Bondhusdalen is a valley located on the north side of the Folgefonna Glacier (Fig. 1). Several areas on the valley’s slopes are covered in loose rocks from previous avalanches. The areas of interest include Gråura (Fig. 2) and the avalanche following the river up to the glacier, which will be referred to as the River Valley Avalanche (RVA) (Fig. 2 and 3).
The reason for the high abundance of avalanches in the valley is because of the steep slopes of Husafjellet. This can be a
hazard for hikers if the avalanche reaches the track. To find out the risk, we can calculate the run-out distance using this equation height/length = 0.4. 0.4 is the value used for large rock avalanches. The height of Husafjellet is 1068 m, and the release area would be where the steepest part is, which is at approximately 900-1000 m.
Using the scale on norgeskart.no, the distance from the release area to the hiking track can measured to about 1100 m, which means that this track is a hazardous area regarding rock avalanches.
The RVA is the larger avalanche of the two, as seen in Fig.4. The volume is calculated using the average length (519 m), average width (275 m), and a rough estimate of the average thickness using topographic contours (35 m). This gives a volume of about 5
million cubic meters.
Potential triggering factors of this avalanche are most likely related to the last ice age. As glaciers retreat, buoyancy will cause uplift of the land, and this can cause shaking, triggering weaker and more unstable parts of the mountain. Terje Erstad, 2013, says there has been attempts to date the avalanche, but to no success. However, they do believe it occured shortly after the retreat of the glaciers in correspondence with the rapid decrease in pressure. The avalanche is also thought to be what created Bondhusvatnet, damming up glacial water as the rocks fell across the valley from Husafjellet.
Gråura is most likely a younger colluvial fan, as the area is not as vegetated as the RVA. Its boulders are all angular, with the largest ones at the toe of the fan.
