Jökull - 01.12.1980, Side 31

abnormal ones mentioned earlier. The very
high content of material almost identical to
the Grímsvötn volcanic ash was observed in
this peak.
Fig. 7 shows sediment transport curves for
fine and coarse silt in 1976. There is only a
minor quantitative difference between the
sampling stations except, as already stated,
the Sl-samples from the bridge yield greater
quantities of the coarse sediment. No abnor-
mal sediment peak was observed.
SEDIMENT DEPOSITED
IN THE FLOOD CHANNELS
After the jökulhlaup of 1972 most of the
flood channels were strewn with black, fine-
grained material (see Tómasson 1974, Fig. 7.).
This layer consisted mainly of the sand and
coarse silt grain size group (see histograms of
layer 1 in Fig. 8). The layer was generally
about 10 cm thick. No such layer could be
found in the flood channels after the jökul-
hlaup of 1976. The deposition of such a
material in a channel with velocity exceeding
5 m/s is against hydraulic rules. It should take
place at only l/100th of the actual velocity of
the jökulhlaup (Hjulström 1935).
After this jökulhlaup the channels at the
outlet were also strewn with deposits of ice
fragments partly hidden in black sand and
still finer material. In the field this was
assumed to be glacier ice. A few samples were
collected and laboratory analysis of one of
them revealed that it contained identical
Fig. 7. Coarse and fine silt in
suspended load in Skeiðará jök-
ulhlaup 1976. Key: 1. coarse silt
at glacier outlet; 2. coarse silt.at
bridge, Sl-samples; 3. coarse silt
at bridge, S3-samples; 4. coarse
silt 10 km downstream of
bridge; 5. fine silt at glacier
outlet; 6. fine silt at bridge,
Sl-samples; 7. fine silt at bridge,
S3-samples; 8. fine silt 10 km
downstream of bridge. — Mynd
7. Aurrit mós og mélu í Skeiðarár-
hlaupi 1976. 1. Mór við Útfall. 2.
Mór við brú Sl. 3. Mór við brú S3. 4.
Mór 10 km neðan brúar. 5. Méla við
Útfall. 6. Aléla við brú S/. 7. Méla
við brú S3. 8. Méla 10 km neðan
brúar.
kg/s
JÖKULL 30. ÁR 29