Jökull - 01.12.1983, Page 133

Comparison of the maxirnum and minimum of
month values clearly shows whether the dominant
component is due to the spring-fed or direct run-oíf
rivers. In spring-fed rivers the difference is small,
e.g. in the spring-fed Ytri-Rangá at Arbæjarfoss the
maximum month is 8.8% and the minimum month
7.8% of the annual value. In the case of direct
run-oíf rivers the difference between seasons is
greater, e.g. the table shows that the Bessastadaá
river (Eastern Iceland) transports 36% of the
yearly flow in May, but only 2% in February.
© Mean yearly flood MHaQ. By yearly flood is
meant the largest flood in each year. The larg-
est flood is that tlood which has the highest
peak discharge, irrespective of the total
amount of water in the flood. The latest year
which the table refers to is 1980. Before the
automatic water leveí recorder was put into use
it was usually impossible to decide on the
highest peak discharge accurately so that the
mean flood size does not refer to these years.
© Largest measured flood, HQ i.e. the largest
tlood in the period ofrecords. In addition to the
flood being given as m'/s, the run-oíf, Hq per
km-> is also shown. ve = winter. ha = autumn
© Calculated floods. There are two columns
which should show what size of floods can be
expected at 50-year and 100-year average
intervals (return period). The user of run-off
data to wltich floods are of most importance is
the Public Roads Administration; this body
regards the problem as of prime importance
and has investigated the records of floods
specially. The size of 50-and 100-year floods
are calculated by L. Al. Thorsteinsson,
engineer of PRA. It should be noted that these
values are not to be regarded as the so-called
design flood of that authority.
It should be clear that it is important to classify
floods by type. The repetition intervals ofparticular
types are very variable. Investigation concerning
Fig. 7. Jökulhlaup i.e. Glacier burst. Skaftá river on 7,h Jan. ’82. Noisy water, ice run. The river has within
one and a halfday increased from 50 to 750 m3/s. (Photo RAx).
7. mynd Jökulhlaup í Skaftá. 7. jan. 1982.
JÖKULL 33. ÁR 131