Jökull - 01.12.1983, Side 134

floods in spring-fed rivers is a special topic depend-
ing predominantly on frost in the ground.
Nowadays there is easy access to the flood
research literature ofmany countries. As a sequel to
the International Flood Conference in Leningrad in
1967 UNESCO has issued a publication dealing
with the experience of many countries. As regards
flood research from the climatological point of view
reference is made to a publication of VVMO:
Estimation of Maximum Floods, technical note no.
98.
It is usual to give the run-offas 1/s, but here m3/s
have been employed so that neighbouring columns
do not have figures which are too alike, thus avoid-
ing frequent confusion. The reader should note that
one only needs to omit the decimal point to obtain
the figures as 1/s per km~, with which most users of
hydrological data are familiar.
As regards type, reference is made to the section
on Flood types. So that it is made clear that large
autumn and winter-floods are to a great extent re-
stricted to certain parts of the country, the abbrevi-
ations ve = winter, vo = spring, and ha = autumn,
have been used.
The causes of geological-event Jloods are as follows:
At several places in the table a geological-event
flood is indicated. These need special explanation.
The cornice avalanche in the Bægisá river gorge
some distance above the recording station dammed
the river for a while, the accumulated water then
forcing through in a sudden burst. The flood in the
Jökulsá á Dal river at Hjardarhagi was associated
with surging of Brúarjökull glacier; note that on
lOth October 1964, the date of the flood, an auto-
matic recorder had not been installed at Brú a
Jökuldal. On the rivers Hvalá in Ofeigsíjördur,
Eyvindará in Hérad and Tungnaá in Rangárvalla-
sýsla ice dams gave away causing floe-run in associ-
ation with rain- and meltfloods. The largest flood on
the Vestri-Jökulsá in Skagafjördur is designated
type 5; here a large step-burst exceeded all other
flood types.
The run-off values for geological-event floods and
man-made floods are given in brackets since these
floods are independent of the size of drainage area.
In a similar way the run-offvalues ofrivers inastate
ofjökulhlaup or step-burst are of little value.
The largest flood which has occurred in Iceland
since systematic hydrological investigations began
in 1947 was the GrímsvötnJökulhlaup of 1954. This
was 10,500 m3/s ± 20%. The Grímsvötn Jökul-
hlaups don’t really belong in the table, as observ-
ations elsewhere in this paper suggest.
The ratio HQ/MQ gives some indication of to
what extent a river is a flood river.
ÁGRIP
FLÓÐ OG FLÓÐHÆTTA
Sigurjón Rist, Orkustojnun
Flóðum í dragám, lindám og jökulám er lýst á 50
vatnasvæðum víðsvegar um land og þeim skipt í
eftirtalda sjö flokka: 1. regnflóð, 2. leysingaflóð, 3.
regn- og leysingaflóð, 4. jökulhlaup, 5. þrepahlaup,
6. mannvirkniflóð og 7. viðburðaflóð. Flóðahættu er
lýst lauslega á nokkrum stöðum hér á landi, en
allítarlega á vatnasvæði Hvítár og Ölfusár (Greinin
er að mestu þýðing á grein höfundar Flóð og flóða-
hætta í bókinni Eldur er í Norðri, útg. Sögufélagið
1982).
132 JÖKULL 33. ÁR