Jökull - 01.12.1974, Síða 7

Fig. 3. Changes in the
water level of Grimsvötn
at Depill since 1954
('Thorarinsson, 1965; Ei-
riksson, 1972; Björnsson
and, Hallgrimsson, 1974).
The height of the water
level immediately after a
jökulhlaup was first mea-
sured in 1972. The total
drop of the water level
was tlien 105 m.
Mynd 3. Breytingar á
vatnsborði Grímsvatna
frá 1934.
usual ice cap shape would result. Ice and water
would flow from the cap’s central area, and
presumably no jökulhlaup would occur.
Although the process of jökulhlaups has not
been explained, one knows by experience how
to forecast them (Thorarinsson, 1974). During
the last two decades jökulhlaups could be fore-
cast as occurring within a few months of the
water level of the Grímsvötn lake reaching a
critical level. People usually sense a strong sulph-
urous smell, some days before the discharge be-
gins to increase marking a change of chemicai
composition in the rivers on Skeidarársandur.
Water from the geothermal area in Grímsvötn
lias tlien reached down to Skeidarársandur. The
colour of the rivers changes, sometimes even
before the jökulhlaup starts. At the same time
crevasses form around the edge of Grímsvötn.
The discharge of the rivers often suddenly de-
creases, showing that some changes in the sub-
glacial waterway are taking place; sometimes
in winter the rivers become completely dry,
which is a sure sign of a jökulhlaup starting
within a few days.
The jökulhlaup usually emerges from some
10 tunnels but the main water volume runs
from 3 or 4 rivers. Two of the tunnels are
situated at each side of the glacier front, the
rivers Skeidará and Súla, a logical position for
tunnels in a piedmont glacier. The outlet Sand-
gígjukvísl and Blautakvísl are located in the
central part of the glacier front, Fig. 1. The
jökulhlaup does not usually start at the same
time in all of the rivers. The jökulhlaup in
Súla often starts 1 to 2 days later than the one
in Skeidará (Thorarinsson, 1974).
It is a clifficult task to measure the discharge
on Skeidarársandur. Since 1954 the discliarge
has been estimated using the Manning formula
at places where the main rivers flow in rela-
tively well-defined channels (Rist, 1955, 1973).
Fig. 5 shows the characteristic form of the
hydrographs. The discharge increases approxi-
mately exponentially and drops down rapidly
after having reached a peak.
During the last three to four decades jökul-
hlaups from Grímsvötn have occurred approxi-
mately twice each decade (1941, 1945, 1948,
1954, 1960, 1965, 1972) and the total water
volume in each jökulhlaup has been estimatecl
at roughly 3—3.5 km3; the antecedent water-
level change was 80 m to 105 m. Earlier in this
century (and indeed from about 1600 A.D.,
Thorarinsson, 1974) one jökulhlaup with a
water volume estimated to be double that of
recent jökulhlaups occurred each decade (1903,
1913, 1922, 1934, 1938 (an exception, see later
p. 22)); the antecedent water-level change has
been estimated as 150 m to 200 m. Information
about the settlement on Skeidarársandur before
the catastrophic eruption from Mt. Öræfajökull
1362 A.D. indicates that early jökulhlaups had
less water volume than those of the early twen-
tieth century. This led Thorarinsson (1953) to
observe that the frequency and water volume
of the jökulhlaups depends upon the thickness
JÖKULL 24. ÁR 5