Jökull - 01.12.1983, Blaðsíða 121

Floods and Flood Danger in Iceland
SIGURJÓN RIST
Hydrological Survey, National Energy Authority, Grensásvegi 9,
108 Reykjavík, Iceland
ABSTRACT
RunoJJ data are given for 50 drainage areas all over
Iceland, describing jlood characteristics in direct runoff
rivers, spring-fed rivers and glacier rivers. Seven types of
Jloods are defined: rain floods, melt floods, rain and melt-
floods, jökulhlaups, step-bursts, man-made Jloods and geo-
logical event floods.
Representative flood areas are described and possible
Jlood protection is discussed. Special attention isgiven to the
Hvítá— Olfusá area, which is the most seriously threatened
Jlood area in Iceland.
INTRODUCTION
Floods play an important role in hydrology, de-
stroy lives and damage land and other property.
Although floodplains are not densely populated in
Iceland, flood studies are important for future land
use planning. The present paper discusses mapping
of potential flood areas in Iceland and classifies the
observed ílood types. Further, flood danger is
described in the most threatened areas, and mea-
sures for flood protection are discussed.
ICELANDIC FLOODPLAINS
Iceland is a convenient model for earth and
hydrological studies. The chain of events is rapid
and clear. The most typical floodplains are those of
the old rivers of northern and eastern Iceland, e.g.
the lowlands of Skagafjördur and Eyjafjördur and
Utmannasveit in the East by the Selfljót river. We
may consider the lowland of Eyjafjördur inland of
Akureyri as an example. The farmers of Eyjafjördur
have situated their farms up on the mountain sides
on either side of the plains of Eyjafjördur. They
have realized, that the plain is an area where the
river dominates in flood condition.
But how do the large floods happen on these
floodplains? It is apparent that the large flood is the
result of the interplay of many factors. The first
stage is the large flood of the river in the valley of
Eyjafjördur. That alone is not sufficient to flood all
the lowland. The sea level at the estuary at the head
of the fjord is very important. A high sea level is
sufficient to prevent flow many kilometres inland.
A high sea level can be the result of various condit-
ions, as is generally known, e.g. high tide of the tidal
stream, on-shore wind, low atmospheric pressure.
The situation can also be such that the flow to the
sea is hindered because the river bed is in a bad
condition, i.e. it has an unusually poor transport
ability due to ice, sediment, or that the flow is even
hindered by growth or constructional operations as
well as many other factors. Some of these factors can
be calculated as regards their frequency and the rise
in water level which they cause. In other cases
figures do not apply so that the total effect becomes
unclear e.g. what factors work together to cause a
rise in water level and what factors do not? This long
Iist should be sufficient to show that it is clear that a
variable and usually long period of time elapses
between large floods. The water level alone is not
sulficient in this case to show what the flow is each
time. In other words: The connection between
water level and flow is broken and confused by
various obstacles. For this reason flow recording
stations are situated above the floodplains, where
the same water level always gives the same flow (in
m'Vs). On large rivers abroad it also proves
economical for other reasons to have the flow re-
cording stations above the floodplain and in fact a
good distance away from them, so that it’s possible
to give flood warnings well in advance.
Throughout almost the entire history of Icelandic
farming, floodplains have been the greatest and best
hay producing land, but half a century ago they
suddenly lost their value. It is pretty certain that
JÖKULL 33. ÁR 119