Jökull - 01.12.1969, Síða 133

there is a serious and hidden danger o£ flood-
ing because then they have lost their natural
ability to brake down a sharp flood peak.
ítem 3 includes the essential factors which
lead to flooding. Major floods result from
strong coordination of all external factors.
Tlie discussion so far has focussed on short-
range prediction of flooding (few days or
weeks). Now fong-range prediction of flooding
wili be discussed. For such predictions the
natural time sequence of externaf factors lead-
ing to flooding is equally important as for
short-range predictions. If climatic changes lead
to increasing clanger of flooding, they must
first of all fulfill itern I and have a positive
influence upon item 2. It is of primary im-
portance that the climatic change causes either
more rapid or bigger temperature fluctuations
or more continuous rainfall than takes place
presently, or both. In order lo make reason-
able predictions, it is necessary lo stndy separ-
alely the expectecl itifluence of climatic changes
on each of the external factors.
Meteorologists have pointed out during this
symposium that changes in the weather can be
more rapid during periocls of colder climate
than during periods of warmer climate. Danger
of floocling is expected to increase with cool-
ing climate because then there are potential
conditions for sudden invasion of warm and
moist air masses to procluce heavy rainfall on
frozen ground.
For long-range prediction it is not sufficient
to consider meteorological factors only but also
their influence upon vegetation. Wind erosion
and destruction of vegetation increase the
danger of flooding, but wooclen country de-
creases such danger much. It seems likely tlrat
luiman activities are more important in cutting
ditches, and drying bogs during periocls of
warmer climate, as during this preceeding
period, and therefore they increase the danger
of flooding. Thorarinsson (1969) has discussed
glacial bursts in detail and shown that climatic
changes lead to glacial bursts in some localities
but cause them to cease in others. A good
example of the effect of such changes is the
ice dammed lake in the southern part of Hofs-
jökull as pointed out by Kjartansson (1969).
It is not essential to know if a flood results
frorn an overall climatic change or from ex-
treme fluctuations in the dominating climatic
conditions. It is of primary importance to be
able to predict the flooding. Increasing know-
ledge of the problem can be obtained most
rapidly by studying with the aid o£ a com-
puter the correlation of the weather and water.
However, reliable observational data is a necess-
ary basis for such a study.
There is a reason to look at the thaw in
March of 1948 and ask how intense such thaws
can be in wintertime. It is also interesting to
study the late spring thaw in 1949, the heavy
rainfall in northern Iceland on July 6, 1954,
which led to land sliding in Nordurárdalur in
Skagafjördur, and also the deep thaw which
rushed across the country on February 2, 1956.
Here, only one flood will be discussed in
some detail, the flood in Ellidaár last winter,
February 1968. The City Engineer of Reykja-
vík asked the building constructing company,
Almenna byggingafélagid, to collect detailed
data of this flood and nrake a first plan ot
what preparations could be made in the upper
part of the drainage to reduce the danger of
flooding. The following points of interest have
been extracted from this report (Jónatansson
1968):
“The flood which is considered to be 215—
220 ms/sec, is substantially heavier than any
other known and measured major floods for
a period of 40 years.
In view of the fact that tliis flood was
nruch heavier than otlrers recorded, it ap-
pears valid to call this a 100 year flood, i.e.
a flood which can be expected every 100
years although probability calculations sugg-
est that floods of this magnitude would not
be that frequent.
According to reports of the Meteorological
Office the heaviest 24 hours precipitation in
1968 occurred during the flooddays of Fe-
bruary, as much as 37.8 mm. The heaviest
precipitation nreasured in Reykjavík between
1924 and 1964 was 56.7 nrm which occurred
on March 5, 1931, i.e. in the sanre season
but the precipitation was tlren 50% heavier.
It is considered, and approved by the
Meteorological Office, that a precipitation
of 70 rnm or more during 24 hours can be
expected once every 100 years or double the
JÖKULL 19. ÁR 129
ti