Jökull - 01.12.1976, Síða 82

Grímsvatnahlaupið 1976
SIGURJÓN RIST,
VATNAMÆLINGAR, ORKUSTOFNUN
ABSTRACT
A jökulhlaup from Grímsvötn into Skeidará
commenced on Sept. 4, 1976. The flood reached
its peak on Sept. 22 and was over on Oct. 2
(Fig. 1). The total water volume amounted to
2.4 kms. The last burst before this one occurred
in March 1972 and totalled 3.2 km3. The
volume ratio is in agreement with the ratio of
accumulation periods which were 6 years and 5
months for the 1972 burst and 4 years and 5
months for the 1976 burst.
The river Skeidará yielded similar volume of
water in both bursts, but in 1972 the river Gigja
yielded about 0.8 kms in addition to the water
that came to Skeidará.
The recent bridge over Skeidará, which was
built in 1974, offered now favorable conditions
for observations. In the strongest flow channels
the water velocity was too high for conventional
current meters. The meter could not be kept at
a fixed position. In these channels the velocity
was measured with the aid of floats. A wooden
vertical pole with crossed vertical plates, loaded
with bag of gravel at the lower end and a buoy-
ant float at top was thrown into the current
and the time required by the float to draw out
a fine thread of 50 or 100 m length was re-
corded. The pole is carried downflow with the
average water velocity at the depth of the cross-
ed vertical plates. By varying this depth the
velocity could also be measured as a function
of depth in the water. The poles could not be
retrieved and one pole was thus needed for
each measurement. The measurements were very
tedious as the river bottorn was rapidly chang-
ing.
A strong smell of hydrogen sulphide ivas felt
during the time of the burst. This is usual but
the smell was stronger than during the latest
previous bursts.
The new construction on Skeidarársandur, the
bridges and the dams withstood the flood witli-
out damage. This time no icebergs broke off
the glacier and no experience could therefore
be obtained, how tlie bridge would withstand a
flood with colliding icebergs. The water current
eroded gravel from the pillars of the bridge, e.g.
pillars no. 19 and 20, but this excavation filled
again when the erosion moved towards west.
The lowering of the ice surface at Grímsvötn
was observed during the latter half of the burst.
Attempts to reach Grímsvötn at the beginning
of the burst failed because of weather condi-
tions. A goocl agreement tuas obtained between
the volume of the water carried by Skeidará and
the decrease of stored water in Grímsvötn.
AÐDRAGANDI
Þegar Ragnar Stefánsson bóndi í Hæðum í
Skaftafelli kom út á hlaðvarpann árla morguns
4. september 1976, sló fyrir hægum vestan and-
vara sem bar að vitum hans kunnan þef. Það
var ekki um að villast, brennisteinslykt var í
lofti. Lykt sú er öræfingar nefna jöklafýlu.
Ragnar grunaði strax, að hér væri á ferðinni
fyrsti boðberi um það, að Grímsvatnahlaup
væri að hefjast. Grímsvötn hlupu síðast í mars
1972 og með hliðsjón af tímalengd milli fjög-
urra síðustu hlaupa, var ekki að búast við
hlaupi, fyrr en upp úr miðju ári 1977. Spurst
höfðu þó út þau tíðindi, að sennilega yrðu
Grímsvötnin með fyrra fallinu. Vorleiðangur
Jöklarannsóknafélags Islands mældi stöðu vatns-
borðs í Grímsvötnum 1425 m y. s. hinn 20. júní
1976. Helgi Björnsson jöklafræðingur taldi lík-
legt að búast mætti við hlaupi, þegar kornið
væri fram í mars 1977.
Ragnar sá ekkert óvenjulegt við Skeiðará.
Hún var mikil og dökk af korgi eins og vera
bar eftir úrkomur og hlýindi. Hvarvetna var
vöxtur í vatni. „Þetta skýrist næstu daga,“ sagði
Ragnar.
Flokkur vegavinnumanna bjó í skálum suður
80 JÖKULL 26. ÁR