Pascal Locat

Numerical analysis of the mobility of the Palos Verdes debris avalanche, California, and its implication for the generation of tsunamis

Abstract Analysis of morphology, failure and post-failure stages of the Palos Verdes debris avalanche reveals that it may have triggered a significant tsunami wave. Our analysis of the failure itself indicates that the slope is stable under aseismic conditions but that a major earthquake (with a magnitude around 7) could have triggered the slide. A post-failure analysis, considering the debris avalanche as a bi-linear flow, shows that peak velocities of up to 45 m/s could have been reached and that the initial movement involved a mass of rock less than 10 km wide, 1 km long and about 50–80 m thick. Initial wave height estimates vary from 10 to 50 m. Tsunami waves propagating to the local shoreline would be significantly smaller. Such a range demonstrates our lack of proper knowledge of the transition from failure to post-failure behavior related to mass movements. Further investigations and analyses of terrestrial and submarine evidence are required for a proper hazard assessment related to tsunami generation in the Los Angeles area.

Inventory of Large Landslides in Sensitive Clay in the Province of Québec, Canada: Preliminary Analysis

The Ministere des Transports du Quebec is currently conducting a comprehensive inventory of large historical landslides that have occurred in sensitive clays in Quebec. One hundred and eight cases have been identified during the period from 1840 to 2012 through historical documents or aerial photos. Detailed data have been collected for numerous sites with respect to pre- and post-failure topography, soil properties, the position of the failure surfaces and the groundwater flow regime. In addition, piezocone soundings have been conducted at approximately 50 other undated large landslide sites in various regions in Quebec. This article summarizes the preliminary analysis of these data.

Characterisation of a Submarine Flow-Slide at Pointe-Dufort, Saguenay Fjord, Quebec, Canada

The submarine flow-slide of Pointe-du-Fort is situated on the south shore of the Saguenay Fjord, near the mouth of the Baie des Ha!Ha!, Quebec, Canada. About 1.5 million m3 of material, constituted of clayey silt rhythmites and thin sand layers, were involves in the slide. Surface of rupture would have reach a till contact. Seismic surveys over the displaced mass revealed a multiphase accumulation of the debris. Stratigraphic position of the debris link the event to the 1663 (Ms~7) earthquake. The slide was approximately dated to be over 260 years old (from present day) using sedimentation rates.

Linking Geotechnical And Rheological Properties From Failure To Post-Failure: The Pointe-Du-Fort Slide, Saguenay Fjord, Québec

The Pointe-du-Fort submarine mass movement likely took place at the time of the with a run out distance of 1070m and a final flow thickness of 10-15m resting on a slope of 1.4 degrees. The slide involved about 1.95 Mm 3 of clayey sediments from an original slope of 24 degrees. The slide took place in normally consolidated sediments composed of stratified low organic Laflamme Sea clay at the base overlain by progressively more organic rich recent sediments. In situ strength testing and sampling on the tidal flat, morphological analysis and remolded strength of the debris lobe can be related to rheological tests to model the mobility of the debris. For the first time, it has been possible to link the mobility of a submarine slide with the characteristics of the sediments at the time of failure with no need to consider water content increase to explain the observed mobility.

The 1908 Disaster of Notre-Dame-de-la-Salette, Québec, Canada: Analysis of the Landslide and Tsunami

The landslide of 1908 in Notre-Dame-de-la-Salette, Quebec, was the deadliest event occurring in sensitive clays of Eastern Canada, causing 33 deaths. Of these, 26 are associated with the tsunami generated impact of water and ice on the opposite bank. A LiDAR survey of the sector, and a geotechnical investigation were carried out respectively in 2009 and 2010 to characterize this landslide. Covering an area of 6.5 ha, the soil mass carried is estimated at nearly 1.2 million m3. The paper describes the event, reports the results of the investigation and discusses the tsunami caused by the debris of the landslide. The tsunami approach includes modeling both the kinematics of the slide and the wave.

Parametric Analysis of the Mobility of Debris from Flow Slides in Sensitive Clays

Sensitive clays are prone to various types of landslides. Among these are flow slides that are able to affect hectares of land. Moreover, debris from these flow slides has a high mobility with run out distance of hundreds of meters, even in relatively flat areas, are quite common. In the context of hazard mapping, mobility of the debris is also an important factors to consider. In this context, a parametric analysis using simplified geometries is undertaken in order to evaluate the run out characteristics of these flows (such has the length of the run out area and the lateral spread of the debris) as a function of the rheological parameters such as the yield stress and viscosity. In order to proceed with the parametric analysis, a newly developed 3D numerical model was used.

Characterization and Post-failure Analysis of the 1980 Landslide in Sensitive Clays at Havre-St-Pierre, Québec, Canada

Early in the morning of April 3rd, 1980, approximately 1.15 × 106 m3 of soils from the coastal bluffs spread out over the tidal flat along the North Shore of the Jacques-Cartier Strait in the Gulf of St. Lawrence. This landslide had a width of 410 m and had retrogressed back up to 110 m from the slope crest, cutting through Highway 138, 9 km west of the town of Havre-St-Pierre. In this area, the coastal bluffs are approximately 23 m high and consist, from the bottom to the top, of a thick marine sensitive clay unit from the Goldthwait Sea, overlain by 3 m of sand and 3 m of peat on top. The debris morphology has a “thumbprint-like pattern” characterized by unbroken blocks of intact clay forming elongated ridges, surrounded by a mixture of remoulded clay, sand and peat, aligned parallel to the bluff and arched in the direction of the flow. The absence of obstacles and the non-channeled character of the flow path of the Havre-St-Pierre landslide provide a good opportunity to characterize and analyze the post-failure stage of this large landslide in sensitive clays.

Development of a Long Term Monitoring Network of Sensitive Clay Slopes in Québec in the Context of Climate Change

The Government of Quebec recently initiated the deployment of a vast groundwater pressures monitoring network in postglacial marine clays to document their variations in time and improve our understanding of the relationship between failure initiation and climate in clay slopes. This project aims at evaluating the impacts of climate change on clay-slope stability and how it can be integrated in landslide risk management to improve public safety. Hydrogeological data will be acquired at sites located throughout the Quebec Province’s post-glacial clay deposits to create a public georeferenced index of typical hydrogeological conditions. The project goes beyond the characterization of groundwater pressures and their variations in clay slopes. Indeed, slope deformation will be measured at several sites. Also, two sites in flat terrain will be instrumented in order to evaluate mechanical properties of clay layers in simple 1-D conditions and groundwater recharge. The unsaturated clay crust in slopes susceptible to superficial landslides will be characterized and instrumented. The current lifetime of the monitoring project has been set to a period of 25 years.

Reply to the discussion by Olsen and Stuedlein on “Use of terrestrial laser scanning for the characterization of retrogressive landslides in sensitive clay and rotational landslides in river banks”Appears in Canadian Geotechnical Journal, 47(10): 1164–1168.

In their discussion about aerial laser scanning (ALS) and terrestrial laser scanning (TLS), Olsen and Stuedlein present some issues than we did not address in our paper (Jaboyedoff et al. 2009a). Our study deals with compact landslide bodies and we had to overcome many related problems, such as shadowing, vegetation, site accessibility, and safety issues. In contrast, Olsen and Stuedlein deal with linear, clean features (coastal cliffs), which enable a completely different way of working. Nonetheless, their remarks are relevant and they were not addressed in our paper because it was beyond the scope of our study. Studies on landslide volumes and mechanisms do not in general need a high accuracy, while it is crucial for landslide movement monitoring. In their discussion, Olsen and Stuedlein give some examples of TLS applications in landslide studies in addition to those highlighted in our article. Most of them relate to coastal erosion and rockfalls in cliffs, and only a few relate to landslides. For the sake of completeness of the literature overview, TLS has also been widely used for