based on geophones has also been designed and installed by NGI in Lauvjuvet, in Vinje, Telemark, a mountainous area in southern Norway. The system was in place in the autumn of 2017. Here, the County Road 362 has frequently been blocked by avalanches over the years.
This region experienced very heavy snowfall in the first part of February 2018, which resulted in several local avalanches. The probability of an avalanche in Lauvjuvet is therefore quite high at present (mid February 2018).
Yme Kvistedal receives notification by e-mail when an avalanche is registered in one of the areas where he has installed the geophone monitoring system. As a curiosity he tells that he was notified when an earthquake off the coast of Western Norway occurred in november 2017. That incident generated vibrations in the rock mass over very long distances, and these vibrations were registered by the geophones in Lauvjuvet in Telemark.
-That shows how sensitive these instruments are for detection of vibrations in the ground. Even small avalanches will be registered and notified, tells Yme.
Already in the 1980 and -90 's, the Public Roads Administration mounted and tested geophone based systems to register avalanches and notify travellers. In 2013 an evaluation of these old warning systems was carried out to assess whether the time was ripe to further develop and improve this technology. The results of this evaluation showed that the existing geophone systems do not work satisfactorily.
- A positive feature with the geophone system is the simplicity, and also that the sesnsors and equipment is robust. But the technology and equipment from 1980's is now too old and not operationally reliable, explains Gunne Håland.
Based on the conclusions of the evaluation it was therefore decided to develop and design a modern geophone system, and a contract with NGI was signed. Sandeslia was selected as a suitable test site because of the relatively low annual daily traffic, its favorable topographic conditions and the high frequency of avalanche events.