A multidisciplinary study of shoreline landslides:From geological development to geohazard assessment in the bay of Trondheim, Mid-Norway

Abstract

The periodic occurrence of landslides in near-shore environments is common worldwide and poses a constant threat to coastal communities. Multidisciplinary studies on a regional scale have shown to be fruitful in understanding landslide processes in the submarine realm and on land, but have seldom been performed in near-shore areas. The present study integrates geological, geotechnical, geophysical and geomorphological data from land and sea in order to better understand near-shore mass wasting processes, their origin and their development. The study area for the work presented in this thesis is situated in Mid-Norway along the shoreline of Trondheim. Here, the geological and historical records illustrate that landslides are recurrent phenomena. In the last 15 years, land reclamation along the shore has been intensified for construction work and building activity. This has increased the concerns about the stability of the fjord-deltaic sediments and has highlighted the need for a better understanding of the mass movement processes.

The integration of high resolution seismic surveys and detailed bathymetric maps acquired in the fjord, along with sedimentological, geotechnical and 14C datings data from both land and fjord, have given new inputs for establishing the stratigraphy and geological development in the bay of Trondheim. The stratigraphic model shows that bedrock and possibly till is overlain by thick glacio-marine and marine clay deposits, covered by fjord-deltaic sediments. The three main units recognized in the upper part of the valley fill are from bottom to top: (1) prodeltaic, fjord marine sediments composed of silty sands and silts with decimeter- to meter-thick clay-rich beds, (2) steeply dipping delta foresets generally composed of loosely packed medium to fine sand that pinch out into the fjord, and (3) fluvial, littoral and sub-littoral deposits mostly composed of sandy and gravelly material.

The geological model was combined with detailed morphological analyses of slide scars and limit equilibrium analyses in order to understand landslide failure mechanisms in the near-shore area. Results have shown that many of the investigated landslides start as translational failures with glide planes associated to clay-rich beds in the prodeltaic sequence. These beds were deposited rapidly from turbiditic flows originating from large landslides of terrestrial origin. The provenance and mode of deposition of the weak layers explains their clay-rich nature with higher water content, greater plasticity, higher sensitivity and lower shear resistance compared to the surrounding, more slowly deposited successions.

The main pre-conditioning of near-shore landslides in the area of Trondheim is a result of longterm loading onto the weaker clay-rich beds by delta progradation, beach sedimentation and manmade fills. Over-steepening from erosion/sedimentation and unfavourable groundwater conditions are other factors that also play an important role for destabilizing the shoreline slopes. Once failure is initiated, the mobility and retrogressive behaviour of the landslide masses depends on the surrounding stratigraphy and sediment properties, orientation of the layers and the morphology of the seafloor. In the western part of the embayment, the displaced masses tend to accelerate into a large channel system from the delta front and towards the deep fjord basin. These flows are erosive along the channel and may be tsunamigenic.

Finally, the results presented herein illustrate the importance of detailed morphological analyses, combined with a robust geological model including the physical/geotechnical properties of sediments on- and off-shore, in order to perform proper shoreline slope stability assessment