In the new report, commissioned by the Norwegian Waterways and Energy Directorate (NVE), NGI’s tsunami experts used new and better methods to calculate run-up heights following rockslides. Factoring in probabilities in all stages means that previously designated danger areas can be reopened.

“There are four locations where rockslides could generate tsunamis in Lyngen. They are Jettan, Indre Nordnes and two locations near Revdalsfjellet. The volume for each rockslide has been estimated by NVE", explains Sylfest Glimsdal, NGI's project manager on the newly released report.

In the new report, NGI has also worked out the greatest possible run-up height in different locations along the fjord, but the new calculations show that the likelihood of such ‘worst-case scenarios’ is vanishingly small.

The county governor of Troms said in a statement that this is very pleasing in terms of the scope for building and living in the municipalities of Lyngen, Storfjord and Kåfjord.

Nordnesfjellet NGI Flodboelgeberegninger E6

The E6 passes beneath the unstable mountain slopes of Jettan and Midtre Nordnes. (Photo: NGI)

Thousands of calculations

In the calculations done by Sylfest Glimsdal and the team at NGI, they varied the impact velocity and the shape of the rockslide when it enters the fjord. They also varied the underwater run-out length of the slides.

"If you combine all choices of parameter values and remember that we need to calculate the run-up at 31 locations altogether, you get a total of 600 scenarios and 18,600 calculations. We assigned a probability to all these parameter values, and ran what are known in the trade as probabilistic analyses”, Sylfest explains.

“The probability for each parameter value in all 600 scenarios was determined from models, recorded historical events and experience, and combined with the likelihood that the rocks could slip. Every point in a run-up area is then given a probability of getting flooded. For landuse planning, this is then linked to the probabilities assigned to the various safety classes in the Norwegian Planning and Building Act. These are complex calculations, but we have focussed on presenting a tool that is simple and straightforward to use”, says Sylfest.

Contingency planning

The report concludes that, with probabilities in line with the Norwegian Planning and Building Act, run-up heights are significantly reduced from what was previously assumed.
If one were to choose a location along the fjord, and look at the run-up for the largest tsunami scenario that might occur there, this scenario will happen very little likelihood. However, if you go downwards closer to the shoreline, the probability is much greater because there are many different rockslide and tsunami scenarios, which could cause a tsunami to reach this point.

“As part of the report we created a new series of hazard maps for the Lyngen area. We started by considering the municipalities’ need for land use planning and the possibility of granting building permits according to the Norwegian Planning and Building Act. Analysing worst-case scenarios is not so interesting when it comes to land use planning. On the other hand, there is useful information for use in local and regional contingency planning”, explains Carl B. Harbitz, Head of Section for RiSC – Risk, Slope stability and Climate adaptation – at NGI.

The county governor of Troms is now convening a meeting with NVE, the police and the Norwegian Public Roads Administration to look at new contingency options.

Decision-making tool

For more than 15 years, NGI has had a dedicated group of tsunami experts, with a special focus on tsunamis caused by rockslides and earthquakes. NGI is now reaping the rewards of this long-term effort in the form of major consulting jobs and R&D grants for probabilistic hazard and risk analyses of tsunamis. The group is also engaged in international projects and acts as consultants to the United Nations Office for Disaster Risk Reduction (UNISDR), among others.

"Politicians and public authorities are given a much better tool for making decisions about everything from spatial planning to contingency measures in the municipalities concerned. In addition, NVE has very extensive systems for monitoring the mountain slopes and warning the population, ensuring that timely contingency measures and evacuations can be initiated. At the end of the day it is about both citizens and society being able to live and build on safe ground", says Carl.