Jökull - 01.12.1957, Qupperneq 25

Fig. 2. Discharge graplis of some limno-glacial and volcano-glacial jökulhlaups occurring in Ice-
land within the last few decades. Vertical scale logarithmic. The volume figures show the total
volume of water discharged by each hlaup.
Línurit af gangi nokkurra jökulhlaupa. Tölurnar sýna heildarvatnsmagn hvers hlaups.
western Vatnajökull, ab. 7 km NW of Vatns-
liamar in Grímsvötn. My graph is reconstructed
from Rist’s graph that was based on his measure-
ments of the discharge (Rist and Thorarinsson
1955a).
What is most striking in the diagram (Fig 2)
is the difference between the Kötluhlaup graph
and the other graphs. Although this Kötluhlaup
was very small compared with normal ones I
think tliat it may be regarded as representative
for the Kötluhlaup type, and as such it is a
clue to a better understanding of these hlaups.
Characteristic for this type is the very great dis-
charge at the euphemeral maximum of the
hlaups compared with the total volume of water
discharged. The max. discharge of this hlaup,
reached within an hour after its visible start
from the glacier front, was ab. 3000 m3/sec or
ab. 50% of the max. discharge of the hlaup
from Grænalón 1939, whereas the total volume
of the discharged water was only 28x10° m3
or ab. 2% of the total discharge of the Græna-
lón hlaup.
From this diagram we can roughly calculate
that a Kötluhlaup of the same type as that of
1955 but with a maximum discharge of about
50.000 m3/sec, or as the biggest Skeidarárhlaups,
would result in a total discharge of only ab.
0.4 km3 of water, or little more than 5% of
the total volume of a normal Skeidarárhlaup,
and even if the euphemeral discharge maximum
rose as high as to 200.000 m3/sec the total
volume would not be rnore than ab. 2 km3 or
ab. one fourth more than that of a normal
Grænalónshlaup, the max. discharge of which
is only ab. 6000 m3/sec. Bearing this in
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