Tuesday, 5 November 1996 (10:30)
Jokulhlaup started this morning quite abruptly.
The growth is fast - small islets are floating from the glacier
stranding on the sand before reaching the bridges.
The water is presentlly flowing in the east part of Skeidararsandur
plain. Estimated flow is 6000 m3/sec, more than the peak of last
summer's jokulhlaup which was 3000 m3/sec.
Water is coming from just east from the middle of the glacier and has
not covered the sandur plain, which is 25 km broad from Skaftafell to
Lomagnupur fell.
Road is closed, as the floodwater has cut through the road. Sand is changing from gray to black. Weather is -6C, bright and sunny.
Tuesday, 5 November 1996 (14:00)
Large jokulhlauphlaup is now in progress. The start was much more sudden
than anticipated - more sudden than was even imaged be possible.
At 0800 all was quiet but as the flood started the increase in flow is
estimated to have been 80-100 fold in less than 2 hours in the
beginning.
Hlaup started in the E part of sandur plain. Water flowed from the spout of the glacier and up through fissures in the glacier itself. Road is cut both sides of main bridges, increasing flow endangers the main structures. Bridges are built to take 10-12.000 m3/s of water. Electric lines over the Skeidararsandur plains are in danger, elcectricity stopped W of sandur before 1200, now is back from lines W of Skeidararsandur.
Rivers from W
11.00 14:00
Apsvotn not started
Sandgigjukvisl grey to muddy bridge fell at 13:00
Saeluhusakvisl lot of water flow over bridge
ice floats
Skeidara strong current flow around bridge ends
Flow grows extremely quickly and the amount of water in the Grimsvotn
caldera waiting to flow down to the sandur plain is 3 km3.
Gigja bridge fell just before 1300 is the first bridge to be destroyed
in the flood. It means that the ring-road is now cut as Gigja was one
of the main bridges over the Skeidararsandur plains. Repairs will take
months and hopefully this will be the only one.
The flood is digging at the ends of Skeidararbridge but it is not yet in
danger - the flood is just starting!The Skeidararsandur has been closed from 23-08 since the eruption started. A policeman who opened the road at 08:00 this morning was having coffee E of the sandur when he was summoned out again to close the road. He drove west over the sandur plains and just made it all the way as water had already started to flow over the road. This was just after 0800.
(I have heard that a photographer from Nat.Geogr.Mag. who had waited for 3 weeks for the jokulhlaup to start left Iceland this morning).
--------------------------------------------------------------------- Helgi Torfason Orkustofnun (National Energy Authority) Grensasvegur 9, IS-108 Reykjavik, Iceland ---------------------------------------------------------------------
10:am
The road across Skeišarįrsandur has been closed. The jökulhlaup now surrounds the 900 m long
bridge across Skeišarį which has reached a flow rate of 6000 cubic m/s within the first 2 hours.
Icebergs are breaking off the glacier snout and are being carried down with the flow. The maximum
tremor amplitude is around 160 units.
13:am
According to radio reports, the bridge across one of the glacier rivers (Gķgjukvķsl) has now
perished in the flood. The powerline is down and the telephone fiber cables have been cut by the
flood. The maximum tremor amplitude is around 240 units.
18:am
The bridge across Sęluhśsakvķsl has been demolished by the flood and the longest bridge in
Iceland, the 900 m long Skeišarį bridge is severly damaged as well, and not expected to last much
longer. Glaciologists surveying the flood from an aircraft at 15 h estimated the flow rate to be
around 25.000 cubic m/s. The maximum tremor is now 270 units. The Skeišarį bridge is worth 10
million US$. The total damage to constructions today is estimated at about 15 million US$.
November 6.
9:00 GMT
The jökulhlaup culminated at 22:30 hours last night. At that time close to 45.000 cubic m/s flowed from Grķmsvötn, along a 50 km long path beneath the outlet glacier Skeišarįrjökull out to the alluvial plane, Skeišarįrsandur. The high frequency tremor recorded at Grķmsfjall stayed fairly constant throughout the night and still has a maximum of 230 units. Although damaged, close to
700 m of the 900 m long Skeišarį bridge is still standing but the bridges across Sęluhśsakvķsl and Gķgjukvķsl washed away. Skeišarį is the easternmost river draining this region whereas Sęluhśsakvķsl and Gķgjukvķsl are the central bridges. The westernmost bridge, across Nśpsvötn stayed intact. The initial (main) phase of the jökulhlaup, which begun at 8 h yesterday morning with a 3-5 m high water wave, flowed down the easternmost and central part of the
Skeišarįrsandur plain, not reaching Nśpsvötn until afternoon yesterday. Many large icebergs (up to 200 tons) are scattered on the plain.
10:00 GMT
Correction: Latest reports claim that the bridge across Sęluhśsakvķsl is still standing and that the
jökulhlaup has decreased markedly.
November 7.
12:am GMT
The jökulhlaup has finished. The flowrate in the glacier rivers on Skeidarįrsandur is now at 400 cubic m/s. At least 3 cubic km of water thick with ash and other fragments from the eruption has wedged its way from Grķmsvötn, along the bottom of the glacier out to sea, within the last 52 hours. The water has left many huge icebergs, some are 10-15 m high weighing up to 1000 tons. The bridge across Sęluhśsakvķsl was engulfed by the water for some time on November 5th but emerged with only minor damage. The high frequency tremor recorded at the seismic station on Grķmsfjall, on the southeastern rim of the Grķmsvötn caldera, has decreased to less than 10 units. However, the instrument still records numerous icequakes which occur within the glacier as it is still being cracked in response to the sudden lowering of the iceshelf on the caldera lake.
Since 1938, however, there have been two, or even three, jökulhlaups per decade with correspondingly smaller volumes, 0.5-3.5 km³, and a maximum discharge rate of 1,000-8,000 m³/s. Such jökulhlaups took place in 1939, 1941, 1945, 1948, 1954, 1960, 1965, 1972, 1976, 1982, 1983, 1986, 1991 and 1996. The 1996 jökulhlaup is thus the largest jökulhlaup since 1938. The change in water level of the Grķmsvötn caldera lake in the larger jökulhlaups has been estimated at about 150 m whereas the more recent jökulhlaups were followed by a smaller drop in water level, i.e. 80-100 m. The decrease in jökulhlaup volumes after 1938 is caused by declining geothermal power of the high-temperature geothermal area under Grķmsvötn. The declining geothermal power is caused by low volcanic activity as the geothermal activity thrives on repeated dike injections from a crustal magma chamber situated within the uppermost 5 km of the crust beneath Grķmsvötn.
How the triggering of the 1996 jökulhlaup differs from the previous jökulhlaups.
There are two ways to trigger a jökulhlaup. In general the jökulhlaups occur when the water level of Grķmsvötn has risen to the critical level which enables it to brake the ice barrier at the eastern side of the lake. This level is slightly flexible from one jökulhlaup to the other, depending on the ice thickness at each time. As the water level rises in Grķmsvötn the width and and height of the potential barrier are reduced. The regular jökulhlaups have taken place before the the water pressure at the margin of the lake reaches the overburden pressure of the ice dam. The water has thus managed to melt a path through the barrier before it can lift it. The time it takes to melt the barrier and trigger a jökulhlaup is uncertain as the lake level has not been measured continuously throughout the years. We know, however, that it takes more than 4-5 weeks as the current jökulhlaup did not manage to melt its way through the barrier before the water pressure overcame the glacier overburden pressure. During the last 5 weeks, the lake level rose in a few days to the draining level of the last regular jökulhaup, in April this year, and continued to rise until the water pressure became sufficient to lift the glacier dam and wedge its way at the base of the glacier, down to the Skeišarįrsandur plain. The meltwater from the eruption also has higher temperature (10 degrees C) than the regular geothermal water (0 degrees C) which enabled the current jökulhlaup to melt its way at the base of the glacier much faster than a regular jökulhlaup. The combined effect of higher pressure and temperature generated a 5 m high initial wave which propagated 50 km at the base of the glacier in about 11 hours. The recent jökulhlaup therefore culminated much faster than a regular jökulhlaup as the Grķmsvötn lake was almost emptied in 3 days.
Damage caused by the jökulhlaup
The bridge across Gķgja was totally demolished.
The 900 m long Skeišarį bridge was badly damaged, as 200 m washed away.
The bridges across Sęluhśsakvķsl and Nśpsvötn still stand
The road across the Skeišarįrsandur alluvial plane was severly damaged as well.
More than 5 poles of the power line across the alluvial plane disappeared.
The optical telephone wire was damaged.
From Magnus M. Halldorsson at the University of Iceland