Jökull - 01.12.1980, Page 34

diagram, layer 2, in Fig. 8, consisting mainly
of gravel and boulders mixed with sands as the
finer part. The reason why a similar layer to
layer 1 formed in 1972 was not formed in 1976
can be explained by the difference in air
temperature. In Fig. 9 the air temperature at
Fagurhólsmýri during both of the jökulhlaups
is compared. In 1972 air temperature was near
0°C, fluctuating considerably above and
below freezing point, but in 1976 the
temperature was near to 8°C, varying but
without falling at any time to near 0°C. Thus,
anchor ice should have formed in considerable
quantities in the jökulhlaup of 1972. This was
impossible in 1976. The irregular quantitative
spatial distribution of the sediment
concentration in 1972, shown in Fig. 6, could
also be explained as a consequence of anchor
ice formation whereby suspended sediment of
all grain sizes is stuck to the bottom and even
in the bottom layers. This explains the great
difference encountered in the distribution of
both fine and coarse sediment. In the jökul-
hlaup of 1976 the spatial distribution was far
more uniform, divergences were found only in
the coarse part of the sediment. Layers of fine
sand and coarse silt in ancient alluvial
sediments as well as clay fillings in coarse
gravel may have been formed in a similar way.
The drying-up of pools is also a plausible
explanation of these occurrences but their
frequency renders both explanations
necessary. Most of the thick layers of this type
can be assumed to have been formed by
anchor icing, but layers of 1 cm thickness or so
are more likely formed by drying up of pools.
ACKN OWLEDGEMENT
The authors thank Mr. Gudmundur Vigfússon,
systems analyst at the National Energy Authority, for
the computer calculations on which sediment rating
curves are based.
REFERENCES
Björnsson, H. 1974: Explanation of jökulhlaups
from Grímsvötn, Vatnajökull, Iceland.
Jökull, 24: 1 — 26.
Hjidström, F. 1935: Studies of the morphologi-
cal activity of rivers as illustrated by the
River Fyris. Meddelanden frán Uppsala
Universitets Geografiska Institution, Ser. A.
N:o 10: 121-527.
Rist, S. 1955: Skeiðarárhlaup 1954. Jökull, 5:
30-36.
— 1962: Þjórsárísar. Jökull, 12: 1 — 30.
— 1973: Jökulhlaupaannáll 1971, 1972 og
1973. Jökull, 23: 55 — 60.
— 1976: Grímsvatnahlaupið 1976. Tökull, 26:
80-90.
Thorarinsson, S. 1974: Vötnin stríð. Saga
Skeiðarárhlaupa og Grímsvatnagosa.
Bókaútgáfa Menningarsjóðs, Reykjavík,
254 pp.
Tómasson, H. 1974: Grímsvatnahlaup 1972,
Mechanism and sediment discharge. Jök-
ull, 24: 27-39.
Manuscript accepted lOth October 1980.
ÁGRIP
SAMANBURÐUR A AURBURÐI
í SKEIÐARÁRHLAUPUM
1972 OG 1976
I grein þessari er stuttlega lýst sérkennum
Skeiðarárhlaupa. Magn uppleystra efna í
Skeiðará eykst frá um 50 — 90 mg/1 við venju-
legar aðstæður í um 300—400 mg/1 i hlaup-
um, og hlutfallið dökkt gler/ljóst gler í svifaur,
sem hefur verið um 10—20 á milli hlaupa,
margfaldaðist í þremur síðustu hlaupum, en
það eru einu Skeiðárárhlaupin, sem við höfum
haft sýni úr til bergflokkunar (2. mynd).
Birt er yfirlit yfir mælingar á magni svifaurs
í Skeiðarárhlaupum (Tafla 1). Fram til 1972
höfðu nær eingöngu verið tekin sýni við bakka
í venjulegar flöskur án sýnataka (F-sýni).
Vegna fyrirhugaðrar vega- og brúargerðar á
Skeiðarársandi voru miklu fleiri og vandaðri
svifaurssýni tekin úr Skeiðarárhlaupinu 1972
en úr fyrri hlaupum, og voru sýni þá tekin við
bakka í sýnataka (S3-sýni) á nokkrum töku-
stöðum. I hlaupinu 1976, eftir að Skeiðará
hafði verið brúuð, var miklu betri aðstaða til
sýnatöku en áður, þvi að þá var unnt að taka
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