Lahar flows down the Eyjafjallajökull south slopes
On wednesday May 19th, rain-triggered debris flow filled the channel of Svaðbælisá river and caused swelling of other rivers in the area. All the rivers from Holtsá to Skógá became colored by mud. The event was first detected about 9 o'clock in the morning by heavy noise at the farm Thorvaldseyri. When the farmer reached the river at the levee´s a few minutes later the levees were almost overrun by concentrated debris flow. He describes the flow like wet concrete, which is typical for lahar flow with sediment concentration of 60 % or more. The farmer estimated that the top discharge was over at 10 o´clock.
Scientists from Meterological office and a student from Earth Science Institute estimated the sediment load up to 50 % at 14 o´clock. According to this, the flow had then undergone downstream transformation to hyperconcentrated flow. About two hours later the flow was even more diluted and had transformed to muddy streamflow with less than 20 % sediment load. The flood level from the existing water surface was 50 cm but there was still excess water in the channel. Pulses or waves in the flow were detected every 1-2 minutes.
The debris flow originated in the slopes of the glacier as verified by radar image taken from TF-Sif . It shows a sliding area of a few square kilometers, and two smaller areas. Heavy rain on the glacier has saturated the ash-layer on the glacier and triggered this extensive sliding of volcanic mud down the slopes to the draining rivers of the area.
|Photograph taken 19 May at 16:57.
SAR Radar image from TF-SIF, taken 19 May at 11:03. Copyright: The Icelandic Coast Guard.