Traces, causes, and consequences of the Glacier Lake Outburst Floods from Austerdalsisen, Mo i Rana, between 1941 to 1958.

Abstract

Glacier lake outburst floods (GLOFs) present themselves as large-scale natural hazards being highly destructive and potentially life-threatening. This study investigated traces, causes and consequences of the GLOFs that unfolded at Austerdalsisen before, within and after the Little Ice Age (LIA). The fieldwork was conducted in Røvassdalen, Svartisdalen and Austerdalen during July-August 2023. Excavations in Røvassdalen and Svartisdalen were conducted to study the characteristics of the GLOF sediments, as well as suggesting the approximate coverage of the GLOFs. Cores were extracted from the Lake Litl Røvatnet to provide a more in-depth study of the GLOF sediments by the use of laboratory methods such as magnetic susceptibility (MS), X-ray fluorescence (XRF) and computed topography (CT). Remote sensing was utilized to calculate GLOF coverage in Røvassdalen and the approximate GLOF volume of 1954. Measurements of MS, XRF, loss-on ignition (LOI), dry bulk-density (DBD) and radiocarbon dating (C14) from Jansen et al (2018) provided crucial data for this thesis.

The excavations, substantiated by flood coverage LIDAR-calculations, revealed a likely maximum extent of the GLOFs in Røvassdalen. Due to topographical differences and the nearby fluvially active Blakkåga river, the GLOFs did not cover the whole north-eastern side of Røvassdalen. The presence of a topographical narrowing at Bjørnneset further south in Røvassdalen, amplified the GLOFs up-valley by limiting the drainage rate, causing the floodwaters to reach a height of up to 3 meters above Litl Røvatnet.

MS, XRF and CT data from the cores of LITROV-123 and LILROP-123 revealed two distinct GLOF periods; a long-lasting period of minerogenic sedimentation before-, and a shorter period after the LIA, compared with Jansen et al’s. (2018) MS-, XRF-, LOI-, DBD- and C14-data. These minerogenic layers consist of densely packed finer-grained sediments that were deposited at Litl Røvatnet, due to it being an efficient sediment trap.

The existence of major glaciofluvial terraces on the western side of Austerdalsvatnet indicate a change in drainage points from the glacier. Although it presently drains through the eastern overflow gap at 212 m a.s.l. because the glacier has retreated too far to dam up the lake, it used to drain through the western overflow gap at 273 m a.s.l. in between the GLOFs. Jansen et al (2018) provides a reconstruction of the glacier fluctuations since before the LIA, giving valuable insights in the history of GLOFs in the area.

Because of the known amounts of GLOFs between 1941 and 1958 (16), an average sedimentation rate of 1.0125 cm was calculated. This average was then used to roughly approximate 82 potential pre-LIA GLOFs.

LIDAR imagery with a resolution of 1 meter was used to accurately calculate the volume of Austerdalsisen before the GLOF in 1954.

Finally, numerous newspaper articles from the time of the GLOFs were examined to find out how the local population was affected. These articles reveal a lack of urgency from the governing bodies, as well as a lack of funding, to regulate the destruction of the GLOFs. It was not until the possibility of an imminent life-threatening GLOF that funding for a relief tunnel was given.