Jökull


Jökull - 01.01.2021, Side 13

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
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