Geohazard Assessment related to Submarine Instabilities in Bjørnafjorden, NorwayC.S. Forsberg, B. Carlton, M. Vanneste, C.F. Forsberg, S. Knudsen, F. Løvholt, T. Kvalstad, G. Sauvin – NGI H. Kjennbakken*, M.A. Mazhar**, S. Degago – SVVH. Haflidason – UiB

Currently:* Norconsult** COWI

Outline

IntroductionAvailable data1D analyses2D analysesLandslide run-out analysesAge-datingSummary

Intr

oduc

tion Framework: Ferry-free Coastal Highway (E39)

© NPRA

Bjørnafjorden – Side-anchored floating bridgeIn

trod

uctio

nAnchoring of four pontoons with eight anchors each 32 anchorsRedundancy: two anchors in one cluster out of play

Images © NPRA

Bjørnafjorden – Multi-span suspension bridgeIn

trod

uctio

n2 towers supported by floating platforms anchored with 16 tethersRedundancy: Two tethers out of play

Images © Baezeni/NPRA

Implications for design (Risk)In

trod

uctio

nMass movements (and their consequences) are the main geohazardFairly unique situation and some complications:

Many possible impact pointsSource areas can be found all over, from all anglesRelocation and avoidance not realistic

Image © Baezeni/NPRAImage © NPRA

Key Data AvailableDa

ta

Geophysical data:• Multibeam echosounder seafloor topography

• Sub-bottom profile data sub-seafloor information

Geotechnical data:• 5 locations with CPTU

and/or sampling data

1 age dating location

Bathymetry & slope angleDa

ta

Bathymetry & sediment thicknessDa

ta

Bathymetry & mapped landslide featuresDa

ta

UtløpsområdeKildeområde

1D analyses – Screening Tool1D

ana

lyse

s

2D Plaxis Analysis, incl. Retrogression2D

Ana

lyse

s

Bedrock/stiff material

Marine clay

Water

2D Plaxis Analyses2D

Ana

lyse

s

Landslide Back-analysisLa

nd sl

ide

runo

ut

Back-analysisLa

nd sl

ide

runo

utNo rheological parametersSome guestimates:

Evacuation ~ 20,000 m2

Deposition ~ 45,000 m2

Run-out ~ 300-350 mHeadwall ~ 5 mThickness < 10 mVolume (displaced) ~ 100,000 m3

Bedrock/stiff materialMarine clay

Water

Land

slid

e ru

nout Landslide dynamics – Back-analysis

Landslide dynamics – Quasi-3DLa

nd sl

ide

runo

ut

Landslide dynamics – forward modellingLa

nd sl

ide

runo

ut

Age dating resultsCore descriptionsEvents 1 & 2: debris flow or mud flow deposits

SummaryAs part of the geohazard assessment we conducted:1. 1D static and pseudo-static and fully dynamic slope stability analyses2. 2D slope stability analyses for 40 profiles 3. Back-analysis of a palaeolandslide;4. Landslide dynamic simulations calibrated using results from the back

analysisResults show that the soils in many locations are unstable and could cause debris flows with long runout distancesGeophysical data support this interpretationNeed more information about the timing and triggering mechanisms of the landslides

Acknowledgments

The audience for kindly listening

Dof SubSea for geophysical data acquisition

NPRA (SVV) for permission to publish this work

NGI colleagues: Dieter Issler, Jihwan Kim, Noel Boylan, Amir Kaynia

GMT – Generic Mapping Tools

NGI’s R&D funds for developing in-house routines (screening tool, landslide dynamics, …)