River erosion, landslides and slope development in Göta River: A study based on bathymetric data and general limit equilibrium slope stability analysis

There can be many different triggers and combinations of factors that set of a clay landslide. One of those factors is river erosion, which through the removal of soil material causes changes in the stress distribution in the adjoining soil. These changes might be large enough to unsettle the static equilibrium that makes a slope stable, and subsequently cause a slope failure and landslide. Even though river erosion generally is regarded as a major factor when it comes to landslides along Göta River, relatively little is known about how the river erosion more precisely affects the geometry of a slope profile, and how these geometric changes affects the slope stability factor of safety. This thesis examines the characteristics of the river erosion in Göta River, including estimations of erosion rates over time, by analysing and comparing geophysical and bathymetric data from 2003 and 2009. The bathymetric data has also been used in conjunction with a topographic land model in a geographical information system to study and classify different types of slope geometries along the river. Based on previous geotechnical field investigations at two locations along the river, a couple of different slopes where modelled in the geotechnical software Slope/W to determine how the slope stability factor of safety varies with the established characteristic erosion in this thesis. The method used in the slope stability calculations was the general limit equilibrium (GLE) method. The different elements in this thesis span many disciplines within earth science and engineering. Hence the result can be seen as an example of how different data, methods (such as the use of geographical information systems and geophysical data) and disciplines have been combined to produce a general theory on the geomorphologic development of slopes in Göta River, and in particular how the river erosion affects the geometry and stability of the slopes in the river. One of the more specific results in this thesis show how river erosion in Göta River mainly occurs along trenches on the river bottom and along the slope toe, and that the erosion is localized and unevenly distributed and seemingly not a direct function of the hydraulic shear stress caused by the water flow. This is in contrast to some previous notions based on turbidity measurements where the river erosion has been thought of as a more uniform process occurring all over the river bed, which leads both to overand underestimation of erosion rates depending on the location.