Hazard mapping for the eastern face of Turtle Mountain, adjacent to the Frank Slide, Alberta, Canada

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1 Hazard mapping for the eastern face of Turtle Mountain, adjacent to the Frank Slide, Alberta, Canada C.R. Froese Alberta Geological Survey/Energy Resources Conservation Board, Edmonton, Canada M. Jaboyedoff and A. Pedrazzini Institute of Geomatics and Risk Analysis, University of Lausanne, Switzerland O. Hungr Department of Earth and Ocean Sciences, University of British Columbia, Vancouver, Canada F. Moreno Alberta Geological Survey/Energy Resources Conservation Board, Edmonton, Canada ABSTRACT: Since the occurrence of the Frank Slide in 1903, studies have identified potentially unstable volumes on Turtle Mountain adjacent to the original slide source. In the 1930 s, a mass of 5 million m 3, known as South Peak, was identified as the most unstable. Limited monitoring was carried out for close to 70 years. The monitoring system was upgraded at the turn of the millennium, as part of a comprehensive risk management program, concerning identified elements at risk in the valley. Intensive monitoring has now been carried out for 6 years and an early warning system implemented. With the acquisition of airborne LiDAR in 2005, a computer based structural mapping tool, COLTOP, was applied to better understand the structural controls on the Frank Slide, South Peak and other portions of the eastern face of Turtle Mountain. Recent studies by Jaboyedoff et al (2009), Froese et al. (In Press) and Pedrazzini et al. (2008) have highlighted structural controls on instabilities on the eastern face of the mountain that differ from those previously known for both the South Peak and for another portion of the mountain, Third Peak, which had not been identified in previous studies. The computer based structural models were confirmed with field structural mapping and a Sloping Local Base Level (SLBL) technique was used to estimate scenarios implying different volumes of potential future instabilities. To understand the level of activity of these volumes, an expanded monitoring network was installed in 2007 and It is expected that these portions of the mountain are moving at a very slow rate (sub-millimeter to millimeter per year) and conclusive evidence as to the level of activity may take years to ascertain. In order to better understand the extent of the potential impact of the hazards posed by these various unstable volumes (scenarios), dynamic runout analyses have been undertaken. A three-dimensional dynamic model has been calibrated using the experience from the Frank Slide and other regional rock avalanches and applied to the volumes derived from the SLBL to better define areas that are potentially susceptible to impact from landslides below the eastern face of Turtle Mountain. The results will be used to improve contingency plans within the hazard area. 1 INTRODUCTION After the 1903 Frank Slide, focus of landslide hazards specialists concerned with the locality shifted to potentially unstable areas on other portions of the eastern face of Turtle Mountain (Figure 1). Initially concerns regarding the stability of the North Peak (Daly et al, 1912) led to the relocation of the remaining portions of the Town of Frank. It was not until the 1930 s when studies by Allen (1931, 1932, 1933) mapped a series of deep fractures around the South Peak of Turtle Mountain and identified two different danger zones based on empirical estimates of run out for a 5 million m 3 volume of rock from the South Peak of the mountain 2 HAZARD CHARACTERIZATION Between 2003 and 2005, under a risk-management initiative of the Alberta Geological Survey, a stateof the art monitoring and warning system was designed and installed on the South Peak of Turtle Mountain (Froese et al., 2005, Moreno and Froese, 2006) in order to characterize and provide warning related to deformations along a series of deep fissures. The implementation and design of the monitoring system assumed that the rock slide volume and kinematics were as identified by Allen (1931) and sensors were installed to map movements consistent with these assumptions. The initial focus

2 Figure 1. Photo of the eastern face of Turtle Mountain showing the main four zones discussed in this paper. was to develop the monitoring infrastructure and provide upgrades to increase reliability of the system. With developing knowledge of the hazards and an increase in financial resources, more detailed studies were undertaken to expand the understanding of the stability of the mountain as a whole using both conventional field mapping and newly available remote sensing techniques. The most valuable piece of data used to characterize instabilities on the mountain has been the airborne LiDAR survey collected in June The specifications of this data are described in more detail by Sturzenegger et al (2007). By utilizing the 1 metre high resolution DEM (HRDEM) derived from interpolation of the ground returns, hill shade models were derived that not only highlighted the visible morphology of the mountain but also allowed for high level structural mapping of the features on the mountain. Colors orientation, zone selection to perform statistics and fault drawing could be performed but only in a 2D view (shaded relief view) (Figure 2). 2.1 DEM analysis The analysis of the topography, in particular in unvegetated rocky outcrops, reflects the structural features (joints, faults) affecting the slope (Jaboyedoff et al. 2005). Using the orientation of each of a series of quadrilateral surface segments (cells), a DEM can be represented by a 3D shaded relief that displays color representation of dip and dip direction by means of a Schmidt Lambert projection. In the current version of COLTOP 3D (Jaboyedoff et al, 2009), this is performed by plotting the normal vector of each DEM cell. The result is a colored shaded relief map combining slope and slope aspect in a unique representation. The slope orientation is coded by the Intensity Hue-Saturation system (HSI). Figure 2: (A) Example of colored DEM grid. Explanation of vector computation representing the center of one cell. (B) Color coding principle of a pole represented in a lower Schmidt-Lambert stereonet (from Jaboyedoff and Couture, 2003). Grid data, as well as unstructured point cloud data, can be represented and analyzed in 3D using the new COLTOP 3D version (Jaboyedoff et al. 2009). The slope morphology on the eastern limb of the anticline is significantly influenced by the orientation of the bedding planes, in particular those observed

under the Third Peak. As slope overburden deposits and vegetation are abundant, especially in the lower part of the slope, the utility of the DEM to detect structural features in this zone is reduced.

Yellow joint orientation (020/45) has also an important influence in slope morphology, in particular in the lower part between South Peak an Third Peak.

3 under the Third Peak. As slope overburden deposits and vegetation are abundant, especially in the lower part of the slope, the utility of the DEM to detect structural features in this zone is reduced. However, five main orientations have been detected (Table 1) using COLTOP 3D: Medium violet (Figure 3) represents the predominant bedding orientation under Third Peak area. Yellow joint orientation (020/45) has also an important influence in slope morphology, in particular in the lower part between South Peak an Third Peak. Red joint orientation (060/55) forms the main rocky outcrops and sub-parallels the slope surface. 2.2 Volume Estimation Based on the structures highlighted during field mapping and the COLTOP3D analyses, various zones of potential instability were determined. Their volumes were estimated and preliminary kinematic analyses undertaken to determine whether or not they were susceptible to movements. The volume estimation was performed using the Sloping Local Base Level (SLBL) method (Jaboyedoff et al, 2006). This method allows to identify a surface, above which a rock mass is assumed to be kinamatically capable of detachment. The SLBL method applied to a 3D surface (DEM) consists of replacing the altitude z ij of a DEM node by the mean value of the highest and the lowest node altitude among the four direct neighbors, when the altitude z ij is greater than the mean value (Jaboyedoff et al, 2006). When the difference between subsequent iterations of this process is near to zero, the analysis is stopped. This procedure allows determining a straight line between two fixed nodes. The described analysis is linear. A quadratic-like surface is obtained when a tolerance is added to the mean value obtained by the highest and the lowest neighbor at each node. This is shown schematically on Figure 4. Figure 4: The Sloping Local Base Level (SLBL) concept: A) Results of a linear SLBL computation without a tolerance, the result is a straight line. B) Using a tolerance, a curved surface can be obtained. Figure 3: A 3D view of the eastern face of Turtle Mountain, showing discontinuity orientations detected using COLTOP 3D. Table 1: Mean values of discontinuity sets detected in the eastern limb. Name (color / variation) Dip Direction Dip J1 (yellow,+/-10 ) J2 (red,+/-10 ) S0 (medium violet,+/-10 ) J3 (light blue,+/-10 ) J4 (dark blue,+/-10 ) For the South Peak area, the COLTOP and preliminary kinematic analyses were used to define two distinct zones: Upper South Peak and Lower South Peak. For the Upper South Peak, studies by Froese et al. (in press) highlighted structural control of joints J2, J3 and J6/SO on the deformations of a large wedge and subsidence feature and the control of J1 on toppling from the eastern face of the Upper South Peak. This is shown in Figure 5. For the Lower South Peak, a variety of potentially unstable volumes were estimated, ranging from 0.12 to 1.89 million m 3. These features were predominantly influenced by the intersection of Joints J2 and J3 and are shown in Figure 5. In the Third Peak area, the potentially unstable volumes on the Upper Third Peak were considered to be very small and therefore the focus was on estimating kinematically feasible mechanisms and volumes on the Lower Third Peak. Of specific interest in the Lower Third Peak area was the volume of a potential deep-seated slope deformation feature, (DGSD) outlined in the field by a large crack with accumulated displacement of about 20 cm, which was estimated to have a volume of 2.59 million m 3. Altogether, three potentially unstable zones were identified on the Lower Third Peak and are shown on Figure 5. The potential instabilities identified by the abovedescribed structural analysis were used in runout analyses described in the following.

4 Figure 5. Outlines of the twelve unstable zones indentified from the combined COLTOP/SLBL analysis of the Third and South Peak areas. 3 RUNOUT ANALYSIS 3.1 Model Calibration Given the relatively small volume of the potential rock avalanches and the characteristics of the upper slope of Turtle Mountain, it is considered that the motion of these landslides is most likely to remain frictional (Hungr et al., 2005). In order to develop a predictive model for the twelve unstable volumes identified using COLTOP and SLBL calibration back-analyses were carried out of nine rock avalanche cases. The cases were selected to represent rock avalanches that traversed relatively dry slopes, free of deep, saturated soil deposits. Four of these originated in limestone rock and five in strong igneous or metamorphic rocks. All the cases ran out over steep mountain slopes, but only those larger than 10 million m 3 reached valley bottoms. Table 2 gives the (bulked) volumes of the rock avalanches and the Bulk Friction Angle values (see Hungr, 1995) required to produce the observed runout in each case. The back-analysis clearly indicates an inverse relationship between volume and the friction angle, as shown in Figure 6. Such a trend has been observed in previous analyses. The reasons for it are presently not clear, but may include: 1) increased likelihood for the larger slides to encounter unconsolidated Table 2. Rock avalanche cases selected for calibration Location Volume (m3) Bulk Friction Angle ( ) Afternoon Ck 0.7 M 35 Thurwieser 1.9 M 26 Jonas North 2.4 M 25 Jonas South 3.7 M 26 Madison 33 M 15 Frank, M 14 ValPola 59 M 18 Hope M 22 Diablerets 73 M 17 Reference Strouth et al., 2006 Sosio et al., 2008 Bruce, 1978 Bruce, 1978 Cruden and Krahn, 1978 saturated material on lower slopes, 2) greater intensity of possible undrained loading of material overrun by a larger slide, 3) more intensive grain crushing and destruction of rock asperities in the larger events.

5 A semi-logarithmic lower-limit envelope was drawn to the calibration data in Figure 6. This envelope was used to select bulk friction angles used in forward analysis, given the volume of each detachment. As a result, the larger rock avalanches are more mobile. Analyses were completed using both a threedimensional model DAN 3D and a pseudo-threedimensional model DAN. Descriptions of both models can be found in McDougall (2008). Usually, both models give comparable results when applied to the same travel paths with the same resistance parameters. However in this case, the DAN analyses were used to examine potential mobility of the rock avalanches in the event that less lateral spreading may take place than implied by the 3D model. Figure 7. 3D prediction of the runout of the 6.59 m 3 LSP-2 rock avalanche from the South Peak. The red areas indicate the final position of the 3D deposit, with 5 m contours. The black line on the map shows the location of the profile used in the 2D analysis. The thick, short black line shows the 2D runout. Table 3. Potential source volumes identified by the UL group. Figure 6: Calibration analysis, correlation between landslide volume and the back-calculated bulk friction angle (including pore pressure effects). Symbol Location (UL) Expanded Volume (m3) Bulk friction angle ( ) 3.2 Results of Analyses The final distribution of debris for each of the 12 rock avalanches (Table 3) was predicted by DAN 3D analysis as shown by an example in Figure 7. All of the rock avalanches are essentially completed in less than 1.5 minutes, similar to the 1903 Frank Slide. The maximum velocities are in excess of 50 m/s (180 km/h). The deposit thickness, shown in Figure 5 in 5 m contour intervals, is typically up to 10 m, but ranges up to 20 m. Figure 8 shows envelopes of maximum runout resulting from all 12 potential rock avalanches, as analysed in 2D and 3D. The 2D maximum runout distances are typically somewhat longer than the 3D results. The reason for this is that the 2D analyses were conservatively forced to travel on relatively narrow paths and therefore, their energy is more confined. Such a situation could actually arise in reality, as the rock slide mass may remain somewhat coherent in the initial stages of its movement, in contrast to the 3D model that assumes instant, fluidlike lateral spreading. LSP-1 South Peak Lower Instability 1.89 M 25.0 LSP-2 Lower South Peak Total 6.59 M 20.0 LSP-3 South Peak Slice M 36.0 LSP-4 South Peak Slice M 30.4 LSP-5 South Peak Slice M 29.7 LSP-6 South Peak Slice M 26.2 USP-1 Subsidence Upper South Peak Zone 1.99 M 24.5 USP-2 Upper South Toppling Peak Zone 0.30 M 31.0 USP-3 Upper South Wedge Peak Zone 1.37 M dP-1 Third peak DSGSD 2.59 M dP-2 Third Peak GPS Station dP-3 Third Peak Sackung Zone 1.37 M 26.3 The direction of movement of the 3D slide and the assumed 2D profile did not often agree.

Over most of the area, the largest runout is determined by LSP-2, the total failure of Lower South Peak, with a volume of 6.6 million m 3. affected population is also ongoing.

6 Over most of the area, the largest runout is determined by LSP-2, the total failure of Lower South Peak, with a volume of 6.6 million m 3. affected population is also ongoing. On an annual basis the most recent results are not only published (Moreno and Froese, 2008) but also presented in public meetings to the municipal officials and residents in the affected zones. Frequent updates are also provided on the Alberta Geological Survey website at 5 ACKNOWLEDGEMENTS The authors are grateful to our colleagues at the Alberta Geological Survey, University of Lausanne and the University of Alberta for assistance in the field and ongoing discussions on the hazards on Turtle Mountain. 6 REFERENCES Figure 8. Envelope of hazard areas for 12 potential rock avalanches from the South Peak and Third Peak of Turtle Mountain. Full line: 3D analyses. Dotted line: 2D analyses. 4 HAZARD MANAGEMENT With the establishment of new zones that are potentially susceptible to runout, management and communication of the hazard and risk is underway. As the potentially unstable structures have only been identified at this point and there is no information on the spatial and temporal characteristics of the deformations, a monitoring system has been installed on the lower Third and Lower South Peak areas, in order to characterize the movements. This system consists of an array of overlapping global positioning system (GPS) monitoring points (both continuously monitored and periodically monitored) and a series of 20 mirror prism which are monitored via robotic total station from across the valley. As it is expected that deformation rates are likely in the millimeter to sub-millimeter level, many years of continuous monitoring will likely be required to gain confidence in the displacement trends. Communication of the risk associated with these hazards to the Allan, J.A Report on the stability of Turtle Mountain, Crowsnest District, Alberta. Department of Public Works, Edmonton, Alberta. 14 p. Allan, J.A Second report on the stability of Turtle Mountain, Crowsnest District, Alberta. Department of Public Works, Edmonton, Alberta. Alberta Provincial Archives. 25 p. Allan, J.A. (1933): Report on stability of Turtle Mountain, Alberta and survey of fissures between North Peak and South Peak; Alberta Department of Public Works, Alberta Provincial Archives, 28 p. Bruce. I., ( 1978). The field estimation of shear strength on rock discontinuities. Ph. D.Thesis. U. of Alberta, Edmonton: 308 p. Cruden, D.M. and Krahn, J., Frank Rockslide, Alberta, Canada. in B. Voight (ed.),rockslides and Avalanches,Vol. 1, pp Amsterdam: Elsevier. Daly, R.A., Miller, W.G. and Rice, G.S Report of the commission appointed to investigate Turtle Mountain, Frank, Alberta. Geological Survey of Canada Memoir p. Froese, C.R., Murray, C., Cavers, D.S., Anderson, W.S., Bidwell, A.K., Read R., Cruden, D.M., and Langenberg, W Development of a Warning System for the South Peak of Turtle Mountain. in Landslide Risk Management, O. Hungr, R. Fell, R.R. Couture and E. Eberhardt, A.A. Balkema, Leiden, Netherlands, p Froese, C., Moreno, F, Jaboyedoff, M. and Cruden, D. (In Press). 25 Years of Movement Monitoring on the South Peak of Turtle Mountain (Alberta, Canada): An Understanding of the Hazard; Canadian Geotechnical Journal (Accepted for Publication)

7 Hungr, O., A model for the runout analysis of rapid flow slides, debris flows and avalanches. Canadian Geotechnical Journal, 32(4): Hungr, O. and McDougall, S Two numerical models for landslide dynamic analysis. Computers and Geosciences, Hungr, O., and Evans, S.G., 1996, Rock avalanche runout prediction using a dynamicmodel: Trondheim, Norway, Proceedings, 7th International Symposium on Landslides, v.1, p Jaboyedoff, M., Couture, R. and Locat, P. (in press): Structural analysis of Turtle Mountain (Alberta) using digital elevation model: toward a progressive failure by toppling of gently dipping wedges; Geomorphology. Hungr, O., Corominas, J. and Eberhardt, E., 2005.State of the Art Paper #4, Estimating landslide motion mechanism, travel distance and velocity. In Hungr, O., Fell, R., Couture, R. and Eberhardt, E., Eds.. Landslide Risk Management. Proceedings, Vancouver Conference. Taylor and Francis Group, London. O. Hungr Geotechnical Research South Peak of Turtle Mountain, Frank, Alberta: Runout analyses of potential landslides. Report prepared for Alberta Geological Survey. 13 p. Jaboyedoff, M., Couture, R., and Locat, P. (2009). Structural analysis of Turtle Mountain (Alberta) using Digital Elevation Model: Toward a progressive failure. Geomorphology, Volume 103, Issue 1, 1, Pages Jaboyedoff M. and Tacher L., Computations of landslide slip surface using DEM, Geological Society of London, William Smith Meeting (2004). Jaboyedoff, M., Couture, R Report on the project COLTOP3D for March 2003: stay of Michel Jaboyedoff at GSC - Ottawa. Quanterra administrative document - Activity report - RA01. Hungr, O. and McDougall, S., 2008 Two numerical models for landslide dynamic analysis. Computers & Geosciences. In Press. Hungr, O., Corominas, J. and Eberhardt, E., 2005.State of the Art Paper #4, Estimating landslide motion mechanism, travel distance and velocity. In Hungr, O., Fell, R., Couture, R. and Eberhardt, E., Eds.. Landslide Risk Management. Proceedings, Vancouver Conference. Taylor and Francis Group, London. Pedrazzini, A., Jaboyedoff, M., Froese, C.R., Langenberg W., Moreno, F Structure and Failure Mechanisms Analysis of Turtle Mountain. In : J. Locat, D. Perret, D. Turmel, D. Demers et S. Leroueil, (2008). Proceedings of the 4th Canadian Conference on Geohazards : From Causes to Management. Presses de l Université Laval, Québec, 594 p Sosio, R., Crosta, G.B. and Hungr, O., Complete dynamic modeling calibration for the Thurwieser rock avalanche (Italian Central Alps). Engineering Geology (in press). Strouth, A., Eberhardt, E. and Hungr, O., The use of LiDAR to overcome rock slope hazard data collection challenges at Afternoon Creek, Washington In Golden Rocks 2006, The 41st U.S. Symposium on Rock Mechanics (USRMS): "50 Years of Rock Mechanics - Landmarks and Future Challenges.", Golden, Colorado, June, p. Sturzenegger M., Stead, D., Froese, C., Moreno, F. and Jaboyedoff M., (2007): Mapping the geological structure of Turtle Mountain, Alberta: A critical interpretation of field, DEM and LiDAR based techniques. In Eberhardt, E., Stead, D and Morrison T. (Eds.): Rock mechanics: Meeting Society s Challenges and demands (Vol. 2), Taylor & Francis. pp

Hazard mapping for the eastern face of Turtle Mountain, adjacent to the Frank Slide, Alberta, Canada

Hazard mapping for the eastern face of Turtle Mountain, adjacent to the Frank Slide, Alberta, Canada C.R. Froese Alberta Geological Survey/Energy Resources Conservation Board, Edmonton, Canada M. Jaboyedoff