Jökull - 01.12.1967, Qupperneq 9

Jökulhlaups from the Ice Cover of Mýrdalsjökull
on June 25, 1955 and January 20, 1956
SIGURJÓN RIST,
HYDROLOGICAL SURVEY, NATIONAL ENERGY AUTHORITY, REYKJAVÍK, ICELAND
ABSTRACT
On June 25, 1955, a jökulhlaup (glacial
burst) broke out from Mýrdalsjökull and flow-
ed inlo the rivers of Múlakvísl and Skdlm.
Simultaneously two cauldrons formed in tlie
ice cover where the volcano Katla is located
but no eruj)tion was observable. The volume
of both cauldrons was 28 Gl ± 15% (1 Gl —
208 m3). The volume of the jökulhlaup water
was similar. It is consiclered that the water
either originated as surficial melt xoater accu-
mulating in a depression of an unknown capa-
city or as melt water due to subglacial volcanic.
activity.
Anolher jökulhlaup from Mýrdalsjökull fell
into the Múlakvísl on January 20, 1956. Here
the jökulhlaup water was of differe?it origin,
namely from ice dammed lakes in the rnoun-
tains at the edge of the ice cover. The volume
of the hlaup water was of the order of magni-
tude of 3.5 Gl.
INTRODUCTION
In the previous article on the thickness mea-
surements of Mýrdalsjökull in June 1955 it
was mentioned that a jökulhlaup broke out
from the ice cover while the expedition was
staying there. A preliminary report of this
jökulhlaup was given by Thorarinsson and
Rist (1955) and Thorarinsson (1957).
THE JÖKULHLAUP IN 1955
According to eyewitnesses it can be conclucl-
ed that the jökulhlaup set off at 20.00 hours
(GMT) on June 25, 1955, emerged in Rjúpna-
gil at the western corner of Höfdabrekkujökull
(synonym with Kötlujökull), a glacier migrat-
ing ESE frorn the ice cover of Mýrdalsjökull,
(see Fig. 1 in the preceding article p. 239)
and reached maximum in the Múlakvísl by the
bridge on the main road at 21.00 hours and
swept it away. This bridge was 54 metres long
and strongly built of reinforced concrete. The
water level rose at the bridge 6.6 metres
and a large amount of ice fragments was trans-
ported with the hlaup water. (It should be
pointed out tliat at this time the bridge across
the Múlakvísl was east of Höfdabrekkuheidi
and north of Selfjall. After the jökulhlaup it
was rebuilt where it is now 8 km farther south,
Fig. 1. The river Múlakvísl at the foundations
of the bridge, which was swept off by the
hlaup on June 25, 1955. Denudation on the
slope shows tliat the maximum water level was
more than 6 m above the normal.
Photo S. Rist June 28, 1955.
Mynd 1. Brúarstœðið við Múlakvisl, þar sem
brúna tók af i hlaupinu 25. júní 1955. Rof í
hlíðinni handan drinnar sýnir, að mesta vatns-
heeð hefur verið rúmum 6 m ofar en eðlileg
hœð. Ljósm. S. Rist 28. júni 1955.
JÖKULL 17. ÁR 243