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• 1. [Article] Holocene History of Deep-Seated Landsliding in the North Fork Stillaguamish River Valley From Surface Roughness Analysis, Radiocarbon Dating, and Numerical Landscape Evolution Modeling

  Documenting spatial and temporal patterns of past landsliding is a challenging step in quantifying the effect of landslides on landscape evolution. While landslide inventories can map spatial distributions, ...

  Citation × Citation Citation Abstract Copy Title: Holocene History of Deep-Seated Landsliding in the North Fork Stillaguamish River Valley From Surface Roughness Analysis, Radiocarbon Dating, and Numerical Landscape Evolution Modeling Author: Booth, Adam M., LaHusen, Sean R., Duvall, Alison R., Montgomery, David R. Year: 2017 Documenting spatial and temporal patterns of past landsliding is a challenging step in quantifying the effect of landslides on landscape evolution. While landslide inventories can map spatial distributions, lack of dateable material, landslide reactivations, or time, access, and cost constraints generally limit dating large numbers of landslides to analyze temporal patterns. Here we quantify the record of the Holocene history of deep-seated landsliding along a 25 km stretch of the North Fork Stillaguamish River valley, Washington State, USA, including the 2014 Oso landslide, which killed 43 people. We estimate the ages of more than 200 deep-seated landslides in glacial sediment by defining an empirical relationship between landslide deposit age from radiocarbon dating and landslide deposit surface roughness. We show that roughness systematically decreases with age as a function of topographic wavelength, consistent with models of disturbance-driven soil transport. The age-roughness model predicts a peak in landslide frequency at ~1000 calibrated (cal) years B.P., with very few landslide deposits older than 7000 cal years B.P. or younger than 100 cal years B.P., likely reflecting a combination of preservation bias and a complex history of changing climate, base level, and seismic shaking in the study area. Most recent landslides have occurred where channels actively interact with the toes of hillslopes composed of glacial sediments, suggesting that lateral channel migration is a primary control on the location of large deep-seated landslides in the valley. Title: Holocene History of Deep-Seated Landsliding in the North Fork Stillaguamish River Valley From Surface Roughness Analysis, Radiocarbon Dating, and Numerical Landscape Evolution Modeling Author: Booth, Adam M., LaHusen, Sean R., Duvall, Alison R., Montgomery, David R. Year: 2017 Documenting spatial and temporal patterns of past landsliding is a challenging step in quantifying the effect of landslides on landscape evolution. While landslide inventories can map spatial distributions, lack of dateable material, landslide reactivations, or time, access, and cost constraints generally limit dating large numbers of landslides to analyze temporal patterns. Here we quantify the record of the Holocene history of deep-seated landsliding along a 25 km stretch of the North Fork Stillaguamish River valley, Washington State, USA, including the 2014 Oso landslide, which killed 43 people. We estimate the ages of more than 200 deep-seated landslides in glacial sediment by defining an empirical relationship between landslide deposit age from radiocarbon dating and landslide deposit surface roughness. We show that roughness systematically decreases with age as a function of topographic wavelength, consistent with models of disturbance-driven soil transport. The age-roughness model predicts a peak in landslide frequency at ~1000 calibrated (cal) years B.P., with very few landslide deposits older than 7000 cal years B.P. or younger than 100 cal years B.P., likely reflecting a combination of preservation bias and a complex history of changing climate, base level, and seismic shaking in the study area. Most recent landslides have occurred where channels actively interact with the toes of hillslopes composed of glacial sediments, suggesting that lateral channel migration is a primary control on the location of large deep-seated landslides in the valley. Close

• 2. [Article] Mapping of Unstable and Potentially Unstable Slopes in Sogne og Fjordane

  For the past three years NGU has worked on 25 unstable and potentially unstable rock slopes in Sogn og Fjordane. In addition Fjærlandsfjord, Hyenfjord and Årdalsvatnet were systematically mapped for deposits ...

  Citation × Citation Citation Abstract Copy Title: Mapping of Unstable and Potentially Unstable Slopes in Sogne og Fjordane Author: Hermanns, Reginald L., Fischer, Luzia, Oppikofer, Thierry, Bøhme, M., Dehls, John F., Henriksen, H., Booth, Adam M., Eilertsen, R., Longva, O., Eiken, Trond Year: 2011 For the past three years NGU has worked on 25 unstable and potentially unstable rock slopes in Sogn og Fjordane. In addition Fjærlandsfjord, Hyenfjord and Årdalsvatnet were systematically mapped for deposits of prehistoric and historic rock slope failures onshore and with help of a bathymetry. Mapping on land included structural mapping of ten sites by on-site field mapping and nine sites by remote structural mapping using terrestrial laser scanning technology (TLS). Field work also included periodic monitoring of 14 sites using differential Global Positioning Systems (dGPS) and TLS at 4 sites. Synthetic Aperture Radar was app lied for the entire county but slide velocities could only be mapped out at one locality at Osmundneset (Gloppen municipality). A large amount of work was carried out on the slope east of Flåm in the Aurland valley, and results have been reported separately (NGU 20 11 .025). Three large instabilities have been discovered or taken into the monitoring program. These are Osmundneset in Gloppen municipality, Skrednipa in Sogndal municipality, and the Ovris valley in Vik municipality. The largest movements with 1.5 cm horizontal and 1.5 cm vertical displacement were measured on the instability with a volume of approx. 1 Mm3 in Ovris valley. Opening of cracks has been measured at that site also in the 1960’s, 1970’s and 1980’s. Our data suggest a slight acceleration of this instability. Acceleration was also documented for a 100.000 m3 large instability called Lifjellet, although velocity of that site is less than half of the velocity of the block in Ovris valley. However at that site a collapse of a rockslope with a volume of 25.000 to 30.000 m3 occurred only 19 years ago. Installation of continuous monitoring and early-warning systems should be considered at both of those sites as well as at similar sites where relatively small instabilities that might fail without a long acceleration phase are positioned above settlements (Gråberget in Høyanger municipality). Similarly all other monitored instabilities in Sogn og Fjordane are in the order of mm/yr and not considered to be critical on a short term. However, periodic monitoring has to be continued. Cosmogenic nuclide dating (CN) has been applied to determine ages of rockslide deposits in Fjærlandfjord (Sogne municipality) and at the slope E of Flåm (Aurland community), resulting in Late Pleistocene and Holocene ages. CN dating has also applied to the sliding planes at Skjeringahaugane (Luste municipality). The results indicate that the movement initiated at the beginning of the Holocene and is progressive. Long term slip rates are in the same order of slip rates measured by dGPS. Title: Mapping of Unstable and Potentially Unstable Slopes in Sogne og Fjordane Author: Hermanns, Reginald L., Fischer, Luzia, Oppikofer, Thierry, Bøhme, M., Dehls, John F., Henriksen, H., Booth, Adam M., Eilertsen, R., Longva, O., Eiken, Trond Year: 2011 For the past three years NGU has worked on 25 unstable and potentially unstable rock slopes in Sogn og Fjordane. In addition Fjærlandsfjord, Hyenfjord and Årdalsvatnet were systematically mapped for deposits of prehistoric and historic rock slope failures onshore and with help of a bathymetry. Mapping on land included structural mapping of ten sites by on-site field mapping and nine sites by remote structural mapping using terrestrial laser scanning technology (TLS). Field work also included periodic monitoring of 14 sites using differential Global Positioning Systems (dGPS) and TLS at 4 sites. Synthetic Aperture Radar was app lied for the entire county but slide velocities could only be mapped out at one locality at Osmundneset (Gloppen municipality). A large amount of work was carried out on the slope east of Flåm in the Aurland valley, and results have been reported separately (NGU 20 11 .025). Three large instabilities have been discovered or taken into the monitoring program. These are Osmundneset in Gloppen municipality, Skrednipa in Sogndal municipality, and the Ovris valley in Vik municipality. The largest movements with 1.5 cm horizontal and 1.5 cm vertical displacement were measured on the instability with a volume of approx. 1 Mm3 in Ovris valley. Opening of cracks has been measured at that site also in the 1960’s, 1970’s and 1980’s. Our data suggest a slight acceleration of this instability. Acceleration was also documented for a 100.000 m3 large instability called Lifjellet, although velocity of that site is less than half of the velocity of the block in Ovris valley. However at that site a collapse of a rockslope with a volume of 25.000 to 30.000 m3 occurred only 19 years ago. Installation of continuous monitoring and early-warning systems should be considered at both of those sites as well as at similar sites where relatively small instabilities that might fail without a long acceleration phase are positioned above settlements (Gråberget in Høyanger municipality). Similarly all other monitored instabilities in Sogn og Fjordane are in the order of mm/yr and not considered to be critical on a short term. However, periodic monitoring has to be continued. Cosmogenic nuclide dating (CN) has been applied to determine ages of rockslide deposits in Fjærlandfjord (Sogne municipality) and at the slope E of Flåm (Aurland community), resulting in Late Pleistocene and Holocene ages. CN dating has also applied to the sliding planes at Skjeringahaugane (Luste municipality). The results indicate that the movement initiated at the beginning of the Holocene and is progressive. Long term slip rates are in the same order of slip rates measured by dGPS. Close