Jökull - 01.12.1983, Page 124

insufficient. Five thousand people lost their lives
(Mead 1919). Many thinking men awoke to the
realization that mankind faced a new flood danger.
During the last decades there have been several
dam disasters throughout the world, e.g. in South-
ern Europe.
In Iceland we have only experienced a kind of
warning as far as this kind of ílood is concerned. On
the 17th June 1959 a cofferdam on the river Sog
below Lake Thingvellir gave way. (Jónsson 1961).
The flow reached 378 m3/s. The maximum flow in
the river Sog apart from this event is 174 m3/s.
Another instructive event: in the last few days of
February 1968 there were large floods in the rivers
of South-west Iceland due to rain and snow melting
when the flood was at its height in the river Ellidaár
area an earth dam below Lake Ellidavatn gave way.
The flow in the river became 150 m3/s and flowed
over the earth dam causing the sods, already riddl-
ed by mink burrowing, to be carried away. Because
of this failure the flow in the lowest part of the river
increased by 70 m3/s, and caused the largest flood
known in the river Ellidaár. It is belived that on one
previous occasion flow in the river Ellidaár reached
160 m3/s, that was in February 1927. (Jónsson 1961).
Protection against this type of flood entails know-
ledge and careful monitoring.
7. Geological-event Floods
Geological-event floods are floods which are the
result of unexpected, usually sudden natural haz-
ards, e.g. eruption, earthquakes, landslides, aval-
anches etc. Here two floods from recent years will be
discussed. At the beginning of the Hekla eruption of
1947 a flood-wave surged into the river Ytri-Rangá.
Measurements of the flood channel suggested that
at its peak the flood had been 880 m3/s at Hringl-
andahraun, 180m3/s atÁrgil and 123m3/satHella,
while the total amount ofwater was 1.9 Gl. (Kjart-
ansson 1951).
The later flood was the result of rockslide. On the
15thjanuary 1967 a 15 million m3 rock face fell from
Innstihaus (Eyjafjalla-glacier) into Lake Steins-
holtslón (Kjartansson 1967). This triggered offa very
large flood wave. The wave soon dispersed on the
extensive sandur, but at the Markarfljót bridge it
Fig. 2. Flood areas of Iceland (Drawn by Hydrological Survey, National Energy Authority).
2. mynd. Flóðasvœði íslands.
122 JÖKULL 33. ÁR