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Extreme precipitation on dry ground in western Norway –characteristics of induced landslides call for adaptation of theNorwegian practice in landuse planning
Denise Christina Rüther1, Lena Rubensdotter2,3, and Heidi Hefre4
1HVL, Department of Environmental Sciences, Røyrgata 6, 6856 Sogndal, Norway
2NGU, Geohazards and Earth Observation, Leiv Eirikssons vei 39, 7040 Trondheim, Norway
3UNIS, Department of Geology, P.O. Box 156, 9171 Longyearbyen, Norway
4NGI, Norwegian Geotechnical Institute, Sandakerveien 140, 0484 Oslo, Norway

Abstract. Following a particularly dry summer, a torrential rain event struck Western Norway on Tuesday
30 July 2019. The resulting floods and shallow landslides caused one fatality and severe damages to public
and private infrastructure in the former Jølster municipality. Building on earlier work, in which we identified
characteristics of the shallow landslides induced by torrential rains on unsaturated soils, we here present
suggestions for adaptation of the Norwegian practice in landuse planning.

1 Introduction
Historically in Norway, the majority of shallow landslides
occur after prolonged and/or intense rainfall, sometimes
combined with snowmelt; commonly during spring andautumn [1,2]. Whilst more research is needed, it isgenerally assumed that shallow landslides in Norway are
primarily released in fully saturated soils as slope-specificporewater pressure thresholds are exceeded [3]. In
contrast, during the Jølster event soils were not fully
saturated, instead failures largely occurred due to locallyhigh porewater pressures at depth, as short intense rainfall
on dry ground led to surface water infiltration in open
cracks in the surface cover, often at soil-bedrock or soilboulder contacts [4].
1.1 The Jølster event
In July 2019 the weather in Western Norway was
relatively warm and dry, resulting in low to very low
groundwater levels in areas not affected by snow or
glacier melt [5]. Unusually warm air masses dominated
Southern Norway in the week prior to the extreme eventon 30 July, whilst colder air masses approached from theeast and northeast the day before the event. This createdatmospheric instabilities which intensified as the air
masses moved westwards and absorbed humidity from theglaciated and still snow-covered areas in the southern
Scandinavian Mountains [6]. The rain gauge at
Haukedalen, 16 km south-southeast of Vassenden,
recorded 43.6 mm in 3 hours, exceeding the 200-year
event magnitude [6]. However, weather-radar datasuggest much higher intensities in the mountains aboveSlåtten and Vassenden (Fig. 1A) focused to a narrow time
window from 3 to 5 pm.

Fig. 1. Landslides and rainfall in Jølster 30 July 2019.
A: Accumulated rainfall estimated from weather radar 2-8pm
with shallow landslides [7] B-D: Close-up of terrain and slope
angles with detailed mapping of the largest shallow landslides[4].

The intense deluge triggered >120 landslides andfloods which damaged public infrastructure and private
property in a 10 km radius around Vassenden (Fig. 1A).
The extreme event resulted in one fatality, 150 people© The Authors, published by EDP Sciences. This is an open access article distributed under the terms of the Creative Commons Attribution License 4.0(https://creativecommons.org/licenses/by/4.0/).

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evacuated from the area and the closure of Highway E39
for several hours. Following the event, 17 million kroner(ca. 1.65 million euro) were spent on debris removal andbuilding of safety measures; payments from private
insurances not counted.
120 shallow landslides triggered by this torrential
rainfall event were identified based on Sentinel-2 NDVI(Normalized Difference Vegetation Index) images in a 10
km radius around the town of Vassenden (Fig. 1A; [7]).
52 of these, including the most destructive debris flows
and avalanches (Fig. 1B-C), were studied in greater detailand presented in [4]. The findings are shortly summarized
in the following paragraph.

less destructive compared to counterparts released in fully
saturated soils.
Many landslide paths were densely forrested
illustrating the rare occurrence of mass movements on
those slopes. Therefore, timber was an important
ingredient of the total landslide masses. In the Slåtten area
(Fig. 1C) logs accumulated during the first more viscous
pulse functioned later to protect the settlement from asecond, more liquid, landslide pulse. Landslide deposits
from the larger Jølster landslides were very diverse: fromthin diamicts with angular boulders, over thin unsorted tolayered diamicts, to almost isolated freshly eroded
cobbles and boulders with basically no sediment matrix
deposited on relatively undisturbed topsoils andvegetation. The last category differs significantly from thenormally recognised landslide deposits, and may after few
years likely be misinterpreted as rockfall deposits rather
than shallow landslides.

1.2 Characteristics of landslides induced bytorrential rain on unsaturated soil
Commonly, autumn and spring storms can induce shallow
landslides spread over larger areas in Norway [2]. In
contrast the torrential rainfall in Jølster triggered amultitude of shallow landslides in a spatially confinedarea (< 160 km2) during the warm summer month.
The landslide source areas in Jølster are situated in theupper hillslopes, often at the transition between barebedrock and thin (< 0.5-1 m) soil cover and primarily ator above the tree line. Many landslides are initiated at thefoot of larger or smaller cliffs. Notably, and importantly,
many of the starting points were not recognized in existingsusceptibility and hazard zone maps. These maps are allbased on combinations of field observations of deposits
and tracks from previous events, scattered historical
records and computer models based on digital terrain data.
While some landslides were subsequently channelized
into known flow paths, several landslides paths occurredvery unexpectedly considering geomorphology andknown previous events.
Some open fissures observed in the topsoil around anddirectly above 2019 backscarps could traditionally beinterpreted as retrogressive soil failure, but fissures werealso observed further away from the backscarps andbetween landslides. Therefore, we argue that they likely
initiated prior to the event, due to the prolonged dry spell.
The time from rain shower onset until triggering of the
majority of shallow landslides was barely 1-2 hours,suggesting that only the topsoil (0.1-0.3 m) was saturated
and that intense surface runoff instead infiltrated directly
through pre-existing fissures. This mechanism facilitated
rapid water infiltration and localized build-up of waterpressure at depth corresponding to typical observed scarp
thickness (0.6-1.2 m), at stratigrahic transitions, as well as
soil-bedrock and soil-boulder contacts.
Point field observations and subsequent volume
analyses from pre- and post-landslide terrain models
further indicate that landslide surface sedimententrainment outside the main channels was restricted toroughly 0.2 m below original surface. This may be related
to the depth of transition from saturated to unsaturated
topsoils. Due to the low sediment entrainment, the waterto-solid ratio of the mobilised landslide/debris flow
material remained high. The observed shallow landslides
are thus more liquid, have longer runouts and are slightly

2 Natural hazards in Norwegian landuse
planning
Considerations of natural hazards in Norwegian landuse
planning are regulated through the Planning and Building
Act §§ 28-1 and 29-5 [8] and in Regulations on technicalrequirements for building works (TEK17) § 7 [9]. The
toolbox for natural hazard evaluation in spatial planningconsists of i) seamless national susceptibility maps
establishing potential danger and ii) local hazard zone
maps which verify and quantify real danger based on mass
movement probability. Since 2009 hazard zone evaluation
has been required for planned building projects within asusceptibility zone. The Norwegian Water Directorate(NVE) works on hazard zone mapping of the most densely
populated areas within selected municipalities; currently
only 2.6% of Norway’s surface area is analysed.
Hazard zones in Norway are defined based on the largest
nominal probability for a mass movement hitting anddamaging individual buildings:i) For infrastructure belonging to safety class S1(garages, boat houses) annual probability must be≤ 1/100.
ii) In safety class S2 (residential houses with max. 10
units, work and residential houses for a max. of 25
persons) annual probability must be ≤ 1/1000.
iii) In safety class S3 (residential houses with more than
10 units, work and residential houses for more than 25
persons, schools, kindergartens, nursing homes andinstitutions for emergency preparedness) annual
probability must be ≤ 1/5000.
Compared to other countries’ legislations, these arelong return periods to consider and consequently
consultants have less use of modern climate and event
databases to inform evaluations. Geological andgeomorphological observations thus become increasingly
important for mass movements with very long return
periods. Unlike the Swiss practice in landuse planning[10], intensity or energy at impact and damage potential
are only considered indirectly.

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2.1 Uncertainties in identification of sourceareas

produces the overall best results. For the Slåtten caseconservative Voellmy parameters (=3000 m/s2; µ=0.05)
and no erosion gave more realistic results which isconsistent with highly liquid debris flows subject to very
low frictional resistance and restricted sedimententrainment due to dry soil. If consultants simulate withdifferent starting zones and sets of parameters
corresponding to rain-on-dry-ground vs rain-onsaturated-soil scenarios, such modelling results could giveindications for the construction of maps which show theintensity or energy at impact.
Landslide paths with long return periods arecommonly densely forested. Whilst modelling would
have central role in quantifying runout length andintensity, it should be mentioned that any heterogeneity inthe landslide mass, and in particular timber, willcomplicate landslide flow dynamics. In Jølster, we haveseen examples of how logs function as a barrier, boththrough uprooting of single trees and as log walls in thedeposition zone. Resulting changes in landslide pathdirection are near impossible to predict. In other historic
events in Norway logs created the biggest damage inotherwise harmless clay-rich landslides, stressing theunpredictable nature of landslide masses with high timber
content.

The current methodology for shallow landslide
susceptibility mapping in Norway [11] identifies potential
starting points based on three topographic characteristics:
slope angle, planar curvature and upslope catchment area.
The national terrain model which was used for the present
dataset had variable original resolution of 5, 10 and 25 m,
subsampled to a 10 m raster dataset. The National
quaternary geological dataset at varying scale of
1:250 000 to 1:50 000 was used as additional criteria
together with a very generalised regionalisation ofmorphological traces of previous events. The fact that
Jølster event starting points were largely not recognized isprobably due to a combination of the following: i) many
shallow landslides occurred in areas mapped as bare rockin the quaternary map, ii) several shallow landslides werereleased from hillslope with little planar curvature (> -0.5
per 100 m) and iii) some shallow landslides do not fulfil
the requirements regarding upslope catchment (> 5 000m2). The low general resolution of the terrain model used
(10 m) will also make detection of smaller topographic
features inaccurate and some potential starting points may
not be detected at all.
Identification of potential source areas for shallow
landslides in hazard zone mapping according to
Norwegian TEK17 is done based on study of historic
events, climate analyses, GIS-based terrain analyses(based on Lidar-derived terrain models) and fieldwork.
However, looking in greater detail, many of the Jølster
event source areas would still not be considered typicallandslide starting zones based on the thin soft-sediment
cover (< 0.5-1 m) often on narrow ledges or on open
slopes, and might not have been investigated further
during hazard evaluation. The Vassenden debris flow
(Fig. 1B) is a good example of such an unexpected startingzone high up on an open slope without obvious catchment
or initial sediment. Many of the Jølster landslides had
comparably small starting volumes, but due to
exceptionally rainfall intensities and runoff over steep tovery steep upper transport areas (30-45 and 45-60
degrees, see Fig. 1B-D), momentum was neverthelesssufficient to sustain large debris flows and avalanches
down to the valley bottoms.

2.3 Uncertainties in identification of debris flow
deposits
In Norwegian hazard zone mapping, the mass movementwith the longest runout and highest impact is ruling theplacement of hazard zone borders, whilst hazard zones areconstructed for each mass movement type and then
combined to synoptic hazard maps in other countries [e.g.
10]. Thus, field investigations, together with digital map
sources and modelling results often inform the decision
which mass movement type is determining hazard zoning.
As before mentioned, we found a somewhat unexpectedcategory of landslide deposits: almost isolated cobbles
and boulders with little or no matrix, sometimes
accompanied by tree logs, deposited on undisturbed
topsoil and vegetation. Angular boulders will likely be themost long-lived superficial remains of these deposits andmay be misinterpreted as rockfall rather than debris flows
deposits. In the Norwegian approach for hazard zone
mapping a falsely interpretated origin of boulder material
may have far-reaching consequences.

2.2 Uncertainties in landslide paths prediction
According to Norwegian TEK17 hazard zones mustreflect areas where mass movements potentially harm
humans or hit and damage buildings, but little guidance isprovided on how much damage may be acceptable. In
addition to field observations and the study of digital map
sources, consultants are obliged to simulate landslide
paths with dynamic models, in many cases with
RAMMS::Debrisflow. Taurisano [12] has used theSlåtten shallow landslides as one of eleven case studies todetermine the best set of parameters in RAMMS for nonchannelized shallow landslides in Norwegian conditions.
He concludes that the use of standard Voellmy parameters(=200 m/s2; µ=0.2) and consideration of erosion indensely packed sediments (erosion rate 0.013 m/s)

2.4 Suggested adaptations of the Norwegian
legislation and practice in landuse planning
The Norwegian legislation and practice in landuse
planning is developed based on historic mass movement
events and has been well suited to tackle debris flow
hazard in the past. The Jølster landslides triggered bytorrential rain on dry soil can be considered a hitherto rarecategory of events in Norway which is expected to
become more frequent due to a warming climate [13]. In
our opinion the Jølster events challenges existinglegislation and practice in landuse planning and sheds
light on its shortcomings.

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To begin with, we want to address an apparent
contradiction in the review of the Norwegian landuse
approach presented above. On the one hand, Norwegian
legislation demands the consideration of long return
periods of 5000-, 1000- and 100-years; it can thus beconsidered a conservative approach. This is amplified by
the current version of conservative susceptibility maps.
Hazard zoning restricts building activities in many
mountainous municipalities and inhibits local economy
and infrastructure development. On the other hand, ouranalysis of the Jølster event may suggest that the
Norwegian approach is not conservative enough tocapture this type of landslides induced by torrential rainon dry ground as observed starting points were largely notrecognized in susceptibility and hazard zone maps. To
resolve this contradiction, we suggest a series ofadaptations to the Norwegian legislation and practice inlanduse planning.
In our opinion, there is a need to adjust legislation sothat the considered return periods for mass movements insteep terrain in Norway are shortened considerably. This
would allow evaluations to be based more strongly on
climate records and reduce the necessity of expert
guessing. Another required change which is already on
the way for avalanches and shallow landslides is the
production of revised susceptibility maps to make themless area demanding.
The main challenge with debris flows induced byextreme summer rainfall is that we seem to be unable topinpoint starting points and rather come to the fatalistic
conclusion that soil failure must be expected on any slope
which is steep enough. Therefore, in the framework of theexisting Norwegian approach, we suggest them to betreated more like rockfall. Belonging to the category ofmass movements which occur randomly along a
mountainside, the probability for a rockfall hitting aproperty is considered for a 30-meters-wide foothillsection. Since the cross-sectional area affected by a debris
flow is wider than for rockfalls, the foothill section should
be increased to at least 100 meters for landslides in rainon-dry-ground scenarios. This simple adaptation of theNorwegian practice in landuse planning suggested above,
could be set into place promptly.
In the longer term, we suggest more fundamental
changes to the Norwegian approach in landuse planning.
Landslides induced by torrential rain on dry soil are rare
events with long runouts and possibly lower energy at
impact. These characteristics could be tackled more
adequately with a clear differentiation of the mass
movement probability, intensity and resulting hazard. At
present probability of the various mass movement types
and intensity are only considered indirectly by consultantsin one single hazard map showing the combined effect of
all mass movement types which have the potential toendanger human lives and health or cause large economic
loss. Further, the maximum allowed annual probability isdetermined based on the function of each individual
building in Norway, while countries like Switzerland andSouth Tyrol follow an aggregated approach in whichhouses belonging to densely or sparsely populated areas
are defined as common objectives. The aggregatedapproach may help consider rare events more adequately.

A final shortcoming of the Norwegian system that wewould like to highlight, is the lack of communication andawareness of the residual risk. Buildings belonging tosafety class S3 should be placed outside the threeconsidered hazard zones, where the accepted annual
probability is no greater than 1/5000. In the public opinion
however, there seems to be a misconception that is nohazard whatsoever beyond these zones.

3 Conclusions
The characteristics of the Jølster event with heavy rain on
dry ground highlight the shortcomings of the Norwegian
legislation and practice in landuse planning. Many of the
Jølster event landslides started outside existing national
susceptibility zones and in places not previously known
for similar processes. We suggest changes to thelegislation as well as short- and long-term adaptations of
the Norwegian practice in landuse planning.

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