Jökull


Jökull - 01.12.1967, Side 9

Jökull - 01.12.1967, Side 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
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