June 28th 2017

Uncovering major landslide activity

Clay terrain characteristic of Byneset in Trondheim municipality. The clay was deposited as the ice retreated after the last ice age. Photo: NGU
We must rapidly expand our knowledge of past landslide history in Trondheim, Norway. With the help of advanced, modern laser technology scientists have uncovered traces of several hundred landslides in the municipality.

"Yes, through the ages there have been many more landslide events in Trondheim than we once thought," says researcher Inger-Lise Solberg at the Geological Survey of Norway (NGU).

The new information is already displayed as linear symbol on the NGU's digital map services, which also displays ravines. "Overall, the update will benefit both those who engage in mapping and spatial planning," says Solberg.

Digitally 'stripping away' the landscape

Laser measurements from an aircraft, known as airborne LiDAR scanning, allow us to "see through" forest canopies and vegetation providing us with a better overview of the landscape than we can get with either aerial photos or mapping on foot.

We tested to see how a previously mapped area could be updated in Trondheim, without conducting detailed field work. We were truly surprised. About 50 major landslide scars had been mapped before, but the large scale of the map did not permit us to draw in smaller scars. This new method has allowed us to register over 600 traces of landslides in marine deposits in the municipality. Several of those which were in the registry from before have also been plotted in greater detail, tells Solberg. In addition, the boundaries between various types of surficial deposits have also been improved.

The illustrations below clearly show that NGUs Quaternary maps can be improved by use of the LiDAR data:

a) Here is a typical laser image, taken of Tiller and Sjetnemarka in Trondheim municipality, where there have been several landslides throughout history.
b) This is the initial Quaternary map of the distribution of deposits in the same area, overlaying the LiDAR data  Blue indicates marine deposits, part of which is clay.
c) The image overlay allows a better view of the landscape, so that researchers can include features such as ravines and more detailed and/or new landslide scars.
d) A new, updated map completed, with both ravines and landslides scars drawn in.

The map corresponds with the terrain

Reseacher Inger-Lise Solberg.

Most of the landslides are naturally triggered over the course of many thousands of years. Some are also completely or partially hidden, as the result of land leveling, landscaping and building activity. "To make sure, we have been out to check a number of new entries with our own eyes, too, to see that the map corresponds with the terrain," said Solberg.

Now researchers have gone on to Trondheim neighbouring municipalities, in Gauldalen and Klæbu. "In Klæbu we see that landslide activity and active ravines have affected nearly an entire section of the municipality where there are marine deposits," said Solberg.

Updated digital maps

Up-to-date and more detailed information will benefit those who produce landslide hazard maps and for spatial planners working with municipalities and counties.

In addition, NGU maps now include linear symbols to indicate all known landslide scars and ravines in all the digital Quaternary maps at the scale 1:50,000.