Subaqueous Mass movements in Lake Mjøsa, Norway
Abstract
Lake Mjøsa is Norway’s largest lake (117 km long, area: 365 km2, greatest depth 468 m). It lies in a depression formed by glacial over-deepening of the southward extension of the Gudbrandsdalen glacial valley. It is therefore characterized by typical glacial ”U-shaped” cross sections. In the study area it almost annually covered by winter ice. The present study arose from pipeline breakages near the city of Gjøvik that ROV inspections demonstrated were caused by subaqueous mass movements. The purpose of the study was suggest new pipeline routes based on slope stability evaluations from morphological analyses supplemented by simple geotechnical testing of core samples. Data consisted of a multibeam bathymetric survey of the study area, a grid of sub-bottom profiler lines (SES 2000) and 9 gravity cores. The morphological analysis of the multibeam data revealed that lake bed has slopes between ~15° and ~35° from the coastline and into the central deep (~250 m in the study area). Numerous flow paths were identified as were areas offshore from river mouths with clear evidence of downslope mass transport. Several slide scarps from 1 to 3 m were also discovered. Slope stability analyses reveal safety factors close to 1 for the steepest gradients. The submarine slides are therefore thought to have been caused by density currents of cold water formed during winter. This dense water flowed down slope along the flow paths and is beleived to erosively undercut the soft seafloor sediments which thereby lost their downslope support. New pipeline corridors were drawn to avoid such scenarios.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2010
- Bibcode:
- 2010AGUFMOS13E1294F
- Keywords:
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- 3022 MARINE GEOLOGY AND GEOPHYSICS / Marine sediments: processes and transport;
- 3070 MARINE GEOLOGY AND GEOPHYSICS / Submarine landslides