Norway is in the process of change due to the climate. Increased precipitation means greater risk of slush-flows and flooding. It is therefore important to locate NVE, (The Norwegian Water Resources and Energy Directorate), is responsible for preparing risk-zones maps for three types of threat: quick-clay slides, flooding and slides in steep terrain (both soil slides, rock falls and avalanches). Every year, several Norwegian communes with steep mountainsides are mapped for dangerous locations so they may plan for secure building sites and infrastructure.

NGI has, on assignment from NVE during 2018, carried out extensive site surveys in such areas. Years of experience in evaluating slide terrain, together with local knowledge and advanced technology, culminate, with site surveys, in a solid basis for new slide risk-zone maps for 2019.

Early in 2018, NGI was allocated eight out of fourteen assignments from NVE for slide risk assessment. These cover the following Norwegian communes:

  • Søndre Land, Oppland
  • Sirdal, Rogaland
  • Førde and Jølster, Sogn and Fjordane
  • Eid, Sogn and Fjordane
  • Sande, Møre and Romsdal
  • Volda, Møre and Romsdal
  • Stranda, Møre and Romsdal
  • Berg, Lenvik, Tranøy and Torsken at Senja, Troms (will become one commune in 2020)

Working together with the communes, NVE designates which areas to survey. The assignment consists of assessing slide risk and preparing risk-zone maps in each of the areas. Slide risk encompasses all slide types that may trigger in steep terrain, i.e.: soil and rock slides, rock falls, flood slides, avalanches and slush flows.

Airborne and by foot

– We usually begin such surveys in early spring whilst snow is still visible in the mountains. Observations via helicopter provide a good overview and allow useful photography. Sometimes drones are also used for this purpose. An inspection on foot follows, explains senior specialist Ulrik Domaas, who has worked 38 years with avalanches and slide risk mapping at NGI.

– All such projects involve substantial fieldwork where we explore the terrain and record traces of sliding activity; whether this be misplaced boulders, vegetation destroyed or with restricted regrowth together with traces of differing types of slides in uncompacted soil deposits. We consider the entire picture of sliding activity since the previous ice age, says Domaas.

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Photo from Skredestranda on the south side of Hornindalsvatnet, Sogn and Fjordene. The mountainside is prone to both landslides, flood slides and avalanches. A large avalanche crashed down into the fjord last winter, blocking the road.

Listen to local knowledge

An important part of this work is talking to locals with personal experience and/or knowledge of recent, older or historic slide events. Old archive material must also be examined. NGI also checks for site activity, large construction projects or other human activity that may influence slide conditions. All this information is collated with the national slide database, which contains information on both new and old slides.

100 km of mountainsides in one project

Senior engineer Heidi Hefre leads the work of preparing risk-zone maps for Førde and Jølster communes, one of the eight NGI assignments from NVE. In these two communes alone, there are 28 areas to receive risk-zone maps.

– In Førde and Jølster, there are in total about 100 km of mountainsides to be surveyed, assessed and mapped. This means many days out in this terrain observing. There is also a lot of new technology involved. Everything is registered on iPad now, an important field tool – no more registering on paper, says Hefre.

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An iPad in the field provides us with all the maps we need. An inclination map, above, where yellow, red and grey colours show steep areas, (grey indicates steep cliffs where the possibility of rockfalls must be evaluated). All observations are plotted in the iPad as points and lines, (photos too), together with a brief description. Black thin lines on the iPad show the survey observer's track and figures indicate the number of points registered. Buildings are coloured according to their slide risk class, (yellow = dwelling, and green = garages and other buildings with infrequent human presence).

Senior engineer Kjetil Sverdrup-Thygeson, a colleague of Hefre, is an expert on remote analysis and GIS, (geographic information system). He has developed three different types of maps which are used to register information in the field. These maps contain different kinds of data, such as terrain slope and, so called, shadow relief of the surface – all based on a detailed surface model from LiDAR-data, (an optical remote measurement technique). These maps are patched into the national database for slide events, making it easy to obtain historic information on slides when registering out in the field.

What may be built and where?

The end product is a series of risk-zone maps showing which areas have the highest slide risk. These are graded into zones according to estimated nominal annual probability, (repetition interval), of respectively 1:100, 1:1000 and 1:5000.

1:100 indicates a probability that on average one slide will trigger every hundred years if considering a timespan of several hundreds of years. Accordingly, 1:1000 and 1:5000 indicate the probability of one slide per thousand years or five thousand years on average.

Communes use slide zone and risk category, amongst others, in planning land utilization. What should be allowed to be built and where? This is regulated in the Planning and Building law and technical regulations. For example, the requirements for building a hospital and other critical infrastructure state that the slide risk must be at the lowest possible level, i.e. 1:5000. For housing, the requirement is 1:1000, whilst 1:100 applies to buildings not used for work or residence.

Risk-zone maps have symbols indicating slide type dimensions, i.e. avalanche, slush flow, flooding, rockfall or soil- or rock slide.

Discretionary assessments

– Often we must evaluate many factors and weigh many of them up against each other when assessing slide risk, says Håkon Heyerdahl, Head of section for Avalanches and Rock Slides at NGI.

- As an example, sometimes a terrain slope and topography call for a high slide or avalanche risk, but conditions such as local climate or dense forest will in practice imply a low risk. That's why it's important to make approximations when preparing a risk-zone map. In addition, we must be able to make advanced analysis and interpret model results. In order to deliver work of high quality, it is crucial to combine practical field experience, theoretical knowledge and sound judgement. Even with the advent of modern technical aids and new, effective methods, slide risk assessment is a matter of experience, says Heyerdahl.

NGI's slide and avalanche experts contribute to a range of research projects. They also improve and develop their competence through active participation in international conferences, writing technical articles and exchanging experiences with colleagues all around the world.