Jökull - 01.12.1967, Page 10

Discharge in m^s
i.e. at the southeast corner of Höfclabrekka.)
At the place where the old bridge was and
downstream from it the Múlakvísl flows in a
gorge borclered on either side by cliffs with a
fairly even stream becl making flow measure-
ments favourable. The width of the streambecl
was 60 metres and the depth of water 4.8
metres when the hlaup was at maximum. The
measurement was carried out on a 246 metres
long stretch where the fall was 1.80 metres.
Usittg roughness coefficient of M = 40 in the
Manning formula a flow of 2500 m3/sec ± 25%
was obtained.
A smaller part of the jökulhlaup broke out
a little farther east or more precisely 3.5 ktn
east of Rjúpnagil. The water did not come
to the surface at the edge of the glacier but
about 100 metres above its snout. The flow
rate and total volume of this burst was less
than that of the more westerly burst as seen
in Fig. 2. When the more westerly jökulhlaup
water, flowing into the Múlakvísl, was at
maximum some of the water came also to the
surface in the glacier 100—200 metres above
its edge, but the greater part flowecl through
tlie water channel emerging at its snout.
The more easterly burst water falling into
the Skálm flowed across flat alluvial plains
from the glacier. Thus viewecl front an aero-
plane the flow rate ol' the burst appeared high
ancl dangerous to the inhabitatecl area of Alfta-
ver. A little above the bridge on the Skálm the
water flowed into a straight river bed although
rather shallow and with an even lava bottom.
At this spot it was possible to measure the
flow rate. The flood swept away the bridge
on the Skálm. Later it was rebuilt on the same
spot.
THE VOLUME OF THE CAULDRONS
The expedition carrying out measurements
of the thickness of Mýrdalsjökull could not
study the changes on the ice cover simultaneous
with the jökulhlaups due to bad weather ancl
poor visibility.
On July 4 tlie author went onto Mýrdals-
jökull on foot with two farmers from the neigh-
Fig. 2. Hydrograph of the jökulhlaup on June 25, 1955. The total volume of the ice cauldrons
is compared to the surplus water discharged by the rivers Múlakvísl and Skálm.
Mynd 2. Linurit yfir rennsli jökulhlaups i Múlakvisl og Skdlm 25. júni 1955. Rúrnmdl ketil-
siga er borið saman við magn hlaupvatnsins.
244 JÖKULL 17. ÁR