Jökull - 01.12.1983, Síða 123

lava covered in sand can become impermeable and
result in surface run-oíT, but rough lavas and crater
areas can never become watertight.
Most of the large and dangerous íloods on earth
are rain- and meltfloods.
4. Jökulhlaups
The jökulhlaups (glacier bursts) are of two diífer-
ent orgins, the more common examples being caus-
ed by sudden outbursts of accumulated water from
ice-dammed lakes, which are located in side valleys.
The other jökulhlaups are caused by subglacial geo-
thermal or volcanic activity, and may be cata-
strophic. The jökulhlaup can occur at any time of
the year. The main aspects of research on the jökul-
hlaups concern forecasting their return time, their
peak discharge and the total water volume in-
volved.
The jökulhlaups from Katla and Grímsvötn,
which are caused by volcanic orgeothermal activity
are by far the largest íloods in this country and
indeed are large by any standards. In the three last
decades the ílood peaks of the Grímsvötn Jökul-
hlaups (Skeidará-Jökulhlaups) have only been from
10 to 2 thousand m3/s, and have steadily decreased.
Jökulhlaups connected with eruptions from earlier
periods have been estimated at 4 to 7 times greater.
The maximum of each Grímsvötn Jökulhlaup is
reached gradually. The water reservoir is as far as
50 km in under the glacier. On the other hand the
Fig. 1. Seen east towards Ól-
afsvellir at Skeid from the
Hvítá river in ílood 24th
February 1961. Gateposts at
Utverk stand up above the
flood, as well as fence posts
just visible above the flood
water. Hills and ridges to the
east in Rangárvallasýsla can
be seen in the background
(Photo. S. Rist).
1. mynd. Horft austur til Ól-
afsvalla á Skeiöum utan af Hvítá í
flóði 24.febráar 1961. Hliðstólpar
hjá Utverkum standa upp úr
flóðinu, einnig yddir á girðingar-
staura upp úr vatnshapinu. Hteða-
drög og ásar austur í Rangárvalla-
sýslu eru í baksýn. (Ljósm. S.
Rist.)
Katla Jökulhlaups from Mýrdalsjökull are very
sudden and the first flood surge is very high.
The causes and explanations of the jökulhlaups
will not be discussed further here. They have been
discussed in detail; the readers are referred to
publications by dr. Sigurdur Thorarinsson. (Thor-
arinsson 1974). As regards observations and mea-
surements on Skeidará the readers are referred to
this journal where the jökulhlaup reports are
published (Rist 1955, 1959, 1970, 1973, 1976a),
1976b), Bjömsson 1974). In jökulhlaups the
sediment load can reach 10 kg per m3 where as the
normal load in a jökulhlaup is 1 to 4 kg/m3.
5. Step-bursts
Step bursts are sudden floods during frost pcriods
in the winter. While the river is freezing over, ice
ridges or ice dams form on the bedrock bottom;
across wide and shallow channels. Above the ice
ridges water accumulates. If an ice ridge breaks, the
next one downstream ofit is in danger and so on. A
large floodwave can therefore be produced; this is
the step-burst. (Rist 1961,1962).
6. Man-made Floods
Floods connected with man-made constructions
and faults in them, as well as direct interference by
man are stealthy and mostly very sudden floods. In
1889 a dam in Johnstown, Pennsylvania, was
washed away because the spillway overflow was
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