Iceland volcano watch stands down

Jökulhlaup from Grímsvötn subsides

The earlier concern about meltwater flooding from the Grimsvotn glacial lake in Iceland which could have signalled that the volcano underneath was about to erupt has subsided.

Bridge over Gígjukvísl

Figure 1. The jökulhlaup from Grímsvötn: image vedur.is

The jökulhlaup (glacial outburst flood) from Grímsvötn that began near the end of October is now coming to an end. The flood reached a maximum level shortly after noon on November 3, and scientists from IMO visited the site on that day to study the effects of the flood on the region adjacent to the ice margin. Two IMO technicians have performed regular discharge measurements on the bridge over the river Gígjukvísl throughout this week (Figure 1) and the results from their measurements are shown in Figure 2.

Figure 2. Discharge (m3/s) measurements at Gígjukvísl bridge: From the curve the total amount of floodwater is estimated 0.45 km3

The discharge curve is typical for jökulhlaups from Grímsvötn that do not result from volcanic activity: Over the course of several days, the amount of water flowing through an ice tunnel at the glacier bed steadily increases. Loss of frictional heat from the floodwater causes melting of the tunnel walls, thereby increasing the flow capacity of the tunnel.

In past centuries, most jökulhlaups from Grímsvötn have entered the course of the river Skeiðará. This time, however, floodwater that emerged from beneath the eastern part of the glacier went westwards along the glacier margin and then entered the river Gígjukvísl. Skeiðará has deposited very large amounts of sediment on the eastern part of Skeiðarársandur plains over the centuries, increasing the elevation of the sandur area there relative to the central part. In addition, the glacier has carved a trench during times of advance. Thus, it was clear that retreat of the glacier over the past 15 years would sooner or later lead to a drastic shift in the direction of meltwater flow from this part of the glacier. In the summer of 2009, this shift occurred and water has ceased to enter the course of Skeiðará.

 

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