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The Extremes of the Extremes: Extraordinary Floods (Proceedings ol'a symposium held at Reykjavik. Iceland. July 2000). IAHS Publ. no. 2 7 1 . 2002. 121 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" . 3 1 3 3 J 1 3 J 3 1 1 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. J J 1 1 122 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: 2 2 P = K-pgz-pv /2 (1) Catastrophic floods in Iceland 123 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 . J 3 3 J J 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. 1 J 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 124 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 JÔKULS  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. 6 6 J J 6 3 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. J 1 125 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 ) 2 J 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. 3 3 1 3 J 1 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. 3 1 3 3 THE HVIT 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 126 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. 6 J J 1 J REFERENCES Bjornsson, H. ( 1 9 8 8 ) Hydrology Iceland, Reykjavik. Bretz, J. H. ( 1 9 5 9 ) Washington Olympia. of Ice Caps in Volcanic Channeled Einarsson, G. ( 1 9 7 9 ) Myrdalssandur Pumice Haraldsson, H. ( 1 9 8 1 ) The Markarfljôt 15. Scabland, Deposit. Sandur Regions. 1-139. Societas Scientarium Islandica, University of 1-57. Division of Mines and Geology, T h e State of Washington, Iôntxknistofnun, Reykjavik. Area, Southern Iceland. 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( 1919) Kôllugosid 1918 og ajleidingarpess. Prentsmiôjan Gutenberg, Reykjavik. T ô m a s s o n , II. ( 1 9 7 3 ) Hamfarahlaup i J ô k u l s â à Fjôllum. Nûlli'irufrœdingurinn 43, 77-98. T ô m a s s o n , II. ( 1 9 9 1 ) Glaciofluvial Sediment Transport and Erosion. In: Y. Gjessing et. al. Artie Hydrology. Present Future T a s k s . N o r w e g i a n National C o m m i t t e e for Hydrology. Report n o . 2 8 . 2 7 - 3 6 . T ô m a s s o n , 11. ( 1993) Jôkulstifluô vôtn â Kili og hamfarahlaup i I Ivitâ i  m e s s y s l u . Nâtti'trufrœdingurinn T ô m a s s o n , H. ( 1 9 9 6 ) T h e Jôkulhlaup from Katla in 1918. Ann. Glaciology. hôrarinsson, S ( 1 9 6 0 ) Der J ô k u l s â - c a n y o n und Âsbyrgi. Petermanns 22, 249-254. Geographische Milt. 104, 1 5 4 - 1 6 2 . 62, 77-98. and