Landslide at Lyngen can cause flood waves

A potential landslide from Nordnes at the transition area between Lyngen and Storfjordenin Troms can generate large flood waves (tsunami).

This is actually the result from a study conducted from NGI on behalf of the Lyngen municipality after an observation period of four years with corresponding in-situ measurements. The measurements have shown that in Nordnesfjellet in Troms, a mountain slope is moving up to about 3 centimeters per year. This threatening/unstable mountain part lies at an altitude of about 600 to 800 meters.

In cooperation with NGU ( Norges Geologiske Undersøkelse), the Norwegian state company for geological investigations, and the Lyngen municipality as the owner, NGI has conducted flood wave analyses based on two different landslide scenarios, the first (mild) scenario with a sliding mass of about 7 million m3 and the second scenario (a worst case scenario), which is nevertheless very unlikely to occur in just one sliding block than in several smaller blocks  considers a sliding mountain slope of 11 million m3.

NGi has first estimated the wave¿s spreading at Lyngen from Skibotn in the south to Olderdalen in northeast. More detailed flood waves studies were conducted for three places, Lyngseidet, Skibotn and Olderdalen  due to the fact that they are considered more vulnerable since they lie by  the seaside and have considerable population density and infrastructure facilities. In addition to that, detailed studies were conducted on ten locations in the nearby and interconnected fjords for maximum flood wave heights along the seaside. These results are tabulated  below.

The calculations have shown that large local variations in the vulnerability of the areas can be observed due to the ground morphology, fjord depth relations and infrastructure locations. The results have also shown, that for both scenarios the areas between the places Råttenvik, Lyngseidet and Oksevik are receiving the highest flood waves. The flood waves are climbing to a height of about 60 m in the immediate vicinity of the landslide areas. The worst case scenario gives also considerable wave heights in a larger area around the landslide origin.

There are undoubtedly dramatic consequences if either of the two scenarios takes place. However, we have to emphasize that we can not quantify the possibility of either scenarios to occur, since there are not any historic data available for the frequency of such landslides since the last ice age. NGI has searched any possible sources for that but has not found any information about large flood waves generated from landslides in Nordnesfjellet that affected this area in the past.

Table 1. Maximum flood wave height and travelling time as well as shoreline/fjord
inclination angle for 13 location affected by the landslide at Lyngen.