Jökull - 01.12.1977, Blaðsíða 79

Remarks on the hydrographs
The hydrographs of the jökulhlaups in Skaftá
add important data to the description of the
jökulhlaup phenomenon.
The start of a jökulhlaup in Skaftá is marked
hy sulphurous smell, as water from the cauldrons
runs from the glacier outlet. This occurs one or
two days before the flood starts at Skaftárdalur,
which is about 50 km from the glacier. In
general, one can expect the hydrographs to be
both damped and deformecl by the storage
effect of the river channel, as the flood runs
frorn the glacier down to the limnigraph at
Skaftárdalur. In front of the glacier, the flood
runs into a 15—20 km long and 2—5 km wide
outwash ])lain. From there, one river channel
runs for 40 km down to Skaftárdalur. At the
beginning of the jökulhlaup, water will disperse
out into the plain where the flood will be ab-
sorbed. Later, the peak flow will l)e conducted
more rapidly through the entire channel. This
effect would deform the rising limb. However,
one can conclude that the maximum discharge
of the jökulhlaups is reached in 2 to 5 days
from the start of the flood. The peak discharge
can be of the order of 1000 m3/s. The steep
rising limb of the hydrographs results from a
very rapid expansion of the water bearing ice
tunnels. The expansion is caused by the melting
of the ice walls. But heat generated by friction
in the flowing water is not enougli to cause
the rate of cnlargement of the tunnels. Warm
water must be drained from the cupola (Björns-
son, in preparation).
At the end of the jökulhlaup the drainage
out of the outwash plain is delayed. However,
one can conclude for the jökulhlaups in Skaftá
that the recession curve is twice as long as the
rising limb. The flood does not abate rapidly
once the flood peak is reachecl.
CONCLUSIONS
The jökulhlaups in Skaftá originate most
likely from a geothermal area. Presumably the
area is a part of a geothermal field which in-
cludes Grímsvötn. The increased strength of
the jökulhlaups since the 1950ies is thought to
be due either to migration of geothermal activity
or to contraction of the Grímsvötn water basin.
Tlie strengtli of the geothermal activity in the
central part of Vatnajökull seems to have been
fairly constant during the present century.
ACKNOWLEDGEMENTS
The author is indebted to Professor Sigurdur
Thorarinsson for his encouragement and valu-
able discussions. The author is also indebted to
Hydrologist Sigurjón Rist, National Energy
Authority for providing hydrological clata and
to Chief Engineer Helgi Hallgrímsson, Icelandic
lioad Authority, for his continuing interest in
the subject.
REFERENCES
fíjörnsson, H. 1974: Explanation of Jökul-
hlaups from Grímsvötn, Vatnajökull, Ice-
land. Jökull 24: 1—26.
— 1975: Subglacial Water Reservoirs, Jökul-
hlaups and Volcanic Eruptions. Jöknll 25:
1-14.
Bodvarsson, G. 1961: Physical characteristics of
natural heat resources in Iceland. Jökull 11:
29-38.
Einarsson, T. 1966: Physical Aspects of Sub-
glacial Eruptions. Jökull 16: 167—174.
Friedman, J. D. et al. 1972: Infrared Emission
from Kverkfjöll Subglacial Volcanic and
Geothermal Area, Iceland. Jökull 22: 27—
43.
Nye, J. F. 1976: Water flow in glaciers: jökul-
hlaups, tunnels and veins. J. Glaciol. 76:
181-207.
Pálsson, S. 1968: Kúðafljót. Goðasteinn 7 (3):
32-40.
Rist, S. 1976: Jökulhlaupaannáll 1974, 1975 og
1976. Jökull 26: 75-79.
Thorarinsson, S. 1974: Vötnin stríð. Alm. bóka-
fél. Reykjavík, 254 pp.
Thorarinsson, S. og S. liist. 1955: Skaftárhlaup
í september 1955. Jökull 5: 37—40.
JÖKULL 27. ÁR 77