Jökull - 01.12.1976, Page 46

Mynd 3.
Þversnið af Skeiðarár-
jökli, sem þrœðir leið
jökulhlauþsins 1935 frá
Grœnalóni niður á Skeið-
arársand. Byggt á upplýs-
ingur frá Sigurði Þórar-
inssyni og korti Trausta
Einarssonar af Skeiðarár-
jökli 1935.
Fig. 3. A cross-section of Skeidarárjökull along the route of jökulhlaups from Grænalón. Illustra-
tion of the potential barrier at the start of the jökulhlaup in 1935. Based on Sigurdur Thorar-
insson (pers. comm.) and Trausti Einarsson’s map of Skeidarárjökull in 1935, published by As-
kelsson 1936.
equipotential curve <p = 0 has moved up above
the glacier bed (Björnsson 1974, 1975, Nye
1976). Figs. 3 and 4 show the position of the
curve cp = 0 at the start of jökulhlaups from
Grænalón and Vatnsdalslón. In both cases the
curve lies beneath the bed outside the lake.
The bed attains a positive potential which in-
dicates that water should be forced along the
bed towards the lake. The lake ought still to be
sealed by a potential barrier of width W and
a threshold value T. For Grænalón W = 7 km
and T = 4—5 bars, but W = 2 km and T = 2
bars for Vatnsdalslón. A similar presentation
for Grímsvötn showed W = 2—3 km and T =
2—3 bars (Björnsson 1974).
Nye (1976) defined a correction term which
might reduce the threshold value T by 2 bars.
When the water level rises in the lake the lifted
ice at the edge does not immediately attain iso-
static equilibrium. The ice is bent upwards ancl
Mynd 4.
Þversnið af Heinabergs-
jökli, sem þrœðir leið
jökulhlaupa frá Vatns-
dalslóni. Byggt á korti
danska Herforingjaráðs-
ins frá 1904.
Fig. 4. A cross-section of Heinabergsjökull along the route of jökulhlaups from Vatnsdalslón. Illu-
stration of the potential barrier at the start of the jökulhlaup in 1898. Based on the map of
the Danish General Stab in 1904.
44 JÖKULL 26. ÁR