Jökull - 01.12.1983, Síða 125

was still 2000 m3/s. Markarfljót was in large flood or
around 500 m3/s, so that the bridge had to with-
stand a flash flood of 2,500 m3/s. Jökulhlaups
coused by subglacial volcanic eruptions may be
classified as geological-event floods.
MODERN FLOOD PROTECTION
Present day knowledge and experience enable
one to be prepared for flood danger, but man-made
constructions are large so they are not put into eflect
except when they fit into the general commercial
schemes, which then largely foot the cost. In this
connection the famous reservoir in the Tennessee
valley in U.S.A. and the High Aswan dam on the
Nile should be mentioned. The Egyptian dam is 110
m high, 5 km long and can dam up 135000 G1 or 135
km3 of water (Britannica 1965) i.e. nearly two years
run-ofT of the Nile. By comparison it should be
noted that the reservoir capacity in Lake Thóris-
vatn is 1 km3. The type of flood danger in the days of
the Pharaohs and which threatened Cairo for
centuries, is no longer present. But now the Nile no
longer fertilizes the plains of the Nile valley. The
sediments remains on the bottom of the reservoir
but this is made up for by manufactured fertilizer
from the power stations at the dam. Soon the Nile
won’t reach the Mediterranean any more, but will
be allowed to evaporate in the desert to the west.
Water shortage is believed by many to be the
greatest emergency facing mankind. It is generally
worst where the flood danger is greatest or nearby.
Destruction of forest, drainage of bogland, over-
grazing, road construction etc. increase the possi-
bility of both water shortage and flood. Two of the
largest floods experienced on the Mississippi river
in U.S.A. in historical time occured in this century,
in 1927 and 1973. Following the 1927 flood there
was quick inter-state reaction and it is now believed
that the main flood dangers have been overcome.
Flow control
Protection against floods is based on protective
dykes in certain places but the future hopes lie in the
interplay of three measures 1) reservoirs, 2) Jlood-
forecast and 3) flow-control. In the last three decades
large control dam schemes have been put into oper-
ation throughout the world. The present degree of
flow-control of many large rivers would hardly have
been possible before the days of computers. In
China many control dams have been built in the
tributary rivers of the Yellow River. In 1957/60 a
100 metre high dam was built in the gorge mouth of
the river itself40 km west of Sanmenshig in Honan.
The reservoir behind the dam is 35 km3. A by-pass
tunnel has been built beside the dam to get rid of the
sediments. In spite of these and other measures and
improvements it seems likely that the reservoir will
become filled by sediments in a few decades, and
then the flood danger is renewed. What can be
done? Planting the countryside with a grass and
forest cover, does two things by reducing the flood
and lengthening the life of the reservoirs.
More and more countries are realizing that they
must examine and appraise their water budget in
conjunction with their neighbouring countries, i.e.
those which are not islands. In the field of co-oper-
ation and organization UNESCO has done very
useful work.
FLOOD BEHAVIOUR OF
ICELANDIC RIVERS
The largest flood in each river is generally of the
flood type no. 3, although this varies from place to
place. 'Fhc maximum flood in each year provides
the statistical basis for a 50- and 100-year flood (see
Table 1)
FLOOD DANGER
Many people drive quickly over Mýrdalssandur,
with the fear of the great flood of 1918 in their
minds. People are to be excused since the surround-
ing wasteland bears evidence of the mercilessness of
the Katla Jökulhlaups, and it doesn’t help having at
the back of one’s mind the fact that Katla next
eruption is overdue. The last five Katla eruptions
and the resulting Katla Jökulhlaups were in 1721,
1755, 1823, 1860 and 1918. (Thorarinsson 1959).
True enough there was a small jökulhlaup from
Katla in 1955, 3 thousand m3/s and 28 G1 in size,
but no visible volcanism. The flood destroyed
bridges on the rivers Múlakvísl and Skálm. Most of
the flood water came from beneath the glacier at the
known outlets of the rivers and there broke offsome
ice blocks from the edge of the glacier. A part of the
flood water however swelled up through crevasses
in the glacier 100 to 200 meters above the glacier
margin but the pressure was not enough to move the
huge ice blocks there (Rist 1967 b).
On the map of flood areas above. (Fig. 2), the
symbols at the foot of Oræfajökull are a reminder of
the catastrophic floods of 1362 and 1727, the latter
resulting in the deaths of three people, and the
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