Rockfalls and rockslides

Rockfalls and rockslides are when one or more individual blocks fall from a steep cliff. Rockfalls are defined as having relatively small volume, and occur more frequently than larger rockslides.

Figure 1: Example of large blocks from a rock fall event that occurred in 2010 along an old road in Kåfjord, Troms. (Photo: Halvor Bunkholt, NGU).

The terms rockfall and rockslide are often used interchangeably. The national landslide mapping plan in Norway defines rockfalls as small, individual blocks (up to 100 cubic metres), which usually do not split up along their path (Figure 1). On the other hand, rockslides have a larger volume (up to 100 000 cubic metres) and the blocks often split up as they travel down the slope (Figure 2). Both types have source areas in steep terrain (generally with a slope greater than 40 degrees) (Figure 3) and the blocks move down slope mainly by falling, bouncing and rolling (Figure 4).

The landscape in Norway is characterized by steep valleys and fjords, with mountains that have been eroded by several glaciations. People residing in these areas and can thus be prone to avalanches in steep terrain. It important to map potential source and runout areas, in order to prevent the construction of residential areas and infrastructure in hazardous areas.

Several methods can be used during mapping. Among other things, it is important to study the historical frequency of landslides using the national landslide database, historical books and by interviewing local residents. During fieldwork, we record the degree of fracturing, activity along the mountainside (signs of recent rockfalls, fresh deposits), estimate of the size of blocks. In addition, we use digital tools, such as aerial photos, high resolution images (eg. GigaPan images) and digital elevation models, to name a few. These tools provide detailed information about the topography, morphological traces in the terrain of previous events (scars, deposits, signs in vegetation etc.) and slope angle. To define runout areas, we often use digital modelling. These tools, together with field observations, are useful for performing detailed analysis of the source and runout areas. It is also possible to perform a more comprehensive structural analysis using measurement of structures along the rock face, to assess possible failure mechanisms (planar failure, wedge failure and/or toppling).

Figure 2: Example of a rockslide that occurred in 1992, by Jimdalen, near Tafjorden. A large block fell (notice the light area) from the extremely steep cliff and formed a large talus deposig along the foot of the cliff. (Photo: Aline Saintot, NGU).
Figure 2: Example of a rockslide that occurred in 1992, by
Jimdalen, near Tafjorden. A large block fell (notice the light
area) from the extremely steep cliff and
formed a large talus deposig along the
foot of the cliff. (Photo: Aline Saintot, NGU).
Figure 3: Rockfall in Fortundalen (Sogn og Fjordane) from 2014, with obvious source area in the uppermost part of the cliff. Along the mountainside we can see fresh tracks of the landslide where the vegetation has been destroyed. Blocks from the even are scattered on the field and stopped just short of the road. (Photo: Freddy Yugsi Molina, NGU).
Figure 3: Rockfall in Fortundalen (Sogn og Fjordane) from
2014, with obvious source area in the uppermost part
of the cliff. Along the mountainside we can see
fresh tracks of the landslide where the vegetation
has been destroyed. Blocks from the even are
scattered on the field and stopped just short
of the road. (Photo: Freddy Yugsi Molina, NGU).
Figure 4: Photo of a rockslide occurring in Børa, in Romsdalen (Møre og Romsdal), in October 2007. Large blocks were observed flying and bouncing down the slope. (Photo: Marte Kvakland, NVE)
Figure 4: Photo of a rockslide occurring in Børa, in Romsdalen (Møre og Romsdal), in October 2007. Large blocks were observed flying and bouncing down the slope. (Photo: Marte Kvakland, NVE)