Jökull - 01.12.1969, Blaðsíða 134

amount which was measured during the
flood in question. As a result of such pre-
cipitation a flood in Ellidaár by Árbaejar-
stífla of 400—450 m3/sec can be expected.
For such a flood to occur it is thought
necessary to have a simultaneous relative in-
crease in precipitation on the whole drain-
age area of the river and other circumstances
comparable with those of February, 1968,
such as air temperature before and during
the flood, equal or more accumulation of
snow and the same human constructions
associated with the river such as roads,
bridges, dams and so on.
It is difficult to estimate or predict the
frequency of floods as the one in question
as so many external factors must coordinate.
It is possible to speak of possible and prob-
able maximum flow in Ellidaár for presently
existing climatic conditions.”
Probably it would be more appropriate to
call this flood 75 year or 50 year flood because
in February, 1927, that is the year before re-
cording started, the flow in Elldaár reached
160 m3/sec according to Mr. Steingrímur Jóns-
son former director of the Reykjavík Municipal
Light and Power Works (Report of S.Í.R.,
1960). For the period 1928—1964 and accord-
ing to item 1 reducing the flood peak the
degree of freezing of the ground was in all
probability below average. The present writer
agrees with Mr. Jónatansson tliat the estimate
of maximum possible ancl probable flood in
Ellidaár is sufficiently high. It should be
pointed out that the magnitude of floods in
question in Ellidaár is only dependent on the
inflow into the lake Ellidavatn, because below
the damgates at the outlet of the lake man-
made floods are possible.
The heavy floods in eastern Iceland in the
autumn of 1968 conform clearly with the
frame of factors causing flooding listed pre-
viously,
1) frost in October,
2) heavy snowfall on November 6 ancl
3) the most important factor, warm weather
and very heavy rainfall on November 11
to 12.
In such floods the drainage reached a peak
for short time of 6000 litres/sec-km2 in streams
and rivers having drainage area of less than
10 km2 in mountaineous country.
When results found in reports of flows in
rivers are studied, it becomes evident that high
flow rates are related to four seasonal weather
conditions which are:
1) Autumn rainfall (Oct.—Nov.)
2) Winter thaw (Feb.—Aíarch)
3) Spring thaw (May—June)
4) Melting of glaciers (July—August).
These results have been discussed by Risl
and Sigvaldason (1968).
130 JÖKULL 19. ÁR
Fig. I. Seasonal flood peaks.