Jökull - 01.12.1974, Síða 32

1. In summer 1971, before the jökulhlaup:
sampling of bottom material and observa-
tions on riverbed morphology and eleva-
tion.
2. During the jökulhlaup: Sampling of sus-
pended sediment, glacier ice floating with
water, bedload sampling and sediment
sampling in the sea.
3. After the jökulhlaup: Extensive sampling
of bottom material and observations on
riverbed morphology and elevation.
Suspended sediment samples were taken at
several localiues along Skeidará, but mainly at
one locality at Gígja. Direct correlation between
sediment concentration and discharge is not
apparent; on the other hand regular changes
from one locality to another — and in time —
are observed.
The grain size classes used in this paper are
as shown in Table 1.
sorting at each locality in the manner that con-
centration clecreases along the banks but in-
creases in the center channels. All this is shown
on Fig. 4.
As shown in Fig. 5 the ratio coarse silt/fine
silt varies with time, i. e. the ratio increases as
the jökulhlaup proceeds reaching maximum at
the maximum flow and again at the later max-
imum in concentration and even later. The
peak at maximum flow is the highest one.
The reason for these variations in the silt
concentration is connected with the origin of the
sediment load, which is from the melted ice of
the subglacial tunnel at the bottom of the gla-
cier. This melting increases until maximum
flow is reached and the normal peak in sedi-
ment load should be reached simultaneously.
The second peak in sediment load is due to
some other reason. From petrographical analysis
it is obvious that the material in the peak is
TABLE 1
English term Icelandic term Grain size, mm Transport mode
clay leir <0.002 wash load
fine silt méla 0.002-0.02 wash load
coarse silt mór 0.02-0.2 suspended load
sand sandur 0.2-2 suspended and bed load
gravel möl >2 suspended and bed load
The amount of sediment of different grain
sizes is calculated separately.
The quantity of clay is usually only about
2% of the total suspended load. Its variation
in time and place is more or less random. The
quantity of about 500,000 tons has been cal-
culated from its average concentration.
Fine silt constitutes a major proportion of
the total load. Its concentration follows a fairly
regular pattern of even spatial distribution, but
varying with time. This variation is shown on
Fig. 3. The concentration reaches maximum at
peak flow or just before it with another very
high and sharp peak on March 26th. The total
amount of fine silt is calculated as 5,000,000
tons.
The coarse silt shows the same variation with
time as the fine silt but also shows substantial
variation from one sampling place to another.
The general trend seems to be a decrease in
concentration away from the glacier and some
30 JÖKULL 24. ÁR
volcanic glass from Grímsvötn. Simultaneously
the concentration of dissolved solids reached a
peak, which can be explained as a result of a
small subglacial eruption producing big quanti-
ties of sancl and silt carried away with the
water. Tliis eruption woulcl have lasted less than
a day. A similar eruption has occurred before
as a small eruption probably took place in 1945
barely reaching the surface (Askelsson, 1959).
The decrease in concentration of coarse silt
away from the glacier is partly due to its sedi-
mentation in the channel. Coarse silt must
therefore be calculated separately for different
places. At the outlet from the glacier this load
amounted to about 14,000,000 tons.
The suspended sediment samples froni Skeid-
ará contained only a small fraction of sand, but
those from Gígja had a much higher concentra-
tion. Yet we know that a lot of sand has been
carried in the jökulhlaup by both the rivers.
The difference between the rivers is due to the