Jökull - 01.12.1974, Page 4

is altered, and ice and water floiv into the Gríms-
vötn depression.
Man could control the jökulhlaups process in
tivo ways—If 20 ?n3/s of ivater ivere continuous-
ly pumped from the lake the seal provided by
the glacier ice would be permanent. The power
of the required pumping station would be 50
MW. Alternatively, a drainage outlet could be
cut through the subglacial rim of the lake or a
spillivay could be cut through Vat?isha?nar or
Grimsfjall at the 1350—1400 m level. The water
level in Grímsvötn would then remain ivell
beloiv the level required to trigger a jökul-
hlaup.
It is possible that due to thiiining of the
glacier and erosion of the subglacial ridge east
of Grí?nsvötn the jökulhlaups ivill become more
frequent and reduced in volume.
SYMBOLS
A area of the water basin
Ag area of the geothermal region
Aj area of the lake
ab subglacial ablation rate due to a normal
geothermal gradient
ag subglacial ablation rate in the geothermal
area
ag glacier surface ablation rate
b glacier surface ice balance rate
B a constant in the ice flow law
c glacier surface ice accumulation rate
d thickness of a subglacial water sheet
D width of the water basin
g acceleration of gravity
hb glacier bottorn elevation
H[ glacier thickness
hs glacier surface elevation
hw elevation of the water level in Grímsvötn
k permeability
L latent heat per unit volume of ice
lj clepth of a vertical hole in the glacier
lw depth of the water in a hole in the glacier
n exponent in the ice flow law
P = pw g (hw — hb), hydrostatic subglacial
water pressure which would exist if there
were a hydraulic connection between the
levels hw and hb
Pj = pigH[, ice overburden pressure
pw subglacial water pressure of a water sheet
(assumed equal to p^)
q = qw -(- qt, filling rate into the lake
q( ice flux from the water basin into the
geothermal area
qw water flux to the lake
S area
Q geothermal energy flux
t time
At period between jökulhlaups
ub velocity of the glacier due to sliding at
the bed
us velocity at the glacier surface due to de-
formation of the ice
v velocity of water in a subglacial sheet
V volume of water in a jökulhlaup
a glacier surface slope
8b regional Bouguer anomaly
density of ice
pw density of water
T shear stress at the glacier bottom
flux density of geothermal energy
rj viscosity of water
INTRODUCTION
Although it is situated in the central part of
Vatnajökull, Grímsvötn (Grímur-lakes) has pro-
Fig. 1. A map of the western part of Vatna-
jökull, showing the position of Grímsvötn, its
water basin (300 km2), and the subglacial route
(50 km) of the jökulhlaups beneath Skeidarár-
jökull, together with the glacier rivers on Skeid-
arársandur. The rivers Skeidará and Súla
emerge at the edge in single outlets whereas the
rivers Sandgígjukvísl and Blautakvísl (6 km west
of Sandgígjukvisl) collect water frorn a large
part of the central glacier edge. The new road
on Skeidarársandur is marked on the map. The
whole water basin lies above the firn line. The
firn line lies at about 1100 m elevation on
Skeidárárjökull. The two cauldrons north-west
of Grímsvötn drain water in jökulhlaups be-
neath Skaftárjökull towarcl the river Skaftá.
Mynd 1. Kort af vesturliluta Vatnajökuls, sem
sýnir legu Grí?nsvatna, vatnasvœði þeirra og
leið jökulhlaupanna undir Skeiðarárjökli niður
á Skeiðarársand.
2 JÖKULL 24. ÁR