Jökull - 01.01.2021, Side 13

The 1918 Katla eruption
Figure 8. Isopach map of Mýrdalsjök-
ull and its vicinity (blowout from Fig-
ure 7). Red dots indicate farms that
where seriously damaged by the tephra
fall. The larger dots indicate farms
that were uninhabitable for >1 year or
permanently laid waste, the remaining
farms recovered within a year (Sveins-
son 1919). For damage by the jökul-
hlaup see Figure 9. Tephra thickness on
the glacier after M. T. Gudmundsson et
al., (this issue). – Þykkasti hluti Kötlu-
gjóskulagsins frá 1918. Rauðir depl-
ar tákna bújarðir sem urðu fyrir tjóni
vegna gjóskufallsins 1918, stærri depl-
arnir tákna jarðir sem fóru í eyði um
tíma eða alveg (Gísli Sveinsson, 1919).
Sjá 9. mynd um tjón vegna jökulhlaups-
ins. Gjóskuþykkt á jökli er samkvæmt
Magnúsi T. Guðmundssyni o.fl. í þessu
hefti.
2000) and down the Kötlujökull outlet glacier. The
jökulhlaups follow the route of the glacier down to the
Mýrdalssandur flood plain. The 1918 Katla eruption
was accompanied by a large jökulhlaup onto Mýrdals-
sandur with an estimated 300.000 m3/s peak discharge
of meltwater, suspended sediment and ice (Tómasson,
1996).
The flood routes and the course of events dur-
ing the floods on October 12 will be briefly de-
scribed below. The compilation is based on Sveinsson
(1919), Jóhannsson (1919), Karlsson (1994), Tómas-
son (1996) and several others. Aerial photographs
from 1945 and 1946 were also consulted.
First phase
The first of the two phases of the jökulhlaup on Oc-
tober 12 flooded a large area (Figure 9). It can be di-
vided into three forks: The southern fork, which ad-
vanced on the western part of Mýrdalssandur, down
Múlakvísl and Sandvatn rivers: the northern fork,
which flowed down Leirá into Hólmsá, Skálm and
Kúðafljót rivers; and a small middle fork that flowed
onto Mýrdalssandur north of Hafursey.
Southern fork: At about the same time as the erup-
tion plume was noticed (∼3 PM) a large flood was
seen advancing towards the south across the Mýr-
dalssandur plain (Figure 9), following the bed of
the Múlakvísl river and that of the Sandvatn river
(Jóhannsson, 1919; Sveinsson, 1919). From Vík, a
wave of brown floodwater carrying large icebergs was
seen flowing along the Múlakvísl river into the sea,
forming a spit of sediment and ice. At Hjörleifshöfði
hill (a former headland) on Mýrdalssandur, the noise
from the floodwater was noticed shortly before 3:30
PM to the east of the hill. By then the flood had
filled the bed of the Sandvatn river and soon after-
wards the flood had surrounded the hill and reached
the sea (Jóhannsson, 1919), 15–20 km from the edge
of Kötlujökull. The flood was very turbulent and cov-
ered by large and small icebergs and blocks except
where the currents were strong. Many icebergs had
stranded on the sandur, other were carried into the sea.
This first phase lasted for 2–3 hours.
JÖKULL No. 71, 2021 11