Jökull - 01.12.1969, Blaðsíða 132

Climatic Trend and Flood Danger
SIGURJÓN RIST,
HYDROLOGICAL SURVEY, NATIONAL ENERGY AUTHORITY, REYKJAVÍK, ICELAND
Changes in the weather and the danger of
flooding — two natural phenomena difficult
to cope with. Professor Linsley of Stanford
University, who lias contributed much to
modern hydrology, considers that analyses and
effects of climatic trends ancl fluctuations are
the most important problems in the field of
hydrology not yet solved (Linsley 1960). Be-
sides, the danger of flooding is the headache
of hydrologists all over the world.
It is customary to speak of 10 years flood,
which means the heaviest floocl appearing on
average every 10 years. In the same way it is
customary to speak of 20 years flood, 50 years
flood, 100 years flood and so on.
Hydrological constructions are generally in-
tended to last for 50—100 years. Therefore it
is essential to possess at the beginning of the
building of such a construction a satisfactory
knowledge of the magnitude and frequency of
the heaviest floods in order to permit reason-
able decision of so-called design floods. In de-
ciding design floods financial aspects of build-
ing costs and probability of destruction are
taken into account in addition to hydrological
conditions.
If the design flood is based on hydrological
and meteorological records including no heavy
floocls, the imminent danger is obvious, when
climatic changes take place that involve in-
creased heavy flooding. In the same way al-
though hardly of the same order of magnitude
valuables would be wastecl if floods of un-
reasonable magnitude were considerecl to be
possible. Therefore the reliability of the re-
cording period lias to be evaluated. In short,
external circumstances leading to flooding are:
1. Impervious grouncl (porous ground be-
cornes impervious through freezing of
interstitial water; of primary import-
ance).
128 JÖKULL 19. ÁR
2.1. Snow (not of primary importance).
2.2. High water level in lakes.
3.1. Temperature above 0° C.
3.2. Rain.
The external circumstances leading to flood-
ing are listed accorcling to the time sequence
in which they occur.
In areas of the Moberg-formation the ground
is very porous. In these areas item 1 of the
external circumstances is only fulfilled if the
ground is frozen. The reasons of items 2 and 3
need not be explained in detail. It is conveni-
ent to use the time sequence of external circ-
umstances leading to flooding for the short
range prediction of flooding. According to the
first item, which is essential, the danger of
ílooding is none in the extensively fissured
lava fields of Odádahraun. It is also practically
none in the river Ellidaár and the lava fields
in the vicinity of Hekla, if the ground is not
frozen.
Item 2 is not considered to be of primary
importance. Flooding can occur when snow is
absent. This item includes accumulation of
water that is available for floocling. This ac-
cumulation must be estimated at each time in
order to predict how heavy the floods will be
as experience has indeed shown. In the spring
of 1949 the spring thaw did not begin in
northern Iceland and the interior highlands
until after the middle of June and then the
heaviest recorded floods took place. At that
time thawing in the highlands and at the
heads of valleys coincided. The same accumula-
tion of snow plays therefore the rnore import-
ant role in flooding the later the spring thaw
sets in. The latter accumulation factor of item
2, high water level in lakes concerns, of course,
all natural lakes, small and big, while it also
includes lakes formed bv dams. When the
water level in reservoirs is kept at maximum