Underwater mass movements in Lake Mjøsa, Norway

Abstract

This study was initiated because underwater mass movements in Lake Mjøsa, Norway had caused utility pipeline breakages. Multibeam bathymetry, sub bottom profiler data and samples were acquired to allow morphological interpretations of the processes leading to the mass movements. The underwater slopes of the lake generally show gradients of 15–20°, but exceed 30° in places. The sediment thickness above acoustic basement interpreted from sub bottom profiler data show that the accumulation rate in the central lake basin is about 2 mm/year. Numerous channels are seen on the bathymetry as well as several slide scarps that are about 2 m high. The channels are interpreted to be caused by dense water cascading, probably induced by winter cooling. Calculations based on geotechnical tests of samples indicate that sediment layers in excess of 2 m have a factor of safety less than 1.5 on slopes above 30°. Triggering of slides may thus be spontaneous due to sedimentation, but may also be triggered by oversteepening due to erosional channeling.