Download Catastrophic floods in Iceland

The Extremes of the Extremes: Extraordinary Floods (Proceedings ol'a symposium held at Reykjavik. Iceland.
July 2000). IAHS Publ. no. 2 7 1 . 2002.
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Catastrophic floods in Iceland
MAI K l R T Ô M A S S O N
National
Energy
Authority,
Grenscisvegur
9, IS-108
Reykjavik,
Iceland
e-mail: [email protected]
Abstract The paper describes four extreme or catastrophic floods in Iceland
and the associated processes and landforms. These floods span the time from
the last Ice Age up to the present century. Catastrophic floods defined as peak
flows larger than 100 000 m s" normally have occurred twice a century in
Iceland for thousands of years. The last event of this kind occurred in 1918
with the eruption of the Katla Volcano underneath the Myrdalsjôkull Glacier.
Several km of water and ice were then transported in an immense glacier burst,
and over 0.9 km of magma was brought to the surface. The peak flow was
estimated to be ofthe order of 300 000 m s" . A catastrophic flood occurred in
the Jôkulsâ â Fjôllum River some 2500 years ago during which the huge Jôkulsâ
Canyon was eroded. The total erosion of solid rock equalled some 0.55 km ,
and the peak flow was likely ofthe order of 500 000 m s" . This is the greatest
of all known catastrophic floods in Iceland. It most likely originated in the
Bârôarbunga caldera in the Vatnajôkull Glacier. A catastrophic flood occurred
about 9500 years ago in the basin of the Hvitâ River in southern Iceland. This
flood originated in an ice dammed lake at Kjôlur, and its peak flow has been
estimated to be of the order of 200 000 m s" .
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K e y w o r d s c a t a s t r o p h i c ; j ô k u l h l a u p ; p e a k flow; l a n d f o r m s ; c a l d e r a ; e r u p t i o n ; c a n y o n ;
ice-dam lake; Iceland
INTRODUCTION
Glaciers form a part of the solid earth, but they react to natural forces much faster than
other parts of "terra firma". When combined with subglacial volcanism, this
characteristic can lead to rapid changes and release of energy of a scale hardly known
elsewhere in nature. The most violent reactions and greatest releases of energy occur
when volcanic activity takes place in ice-covered calderas or from huge ice-dammed
lakes (Tomasson, 1991).
If a catastrophic flood is defined as a flood with peak flow more than 100 000 m s" ,
two floods of this magnitude probably have occurred in each century in Iceland. The
flow in question equals the average flow of the Amazon River. The most recent event
to approach this scale, after the establishment of the Hydrometric Survey in Iceland, is
the Lake Grimsvôtn jôkulhlaup in 1996 (Snorrason et al, 1997). It is estimated to have
reached a maximum flow of 50 000 m s" . This jôkulhlaup was closely observed and
monitored, which has increased our understanding of catastrophic floods.
The catastrophic floods dealt with in this paper are (see Fig. 1) from the Katla
Volcano underneath the Myrdalsjôkull Glacier in 1918; a prehistoric jôkulhlaup from the
Myrdalsjôkull Glacier about 1700 years old flowing towards the north and west; a very
large jôkulhlaup in the Jôkulsâ â Fjôllum River about 2500 years old, and finally a
series of jôkulhlaups from an ice-dammed lake in the Hvitâ River about 9500 years ago.
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Hauteur Tômasson
Fig. 1 Location of the areas of catastrophic flooding in Iceland. 1: River Jôkulsâ â
Fjôllum; 2: Myrdalssandur flood plain; 3: River Markarfljot; 4: River Hvitâ i
Arnessyslu.
T H E L A N D F O R M S C R E A T E D BY C A T A S T R O P H I C F L O O D S
Catastrophic floods create two main landforms, viz. alluvial plains on gentle slopes
and canyons eroded on the steeper slopes. The flood deposited alluvial plains called
sandur in Icelandic are quite similar to other alluvial plains, while canyons cut by
catastrophic floods differ markedly from other canyons both in size and cross-sectional
area. The Icelandic canyons are eroded into two main parent rock types, i.e. hard basalt
on the one hand, and môberg (tuff, breccia or pillow lava) on the other. The erosion
occurs in two ways, i.e. by stones rolling or jumping in saltation over the bottom, or by
break-up of rock by cavitation. The energy of a rock particle in movement is equal to
0.5/wv where m is mass and v is velocity. Rock particles suspended in water moving at
a high speed are rapidly worn down, and the same applies to the parent rock if it is of
similar strength and hardness. Such a process can lead to a decreasing size and amount
of particles in the fluid and decreasing erosional capacity. Cavitation on the other hand
can add to the erosive capacity by breaking the base rock into particles, and this again
leads to still further erosion. According to Bernoulli's equation the necessary pressure
condition to create vapour bubbles in a flowing liquid is:
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P = K-pgz-pv /2
(1)
Catastrophic floods in Iceland
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where K is a constant; p the density; g the acceleration of gravity, z the geometrical
height over a certain zero plane and v is the velocity. At high velocity this pressure
becomes negative forming vapour bubbles which again collapse in a process named
implosion that has the same effect as explosion (Hjulstrôm, 1935).
There seems to be a considerable difference between canyons eroded into basaltic
lava flows on the one hand and môberg on the other, as wide canyons are cut in basalt
but narrow ones in môberg. In the basaltic lava canyons cavitation is more intense as
more sharp edges necessary for cavitation are formed in this rock type. The môberg is
eroded into much narrower canyons as the main erosional agent is particles in
saltation, many of which are strong basalt, which is a major component of the môberg.
The basalt stones can erode many times their own weight in môberg. Active cavitation
also takes place in extensive subglacial tunnels in the critical cross-sections and
thereby erodes both glacier ice and rock.
THE KATLA VOLCANO AND MYRDALSSANDUR
The Katla Volcano caldera is an active volcano within the Myrdalsjôkull caldera
underneath the Myrdalsjôkull Glacier. It has been very active during recent centuries.
Many of the eruptions have been well described by eyewitnesses. The last eruption
began on 12 October 1918. Detailed descriptions from eyewitness accounts were
published in 1919 (Jôhannsson, 1919; Sveinsson, 1919). In the present paper, the
volume of the volcanic material on land is estimated from the difference in metres a.s.l.
between the maps of the Danish Ordnance Survey from 1904 and the map of the US
Army Map Service of 1946. The contour lines of these maps show a difference of
about 1 k m , which is assumed to correspond to the volume of ash deposited on the
land in the eruption. Measurements by Einarsson (1979) indicate that 9 0 % of the
material deposited on the sandur plain is volcanic ash from the eruption. The rest of the
volcanic material was brought down to the beach and extended the shoreline seawards
by 1 km (Tômasson, 1995). Altogether the total quantity of ash is estimated to have
been 2.5 k m . As to the specific characteristics of the ash, its dry density was near to
the value of 1. Xenoliths and rock fragments made up 10% by volume and 2 5 % by
weight. The density of the flood liquid was estimated to be about 1170 kg m" , but was
probably even higher in the beginning of the flood. The erupted material corresponds
to 0.9 k m of solid lava and the volume of the melted ice is assumed to have been 8 k m .
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The maximum discharge was estimated from flow velocity, based on eyewitness
accounts to be about 10 m s" , and the measured cross-sectional area obtained from
flood marks was 27 000 n r , resulting in the main channel carrying 270 000 m V . In
addition there was another channel flowing towards the east. It is estimated that the
catastrophic flood carried at peak discharge 300 000 m s"' of liquid water, suspended
sediment, and ice.
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T H E M A R K A R F L J Ô T RIVER C A N Y O N
About 1700 years ago, a catastrophic flood originating from the Myrdalsjôkull Glacier
flowed towards the north and west through the Innri Emstrur River region and the
Haukw
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Tômasson
Markarfljôt River basin, and then over the Landeyjar alluvial plain down to the sea at
Landeyjar. This flood eroded the Markarfljôt Canyon (Sigurôsson, 1988), which is
among the largest canyons in the country. In this canyon the difference between
erosion in môberg and basalt is obvious. Both in the Innri Emstur River and at the
lower end the Markarfljôt Canyon, there are wide, almost dry canyons. In the reach in
between, the main canyon is eroded in môberg, and is narrow and up to 200 m deep.
The dating of the erosion of the Markarfljôt Canyon is based on a thick sand layer found
in the soil at Landeyjar (Haraldsson, 1981). This layer is assumed to have been deposited
by the flood. The size of this flood has not been estimated, but it is assumed to have
been of the same order of magnitude as the known Katla Volcano catastrophic floods.
THE JÔKULSÂ Â FJÔLLUM RIVER
The greatest catastrophic or extreme flood in Iceland in postglacial time took place in
the Jôkulsâ â Fjôllum River some 2500 years ago. This flood has left obvious marks in
its 170 km channel all the way from the Vatnajôkull Glacier down to its outlet into the
sea (Fig. 1). The channel bedrock consists of basaltic lava of various ages, from
postglacial as well as interglacial periods. On the interior highland plateau, the slope is
gentle, and the flood path is a plucked and eroded rock surface, alternating with huge
depositional gravel- and sandbars. At the highland margins, canyons are cut into the
bedrock. They can be divided into three sections. The uppermost part is eroded into
thick lava flows (250 x 10 m in volume); the middle part, called Svinadalur, is a
basin filled with postglacial lava now greatly eroded; the lowest part of the canyon,
200 x 10 m in volume, is eroded into interglacial lava flows. Parallel to the lower
canyon is the erosional landform, the Âsbyrgi Canyon, 150 x 10 m in volume, eroded
into thinly layered lava flows. The landforms at the Jôkulsâ â Fjôllum River have a
strong resemblance to the channelled scabland and canyons of the Columbia basalt
plateau in the USA. The Columbia Plateau was inundated in catastrophic floods from
the ice-dammed Lake Missoula (Bretz, 1959) in Montana.
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The catastrophic flood in the Jôkulsâ River has been dated with tephrochronology
(Mrarinsson, 1960). According to this dating, no tephra layers older than 2800 years
old are to be found in the flood path, but a tephra layer that is about 2000 years old is
found at the bottom of the soil layer. This indicates that the flooding took place about
2500 years ago. The question of whether or not there has been more than one cata­
strophic flood cannot be answered by teplrrochronology. Since the settlement of Iceland
about 1100 years ago, eight jôkulhlaups are known and documented in annals. They
indicate that jôkulhlaups of some thousands to tens of thousands m s" in magnitude
did in fact occur (J>6rarinsson, 1960). Within the basin of the Jôkulsâ River on the
Vatnajôkull Glacier, there is a very active volcanic area. The most active and probably
largest volcano is the Bârôarbunga Volcano. In the last two decades, the bottom topo­
graphy of a substantial part of the Vatnajôkull Glacier has been surveyed and mapped.
This survey has shown that within the Bârôarbunga Volcano there is a huge caldera
(Bjornsson, 1988). South of it is another well known caldera, the Lake Grimsvôtn. In
Table 1 the two calderas are compared. The conditions obviously allow a much larger
flood from the Bârôarbunga Volcano than from the Lake Grimsvôtn. The drainage area
of the Lake Grimsvôtn is larger, which should favour more frequent floods.
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Catastrophic floods in Iceland
Table 1 Comparison of the Grimsvôtn and Bârôarbunga calderas.
Drainage area (km"'
Caldera (km )
Threshold elevation (m a.s.l.)
Bottom elevation (m a.s.l.)
Volume (km )
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Grimsvôtn
Bârôarbunga
160
24
1200
1050
3.6
80
48
1400
1100
14.4
There are probably two types of jôkulhlaups. For the first type, the discharge
increases slowly to reach peak flow. Such floods are cool, and the subglacial conduit(s)
under the glacier is (are) enlarged through frictional heat and erosion. In the other type
of jôkulhlaup, the flow rate increases very rapidly, and the water is probably relatively
hot absorbing heat from either geothermal or volcanic activity (Snorrason et al, 1997).
The jôkulhlaup from the Lake Grimsvôtn provides a good example. The volumes of
the Lake Grimsvôtn jôkulhlaups during the period from the 1940s to the 1990s were in
the range 1-3 k m (see Table 1), and the peak flows amounted to 1000-7000 m s"
lasting two weeks from outbreak to maximum. The 1996 jôkulhlaup (Snorrason et al,
1997) was associated with a volcanic eruption, increased very fast, was relatively hot
at first, and lasted about one day. Its total volume was 3.6 k m , and its peak flow was
50 000 m s" . Similar processes are responsible for the jôkulhlaup(s) from the caldera
in the Bârôarbunga Volcano. At present the caldera is full of ice, and no water
accumulates at its bottom (Bjôrnsson, 1987). The meltwater must leak out of the
caldera through rock. The drainage area of the caldera is small, and even if there was
no leakage, it would take a whole century to fill the caldera to a level high enough to
trigger a catastrophic flood. With the leakage, it takes centuries. Every now and then
geothermal heat or volcanic activity increases substantially, and meltwater
accumulates in the caldera and escapes as jôkulhlaups, usually resulting in relatively
small floods. But occasionally all conditions are favourable for a catastrophic flood.
The last time such an event occurred was 2500 years ago.
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On the basis of slope and cross-sectional area, this catastrophic flood is estimated
to be of the order of 500 000 m s" . Other fairly big floods have probably taken place
often, but the general picture is that of one catastrophic flood that is much larger than
any previous or later floods. The volume of water in the flood channel at maximum is
estimated to have been about 5 k m , and it took the water 11 h to flow in the wetted
channel from the glacier to the sea (Tomasson, 1973). The total volume of the flood
can be assumed to be about 15 k m and the flood to have lasted for about 1-2 days.
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THE HVITÂ RIVER î ÂRNESSYSLA AND K J Ô L U R
The catastrophic flood in the Hvitâ River was of a different character from those
previously described, which have all originated from volcanic activity within active
calderas. On the other hand, the floods on the Hvitâ River came from ice-dammed
lakes created when the main glacier retreated over the water divide at Kjôlur gradually
damming up very big lakes south of the water divide (Tômasson, 1993). Favourable
conditions for the creation and release of big floods probably lasted for 100-200 years.
This was at the final stage of the last glaciation of Iceland some 9500 years ago. The
Haukur
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Tômasson
average interval between the catastrophic floods presumably was almost a decade, but
the smallest ones may have occurred yearly. The first floods drained towards the north
under a tongue of the glacier west of Kjôlur. But most floods drained towards the south
under the glacier at the Blâfell Mountain, first west of it over the Blâfellshâls and later
east of it. That was the route of the catastrophic floods.
The ice-dammed lakes left shorelines at various levels. The uppermost ones are
formed above the water divide toward the north, but the most distinct ones are at the
level of the water divide. At that level there was a continuous flow toward the north.
Most of the shorelines are at a lower level. Altogether 11 such lines have been
surveyed, but the actual number of shorelines is probably much higher. At the level of
the Blâfellshâls, a fairly clear shoreline is found that indicates a continuous flow for at
least some time. That period ended with the catastrophic flood east of the Blâfell
Mountain, followed by gradually smaller floods as the ice dam east of the Blâfell
Mountain thinned.
The sequence of flood channels is clear from elevation 300 to 50 m a.s.l. Most of it
is on basaltic lava flows of interglacial age. The most conspicuous part of it is the
Gullfoss Canyon. It is cut into the edge of the high plateau, and its total volume is
100 x 10 m . Downstream of the canyon there is a huge gravel bar deposited by the
catastrophic floods. The maximum discharge is estimated to have been of the order of
200 000 m s" , and the volume of the lake may have been 25 k m . From the above it
can be concluded that the lake was cold, and that the flood may have lasted 2 - 3 weeks.
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and