Geophysical Journal International

Geophys. J. Int. (2015) 201, 372–376 doi: 10.1093/gji/ggv026 GJI Geodynamics and tectonics

EXPRESS LETTER Numerical simulation of the December 4, 2007 landslide-generated tsunami in Chehalis Lake, Canada

Jiajia Wang,1 Steven N. Ward2 and Lili Xiao1 1Faculty of Engineering, China University of Geosciences, Wuhan 430074, China. E-mail: [email protected] 2Institute of Geophysics and Planetary Physics, University of California, Santa Cruz, CA 95060, USA

Accepted 2015 January 13. Received 2015 January 11; in original form 2014 November 6

SUMMARY On December 4, 2007, a three million cubic metres landslide impacted Chehalis Lake, 80 km Downloaded from east of Vancouver, Canada. The failed mass rushed into the lake and parented a tsunami that ran up 38 m on the opposite shore, destroyed trees, roads and campsite facilities. Armed with field surveys and multihigh-tech observations from SONAR, LiDAR and orthophotographs, we apply the newly developed ‘Tsunami Squares’ method to simulate the Chehalis Lake landslide and its generated tsunami. The landslide simulation shows a progressive failure, flow speeds up to ∼60 m s–1, and a slide mass stoppage with uniform repose angle on the lakebed. Tsunami http://gji.oxfordjournals.org/ products suggest that landslide velocity and spatial scale influence the initial wave size, while wave energy decay and inundation heights are affected by a combination of distance to the landslide, bathymetry and shoreline orientation relative to the wave direction. Key words: Numerical approximations and analysis; Tsunamis; Fracture and flow; North America. by guest on February 26, 2015

wide and 500–840 m a.s.l, bounds the primary rock failure with 1 REVIEW OF THE 2007 EVENT AND a volume of ∼2.2 M m3,anareaof∼54 000 m2 and an average OBSERVATIONAL DATA thickness of ∼40 m (Brideau et al. 2012;seeFig.1B, polygon 1 On December 4, 2007, a 3 M m3 landslide occurred on the northwest and Fig. 2B). The lower part of the slide surface contributed an shore of Chehalis Lake in southwest British Columbia, 80 km east extra 0.8 M m3 to the rock avalanche (see Fig. 1B, polygon 2). We of Vancouver, Canada (Fig. 1A). The source of Chehalis rock slide reconstructed the geological profile corresponding to cross-section located at the intersection of the main valley slope and the northern A − A in Fig. 1(B) based on field measurements (Brideau et al. sidewall of a prominent gully (see Figs 1A and B, red polygon). 2012; Fig. 2A). The distribution of the slide mass on the lake bed, Chehalis Valley is incised into the contact of late Jurassic island arc derived from orthophotographs, LiDAR and SONAR data sets, pro- rocks of the Harrison Lake Formation and middle Jurassic to early vides tight constraints on landslide model (Roberts et al. 2013; Cretaceous rocks of the Coast Plutonic Complex. The primary slope Fig. 2A). failure initiated at an elevation of ∼800 m a.s.l (above sea level) in Brideau et al. (2012) believe that the rock slide was induced by a bedrock within the Mount Jasper pluton composed of quartz diorite rain-on-snow event that led to rapid development of high pore fluid containing abundant xenoliths of Harrison Lake Formation andesite pressures inside the fractures and a lower effective friction angle (Mahoney et al. 1995). The xenoliths are mechanically continuous along discontinuity surfaces. The failed mass reached an extremely with the quartz diorite that encloses them. The whole rock mass high speed and spawned a tsunami that ran up 38 m on the opposite is cut by several arrays of broadly planar fractures (Brideau et al. shore and travelled down the lake to its outlet, over 7.5 km away. 2012; Fig. 2B) that likely linked up in the initial stages of failure. Based on the description of Roberts et al. (2013), waves washed The landslide slid obliquely into the deep gully, transformed into a the shoreline, striped vegetation up to tens of metres, and cut a fast-moving debris avalanche, and destroyed a 400-m-long section forest trim line around the lakeshore that can still be seen on re- of forest service road. The material that started at the headscarp mote images (see Fig. 1A, yellow line). Water swept several tens travelled 800 m horizontally and 550 m vertically before entering of hectares of merchantable timber into the lake and transferred the the lake (Roberts et al. 2013). Today, looking up from the 240 m broken wood to the outlet. The woody debris formed a dam that elevation of the lake shoreline one sees freshly exposed slip sur- elevated the lake level and exposed a downstream community to faces on the west bank, revealing a long tongue shaped outline the threat of an outburst flood. An 8 month removal project eventu- of the landslide (see Fig. 1B, red polygon). The headscarp, 210 m ally eliminated this hazard. Most of the sites at three campgrounds

372 C The Authors 2015. Published by Oxford University Press on behalf of The Royal Astronomical Society. Simulation of the 2007 Chehalis Tsunami 373 Downloaded from http://gji.oxfordjournals.org/

Figure 1. (A) Location and 2008 image map of Chehalis Lake. Red polygon is the source of Chehalis Lake landslide. Points P1–P4 are monitors for tracking by guest on February 26, 2015 landslide velocity. The yellow line shows the wave impacted area (white colour) between the tsunami forest trimline and lake shoreline. The white rings represent the campgrounds. (B) Frontal photo of Chehalis Lake landslide. Polygon 1 is the primary rock slide source (modified from Brideau et al. 2012). Polygons 1 and 2 comprise the outline of the landslide.

(see Fig. 1A, white rings), including their ancillary facilities were the same size in such a way as to conserve volume and linear mo- severely damaged, destroyed or covered by woody debris. Luckily, mentum. no overnight or day users occupied the recreation areas at the time The simulation first focuses on landslide motion as constrained of the event, particularly as a result of the heavy snowfall and rain by direct observations. It then computes induced water waves given that preceded and possibly triggered the landslide (Brideau et al. assumptions about energy and momentum transfer. For landslides, 2012; Roberts et al. 2013). squares move under the influence of gravity-driven acceleration and suffer decelerations due to basal and dynamic frictions. For waves, a ‘Drag Along’ force is included at the water/landslide interface. Drag Along acts to slow the slide, but also accelerates the water much 2 TSUNAMI SQUARES APPROACH like an anti-friction. Having been accelerated, the water squares ‘Tsunami Squares’ (Wang et al. 2015; Xiao et al. 2015)isa propagate far away and potentially inundate dry land at shore. 2-D dynamic formulation capable of simulating moving fluids and In this paper, we apply Tsunami Squares to simulate the Chehalis materials with fluid-like nature. It evolves from the earlier work Lake landslide and its tsunami. The Chehalis Lake research area ‘Tsunami Balls’ based on long-wave theory (Ward & Day 2006, consists of a 833 × 1125 topographic grid derived from a 10 m res- 2008, 2011). ‘Tsunami Squares’ inherits advantages of the parent olution DEM. A 2009 June bathymetric SONAR survey provides a method such as not needing a specialized computational mesh or detailed contour map of the lake floor that supplements the incom- separate treatment for dry versus wet cells, but it obviates the need plete DEM (Roberts et al. 2013). Lake bottom bathymetry forms for millions of individual particles and can be extended to large a generally parabolic section, 800 m wide and 120 m deep near the spatial scales. Object materials comprise divisible squares that are landslide toe. Generally, the eastern shore down to lake bottom is accelerated, transported, and then re-allocated into new squares of notably steeper than the western one. 374 J. Wang, S.N. Ward and L. Xiao

(Fig. 2C, panel c) the rock mass impacts the water and spawns a giant wave spreading upon the opposite bank. The simulated slide stops on the lakebed with a uniform angle of repose and only a thin layer deposits above water (Fig. 2C, panel d), coinciding with the observed geological profile A−A (Fig. 2A).

4 TSUNAMI SIMULATION Pre- and post-event field observations and remotely sensed images document the wave impacts along the shoreline of Chehalis Lake (Fig. 4B, observed wave run-up heights). To satisfy these 42 run up height values, we set the Drag Along coefficient μda = 0.11 and rerun the program with the same topography and landslide model, but now with water in the Lake to 240 m elevation. Upon impact, the landslide ‘pushes up’ and ‘drags along’ the water to form a spreading Lake Tsunami (Fig. 3A). The generated wave transits rapidly and negatively impacts Chehalis Lake shores kilometres away. Fig. 3 shows dynamics of wave propagation at six typical moments. In panel (A), 10 s (30 s after the rockslide) after the rock mass hits still water, it forms an Downloaded from expanding wave arc. Within 20 s, the first wave reaches the oppo- site bank, runs up to a maximum height of 38 m and then retraces its steps as a reflected wave (panel B). At t = 50 s (panel C), the return wave washes over the slide scarp. By then, the waves in the

upper lake have propagated 2 km north and reached the Chehalis http://gji.oxfordjournals.org/ River Delta, destroying access roads, a boat launch and many camp- sites. In panel (D), from 60 to 110 s, the downstream waves strike Skwellepil Fan and remove vegetation over a large area that still can be seen in remotely sensed images (Fig. 1A, see white region between trimline and shoreline at Skwellepil Fan). After that, the waves pass in front of Skwellepil Fan and dissipate some energy in the shallow and narrow channel. In Panel E after 6 min and ∼8km of travel, waves reach the south end of Chehalis Lake at the entrance of Lower Chehalis River. Interestingly, wave heights actually grow by guest on February 26, 2015 here due to the funnelling and shallowing of the Lake. Ward (2014) published a YOUTUBE movie ‘Lake Tsunami’ that described the case background and contained details of landslide and tsunami Figure 2. (A) Geological profile corresponding to cross-section A−A processes. in Fig. 1(B). (B) Frontal photo of the rock slide scarp. (C) Four frames showing the model landslide at times t = 5, 15, 25, 65 s in a single simulation. A movie of this landslide model is available at: http://es.ucsc.edu/∼ward/chehalis-slide-3d.mov. 5 DISCUSSIONS Landslide shape and impact velocity are critical parameters affect- ing wave heights. Field surveys well-constrain landslide shape. To 3 LANDSLIDE SIMULATION track velocity, we monitor four points along cross section A−A The investigated and observed data in Section 1 supply fairly tight (see Figs 1Aand2A, P1–P4). Fig. 4(A) displays the velocity curves constraints on landslide simulation parameters. In this case, we set at these spots. At point 1, the purple line reflects the initiation of the dynamic friction coefficient μd = 0.003, underwater dynamic fric- oblique rock slide in the first 15 s and reveals a low initial speed (up ◦ –1 tion coefficient μd = 0.02 and repose angle θrepose = 20.5 . Basal to20ms ). The fast-moving debris flow caused by the collision friction coefficient μb is 0 initially. If the slope angle becomes between the rock slide and the south gully wall, passes through point –1 smaller than θrepose,thenμb grows gradually to slow and stop squares 2 with higher speeds up to ∼40 m s and lasts more than 20 s. The in place. With the lake dry, we run ‘Tsunami Squares’ and obtain the red and orange lines plot the velocities at lakeshore (point 3) and landslide simulation in Fig. 2(C). In panel (A), 5 s after initiation, underwater (point 4). The highest simulated speed (∼58 m s–1)at the rock mass slides obliquely down and strikes the right side- t = 28 s at lakeshore point 3 mainly bears on initial wave heights. wall of the gully. The intense collision smashes unbroken blocks Landslide velocities underwater are much lower due to a higher into pieces. Fifteen seconds after the collapse (Fig. 2C, panel b), frictional resistance imposed there. After 60 s, slide mass at points the rock slide mass rapidly disintegrates into a fast-moving debris 1, 3 and 4 has stopped while some abandoned materials continue avalanche, rushes down along the gully and erodes the lower slope to creep on middle slope at point 2 before finally resting in place mantled by Quaternary sediments to a depth of ∼5 m (Fig. 1B, poly- (Fig. 4A, blue line). Points 1–4 launch in sequence with several gon 2). The main disturbance also triggers other rock failures inside seconds delay due to the progressive failure of landslide motion the outline to join the debris flow. With speeds up to ∼60 m s–1 described in Sections 1 and 3. Simulation of the 2007 Chehalis Tsunami 375 Downloaded from http://gji.oxfordjournals.org/ by guest on February 26, 2015

Figure 3. Panels (A)–(E) show the model tsunami at times t = 30, 40, 50, 110 and 350 s after initiation of the landslide. The orange colour tracks the wave impacted area. The yellow frames in right-bottom corner of each panel quantify 3-D view parameters. The brown bar marks the landslide thickness and the blue bar indicates the wave heights. Panel (F) maps elevation of the model waves above lake level at shoreline sites. A movie of this tsunami simulation is available at: http://es.ucsc.edu/∼ward/chehalis-wave-3d.mov.

Fig. 4(B) collects simulated wave run-up heights along the shore- Skwellipil Fan. After passing the fan, the wave decays more slowly line compared with observed data. The curves reflect the wave de- from sites b to c with minor run-up on steep bedrock slopes. Further cay up and down lake. Waves in the upper lake decay faster and die south, run-up increases again along gently sloping fan surfaces. out rapidly due to the short distance to the end of the lake. Down Seven km from the landslide, the wave reaches the south end of lake, the wave decays slowly accompanied by broad variations with the lake where it suddenly ‘rises up’ and rushes onto the Lower distance. Chehalis River Delta. The highest wave run-up in the simulation, 38.2 m, locates at the The wavy curves in Fig. 4(B) extracted from simulated run-up opposite bank 0.8 km away from the landslide. Near the same place, products in Fig. 3(F) do not show systematic differences between 37.8 m run-up was observed. The area between sites a and b suffers the east and west banks south of site b. You might think that wave from the highest wave wash, and impact heights on east bank (red heights along the convex eastern bank should be higher than on the line) are larger generally than those on west bank (blue line). This concave western bank, but wave refraction tends to compensate for suggests that the backwash phase was less energetic than the initial this large scale curvature. On smaller scales however, the waviness on rush. of run-up seems to correlate with shoreline orientation relative to Around site b, wave heights decrease sharply where the lake floor wave direction. Locations oblique to the general trend of the shore is narrowed and elevated by till, colluvium or alluvium deposited at that more directly face the oncoming wave see greater run-ups. 376 J. Wang, S.N. Ward and L. Xiao

Figure 4. Panel (A): velocity history at four tracked spots along cross-section A−A. Panel (B): simulated wave run up heights compared with observed ones (Roberts et al. 2013) on east and west banks. Location (a): landslide source. Location (b): Skwellepil Fan. Location (c): near the south end of Chehalis Lake. Downloaded from

6 CONCLUSIONS REFERENCES Employing reliable field surveys and high-tech observations, we Brideau, M. et al., 2012. Stability analysis of the 2007 Chehalis lake land- simulated the 2007 Chehalis Lake landslide and its generated slide based on long-range terrestrial photogrammetry and airborne LiDAR http://gji.oxfordjournals.org/ tsunami, using a newly developed numerical method named data, Landslides, 9, 75–91. ‘Tsunami Squares’. Landslide simulations reproduced the pro- Mahoney, J.B., Friedman, R.M. & McKinley, S.D., 1995. Evolution of a gressive failure of Chehalis Lake rockslide and debris avalanche Middle Jurassic volcanic arc: stratigraphic, isotopic, and geochemical with reasonable velocities. The slide mass, now sleeping with characteristics of the Harrison Lake Formation, southwestern British uniform repose angle on the lakebed, shows agreement with Columbia, Can. J. Earth Sci., 32, 1759–1776. the geological profile derived from the post-event field survey. Roberts, N.J., McKillop, R.J., Lawrence, M.S., Psutka, J.F., Clague, J.J., Tsunami products suggest that landslide velocity and spatial scale Brideau, M.E. & Ward, B.C., 2013. Impacts of the 2007 landslide- generated tsunami in Chehalis Lake, Canada, in Landslide Science and

drives initial wave size, while the energy decay and inundation by guest on February 26, 2015 Practice, pp. 133–140, eds Margottini, C., Canuti, P.& Sassa, K., Springer. heights are affected by a combination of distance to the landslide, Wang, J.J., Ward, S.N. & Xiao, L.L., 2015. Numerical modeling of the bathymetry, wave propagation direction and shoreline orientation. rapid, flow-like landslides across three-dimensional terrains: a Tsunami The Tsunami Squares method for landslide and tsunami simu- Squares approach to El Picacho landslide, El Salvador, September 19, lations provides a powerful tool for risk assessment and hazard 1982, Geophys. J. Int., in press. mitigation. Ward, S.N., 2014. Lake Tsunami, YOU TUBE MOVIE, Available at: https://www.youtube.com/watch?v=X3LuT1lP00c, last accessed 11 June 2014. Ward, S.N. & Day, S., 2006. Particulate kinematic simulations of debris avalanches: interpretation of deposits and landslide seismic signals of ACKNOWLEDGEMENTS Mount Saint Helens, 1980 May 18, Geophys. J. Int., 167, 991–1004. JW was supported by the project of China Geological Survey Ward, S.N. & Day, S., 2008. Tsunami balls: a granular approach to tsunami (No. 1212011220173) and National Natural Science Founda- runup and inundation, Commun. Comput. Phys., 3, 222–249. Ward, S.N. & Day, S., 2011. The 1963 landslide and flood at VaiontReservoir tion of China (No. 41202247) and (No. 41302230). We thank Italy. A tsunami ball simulation, Ital. J. Geosci., 130, 16–26. the U.S. Geological Survey for providing the 10 m Digital El- Xiao, L.L., Ward, S.N. & Wang, J.J., 2015. Tsunami Squares approach to evation Data. The comments from an anonymous reviewer are landslide-generated waves: application to Gongjiafang Landslide, Three greatly appreciated for their contributions in improving this Gorges Reservoir, China, Pure appl. Geophys., doi:10.1007/s00024-015- manuscript. 1045-6.