Jökull - 01.12.1983, Blaðsíða 122

their value will increase again soon, there being a
need for level land today. An airport which has been
situated on the deltas of Evjafjördur is clear testi-
mony to this fact. As things are at present the sea is
almost entirely the main factor controlling the
water level at the airport, although the variations in
the rivers count to a small extent. It will soon be
essential to understand the quality and limitations
of the floodplain; therefore it is highly important
that all available information on the level and
frequency of floods should be collected and system-
atic measurements commenced. There are many
variables involved; e.g. accurate slope measure-
ment is an urgent item. With good information it
can be expected that mistakes will be avoided.
In the great meltwater flood ofjune 1949only the
very banks stood above the water, and I have heard
that the entire floodplain is submerged once every
70 years. This half myth is not good enough to plan
the future on. Pack-ice dams in the winter are an
important factor which cause a great build up of
water, and the highest water level at each place
needs to be recorded. Reports of floodplains from
everywhere in the country are similar to the descrip-
tion given.
TYPES OF FLOODS
The flow of the main rivers in Iceland has been
measured continuously for some 30 years. Thus,
there are now available so-called long-term flow
series which form the basis for research in the
various fields of hydrology, e.g. floods. (Rist 1947-
1980) It seems natural to classify floods in this
country as follows:
1. Rainfloods
2. Meltfloods
3. Rain-and meltfloods
4. Jökulhlaups (glacial bursts)
5. Step-bursts
6. Man-made floods
7. Geological-event íloods
1. The Rainfall Floods
The rainfall floods need few explanations. These
are the most common floods in Iceland as elsewhere
in the world. In this country they are never the
largest floods except in certain small rivers.
2. The Meltflood
The meltfloods which are most common in the
spring season are then usually called spring floods.
They are the largest floods as far as the amount of
water is concerned. According to the height above
sea level, spring floods are divided into lowland-
and hill floods on one hand, and highland floods on
the other. This has economic consequences
connected with the running of hydropower stations.
The lowland and hill floods usually begin within the
period lOth April until lOth May, but the highland
floods between lOth May to lOth June. In all cases
the periods are unclear and variable from year to
year. The high summer flood of glacier rivers is a
pure meltflood.
3. Rain- and Meltjlood
With the exception of catastrophic jökulhlaups
the combined rain-and meltfloods are the largest
floods in Iceland, as far as the discharge at the flood
peak is concerned. Large floods in Iceland can
occur at any time ofyear, regardless ofwhether it is
mid-summer or the depth of winter. Experience
shows that the spring floods are most effective,
transporting the greatest amount of water to the
sea. On the other hand the highest flood peaks occur
in the sudden rain- and melt-floods of autumn in
small streams and up to average sized rivers. A
similar rule applies, with the exception of glacier
rivers, to large rivers, where the flood peaks of the
sudden, though rather infrequent winter floods,
normally reach higher than the maximum for
spring floods. The total amount of water of the
spring floods is nevertheless considerably greater.
Many are surprised that flood bursts on an autumn
day should rise higher than in spring rainfall when
there is still a lot of snow on the mountains. The
main requirements for and causes of autumn floods
are: 1) A frost period early in autumn, which makes
the ground completely impermeable. 2) A layer oí
snow completely covering the drainage area. 3) A
deep water-laden Atlantic depression crosses the
country, causing sudden temperature changes and
torrential rainfall with strong winds. In the spring,
on the other hand, depressions are generally weak
and the weather has less effect.
In connection with the winter floods it should be
pointed out that floods never occur in spring-fed
rivers unless the ground surface is frozen solid, since
then and only then can surface run-off begin (Rist
1969) of spring-fed rivers catchments can thus actu-
ally become direct run-off drainage areas for a
while. Spring-fed river areas may be subdivided as
regards winter melting; pumice areas and pahoehoe
120 JÖKULL 33. ÁR