THE ROLE OF GEOMORPHOLOGICAL EVIDENCE FOR

First North America Landslide Conference – Vail, Colorado, AEG Publication No. 23
Editors: V.R. Schaefer, R.L. Schuster & A.K. Turner
pp. 583-592
THE ROLE OF GEOMORPHOLOGICAL EVIDENCE FOR SNOW-AVALANCHE
HAZARD AND MITIGATION RESEARCH IN NORTHERN ICELANDIC FJORDS
Armelle Decaulne1 & Thorsteinn Saemundsson2
1
CNRS-UMR6042 GEOLAB, Clermont-Ferrand, France & Natural Research Center of Northwestern Iceland, Saudarkrokur, Iceland ([email protected])
2
Natural Research Center of North-western Iceland, Saudarkrokur, Iceland ([email protected])
Abstract: Snow avalanches are able to transport considerable amounts of debris down into
the lowlands and are a significant sedimentary transport process. However, snow avalanches
have until recently not been regarded as an important sediment transfer dynamic in Iceland.
Although, this debris transport capacity, and the resulting geomorphological evidences should
be used in combination with other methods to evaluate the potential impact of snow avalanches
in inhabited zones.
In NW-, N- and E- Iceland, studies on the geomorphological impact of ground snow
avalanches have been carried out since 1995. These areas count several inhabited areas for
which snow avalanches have been recognized as a major hazard. So far, only historical records
were considered to evaluate the magnitude and frequency of snow avalanches. As most of the
historical data are younger than 100 years, there is a possibility that records do not cover the
return-period of extreme snow avalanches. The geomorphological data compensate this flaw,
with investigations outside present-day inhabited areas, as town planning has altered the deposit
zone surface of former long distance snow avalanches. Obtained data are then transferred to the
inhabited areas by topographic modeling, underlining the location of potentially most
endangered zones in a proactive manner.
INTRODUCTION
Snow avalanches are a major threat for settlements, infrastructures and transportation
corridors in mountainous areas (Hewitt 2004, Zischg et al. 2005). In most Icelandic cases, the
hazardous situation caused by snow avalanches was highlighted afterwards, when the original
small fishing villages spatially extended, chiefly by the second half of the twentieth century
(Decaulne, 2004, 2005). By the end of the 20th century, although the population started to
decline, the number of buildings increased or remained equal. After several catastrophic snow
avalanche events that stroke Neskaupstadur in 1974 (12 persons died), Patreksfjordur in 1983 (4
persons died), Sudavik and Flateyri in 1995 (34 persons died), the latter year was a turning point
for snow avalanche hazard research in Iceland. It highlighted the inadequate avalanche
prevention, preparedness and planning in the country (Bernhardsdottir, 2001). A large
thoughtful work has therefore been undertaken, mainly through the initiative of the Government
of Iceland that created the Avalanche Division of the Iceland Meteorological Office and
appointed it as the competent authority for slope processes studies and related hazard and risk
assessment (Magnusson, 1996). The main used methods are topographical and physical models
(α/β model, PCM model and derived runout index), chiefly based on historical chronicles,
topographical setting of the slope and meteorological conditions in the considered area (refer to
numerous references concerning several communities in the fjords of Northern Iceland on
http://www.vedur.is/snjoflod/haettumat/).
The present paper argues that geomorphological evidences, especially snow-avalanche
transported boulders, are a relevant complement to historical sources in Iceland. Such boulders
indicate the potential furthest reach of snow avalanches, and therefore give indication for risk
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First North America Landslide Conference – Vail, Colorado, AEG Publication No. 23
Editors: V.R. Schaefer, R.L. Schuster & A.K. Turner
pp. 583-592
assessment and damage potential in inhabited areas with the help of function transfer procedure.
This approach offers the possibility to anticipate major destructive events instead of reacting
after the disaster occurs.
ON-ZONE VS. OFF-ZONE CONCEPT
Snow-avalanche paths are easily visible on slopes above settlement areas, mainly from the
starting zone to the lower part of the track. But land-planning and spatial extension of
settlements in northern Iceland have hidden and destroyed most of the geomorphological
evidences in the runout zone of snow avalanches. Constructions have occurred first along the
shore since 1900-1920, and then on the lower slopes since ca. 1960. However, these evidences
are the most interesting for hazard and risk assessment, as they provide relevant data on the
furthest runout distances of the past events. As a consequence, considering only the slope
located above inhabited areas underestimate the longer snow-avalanche runout distance that
might biased the significance of the applied modeling. Most settlements are less than 100 years
old, but the uppermost houses are less than 50-60 years old, which is more or less the duration
of the pertinent documentation on snow avalanches in those areas, while larger snow-avalanches
have a return period exceeding 200 years (McClung and Schaerer, 1993). Geomorphic evidence
of furthest snow-avalanche reach has therefore to be found out of the settlement areas. The “offzone” information will then complete the “on-zone” ones (Figure 1), which are not reliable
enough for accurate hazard assessment (Decaulne and Saemundsson, 2006a).
Figure 1. The “on-zone” - “off-zone” concept that underlines the necessity of considering geomorphological
evidence for snow-avalanche hazard assessment (adapted from Decaulne and Saemundsson, 2006a).
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First North America Landslide Conference – Vail, Colorado, AEG Publication No. 23
Editors: V.R. Schaefer, R.L. Schuster & A.K. Turner
pp. 583-592
THE BUILDING OF THE TOPOGRAPHIC MODEL “OFF-ZONE”
Selected areas “off-zone”
Among all investigated areas, three reference slopes have been selected in central north and
northwestern Iceland as a basis to provide comprehensive and relevant information on the
extreme runout distance of snow-avalanche transported boulders. Although not located far away
from inhabitations, all of them remain quite remote, and furthest deposits from newly fallen and
older snow avalanches are both well visible. All investigated slopes present typical snowavalanche geomorphic work, especially perched rock debris on large boulders within the track
or upper deposition zone (Decaulne and Saemundsson, 2006c).
The Botn in Dyrafjordur, north-western Iceland
The Botn in Dyrafjordur is a wide east-west oriented glacially eroded valley located in the
innermost part of the Dyrafjordur fjord, in northwestern Iceland. Most avalanches are released
from the top of the northern slope of the valley, at ca. 600-660 m a.s.l., as large amounts of
drifting snow accumulates there during the common northerly autumn and winter snow storms.
These avalanche paths are not confined by the topography. The last large snow avalanche
occurred in late October 1995. This avalanche was responsible of huge debris transfer, together
with an important geomorphic work including several tens of shifted boulders and ploughing
marks resulting from the boulder impacts (Kristjansdottir, 1997; Decaulne and Saemundsson,
2006c). As shown on Figure 2A, the snow-avalanche crossed a 12 m deep river canyon. It
transported huge amount of boulders over the river, for example 0.6 m large rounded boulders
from the riverbed up to the other side of the river, together with others (some of them being 3 m
in long axis), and transferring them up to 65 m from the river (Figure 2B). The snow debris went
up to 50 m a.s.l. on the opposite slope.
Reykjarstrond, Central North Iceland
The Reykjarstrond study site is located on the western side of the Skagafjordur fjord, on the
western slope of the Tindastoll Mountain. The area presents a large upper catchment, which
ends on a large colluvial cone that develops from 220 m a.s.l. The area is known for snowavalanche activity, especially during northwesterly snowstorms. The avalanches recurrently cut
the trail that passes the cone, damaging the fences and power lines. Several boulders, up to 2 m
in a-axis, strew the cone surface, still 130 m down the trail (Figure 3). The lichenometric and
vegetation cover observed on these boulders testifies that both recent and old avalanching events
are responsible for their transfer downslope. The most recent rock debris was transferred
downslope during the February 20th, 1999 snow avalanche.
Fnjoskadalur, Central North Iceland
The southern part of the Fnjoskadalur valley is remote from human activity. Its eastern slope
was particularly investigated, as only a narrow trail enables access, offering a more undisturbed
area, known for its high snow-avalanche activity, mainly during northeasterly snowstorms.
Large boulders are scattered in the lower parts of the slope, within a runout zone that could be
almost 400 m in length (Figure 4). Both old boulders and fresh ones can be observed,
distinguished from their lichen and moss covers, attesting for the regular debris transfer by snow
avalanches. The selected zone is located in one of the heaviest snow avalanche areas in Iceland,
identified by the numerous boulders at a long distance from the slope. The autumn 1995 snow
avalanches were recognized to have transferred a large amount of boulders (Th. Saemundsson,
personal observations). The vegetation on slopes, especially where birch covered, helps to
identify the main paths (Decaulne and Saemundsson, 2005).
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Editors: V.R. Schaefer, R.L. Schuster & A.K. Turner
pp. 583-592
Figure 2. The area of Botn in Dyrafjordur in northwestern Iceland. A: aerial view of the avalanching slope, and
B: aerial view of the ploughing marks and shifted boulders across the river, within the lower track and deposition
zones; the ploughing marks are up to 5 m long, and the larger boulders are 3 m in diameter. See next section for α/β
explanations (Photos: Th. Saemundsson).
Figure 3. The Reykjarstrond study site, in the Skagafjordur district; central north Iceland. The snow-avalanche path
passes the trail, located in the middle part of the deposition area (A). Large boulders, up to 2 m in a-axis, are
scattered within the deposition area, even at long distance from the end of the track, i.e. up to 250 m from it (B, C).
See next section for α/β explanations (Photos: A. Decaulne).
Figure 4. The Fnjoskadalur study site, in central north Iceland. The snow-avalanche slope presents a long
concave profile, and the snow-avalanche path reach a long distance within the valley, with a run out zone of almost
400 m in length (A). Large boulders deposited by snow avalanches are scattered all over this area (B, the car, which
is 4 m in length, gives the scale of the rocky debris), the furthest ones being found across the riverbed (C). See next
section for α/β explanations (Photos: A. Decaulne).
The α/β model “off-zone”
The α/β model is an empirical calculation that uses terrain variables to quantify extreme run out
distances using angles α (slope angle from the starting zone to extreme run out position) and β
(slope angle from the starting zone to the position where the slope reaches 10° the first time,
moving down slope), derived chiefly from studies in Scandinavia and North America (Lied and
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First North America Landslide Conference – Vail, Colorado, AEG Publication No. 23
Editors: V.R. Schaefer, R.L. Schuster & A.K. Turner
pp. 583-592
Bakkehoi, 1980; McClung and Schaerer, 1993). The β-contour line has been empirically defined
as the one from which snow avalanche decelerates. The Figure 5A defines the terrain variables
used for calculation of avalanche runout. The model has been validated in several mountain
ranges over the world and points out that α is proportional to β.
In this paper, the α/β model is used to define the maximum reach of snow-avalanche rocky
debris. The α-contour line was recognized on the field, and the α-angle measured with an
inclinometer. The α-contour line refers to the snow-avalanche boulders in this study, not to the
extreme reach of the snow avalanche itself. So the value of the α angle is higher than if
measured from the extreme reach of the snow debris. Therefore, one may keep in mind that the
maximal reach of extreme snow avalanches goes further than the boulders do. The β-angle was
measured on the field (Decaulne and Saemundsson, 2005, 2006c; Decaulne et al., 2007).
In the Botn in Dyrafjordur, the α-angle measured from the furthest debris, located at 30 m
a.s.l., to the top of the starting zone located at 680 m a.s.l., is 23° (Figure 5B). As the β-angle is
29°, the resulting formula is:
α = 0.79 · β
(1)
Similar relationship is obtained from the Reykjarstrond study site, although with different
terrain variables, as the value of the α-angle is 19° and the one of the β-angle is 24° (Figure 6
C). The α/β relationship here is:
α = 0.79 · β
(2)
The α/β model appears to be validated with the topographical analysis from the third study
site, the Fnjoskadalur valley, as a consistent relationship is defined from the terrain variables.
Here the value of the α-angle is 21° and the value of the β-angle is 26° (Figure 6 D), giving a
relation of:
α = 0.78 · β
(3)
The formula (3), which proposes the furthest runout distance for snow-avalanche transported
boulders, is selected to represent the transfer equation that is applied in inhabited areas.
Figure 5. Topographical variables used in the α/β model (A, modified from McClung and Schaerer, 1993), the
parameters and resulting formulas on different terrains in north-western (Botn in Dyrafjordur, B) and northern
Iceland (Reykjarstrond, C, and Fnjoskadalur, D).
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First North America Landslide Conference – Vail, Colorado, AEG Publication No. 23
Editors: V.R. Schaefer, R.L. Schuster & A.K. Turner
pp. 583-592
THE APPLICATION OF THE MODEL BY TRANSFER “ON-ZONE”
Five communities in northwestern Iceland have been chosen to apply the α/β model validated
in remote areas of north Iceland. These inhabited areas have been selected because the slope
shape above the main part of the buildings is simple. It is not interrupted by intermediate bench,
and as it has not been changed by slope modifying protective structures such as large deflecting
dams or snow supporting structures. Therefore it enables applying the α/β model. Large snow
avalanches occurred in all of these towns and villages, with 150 to 1500 inhabitants, and some
of these events have been catastrophic, inflecting severe damage, injury or lives tolls. The model
is applied here without taking in account the successive impacts on buildings that would reduce
the runout distance of the snow-avalanche transported boulders, in order to Figure the potential
furthest reach of the rock debris and the location of the most potential damages for inhabitants.
The model application in those communities will answer the following questions that are of
crucial importance for risk assessment: (1) Which extend do extreme snow -avalanche
transported boulders have? (2) Have such extreme avalanches already occurred? (3) Where are
the maximal potential damages located?
The Sudavik village
In the town of Sudavik, the β-contour line is more than 75 m uphill the settlement.
Nevertheless, several snow avalanches have hit the village in the past, especially from the
southern paths, and the longest known ones reached the sea (Figure 6A). After the January 1995
destructive event (16 people lost their life), the village has been relocated further south. That
area is less exposed to snow-avalanche activity because of the topographic setting of the slope
above, which is chiefly short and inappropriate to gather large amount of snow. In the older part
of the town the upper houses are unoccupied during the wintertime. This part is located within
the runout zone of snow avalanches. According to our calculation, furthest snow avalanches are
expected to completely cross the old residential area, as well as the rocky debris it might
transport. Only two documented avalanches, released in December 1994 and January 1995
could be qualified as extreme, as they reached the α-contour line resulting from the terrain based
model. The main part of the new village seems to be unexposed to snow avalanches, but the five
southernmost houses of the village are located within a potential snow-avalanche run out zone,
even if no event has been recorded there. If the risk of loss of lives is reduced by the village
relocation, large damages to infrastructures are still expected.
The Hnisfdalur village
Only few houses of the Hnifsdalur community are located out of the maximum runout zone of
snow-avalanche transported boulders (Figure 6B). The main recorded snow-avalanche prone
areas hit the northern part of the village, mainly from three active paths. A large snow
avalanche, from the northernmost path, killed 19 people in 1910. The southern part of the
village encountered only one avalanche, which reached the upper houses, causing material
damages. Nevertheless, the topographical setting of the upper slope enables snow-avalanche
formation and release under suitable weather conditions, especially southerly blowing snow. A
large part of the village might be endangered in that case, as shown on Figure 6B, especially the
uppermost buildings located uphill the β-contour line or just below it. Only the northernmost
snow avalanches, triggered during the 1910’s, reached the α-contour line and should therefore
been regarded as extreme. Larger snow avalanches than those recorded in most parts of the
village should be expected. Several large boulders were observed close to the river, at the
extreme reach of snow avalanches originating from the northern slope, supporting the results of
the α/β model application.
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Editors: V.R. Schaefer, R.L. Schuster & A.K. Turner
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Figure 6. Documented snow avalanches and potential maximum runout distance for snow-avalanche transported
boulders, based on the Icelandic validated α/β model, in the communities of Sudavik (A), Hnisfdalur (B),
Bolungarvik (C), Bildudalur (D) and Patreksfjordur (E).
The town of Bolungarvik
Two areas of the town of Bolungarvik are concerned by snow-avalanche hazard (Figure 6C).
In the northern part, residences are at risk and in the southern part a power building and stables
are located in the path of snow avalanches.
In the northern part, the uppermost houses are uphill of the β-contour line, or close to it.
Therefore, these buildings are exposed to the main force of the avalanche. Some large
avalanches entered ca. 200 m within the residential area in 1960’s, before the houses
construction. According to the model applied to snow-avalanche transported boulders, larger
snow avalanches might be expected, entering the present-day inhabited area by ca. 300 m to
reach the α-contour line.
In the southern part, the large snow avalanches that have been recorded in 1974 went over the
α-contour line. These snow avalanches have to be regarded as extreme, and it is unlikely that
snow-avalanche transported boulders reach a furthest distance.
The Bildudalur village
Only two paths are known for their snow-avalanche activity in the village of Bildudalur,
where several slush flows were recorded (Decaulne and Saemundsson, 2006b). The uppermost
houses and other public buildings, such as the primary school are located uphill the β-contour
line (Figure 6D). The furthest expected distance for rock debris transported by snow avalanches
reaches the sea. Although no avalanche activity has been recorded in between the two main
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Editors: V.R. Schaefer, R.L. Schuster & A.K. Turner
pp. 583-592
confined paths, the area is suitable to release snow avalanches as the upper slope gathers
favorable topographical setting for snowdrift accumulation. The residential area below these
unconfined paths is therefore also at risk. In Bildudalur, the whole village is endangered by
snow-avalanche activity. However, the northernmost part of the village is less exposed as a
deflecting dam has been built at the top of the northernmost documented track, diverting the
snow avalanches to the southern part of it, i.e. to the buildings in the middle part of the village.
No extreme avalanche has yet been recorded in the village of Bildudalur.
The town of Patreksfjordur
The slope above the small town of Patreksfjordur has several well-known avalanche tracks
(Figure 6E). Due to the topographic conditions, the upper part of the whole slope could act as
potential source areas for snow avalanches. Two main paths have been recognized as
particularly dangerous, i.e. in the western part of the settlement area, where large snow
avalanches occurred both previous to and after the building of residence and industrial
buildings, and along the brooks in the eastern part of the town where slush flows already caused
fatalities (Tomasson and Hestnes, 2000). Some extreme events already took place in the western
part of the town, but some more extensive events might be expected in other areas, especially
where residences are located above the β-contour line. We note from the Patreksfjordur study
site that the use of the α/β terrain model is not relevant for slush avalanches initiated in
riverbeds. The easternmost slush flow path passed through the α-contour line, and extends to an
area that is not at risk from snow avalanches originating on slopes.
DISCUSSION AND CONCLUSIONS
This paper uses the relevance of snow-avalanche sedimentary transport within the avalanche
path, transferring downslope a mixture of debris from silt size to large boulders, to define the
potential furthest reach of past, present and future snow avalanches, together with the damage
potential upslope from this extreme contour line. The most extreme geomorphological evidences
from the northern Iceland selected remote sites provide major insights on the extreme runout
distances of snow-avalanche transferred boulders.
Historical sources are of essential help for snow-avalanche assessment and mitigation
(Decaulne 2004). By combining writing sources concerning the snow-avalanche events and
those related to residential building construction, it is obvious that the hazard and vulnerability
spatially increased during the last century (Decaulne, 2004, 2005), together with deep socioeconomic changes. During this time, the resident population increased significantly, as well as
the economic values of the threaten areas and the costs for material damages, because most of
the newest parts of the considered towns and villages are located within the snow-avalanche
runout zone. However, in this paper, it has been demonstrated that the documented snow
avalanches do generally not match with the longest ones. As a consequence, larger snow
avalanches than those already recorded have to be expected in most areas, and risk to buildings
and persons is probably more extended than shown from only historical data. Thus, the risk
estimation and zoning (Jonasson et al., 1999; Arnalds et al., 2004), which is derived from runout
indices calculation, and based on the Icelandic data set based on historical sources, presents
several flaws that lead to an underestimation of the snow-avalanche maximum runout distance.
As prevention, protection and mitigation measures are in turn officially designed on the risk
quantification derived from historical data (Decaulne, 2006), the integration of
geomorphological findings could substantially improve the future development of settlements
that could be better controlled, redirected and protected.
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Editors: V.R. Schaefer, R.L. Schuster & A.K. Turner
pp. 583-592
In Iceland, catastrophic snow-avalanche events resulted from unusual weather conditions
during winter and early autumn snowstorms, and from snow-avalanche activity in unusual paths,
as examples show from 1994 and 1995. Afterwards, old tales revealed longest runout distances
than those recorded in written sources (Haraldsdottir, 1998a and b). These new information
conducted to revaluate the previous zoning, and lessons learnt from disasters enable to improve
significantly both the warning system and alert procedure as well as the hazard control
(Colombo, 2000). Therefore, the snow-avalanche hazard and mitigation research should evolve
with time, and lead to changes in the risk assessment by integrating new relevant information.
According to the extreme runout distance extracted from the geomorphological evidences on
selected study areas, the damage potential could extend further downslope than only the
uppermost houses, increasing the area that is most exposed, and will be subjected to diurnal
fluctuation according to the presence of persons or not in the buildings. Geomorphological
evidences provide one more pertinent element to define appropriate coping strategies. The
maximum runout distance definition is subjected to adjustments according to the new findings
incorporation in the risk analyses, as the research on geomorphological evidence is ongoing.
Acknowledgments: This work was carried out with financial support from the Natural
Research Center of North-western Iceland in Saudarkrokur, the Laboratory of Physical
Geography UMR6042 CNRS, Clermont-Ferrand, and the Arctic Research Network, Besançon,
France. The authors acknowledge the anonymous reviewers for their constructive comments.
Corresponding author: Dr Armelle Decaulne, CNRS-UMR6042, Geolab Laboratory of
Physical Geography, 4 rue Ledru, F-63057 Clermont-Ferrand Cedex, France; Tel: +33 4 73 34
68 19; e-mail: [email protected].
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