Jökull - 01.12.1983, Síða 128

Fig. 5. Floodmarks on the bridge abutment of the Ölfusá river. The Hydrological Survey ofthe National
Energy Authority made the marks, fixed in the stone wall at Tryggvaskáli, but the children of Selfoss
painted and decorated the abutment around. There was a 7 cm difference between the maximum water
level position of the 1948 and 1968 floods. Floodmarks are useful for comparison by those carrying out
hydrological research as well as for city planners. (Photo. Hjalti Thorvardarson).
5. mynd. Flóðamerki á brúarvœng Ölfusárbrúar. Vatnamælingar Orkustofnunar gerðu merkin, kl'óppuðu þau í
steinvegginn hjá Tryggvaskála, en börnin á Selfossi máluðu og skreyttu umhverfið. Ath. 7 cm munur var á hœstu
vatnsborðsst'öðu flóðanna 1948 og 1968. Flóðamerki eru notadrjúg til viðmiðunar fyrir mœlingamenn og skipuleggjendur.
(Ljósm. Hjalti Porvarðarson.)
the river for commercial purposes (see the section
on Presenl day flood protection) With this kind of a
short-cut the Hvítá would bypass the second
obstacle and also the third one which is a con-
strictive and ice-damming area south of Hestfjall.
The fourth serious constrictive- and flood-area is
from the estuary to up above Selfoss. Probably it
seems strange in this discussion to go against the
stream in this way -from the estuary and up to
above Selfoss. The reason is, that in the winter ice
begins to collect forming an ice jam in the channel at
the estuary area west of Arnarbæli. In long endur-
ing frost periods the pack ice extends further and
further upstream eventually reaching Selfoss where
the river is full to its banks. At Arnarbæli the river
looks like a valley glacier, the water level increase
there being 4 meters. If the river then floods its
banks it is known as encroachment (Icelandic
ágangur).
As a result of a large amount of spring-water in
the Olfusá the Arnarbæli pack-ice is longer forming
than the pack ice at Urridafoss on the Thjórsá river
and it forms only in long and severe frost periods.
The freeze up will not be discussed here but refer-
ence is made to the literature. (Rist 1967 a). A
dangerous situation arises if a sudden and quickly
rising flood occurs when the river is in bankfull stage
and is frozen over.
Flow measurements on the Ölfusá at Selfoss were
begun on lst September 1950. The mean flow for
the 30 years period is 370 m3/s. The largest flood in
the recording period was on leap year’s day 1968,
2520 m3/s. There was some pack-ice in the channel
which significantly interfered with the flow. The ice
dam having the most effect on raising the water
level at Selfoss was at Thórustadir between the rock
obstacles at Arnartangi to the north and Flugunes
to the South (Snorrason 1968).
126 JÖKULL 33. ÁR