Jökull - 01.12.1974, Blaðsíða 25

pressures (2 bars lower) than cause plastic de-
formation, it is unlikely that plastic deforma-
tion initiates jökulhlaups from Grímsvötn.
CONCLUSION
A broad understanding of the nature of
jökulhlaups from Grimsvötn has been obtain-
ed. This has been possible in spite of the fact
that one has at the present rather incomplete
data about the glacier bottom topography, the
extent of Grímsvötn itself and the mass balance
terms in the water basin. The main reason
for the success is the dominating effect of
Bárdarbunga and Kverkfjöll on the surface
topography of the water basin. The relatively
steep and continuous slope of the water basin
down to Grímsvötn rnakes detailed information
about the subglacial topography unnecessary.
As the glacier surface continues to fall south
of Grímsvötn it is obvious that water has to
escape that way, so the only important part of
the subglacial topography is the subglacial ridge
between Grímsvötn and the valley beneath
Skeidarárjökull. The good correspondence be-
tween seismic and gravity data in this area
made it possible to map the ridge and to de-
monstrate that the jökulhlaups are triggered
by the breaking of a seal.
It does not seem realistic to attempt to drill
á tunnel through the subglacial ridge to pro-
vide a continuous leakage of water out of the
Grímsvötn depression. (The last of all jökul-
hlaups from Grímsvötn would be catastrophic).
The same applies to a local reduction of the
glacier thickness to decrease the volume of the
jökulhlaups. Any ice removed would be re-
placed by increased ice flow from the north.
In the future the jökulhlaups might become
more frequent and reduced in volume clue to
a combination of subglacial erosion of the ridge
east of Grimsvötn and a thinning of the glacier.
An increased activity of jökulhlaups in Skaftá
woulcl presumeably delay the jökulhlaups but
would not reduce the water volumes. On the
other hand, pumping 20 m3/s of water out of
the Grímsvötn basin would guarantee that lift-
ing of the glacier could not occur. However,
such pumpings woulcl require a 50 MW power
station and it would be very expensive to com-
bine a pumping station with a hydroelectric
power station placed beyond the glacier, say 30
km away. A more practical scheme would be to
provide an overflow at a level of about 1350—
1400 m so that the water level in Grímsvötn
never can rise higher. The spillway would have
to be cut through Vatnshamar or Grímsfjall
and would probably have to be 1 km long.
ACKNOWLEDGEMENTS
The author is inclebted to Professor Sigurdur
Thorarinsson for his introduction to the Grims-
vötn problem and for his encouragement. The
author is also indebted to Professor John Nye
for many discussions on the physical aspects
of the problem and for helpful comments and
advice on the manuscript. D.r. Robert Oakberg
read an earlier draft of the paper and made
several helpful suggestions.
The author is obliged to Civil Engineer
Gunnar Thorbergsson, National Energy Au-
thority, lceland, for providing unpublished
data on gravity and geodetic surveys, and to
Civil Engineer Helgi Hallgrimsson, Icelandic
Road Authority, for providing a variety of in-
formation and for his continuing interest in
the problem. I would also like to express my
thanks to the farmer Ragnar Stefánsson at
Skaftafell for an interesting letter on the be-
haviour of the lower part of Skeidarárjökull.
The work was accomplished while the author
was holding a European Council Scholarship
administrated by The British Council.
REFERENCES
Áskelsson, J. 1936a: On the last eruption in
Vatnajökull. Societas Scientiarum Islandica,
XVIII, Reykjavík.
— 1936b: Investigations at Grímsvötn, Ice-
land, 1934—1935. The Polar Record, 11:
45-47.
— 1959: Skeiðarárhlaupið og umbrotin í
Grímsvötnum 1945. Jökull, 9: 22—29.
Björnsson, H., Jökull, in press.
— and Hallgrimsson, M. 1973: Skýrsla um
Grímsvatnahlaup 1972. Mælingar við
Grímsvötn. (A report for the Icelandic
Road Authority.)
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