Alarming and monitoring systems at the Simplon Pass

The Simplon pass culminates at 2006 m over sea level and is one of the principal alpine roads crossing the Alps in the North-South direction. It is extremely important for intra-European goods transportation, as it is open over the winter and is only protected by avalanche galleries and does not cross a tunnel, so that dangerous goods like chemicals can be transported safely all year round on this road. On the 29th of June 2025 at ca. 4.30 PM, following a few days of high intensity precipitation, several massive debris flows originating from the region of the Hübschhorn rock glacier entered the Engi gallery that is normally protecting the road from avalanches in winter and deposited a layer of more than one meter debris over several tens of meters, causing the closure of the road. The Hübschhorn rock glacier had been melting substantially over the last decade so that a combination of high temperature and important cumulated precipitation could mobilize the debris. The road had to be reopened as rapidly as possible and the debris flow channel had to be raised to avoid new damages to the road in case of events, but this meant performing construction works in a region with frequent rockfall and high debris flow risk. The Federal Roads Office mandated Geoprevent to rapidly install both a monitoring system to provide some information about the state of the rock glacier and a multi-component alarm system to detect debris flows and rockfall, close the road and trigger a local alarm on the construction site in case of detections. This works introduces the different components of these complex monitoring and alarming systems and presents some insights about the challenges faced during their installation and operation.