Jökull - 01.06.2000, Síða 81

Oddur Sigurðsson et al.
SUMMARY
The jökulhlaup from Sólheimajökull on July 18,
1999
A very sudden jökulhlaup occurred in Jökulsá á Sól-
heimasandi southern Iceland during the night and
morning of 18th of July 1999. The discharge has been
reconstructed according to eye witness account of the
water level below the bridge (Figure 5). The calculati-
ons include cross-section area and the slope and
roughness of the river bed using Mannings equation.
Discharge at the bridge has been measured regularly
both before and after the event by the Hydrological
Service of the National Energy Authority. Since 21
July, 1999 a water level, temperature and conducti-
vity gauge connected to a real-time warning system is
operated at the bridge.
The jökulhlaup is believed to have been caused by
subglacial volcanic activity. A few days prior to the
event local people had noticed unusual color of the
river but the discharge was not beyond the ordinary.
The smell of the river had turned from the usual sulf-
ur stench to mercaptan.
According to the Meteorological Bureau and
Science Institute of the University of Iceland seismic
unrest was recorded for 20 minutes at 5 o’clock p.m.
on 17th of July and again for 5 hours starting at 22:18
hours that night. The exact origin of neither of these
seismic phenomena has been established.
During reconnaissance flight in the morning of
18th of July a new cauldron was observed in the
glacier of the volume of 0.02 km

as the source of the
jökulhlaup. Water had broken out of the surface of the
glacier 2 km inside the terminus (Figure 2) indicat-
ing explosive progression of the flood. Water was al-
so injected into a glacier dammed gully, Jökulsárgil
(Figure 3), which acted as a buffer on the flood wave
amplitude. The flood broke considerable volume of
glacier ice off the terminus and icebergs were scatt-
ered along the river bed all the way to the sea (Figures
1 and 4).
The sudden rise of the flood has recorded parallels
from the rivers Skeiðará, Skaftá, Kreppa and Jökulsá
á Fjöllum.
HEIMILDIR
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Oddur Sigurðsson, Árni Snorrason og Snorri Zóphónías-
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