Jökull - 01.12.1989, Side 97

Summary
THE VOLUME OF GRÆNALÓN AND
CHANGES IN THE SIZE AND FREQUENCY
OF J ÖKULHLAUPS
A digital map has been compiled of the ice-
dammed lake Grænalón (Fig. 1) and the area and
volume distribution with elevation has been
calculated (Fig. 2). The digital map was derived
from aerial photographs taken on September lOth
1986, and from a section across the lake bed which
was surveyed when it drained completely in Sept-
ember 1935. Surface features were copied from the
AMS-map of 1946. The position of the edge of the
glacier in Grænalón has been almost the same since
1954 (cf. Fig. 3). Therefore, the results given in Fig.
2 are applicable over the last four decades. During
this period, the jökulhlaups have occurred when the
lake lifted a shallow ice barrier at the inlet in the SE
comer of the lake. These jökulhlaups have run
alongside the glacier down to Skeiðarársandur. The
jökulhlaups have occurred once, even twice, in a
year and typical values have been a total volume of
200T06 m3 and maximum discharge of 1,500-
2,000 m3/s. The lake area has varied from 10 to
15 km2 and the lake level from 560 to 580 m a.s.l.
Earlier this century, up to the late 1950’s, the ice
barrier was thicker than it later became (cf. Fig. 4)
and the jökulhlaups escaped through waterways
which were located at the deepest part of the lake's
ice barrier and ran subglacially all the way down to
Skeiðarársandur.
The total volume of v'ater that drained from the
lake in a jökulhlaup in August-September 1986 is
estimated 500T 06 m3. Changes in Grænalón during
this jökulhlaup are shown in Table I.
Grænalón 1947
viewed toward
the west.
Photo Sigurður
Þórarinsson.
Grœnalón 1947
horft til vesturs.
Mynd Sigurður
Þórarinsson.
JÖKULL, No. 39, 1989 95