Appendix C - TSUNAMI ANALYSIS
Contractor 2015-03-26 Trans Mountain Expansion Project Revision Date:
Contractor Tsunami Assessment A Revision No.:
Page 1 of 73 M&N File 9665/55
Trans Mountain Expansion Project
Tsunami Assessment
KMC Document # 01-13283-TW-WT00-MFN-RPT-0008
Rev Prepared by/ Reviewed by/ Approved by/ TMEP Pages Issued Type No. Date Date Date Acceptance/ Revised Date
C-F Tsai M. Jorgensen R. Byres A Issued for Review 2015-03-26 2015-03-26 2015-03-26
TRANS MOUNTAIN EXPANSION PROJECT WESTRIDGE TERMINAL, BURRARD INLET, BC
TSUNAMI ASSESSMENT
Prepared for:
Prepared by:
TRANS MOUNTAIN EXPANSION PROJECT WESTRIDGE TERMINAL, BURRARD INLET, BC
TSUNAMI ASSESSMENT
M&N Project No. 7773‐03
Revision Description Issued Date Author Reviewed Approved B Issued For Review Mar. 26, 2015 C‐F Tsai MJ/RB RB A Progress Draft Feb. 11, 2015 C‐F Tsai MJ/RB RB
Westridge Marine Terminal Tsunami Assessment 3
EXECUTIVE SUMMARY
Kinder Morgan Canada is currently considering expansion of marine facilities at the Westridge Terminal at Burnaby, BC as part of the Trans Mountain Expansion Project (TMEP). The terminal expansion includes the construction of three (3) new jetty berths capable of accepting vessels of varying size and cargo type.
The present study provides a screening level assessment of landslide‐induced tsunami hazards to the proposed Westridge Terminal and vessels berthed at the facility. The overall scope of work includes:
1. Assessment of the characteristics of tsunami waves (amplitude, period, and wave induced currents) generated by potential landslides within the Indian Arm and Burrard Inlets;
2. Evaluation of the plausible threat of potential impacts to berthed vessels;
3. Recommendations for design if the tsunami waves are perceived to be significant enough to result in potential issues to berthed vessels.
CONCLUSIONS
Based on the scenarios investigated, the following conclusions can be made.
Landslide Scenarios
A number of potential landslide scenarios (i.e. location and slide volume) were provided to M&N by BGC Engineering Ltd. These landslide scenarios cover a large range of hazard exposure from no tsunami generation, to an extreme event comparable with some of the largest known historical landslide‐generated tsunamis on record. This large range of scenarios was included to cover the maximum plausible range of outcomes, initially without considering the very low (and as yet unquantified) probability of such events.
Since there are no records of past landslides or tsunamis occurring in Burrard Inlet or Indian Arm, there is currently no means to quantify the probability or return period of the scenarios examined. Nonetheless, it is believed that the risk of such extreme events is very low. A more detailed geological or geotechnical assessment is needed to establish to what extent the investigated landslide scenarios are credible. At present, the extreme condition of landslide scenario 1 is comparable (actually slightly greater) than the mega‐landslide evidenced in Lituya Bay, Alaska which occurred in 1958. The current findings point to the fact that landslides of such magnitude tend to be localized by particular geological conditions conducive to formation of
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large deposits of unstable rock. It is recommended that a geologist be consulted to establish whether that is the case in Indian Arm.
Because no present findings point to large landslides in Indian Arm and Burrard Inlet in the past, and because the area has had (geologically) recent exposure to the 1700 Cascadia fault rupture, which was approximately magnitude 9.0, landslide scenario 1 is at present deemed implausible, subject to further geological investigation. This study can therefore be considered to represent “worst case” conditions. If landslide parameters are revisited and revised downwards, the result would be a substantial reduction in the magnitude of tsunami wave height, and corresponding flow velocities.
Tsunami Wave Formation and Propagation
Regarding the formation of tsunami waves as a result of landslide activity, it can be noted that while the initial wave height produced at the point of landslide impact can be substantial, features of Indian Arm tend to limit the maximum wave heights affecting the project site.
Factors affecting initial tsunami wave heights include the water depth and bathymetry, and the fact that the fjord is quite narrow, with the primary landslide momentum directed across the fjord rather than along its length.
Aspects of the bathymetry, such as the change from deep water in the central portion of Indian Arm to quite shallow water at the southern end down in Burrard Inlet work to impede tsunami wave front propagation. The narrow opening of the inlet at the southern end of Indian Arm further works to disperse tsunami wave energy.
Impacts to Berthed Vessels
The record of simulated water level variations and tsunami‐induced currents at the project site shows that tsunami‐induced water level variations are within the range of typical tides occurring in Burrard Inlet. While tsunami wave propagation would unfold over a matter of minutes as opposed to hours for tidal variations, the water level changes are believed to be slow enough that vessel moorings would be able to accommodate the change.
Likewise, it is found that tsunami‐induced currents are within the range of the OCIMF cases investigated in the mooring analysis (M&N, 2014), and moored vessels would therefore not be particularly prone to tsunami‐related impacts. We therefore conclude that the risk of a vessel being damaged or experiencing a breakaway event from parted mooring lines is very low. We further infer from the results that damage to the facility itself (such as wave impact damage, run‐ up, or overtopping/inundation), is similarly very low.
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TABLE OF CONTENTS
1. INTRODUCTION ...... 9 1.1 Site Description ...... 9 1.2 Project Background ...... 10 1.3 Scope of Work ...... 12 2. DATA REVIEW ...... 13 2.1 Tsunamis From Distant Sources ...... 13 2.2 Categorization of Landslides ...... 14 2.3 Locally Generated Tsunamis ...... 17 2.3.1 Subaerial Landslide Scenarios ...... 20 2.4 Development of Landslide Parameters ...... 25 2.5 Assessment with Analytical Approximation ...... 28 3. DEVELOPMENT OF NUMERICAL MODEL ...... 30 3.1 MIKE‐21 Hydrodynamic Model ...... 30 3.2 Model Bathymetry & Topography ...... 30 3.3 Boundary Conditions ...... 30 3.4 Modeling Approach ...... 31 3.5 Modeling Cases ...... 33 3.6 Comparison With Analytical Approximations ...... 34 3.6.1 Analytical/Numerical Model Comparison ...... 36 4. SUMMARY OF MODEL RESULTS ...... 41 4.1 Tsunami Wave Propagation ...... 42 4.2 Tsunami Wave Attenuation ...... 51 4.3 Results for Modeling Cases ...... 51 5. EVALUATION OF MOORING IMPACTS ...... 57 5.1 Tsunami‐Induced Water Level Variations ...... 57 5.2 Tsunami‐Induced Flow Velocities ...... 60 5.2.1 Comparison of Tsunami‐Induced Flow Velocities and Mooring Analysis ...... 63 6. SUMMARY AND CONCLUSIONS ...... 66 6.1 Discussion of Landslide‐Generated Tsunami Hazard ...... 66 6.1.1 Evidence of Past Tsunami Events ...... 69 6.2 Conclusions ...... 69 6.2.1 Landslide Scenarios ...... 69 6.2.2 Tsunami Wave Formation and Propagation ...... 70 6.2.3 Impacts to Berthed Vessels ...... 70 7. REFERENCES ...... 71
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LIST OF FIGURES
Figure 1‐1: Burrard Inlet, Vancouver, BC (Source: Wikipedia) ...... 9 Figure 1‐2: Westridge Marine Terminal Site Location ...... 11 Figure 1‐3: Rendering of Proposed Westridge Marine Terminal Expansion ...... 11 Figure 2‐1: Tsunami Hazard Categorization, CGEN (2014, Natural Resources Canada, 2009) .... 14 Figure 2‐2: Sketch Showing Tsunami Generation from Submarine and Subaerial Landslide (Nieuwkoop, 2007) ...... 15 Figure 2‐3: Landslides Categorized by Material and Movement Type, BGS...... 16 Figure 2‐4: Morphologic Features Identified in Multi‐beam Swath Bathymetry, GSC OF 7348 (2013) ...... 19 Figure 2‐5: Aerial view of Burrard Inlet and Indian Arm, Landslide Locations 1 to 6, and Modeling Domain ...... 21 Figure 2‐6: Slide Location 1 Elevation Profile ...... 22 Figure 2‐7: Slide Location 2 Elevation Profile ...... 22 Figure 2‐8: Slide Location 3 Elevation Profile ...... 23 Figure 2‐9: Slide Location 4 Elevation Profile ...... 23 Figure 2‐10: Slide Location 5 Elevation Profile ...... 24 Figure 2‐11: Slide Location 6 Elevation Profile ...... 24 Figure 2‐12: Primary Parameters Defining Impulse Wave Characteristics...... 25 Figure 2‐13: Tsunami wave attenuation as a function of direction and distance ...... 27 Figure 3‐1: Model Bathymetry ...... 31 Figure 3‐2: Landslide Scenario 1 Example of Time Varying Bed Level Displacement Input into MIKE‐21 Model ...... 32 Figure 3‐3: Snapshot Showing the Tsunami Wave Generation from MIKE‐21 Model and Laboratory Tests of Bore Formation (Fritz, 2002) ...... 33 Figure 3‐4: Plan view of initial wave height formation, Scenario 1 (left) and idealized bathymetry (right)...... 35 Figure 3‐5: Comparison of water surface elevation profiles for Slide Scenario 1 and idealized Test Case ...... 36 Figure 3‐6: Comparison of MIKE‐21 model tsunami wave length with Heller (2007) analytical method ...... 37 Figure 3‐7: Comparison of MIKE‐21 model tsunami wave height with Heller (2007) analytical method ...... 38 Figure 3‐8: Comparison of MIKE‐21 model tsunami wave amplitude with Heller (2007) analytical method ...... 38 Figure 3‐9: Comparison of MIKE‐21 model tsunami wave celerity with Heller (2007) analytical method ...... 39
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Figure 3‐10: Comparison of MIKE‐21 model tsunami wave period with Heller (2007) analytical method ...... 40 Figure 4‐1: Model output locations ...... 41 Figure 4‐2: Tsunami Wave Propagation – Landslide Scenario 1 ...... 44 Figure 4‐3: Tsunami Wave Propagation – Landslide Scenario 1A ...... 45 Figure 4‐4: Tsunami Wave Propagation – Landslide Scenario 2 ...... 47 Figure 4‐5: Tsunami Wave Propagation – Landslide Scenario 4 ...... 49 Figure 4‐6: Tsunami Wave Propagation – Landslide Scenario 5 ...... 50 Figure 4‐7: Definition of Model Result Statistics ...... 53 Figure 4‐8: Time Series of Surface Elevation at Berth 1 ...... 54 Figure 4‐9: Time Series of Current Speed at Berth 1 ...... 54 Figure 4‐10: Time Series of Surface Elevation at Berth 2 ...... 55 Figure 4‐11: Time Series of Current Speed at Berth 2 ...... 55 Figure 4‐12: Time Series of Surface Elevation at Berth 3 ...... 56 Figure 4‐13: Time Series of Current Speed at Berth 3 ...... 56 Figure 5‐1: General Arrangement Plan for the Proposed Westridge Facilities ...... 57 Figure 5‐2: Range of tsunami‐induced water level variations at Berth 1 ...... 58 Figure 5‐3: Range of tsunami‐induced water level variations at Berth 2 ...... 59 Figure 5‐4: Range of tsunami‐induced water level variations at Berth 3 ...... 60 Figure 5‐5: Magnitude and direction of maximum tsunami‐induced flow velocities ...... 61 Figure 5‐6: East Burrard Inlet Mike21 model – ebb current snapshot, (M&N, 2012) ...... 62 Figure 5‐7: East Burrard Inlet Mike21 model – flood current snapshot, (M&N, 2012) ...... 62 Figure 5‐8: Comparison of tsunami‐induced flow velocities and OCIMF cases adopted in Berth 1 mooring analysis ...... 63 Figure 5‐9: Comparison of tsunami‐induced flow velocities and OCIMF cases adopted in Berth 2 mooring analysis ...... 64 Figure 5‐10: Comparison of tsunami‐induced flow velocities and OCIMF cases adopted in Berth 3 mooring analysis ...... 65 Figure 6‐1: Comparison of slide scenarios investigated with slides recorded worldwide ...... 68
LIST OF TABLES
Table 2‐1: Characteristics of Morphological Features ...... 18 Table 2‐2: Summary of Landslide Characteristics ...... 20 Table 2‐3: Landslide Geometries ...... 25 Table 2‐4: Subaerial Landslide Tsunami Wave Characteristics ...... 29
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Table 3‐1: Modeling Cases and Sensitivity Tests ...... 34 Table 4‐1: Tsunami Wave Attenuation ...... 51 Table 4‐2: Summary of Results at Berth Locations ...... 52 Table 6‐1: Tsunamigenic events in British Columbia in Recent History ...... 67 Table 6‐2: Earthquake Risk, Recurrence, Peak Ground Acceleration, and Magnitude ...... 69
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1. INTRODUCTION
1.1 SITE DESCRIPTION
Burrard Inlet is approximately 25 km long and extends east from the Strait of Georgia to Port Moody (Figure 1‐1). The inlet is composed of the Outer Harbour, the Inner Harbour, the Central Harbour, and the Port Moody Arm. Indian Arm, a 20 km long steep‐sided glacial fjord, extends north from the main inlet.
The waters of Burrard Inlet are sheltered from the open ocean, and the Outer Harbour and English Bay are utilized by vessels transiting to and from Vancouver and as anchorage for vessels waiting to discharge or take on cargoes. Infrastructure along the waterfront is primarily commercial and includes railyards, terminals for container and bulk cargo, grain elevators, and other industry. Much of the shoreline along Burrard Inlet is built out with mixed commercial and residential real estate. Along Indian Arm, the topography is steeper and the area is largely undeveloped.
Figure 1‐1: Burrard Inlet, Vancouver, BC (Source: Wikipedia)
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As a waterway, Burrard Inlet is connected to the Pacific Ocean via the Strait of Georgia. Water depths within the inlet range from approximately 16 to 44 meters on approach to the Westridge facility. The waters of Indian Arm are deeper ranging from 25 to 210 meters. The general bathymetry of Burrard Inlet and Indian Arm is detailed on the following Canadian Hydrographic Service (CHS) Nautical Charts:
• 3481 ‐ Approaches to Vancouver Harbour (1:25,000)
• 3311 – Port Moody to Howe Sound (1:40,000)
• 3495 – Indian Arm (1:30,000)
The hydrodynamics of the inlet are influenced by the constrictions at First and Second Narrows. Both of the narrows are dredged to about 15‐20 m depth for navigation. Currents through the narrows can exceed 5 knots during spring‐tide flood and ebb phases. The First Narrows runs past Stanley Park between the Outer Harbour and the Inner Harbour. The Second Narrows is located between the Inner Harbour and the Central Harbour where Lynn Creek and the Seymour River meet the inlet.
1.2 PROJECT BACKGROUND
The Trans Mountain Pipeline System (TMPL), which has been in operation since 1953 is a 1,150 km pipeline transporting crude oil and refined products to the west coast. The current capacity is approximately 300,000 barrels per day (bpd).
The ongoing Trans Mountain Pipeline Expansion (TMX) Project focuses on expanding the capacity of the pipeline to 890,000 bpd. The expansion includes new pipeline, addition of pump stations along the pipeline, addition of storage tanks at existing facilities, and expansion of the Westridge Marine Terminal.
The TMPL moves product from Edmonton, Alberta, to terminals and refineries in the central British Columbia region, the Greater Vancouver area and the Puget Sound area in Washington state, as well as to other markets such as California, the U.S. Gulf Coast and overseas through the Westridge Marine Terminal at Burnaby, BC. The location of the terminal is shown in Figure 1‐2.
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Indian Arm
Westridge Burrard Inlet Marine Terminal
Port Moody
Figure 1‐2: Westridge Marine Terminal Site Location
The Westridge marine terminal can accommodate barges, and vessels up to approximately 120,000 deadweight tons. In addition to shipping crude oil, the facility receives jet fuel for the Vancouver International Airport. The terminal facilities include three storage tanks with an overall capacity of 290,000 bbl.
Kinder Morgan Canada (KMC) is in the process of expanding marine facilities at the Westridge Terminal, which includes the construction of three (3) new jetty berths capable of accepting vessels ranging from barges to Aframax tankers. Figure 1‐3 shows a rendering of the proposed facility.
Figure 1‐3: Rendering of Proposed Westridge Marine Terminal Expansion
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1.3 SCOPE OF WORK
The present study provides a screening level assessment of landslide induced tsunami hazards to vessels berthed at the Kinder Morgan facility at Burnaby, BC, within the Eastern Burrard Inlet. The overall scope of work includes:
1. Assessment of the characteristics of tsunami waves (amplitude, period, and wave induced currents) generated by potential landslides within the Indian Arm and Burrard Inlets;
2. Evaluation of the plausible threat of potential impacts to berthed vessels;
3. Recommendations for design or further analysis if the tsunami waves are perceived to be significant enough to result in potential issues to berthed vessels.
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2. DATA REVIEW
This chapter summarizes findings of a literature review of prior tsunami studies for the area along with a categorization of landslides and locally generated tsunamis. Based on input from BGC Engineering, six potential subaerial landslide scenarios are investigated and evaluated by analytical methods. The findings are carried over to the numerical modeling effort detailed in Chapter 3 of the report.
2.1 TSUNAMIS FROM DISTANT SOURCES
Canadian shorelines are exposed to tsunamis from a range of far field and near field sources. Distant far field sources include tsunamis generated by seismic events, volcanic activity, and landslides along the Pacific Rim, which includes (in a counter clockwise direction) Alaska, the Aleutian Islands, the Kuril Islands, Japan, the Philippines, New Zealand, South America, and North America. Hawaii, approximately in the center of the Pacific is also a potential source of tsunamis. Along the North American Shelf, the Cascadia Subduction Zone extends to the northern part of Vancouver Island, transitioning into the Queen Charlotte Fault further north. These subduction zones are capable of generating large tsunamis impacting nearshore areas and shores across the Pacific, (Clague et al., 2003).
The primary exposure of the Lower British Columbia to tsunami hazards is along the Pacific seaboard of Vancouver Island, see Figure 2‐1, excerpt from Natural Resources Canada (2009). In the Strait of Juan de Fuca south of Vancouver Island, and upon passage of the Gulf Islands to the north, tsunami amplitudes are reduced by about one half compared with the outer coast, as indicated by both model results and actual tsunamis observed on tide gauges (Leonard et al., 2012), and attenuate further as they enter the Strait of Georgia.
Within the Strait of Georgia, tsunami hazards are low, with a run‐up potential of less than 2 meters. Figure 2‐1 does not cover the eastern part of Burrard Inlet and Indian Arm, but it can be assumed that the primary tsunami hazards in these areas are associated with local landslide activity such as is the case for the other fjords included in the figure. This study therefore focuses on landslide generated tsunamis.
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Figure 2‐1: Tsunami Hazard Categorization, CGEN (2014, Natural Resources Canada, 2009)
2.2 CATEGORIZATION OF LANDSLIDES
Landslides can be categorized into two broad categories, namely submarine landslides and subaerial landslides. Submarine landslides are initiated beneath the surface of the water, while subaerial landslides are initiated above the water and impact the water body during their progression or fall into the water body. The movement of a large slide mass or the impact of the fall displaces the water in the direction of the movement and can lead to generation of a tsunami wave on the surface of the water body. A classic example occurred in Lituya Bay in Alaska, where an earthquake in 1958 triggered a large rock slide that fell into the bay and produced a runup on the opposite shore up to a height of 525 m above sea level (Clague et al., 2003). Another example occurred in 2007 at Chehalis Lake, located 80 km east of Vancouver. In that event, an estimated three million cubic meters of rock broke away from a mountain and slid into the water. Across from the slide on the opposite shore, the tsunami reached 37.8 meters up the slope.
Figure 2‐2 presents a sketch showing differences in tsunami generation between a submarine and subaerial landslide (Nieuwkoop, 2007). For a submarine landslide, a depressions of the water surface is generated behind the landslide while a superelevation of the water surface is created ahead of the landslide. For a subaerial landslide, the water surface is only elevated ahead of the impact location. Once the initial wave field is formed, it propagates outward from the source region. Therefore, the first wave front which reaches the adjacent coast is a trough for a submarine landslide and a crest for a subaerial landslide.
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The present report focuses on subaerial landslide scenarios.
Figure 2‐2: Sketch Showing Tsunami Generation from Submarine and Subaerial Landslide (Nieuwkoop, 2007)
There are several types of subaerial landslide mechanisms to consider as summarized in Figure 2‐3. The current classification of landslides by the British Geological Survey (BGS) follows the scheme based on Varnes, 1978 and Cruden et al, 1996. This scheme terminology is also suggested by the Unesco Working Party on the World Landslide Inventory (WP/WLI 1990, 1993).
The cause of such landslides can vary greatly, and triggering mechanisms can include seismic events, hydrostatic pressure or hydrodynamic forcing (e.g. from heavy rainfall), other external loading, and human activities such as construction or blasting. The primary aspects of tsunami formation are discussed in the following.
The tsunami wave height is known to increase with the speed of the landslide impact at the water surface. Therefore subaerial landslides generally produce larger tsunami waves than submarine landslides of similar volumetric magnitude. In general, the steeper the slope, the faster the speed of impact.
The density and coherence of the landslide affects tsunami wave height generation. Therefore a large block will tend to produce a larger tsunami wave height than a granular mix of material having a more distributed mass and lower bulk density.
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Figure 2‐3: Landslides Categorized by Material and Movement Type, BGS
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Regarding the volume of water displaced, formation of a tsunami wave depends greatly on the characteristics of the time history of ground displacement relative to the surrounding mass of water. Therefore seismic uplift, such as during fault rupture in subduction zones, is one of the primary tsunami hazards because of the potentially large, rapid vertical displacements occurring over a large area. Within Burrard Inlet and along the steep sides of the Indian Arm, distanced from primary fault lines, the primary tsunami hazard is due to subaerial landslides.
2.3 LOCALLY GENERATED TSUNAMIS
This section summarizes findings about past landslide activity in Burrard Inlet and Indian Arm and the potential subaerial landslide scenarios adopted for the analysis.
As a general statement categorizing potential tsunami activity in Indian Arm, Clague et al. (2005) notes that the probability of a landslide‐triggered tsunami in Indian Arm is very low. This, because no such event has occurred since the time of European settlement more than 150 years ago, and no slopes bordering the fjord are known to be unstable.
GSC OF 7348 (2013) evaluates CHS swath multi‐beam bathymetric survey data and identifies numerous areas within Indian Arm exhibiting indications of past and potential landslide activity, including fan delta formations; translational slides; submarine alluvial fan/composite slides; debris flows and turbidity current channels, gullies and lobes; and undifferentiated slides.
Figure 2‐4 summarizes submarine slope failure types and morphologic features from GSC OF 7348 (2013) for Burrard Inlet and Indian Arm. Areas of green color indicate debris flows, purple areas are fan deltas, blue areas represent submarine alluvial fans and composite slides, red areas indicate translational slides, and yellow areas represent undifferentiated slides. The classifications can be categorized as follows:
Debris flows include turbidity current channels, gullies, and debris flow depositional lobes. Debris flows, often categorized as mudslides, mudflows, or debris avalanches are fluid sediment masses that flow as an unsteady, very poorly sorted sediment slurry. These flows typically do not produce tsunami waves.
Fan deltas are characterized by fan shaped deposits often channelized with several types of landslide deposits. The fan deltas consist of sediments derived from an alluvial fan feeder system and deposited mainly or entirely subaqueously at the interface between the active fan and a standing body of water. Fan deltas are constructed by a combination of slope instability processes including debris flows, debris avalanches, turbidity flows and materials settling from suspension. Burrard Inlet has several moderate sized fan deltas and alluvial fans entering from large watersheds. Examples of these are denoted by (A) in Figure 2‐4. The formation of fan deltas
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does commonly not produce tsunami wave activity, although the buildup of unstable deposits may have the potential for subsequent failure and submarine slide activity, which may or may not produce a tsunami wave.
Submarine alluvial fans/composite slides describe complex slides resulting from several events associated with a submarine channel or channels, but where no single, dominant river sediment source is apparent. Submarine alluvial fan/composite slides are distinguished from fan deltas by the lack of a permanent river source and are interpreted to indicate ongoing slide activity of significant magnitude. Depending on the speed of movement and magnitude of the slide mass, slides of this type may produce tsunami wave activity.
Translational slides comprise slides where the landmass moves along an approximately planar surface with little rotation or backward tilting. The moving mass typically consists of a single unit or a few closely related units that have moved downslope as a relatively coherent mass. Examples of pronounced translational slides are indicated with (B and C) in Figure 2‐4. Depending on the speed of movement and magnitude of the slide mass, slides of this type may produce tsunami wave activity.
Undifferentiated slides cover slides where downslope displacement in a single event is evident, but where the form of the slide plane cannot be unambiguously determined, and the planar or rotational aspect of slides has been difficult to establish from the multi‐beam data.
Table 2‐1 summarizes the characteristics of the identified submarine morphological features in terms of their planar extent. It can be seen that debris flows are limited in terms of number and size, while deltaic deposits make up the majority of the identified features (14+8), ranging in size from 2,900 m2 to 721,000 m2. The types of slides having the potential to displace water rapidly enough to possibly produce a wave include the translation and undifferentiated slides, which are relatively limited in number (6+3) and extent.
Table 2‐1: Characteristics of Morphological Features Morphological Feature Count Planar Extent Debris Flows 2 ~ 29,300 m2 Fan Deltas 14 12,500 to 721,000 m2 Submarine Alluvial Fans 8 2,900 to 443,000 m2 Translational Slides 6 13,800 to 70,900 m2 Undifferentiated Slides 3 10,100 to 59,500 m2
Slope failure types such as rotational slides; rock avalanches; bedrock creep; and cones have not been identified in Burrard Inlet and Indian Arm.
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Figure 2‐4: Morphologic Features Identified in Multi‐beam Swath Bathymetry, GSC OF 7348 (2013)
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2.3.1 Subaerial Landslide Scenarios
BGC Engineering Inc. has developed the landslide scenarios adopted as a basis for the present analysis. Figure 2‐5 shows the location of potential landslide scenarios 1 through 6 along Indian Arm and Burrard Inlet. The slide scenarios and slide characteristics are summarized in Table 2‐2. The slide scenarios are hypothetical but representative of locations where slope instabilities could occur or have happened in the past.
Table 2‐2: Summary of Landslide Characteristics Planar Extent Slide Location (m2) Volume (m3) Azimuth (°N) 1 328,408 36,483,961 275 2 67,818 2,678,407 110 3 4,126 24,000 345 4 190,043 12,828,514 270 5 119,287 9,300,400 270 6 82,610 640,000 50
Figure 2‐6 through Figure 2‐11 show the elevation profiles at slide locations 1 to 6. Topographic data shown is based on CDEM data, while bathymetric data is based on NOAA data. The vertical datum is Mean Sea Level. The elevation profile at the shoreline is approximate where data has been interpolated between data sets.
In the figures, the black line represents the profile of the fjord wall above water, while the blue line represents the (seabed) portion below water. The horizontal and vertical scales on the figures are approximately equal. The slide extents are indicated in red with the failure plane indicated by a dash‐dot line. The failure planes are for visualization only, and consequently the slide centroid locations are approximate.
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Figure 2‐5: Aerial view of Burrard Inlet and Indian Arm, Landslide Locations 1 to 6, and Modeling Domain
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Figure 2‐6: Slide Location 1 Elevation Profile
Figure 2‐7: Slide Location 2 Elevation Profile
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Figure 2‐8: Slide Location 3 Elevation Profile
Figure 2‐9: Slide Location 4 Elevation Profile
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Figure 2‐10: Slide Location 5 Elevation Profile
Figure 2‐11: Slide Location 6 Elevation Profile
Table 2‐3 summarizes the overall dimensions of the respective landslide geometries. In the table, the distance is the planar runout distance from the center of the slide origin to the water’s edge. The elevation is the height of the slide centroid relative to the water level. The slope or declination, taken as the ratio of the elevation to the runout distance, is a measure of
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the tsunami potential in relation to the potential energy attributed to the elevated slide mass. The steeper the slope the greater the component of gravitational acceleration affecting the slide mass, while the longer the distance the greater the frictional retardation of the slide.
Table 2‐3: Landslide Geometries Distance Elevation Slope Slide Scenario Width (m) Length (m) (m) (m) (V:H) 1 858 706 1:1.2 500 1,050 2 135 138 1:1.0 420 220 3 61 11 1:5.5 100 50 4 650 351 1:1.9 560 540 5 1,758 985 1:1.8 320 580 6 417 40 1:10.3 450 230
It can be seen that the terrain at slide locations 3 and 6 is relatively flat (Figure 2‐8 and Figure 2‐11). It is anticipated that slides at these two locations would take the form of a slump, rotational slide, debris flow or mud flow (Figure 2‐3), but with insufficient momentum to impact the water and produce a tsunami wave.
2.4 DEVELOPMENT OF LANDSLIDE PARAMETERS
Heller et al. (2009) have developed analytical relations that describe the overall characteristics of tsunami waves generated upon impact from subaerial landslides. Figure 2‐12 defines the primary parameters characterizing the initial wave formed upon impact of the slide mass.
Figure 2‐12: Primary Parameters Defining Impulse Wave Characteristics.
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Parameters characterizing the tsunami wave formation include the maximum wave height, , the maximum wave amplitude , and wave propagation along the radial distance at planar angle and water depth . Additional parameters characterizing the tsunami wave include the wave period , the wave length , and the speed of propagation (celerity) .
The parameters defining the characteristics of the subaerial landslide include the width
, the slide thickness , the bulk slide density , the porosity , the slide mass , the bulk slide volume VS, and the impact velocity at impact angle .
Heller et al. (2009) relate the subaerial landslide impact parameters via the impulse product parameter:
⁄ ⁄ 67⁄ ⁄
Where is the slide Froude number, is the relative slide thickness, is the relative slide mass, and is the slide impact angle defined in Figure 2‐12. The dimensionless quantities are defined as follows:
⁄ ⁄ ⁄
The tsunami wave height as a function of distance from the impact and propagation angle is given by:
3 2 ⁄ , ⁄ 2 3
Where is the impulse product parameter, is the water depth, and is the relative distance of maximum wave amplitude from the impact location.
⁄ The term: describes wave attenuation as a function of angle , and radial distance as defined in Figure 2‐12.
Figure 2‐13 provides a view of the tsunami wave attenuation as a function of direction and distance. At the initial wave formation at the back wall, the wave height is 100% at the peak. The figure shows subsequent attenuation with distance to the sides and front. Because of the forward momentum of the landslide at impact, the attenuation is less in the direction of the impact and more pronounced on the sides.
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Figure 2‐13: Tsunami wave attenuation as a function of direction and distance
The corresponding wave period is given by:
⁄ ⁄ , 15
Where is the tsunami wave height, is the water depth, and the gravitational acceleration.
Additionally, per (Heller et al., 2009), the tsunami wave height is proportional to the base parameters as follows:
. . . . . ∝ , , , ,
This shows that the tsunami wave height is nearly linearly proportional to the slide impact
velocity and to a lesser extent the slide thickness and the slide volume . It can also be seen that the tsunami wave height increases proportionally with water depth , and decreases with radial distance from the impact.
The governing parameter is the slide impact velocity which is given by (Heller et al., 2009):
2 ∆ 1 tan cot
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Where is the gravitational acceleration, ∆ is the drop height of the center of gravity of the slide, is the dynamic bed friction angle, and is the slide impact angle (Figure 2‐12). The first part of the equation describes the free fall velocity determined by equating the kinetic energy of the moving slide to the corresponding energy potential of the slide, i.e.
1 ∆ 2
Where the mass of the slide, , cancels out and the free‐fall velocity emerges as
2 ∆ .
The latter term in the impact velocity equation moderates the free fall velocity by accounting for friction. Consequently, as the dynamic bed friction angle approaches that of the overall slope , the impact speed diminishes. If the dynamic bed friction is greater than the slope angle, the weight of the slide mass won’t be able to overcome the frictional resistance and the slide doesn’t occur.
The value of the dynamic bed friction angle can range from 15° ≤ ≤ 35°, and a value of ≈ 20° may be assumed irrespective of whether the slide mass consists of rock, ice or snow, although the friction angle in reality may have a significant impact on the speed of the slide.
2.5 ASSESSMENT WITH ANALYTICAL APPROXIMATION
Based on the work of Heller et al. (2009), a rudimentary assessment of the potential tsunami wave characteristics can be made.
Table 2‐4 summarizes estimated parameters characterizing the potential tsunami waves for the adopted slide scenarios. The water depth is representative of the approximately deepest depth along the heading of the slide trajectory. The propagation angle represents the planar angle of wave propagation down Indian Arm or along shore in Burrard Inlet toward the project site. The angle is defined in Figure 2‐12 with zero degrees being perpendicular to the shoreline.
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Table 2‐4: Subaerial Landslide Tsunami Wave Characteristics Water Propagation H a T c L Slide Depth (°) (m) (m) (s) (m/s) (m) Scenario (m) 1 140 81 38.8 31.1 41.1 41.0 1,685 2 205 63 30.2 24.1 42.5 47.4 2,014 3 30 ‐ ‐ ‐ ‐ ‐ ‐ 4 140 45 44.2 35.4 42.5 41.5 1,762 5 120 70 41.5 33.2 40.2 38.8 1,560 6 20 ‐ ‐ ‐ ‐ ‐ ‐
The characteristic wave height of the initial tsunami wave is denoted by H in Table 2‐4 and is measured as the height from trough to crest as defined in Figure 2‐12. The amplitude of the tsunami wave, measured from the still water level to the crest of the initial wave is denoted by a in Table 2‐4. The wave period associated with the tsunami wave is labeled T, and the corresponding wave length, L. The speed of propagation, termed the celerity is given as c = L/T.
It can be seen that anticipated wave lengths are typically on the order of 1½ to 2 kilometers, while slide scenarios 3 and 6 would not be expected to produce tsunami waves.
The speed of propagation is directly related to water depth and is therefore lower at sites with shallower water depths. In general, the slides exhibit propagation speeds on the order of 40 m/s.
The tsunami wave heights and amplitudes are more varied as these are dependent on the multitude of parameters summarized in Table 2‐2, Table 2‐3, and Table 2‐4.
It should be noted that the parameters summarized in Table 2‐4 represent only one estimate of the characteristic tsunami wave parameters whereas another basis of assumptions would lead to other estimates.
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3. DEVELOPMENT OF NUMERICAL MODEL
3.1 MIKE‐21 HYDRODYNAMIC MODEL
M&N had developed a two‐dimensional (2D) DHI MIKE‐21 Hydrodynamic Model as part of the Metocean Study in 2012 (M&N, 2012). The existing modeling domain was extended to include the extent of the Indian Arm for the tsunami assessment. The 2D Hydrodynamic Model solves the depth‐averaged shallow water equations and simulates water level variations and flows in response to a variety of forcing functions (DHI, 2012).
The unstructured mesh consists of nearly 505,000 triangular elements. A uniform, high resolution mesh size (approximately 15‐20 meters in length) was used in order to minimize numerical diffusion and avoid underestimation of wave amplitudes.
3.2 MODEL BATHYMETRY & TOPOGRAPHY
Three sources of bathymetry and topography were compiled for the tsunami modeling. Bathymetry (or seabed elevations) for the majority of the modeling domain was extracted from the C‐Map Software of DHI, incorporating Jeppesen Norway’s electronic database of global nautical charts. Bathymetry in close vicinity of the TMEP project site was based on an under‐ water survey by Golder. Land topography behind and around the site was also included to an elevation of 30 meters above mean sea level (Natural Resources Canada, 2013). Figure 3‐1 shows the model bathymetry, referenced to mean sea level (MSL).
3.3 BOUNDARY CONDITIONS
The modeling domain contains one water boundary, west of the Ironworkers Memorial Second Narrows Bridge (Figure 3‐1). Typically, boundary conditions are required to apply physical forcing into the modeling domain through the water boundaries. However, because the run time of tsunami simulation was expected to be much shorter than a typical tidal circulation simulation, the water boundary was implemented as a closed boundary. After some initial runs, it was found that tsunami waves reach the TMEP site within 30 minutes or less so that the variation of tidal levels and tide‐induced currents within the simulation runtime are not significant. In addition, wind does not contribute to tsunami propagation. Therefore, these elements are not considered in the simulation.
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Figure 3‐1: Model Bathymetry
3.4 MODELING APPROACH
The above‐surface portion of the landslide was based on the work of Heller et al. (2009), as summarized in Section 2.4. As the front of the landslide hits the water surface, the effect of the landslide is simulated by generating a time varying bed level displacement. Subsequently, the below‐surface portion of the landslide motion follows the work of Watts (1998) and Grilli et al. (2005). Figure 3‐2 shows a sequence of time varying bed level displacement file used in the model.
Figure 3‐3 illustrates snapshots of the profile across Indian Arm at 15, 20, and 25 seconds after initiation of Landslide Scenario 1. A bore of water rises and propagates with the landslide
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movement. The phenomenon is similar to the laboratory results from Fritz, 2002 illustrated in the black‐and‐white photos included to the right in Figure 3‐3.
Figure 3‐2: Landslide Scenario 1 Example of Time Varying Bed Level Displacement Input into MIKE‐21 Model
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Figure 3‐3: Snapshot Showing the Tsunami Wave Generation from MIKE‐21 Model and Laboratory Tests of Bore Formation (Fritz, 2002)
3.5 MODELING CASES
Table 3‐1 lists the modeling cases conducted in this Tsunami Study. Some general assumptions for the simulation were made as follows:
• A box‐shape for the above‐water mass of landslide was assumed. As a result, the corresponding landslide thickness equals the landslide volume divided by the landslide length and width, as listed in Table 2‐3.
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• The landslide comes down the mountain slope as a block and no landslide transformation occurs in the process. This is a conservative assumption and should be verified with the Project Geologist. • Mean sea level (MSL) was selected as the water level. MSL is 3.1 meters above Chart Datum (CD). • A dynamic bed friction angle of 20 degrees was assumed.
Because Landslide Scenario 1 has the largest volume, it was further investigated for sensitivity tests, including reduction of the landslide thickness by half, at Higher High Water Large Tide (HHWLT, 5.1 m above CD) and at Lower Low Water Large Tide (LLWLT, 0.1 m below CD), and a dynamic bed friction angle of 35 degrees. Generally, the dynamic bed friction angle can range from 15° ≤ ≤ 35°.
Table 3‐1: Modeling Cases and Sensitivity Tests Landslide Scenario Landslide Thickness (m) Tide Level Bed Friction Angle (degree) Modeling Cases based on General Assumptions 1 69 MSL 20 2 29 MSL 20 4 42 MSL 20 5 50 MSL 20 Sensitivity Tests on Landslide Scenario 1 1A 35 MSL 20 1B 69 HHWLT 20 1C 69 LLWLT 20 1D 69 MSL 35
3.6 COMPARISON WITH ANALYTICAL APPROXIMATIONS
Based on Landslide Scenario 1, a comparison was made between the analytical solution methods provided in Heller (2007) and the MIKE‐21 numerical model runs.
Despite the landslide characteristics being the same between the analytical estimates and the MIKE‐21 numerical model, initial runs with the MIKE‐21 model revealed that the initial tsunami wave formation was highly dependent on the bathymetry at the slide impact location. This is because the volume of the slide is comparable to the volume of the surrounding water body. For a slide of this magnitude, it turns out that the width across Indian Arm (approximately 1.5 km across at the location of slide scenario 1) becomes a limiting factor in the maximum wave height formation, whereas an unrestricted landslide impact would develop more uniformly.
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In order to facilitate comparison between the numerical model and the analytical solution provided in Heller (2007), an idealized slide impact scenario was developed in which the water depth was fixed at 140 m and the water body unrestricted (western shore omitted). Figure 3‐4 provides a plan view comparing wave formation for Scenario 1 and the idealized Test Case.
Figure 3‐4: Plan view of initial wave height formation, Scenario 1 (left) and idealized bathymetry (right).
Figure 3‐5 illustrates differences between the model with the actual Indian Arm bathymetry and the idealized Test Case with open water and constant water depth of 140 m.
The solid and dotted blue lines represent profiles of the water surface elevation at two example time steps five seconds apart. The yellow solid and dotted lines represent the corresponding profiles from the idealized Test Case, elevation profiles five seconds apart.
The following differences between the models are apparent. In the case of the idealized bathymetry for the Test Case, the tsunami wave front forms as a distinct wave crest with a wide shallow trough in its wake continuing into a second crest and trough (wave propagation is from
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left to right). The water volume is balanced so that the amount of water in the wave crest (above Mean Sea Level) corresponds to the volume of the trough in its wake.
Comparing with Scenario 1, reflecting the actual bathymetry of Indian Arm, the wave fronts occur at differing distances. This is because the wave propagation speed (celerity) is non‐ uniform in the case of the real bathymetry, which has varying water depth leading to variations in the speed of wave propagation. Also, undulations of the water surface are apparent for Scenario 1 compared to the idealized Test Case. This occurs due to reflection from the shores of Indian Arm, which can be considered to be relatively narrow, 1.1 to 1.8 km, compared to the tsunami wave length, which is on the order of 1.4 to 2.1 km.
The most apparent reflection occurs around the initial slide impact location around 0 to 300 m, where reflection from the opposite shore results in an increase of the wave height due to superposition of the wave radiating out from the impact location and the waves reflected back from shore and the surrounding bathymetry.
Figure 3‐5: Comparison of water surface elevation profiles for Slide Scenario 1 and idealized Test Case
3.6.1 Analytical/Numerical Model Comparison
In the following, the tsunami wave characteristics predicted by the analytical model, Heller (2007), are compared with output from the MIKE‐21 model idealized Test Case. The primary tsunami characteristics considered include the wave length, wave height, wave amplitude, wave celerity, and wave period. The findings are presented in Figure 3‐6 to Figure 3‐10.
Figure 3‐6 shows the analytical/numerical model data for tsunami wave length as a function of propagation distance. The area highlighted in green bounded by the green dotted lines represents the range of wave lengths predicted by the analytical model, Heller (2007). The
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lower bound reflects the wave length of the tsunami wave radiating at angle (refer to Figure 2‐12), while the upper bound represents the wave length of the maximum wave propagating at the azimuth of the slide.
It can be seen that the numerical model results (yellow circles) are in line with the analytical model predictions and close to the results for the wave propagating at angle , which in this case is down Indian Arm.
3,000
2,500 (m)
2,000 Length
1,500 Wave 1,000
Tsunami 500
0 1,200 1,400 1,600 1,800 2,000 2,200 2,400 2,600 Distance (m) Figure 3‐6: Comparison of MIKE‐21 model tsunami wave length with Heller (2007) analytical method
Figure 3‐7 compares the tsunami wave height computed with the numerical model with the range of wave heights predicted by the analytical model. Again the lower dotted line reflects results for wave propagation at angle , while the upper dotted line is representative of the maximum wave.
Again, it can be concluded that the numerical model (yellow circles) produces results that are in line with the analytical model prediction.
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120.0
100.0 (m)
80.0 Heigth
60.0 Wave
40.0
Tsunami 20.0
0.0 1,200 1,400 1,600 1,800 2,000 2,200 2,400 2,600 Distance (m) Figure 3‐7: Comparison of MIKE‐21 model tsunami wave height with Heller (2007) analytical method
Figure 3‐8 compares the tsunami wave amplitude computed with the MIKE‐21 model with that of the Heller (2007) analytical model. In the analytical model the wave amplitude is given simply as: 45⁄ , i.e. 80% of the wave height . The numerical model results confirm this relationship.
120.0
(m) 100.0
80.0 Amplitude 60.0 Wave
40.0
20.0 Tsunami 0.0 1,200 1,400 1,600 1,800 2,000 2,200 2,400 2,600 Distance (m) Figure 3‐8: Comparison of MIKE‐21 model tsunami wave amplitude with Heller (2007) analytical method
Figure 3‐9 compares the speed of tsunami wave propagation (celerity) as computed by the analytical and numerical models. The range of propagation speeds (yellow area) predicted by the analytical model is bounded by the upper and lower dotted lines, representative of the maximum wave and the wave front radiating at angle .
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50.0 48.0 46.0 (m/s)
44.0 42.0 Celerity 40.0 38.0 Wave 36.0 34.0
Tsunami 32.0 30.0 1,200 1,400 1,600 1,800 2,000 2,200 2,400 2,600 Distance (m) Figure 3‐9: Comparison of MIKE‐21 model tsunami wave celerity with Heller (2007) analytical method
The numerical model results are indicated by the red circles, and are within the range of predicted by the analytical model. Compared with the previous results, substantial scatter is seen in the results. This is related to uncertainties in developing the wave celerity from numerical model output, which is taken as the distance of propagation of the wave front over a time step, approximately by: ∆ ⁄ ∆ .
For comparison, the theoretical wave celerity is indicated by the dark grey line. Per Heller (2007), the tsunami wave propagation is governed by the dispersion relation:
2 2
Where is the wave celerity, is the gravitational acceleration, is the water depth, and is the wave length. In the case of a shallow‐water wave, which is characteristic of tsunami wave propagation, the above equation reduces to:
Which expresses that the tsunami wave length is much greater than the water depth, ⁄ 1⁄ 20 (shallow‐water wave). This is the reason why the speed of tsunami wave propagation is wholly governed by the water depth and bathymetric features of the sea floor.
The above equation also expresses that when the tsunami wave amplitude is large, it contributes to the water depth via the term , whereas for a conventional small‐amplitude shallow‐water wave, the celerity would be given by .
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Finally, Figure 3‐10 shows how the MIKE‐21 numerical model output compares with the analytical method in terms of the tsunami wave period. In both cases, the wave period is determined from the relation:
∙
Where is the wave length, is the wave period, and is the wave celerity. Heller (2007), based on data for the wave period and wave celerity, utilizes the above relation to determine tsunami wave length.
60.0
55.0 (s)
50.0 Period
45.0 Wave
40.0
Tsunami 35.0
30.0 1,200 1,400 1,600 1,800 2,000 2,200 2,400 2,600 Distance (m) Figure 3‐10: Comparison of MIKE‐21 model tsunami wave period with Heller (2007) analytical method
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4. SUMMARY OF MODEL RESULTS
Findings from the numerical modeling are summarized in the following. Results for the four landslide scenarios that lead to tsunami wave formation are described in terms of tsunami wave formation and wave propagation, and wave attenuation on a regional scale, covering the area of Indian Arm and Burrard Inlet. Additionally, detailed results are provided at the project site in terms of the exposure of the berths to tsunami waves and tsunami‐induced currents.
Three of four sensitivity tests show that there are nearly no differences by varying tidal levels and the dynamic bed friction angle (1B through 1D). Only the case with half of the thickness (1A) shows a significant reduction of tsunami waves at the project site. Therefore, the results for 1B, 1C, and 1D were not considered further in the study.
Figure 4‐1 shows the model output locations at Berth 1 through Berth 3.
Figure 4‐1: Model output locations
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4.1 TSUNAMI WAVE PROPAGATION
Results for tsunami wave propagation are provided in Figure 4‐2 to Figure 4‐6 for landslide scenarios 1, 1A, 2, 4 and 5. Figure 4‐2 summarizes results for Landslide Scenario 1, which is representative of the largest slide volume amongst the cases investigated. Tsunami wave formation and subsequent wave propagation is illustrated across six snapshots at progressive time intervals. A time step shortly after the initial impact is shown in the upper left figure, and select time steps showing tsunami wave propagation are provided in the following images from left to right, ending with a snapshot of the tsunami wave propagation as it enters the vicinity of the project site. Each time step corresponds to a real time, ∆ of 5 seconds.
In the figures, the light blue color represents the (undisturbed) still water level. Shades of red indicate positive water level displacement (wave crests), while shades of darker blue indicate lowering of the water level (wave troughs). Because the wave height variation from the point of impact to the project site is substantial, the vertical scale is fixed to capture water level variations within 2 meters only in order to emphasize wave crests and troughs.
In the top left image of Figure 4‐2 it can be seen that the initial slide impact produces a near circular wave front which radiates out from the site of the landslide. The water depth is fairly deep in this part of Indian Arm and wave propagation proceeds swiftly across and down Indian Arm. In the second image, top right, it can be seen that the initial tsunami wave front starts propagating down Indian Arm with two wave fronts also propagation north. It can also be noted that the initial wave front at this time has reached the opposite shore across from the landslide impact site. The proportion of the landslide parameters to the water depth is such that the tsunami wave takes the form of a solitary wave or bore, which can be characterized by having most of the wave front in a crest above the mean sea level and a wide, shallow trough in the wake of the wave crest. Examples of typical wave profiles can be seen in Figure 3‐5.
The following two images, center left and right, capture progression of the tsunami wave front down Indian Arm. It can be noted that the initial wave front stretches out into a wider wave front. In the wake of the tsunami, the water motion is characterized by sloshing which occurs because tsunami wave components are reflected off the shoreline and back into the fjord. This is in contract to e.g. tsunami wave propagation in the open ocean where a radial pattern of wave propagation would be expected.
The bottom left image shows tsunami wave propagation just as the wave front passes Belcarra and Deep Cove. From this point on, the tsunami wave propagation is subject to a considerable slowdown, this because the wave propagation speed (celerity) is solely dependent on the water depth. Water depths around the central portion of Indian Arm reach 100 to 200 meters, as deep as 250 meters in some areas. In contrast, the water depth at Belcarra and
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Deep Cove and into Burrard Inlet is limited to 20‐30 meters. The last figure, bottom right, shows that as the tsunami wave front enters Burrard Inlet, the wave front disperses somewhat with two remnants of the wave front headed east and west, respectively.
Figure 4‐3 shows the same sequence for Scenario 1A, which reduces the thickness of Landslide Scenario 1 by half. The wave formation and propagation is similar to Scenario 1.
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Figure 4‐2: Tsunami Wave Propagation – Landslide Scenario 1
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Figure 4‐3: Tsunami Wave Propagation – Landslide Scenario 1A
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The sequence of tsunami wave formation and propagation for Scenario 2 is shown in Figure 4‐4. The landslide volume for this scenario is about an order of magnitude lower than Scenario 1 shown previously. The proportion of the landslide relative to the water body is therefore such that the tsunami wave takes the form of a weakly non‐linear oscillatory wave. The top left and right images showing initial tsunami wave formation and propagation therefore show a more radial progression of the leading wave front with subsequent troughs and crests in its wake.
As described for Scenario 1, as tsunami wave components arrive at the shoreline, reflection occurs, which causes the wave field to become scattered. Because of the limited slide volume, the tsunami attenuates earlier on its path down Indian Arm, and remnants of the tsunami waves are not noticeable in the lower part of Indian Arm past Belcarra and Deep Cove.
Diagrams of tsunami wave attenuation can be found in Appendix A. Wave attenuation coefficients for the project site for the cases investigated are summarized in Table 4‐1.
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Figure 4‐4: Tsunami Wave Propagation – Landslide Scenario 2
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Figure 4‐5 shows landslide tsunami formation and propagation for Scenario 4. The landslide volume for this scenario is about one third of Scenario 1 and thus comprises a substantial slide mass. The proportion of the landslide relative to the water body means that the tsunami wave has characteristics of a weakly non‐linear oscillatory wave to intermediate solitary‐ like wave transition.
The sequence of tsunami wave formation and propagation for Scenario 4 is shown in Figure 4‐5. As seen for the other cases, the wave field quickly becomes scattered due to wave reflection along the shoreline. Again, the bathymetry at Belcarra and Deep Cove acts as a choke point, and only a limited remnant of the tsunami wave front propagation is noticeable east and west in Burrard Inlet. Likewise, a barely noticeable wave front propagates north along Indian Arm.
Figure 4‐6 shows tsunami formation and propagation for landslide Scenario 5. The landslide volume for this scenario is about one fourth of Scenario 1 and comparable to that of Scenario 4, representative of a fairly substantial slide mass. The proportion of the landslide relative to the water depth in the fjord places the tsunami wave with characteristics of a transitional solitary wave.
The sequence of tsunami wave propagation for Scenario 5 is shown in Figure 4‐6. It can be seen that the wave height variation is substantial in the vicinity of the slide impact site due to wave reflection and wave interaction with the complex topography of the area. Due to the long propagation distance (around 17 km), the wave is subject to considerable attenuation on its path down Indian Arm and only a limited portion of the leading wave travels east and west in Burrard Inlet.
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Figure 4‐5: Tsunami Wave Propagation – Landslide Scenario 4
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Figure 4‐6: Tsunami Wave Propagation – Landslide Scenario 5
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4.2 TSUNAMI WAVE ATTENUATION
Tsunami wave attenuation data are summarized in Table 4‐1. The attenuation is a measure of the reduction of the tsunami wave height, measured as the ratio of wave height at the project site relative to the initial wave height at the site of each landslide.
Figures indicating wave attenuation are provided in Appendix A.
Table 4‐1: Tsunami Wave Attenuation Landslide Scenario Tsunami Wave Attenuation 1 3.4 % 1A 3.5 % 2 0.9 % 4 1.2 % 5 1.6 %
The results show that tsunami wave heights at the project site amount to only 1 to 3.5% of the initial wave height at the slide locations.
4.3 RESULTS FOR MODELING CASES
Statistics for modeling results during the 30‐minute simulation times are provided in Table 4‐2, including the maximum and minimum surface elevation, maximum tsunami wave height, approximate tsunami wave period, and maximum current speed. Figure 4‐7 presents an example of a time series of surface elevation. Maximum tsunami wave height is determined from the largest difference between a crest and a preceding trough, or vice versa. Because the corresponding tsunami wave period is not easily defined, a simple Fourier filtering (a toolbox in DHI MIKE‐21) was performed and the frequency (reverse of period) of the corresponding peak energy was used to identify tsunami wave periods.
Time series of surface elevations and current speeds at each of the berth at the project site is provided in Figure 4‐8 through Figure 4‐13.
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Table 4‐2: Summary of Results at Berth Locations Landslide Scenario Berth 1 Berth 2 Berth 3 Maximum Surface Elevation (m, MSL) 1 2.31 2.05 2.49 1A 1.44 1.29 1.50 2 0.24 0.23 0.24 4 0.55 0.53 0.63 5 0.91 0.87 0.89 Minimum Surface Elevation (m, MSL) 1 ‐0.60 ‐0.64 ‐0.82 1A ‐0.43 ‐0.44 ‐0.59 2 ‐0.10 ‐0.09 ‐0.16 4 ‐0.24 ‐0.20 ‐0.30 5 ‐0.30 ‐0.25 ‐0.29 Maximum Wave Height (m) 1 2.70 2.38 2.87 1A 1.63 1.42 1.68 2 0.30 0.27 0.29 4 0.64 0.56 0.84 5 0.92 0.86 0.93 Approximate Wave Period (s) 1 200 200 200 1A 200 200 200 2 125 125 125 4 125 125 125 5 > 300 > 300 > 300 Maximum Current Speed (knots) 1 1.95 1.86 1.92 1A 1.23 1.15 1.02 2 0.17 0.17 0.15 4 0.70 0.67 0.56 5 0.88 0.83 0.63
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Figure 4‐7: Definition of Model Result Statistics
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Figure 4‐8: Time Series of Surface Elevation at Berth 1
Figure 4‐9: Time Series of Current Speed at Berth 1
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Figure 4‐10: Time Series of Surface Elevation at Berth 2
Figure 4‐11: Time Series of Current Speed at Berth 2
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Figure 4‐12: Time Series of Surface Elevation at Berth 3
Figure 4‐13: Time Series of Current Speed at Berth 3
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5. EVALUATION OF MOORING IMPACTS
The planned berths at the project site are numbered from west to east, as shown in Figure 5‐1, with Berths 1 and 2 in a back to back configuration, which share three (3) outboard mooring dolphins. The loading platform of Berth 1 is located more outboard than Berth 2, and Berth 1 and 2 do not share aft mooring dolphins as the pipe racks and walkways interfere with such an arrangement.
Berth 3 represents the western most berth of the proposed expansion plan and has a mooring arrangement identical to that of Berth 2.
Figure 5‐1: General Arrangement Plan for the Proposed Westridge Facilities
5.1 TSUNAMI‐INDUCED WATER LEVEL VARIATIONS
Tsunami‐induced water level variations were output from the numerical model simulations at each of the three berths. The data was extracted at the approximate center of vessels moored at the berth.
Figure 5‐2 to Figure 5‐4 summarize the tsunami‐induced water level variations output from the investigated landslide scenarios. Positive values on the vertical axis for the solid orange
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bars depict the maximum water surface elevation for scenarios 1, 2, 4, and 5 (no tsunami for cases 3 and 6), while negative values indicate the lowest water level. It can be noted that the maximum water levels are larger than the low water levels. This is because the tsunami waves produced in the landslide scenarios propagate much like a solitary or translation wave, primarily consisting of displacement of water above the mean water level.
The transition area depicted for Scenario 1 with a gradient from yellow to white indicates the maximum potential range of water levels for Scenario 1, which places an upper limit on extreme events. As discussed elsewhere, this scenario is at present considered implausible and warrants further input and closure.
The reference datum is the Mean Sea Level (MSL) at zero. For comparison, the tide range is illustrated on the left side in the figures. The dark blue bar represents the high water tide range to Higher High Water Mean Tide (HHWMT), while the light blue bar indicates the range of Higher High Water Large Tide (HHWLT). The dark red bar indicates the range of Lower Low Water Mean Tide (LLWMT), while the light red bar denotes the range of Lower Low Water Large Tides (LLWLT).
Tide Range Scenario 1Scenario 2Scenario 4Scenario 5 3.0
2.0 HHWLT (m)
1.0 HHWMT Variation
0.0 Level ‐1.0
Water LLWMT ‐2.0
LLWLT ‐3.0
Figure 5‐2: Range of tsunami‐induced water level variations at Berth 1
It can be seen that the Landslide Scenario 2 produces the least variation in water levels at the project site, within a range limited to 0.3‐0.4 m. The remaining slide scenarios produce progressively larger water level variations (with increasing landslide volume), but at most a +1.5 m rise of the water level and a decrease in water level by ‐0.6 m. The results are fairly consistent between the three berths, although small differences in water levels can be noted.
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The tsunami‐induced water level variations can be noted to be on the same order of magnitude as the tides in the area. While changes in tide levels occur over minutes to hours, the tsunami‐induced water level variations would occur over a time span of minutes. However, it is likely that the moorings of a vessel at berth would be able to accommodate the change in water level.
It should also be noted that while Figure 5‐2 to Figure 5‐4 show water level variations around MWL for illustration purposes, a landslide event could occur at a higher or lower state of the tide. However, tsunami‐induced water level variations can be expected to be approximately within the tidal range.
Tide Range Scenario 1Scenario 2Scenario 4Scenario 5 3.0
2.0 HHWLT (m)
1.0 HHWMT Variation
0.0 Level ‐1.0
Water LLWMT ‐2.0
LLWLT ‐3.0
Figure 5‐3: Range of tsunami‐induced water level variations at Berth 2
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Tide Range Scenario 1Scenario 2Scenario 4Scenario 5 3.0
2.0 HHWLT (m)
1.0 HHWMT Variation
0.0 Level ‐1.0
Water LLWMT ‐2.0
LLWLT ‐3.0
Figure 5‐4: Range of tsunami‐induced water level variations at Berth 3
5.2 TSUNAMI‐INDUCED FLOW VELOCITIES
Figure 5‐5 summarizes maximum tsunami‐induced flow velocities by magnitude and direction for the landslide scenarios investigated. The dark blue curves are representative of Scenario 1, the purple curves reflect Scenario 5, the red curves Scenario 4, and the orange curves Scenario 2.
In each set of curves, the solid lines are representative of the flow field at Berth 1, the dashed lines portray results for Berth 2, and the dotted lines are representative of Berth 3. Overall, the results for the three berths are comparable in terms of magnitude and direction, although small differences are evident. The direction of flows are shown relative to true North, with the alignment of the berth indicated by the grey vessel outline.
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0° 350° 10° 340° 0.7 m/s 20° 330° 30° 0.6 m/s 320° 40° 0.5 m/s 310° 50° 0.4 m/s 300° 60° 0.3 m/s 290° 70° 0.2 m/s 280° 80° 0.1 m/s Scenario 1A Scenario 5 270° 0.0 m/s 90° Scenario 4
260° 100° Scenario 2
250° 110°
240° 120°
230° 130°
220° 140° 210° 150° 200° 160° 190° 170° 180° Figure 5‐5: Magnitude and direction of maximum tsunami‐induced flow velocities
The results show that while tidally driven ebb flows are approximately parallel to the berths (Figure 5‐6), the tsunami‐induced flows tend to follow the alongshore direction of the inlet, which is approximately west‐southwest to east‐northeast, which is also the case for tidal flood flows (Figure 5‐7).
A comparison between tsunami‐induced flow velocities and the magnitude and direction of currents adopted for the mooring analysis, (M&N 2014) is provided in Figure 5‐8 to Figure 5‐10.
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Figure 5‐6: East Burrard Inlet Mike21 model – ebb current snapshot, (M&N, 2012)
Figure 5‐7: East Burrard Inlet Mike21 model – flood current snapshot, (M&N, 2012)
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5.2.1 Comparison of Tsunami‐Induced Flow Velocities and Mooring Analysis
Figure 5‐8 to Figure 5‐9 provide a comparison between tsunami‐induced flow velocities and the flow velocities adopted for the mooring analysis (M&N, 2014).
In the figures, the horizontal axis reflects the heading, measured clockwise relative to a vessels moored bow‐out, see Figure 5‐1. A heading of 0 and 360 degrees corresponds to the vessel’s bow, while 180 degrees indicates a direction parallel to the berth from the stern. Headings of 90 and 270 degrees are perpendicular to the berth, beam to moored vessels.
The colored vertical bars in the figures reflect the maximum tsunami‐induced flow encountered at the particular heading at intervals of 10 degrees. The tan colored bars summarize results for Scenario 1, the yellow bars results for Scenario 2, while the green and dark green bars summarize the maximum flow velocities by heading for Scenario 4 and 5. The dark blue dotted line indicates the envelope of load cases studied in the mooring analysis (M&N, 2014).
0.00 0.29 0.21 0.00 0.09 0.14 0.12 0.31 0.16 0.33 0.29 0.28 0.32 0.24 0.42 0.15Scenario 0.12 1A 0.10Scenario 0.15 2 0.14Scenario 0.39 4 0.40Scenario 0.32 5 0.17OCIMF Envelope 0.30 0.23 0.15 0.44 0.00 0.08 0.13 0.16 0.14 0.45 0.42 0.09 0.21 0.35 0.14 Bow Stern Bow 0.00 0.36 0.00 0.00 0.14 0.13 0.10 0.28 0.38 0.32 0.24 0.17 0.16 3.0 0.00 0.18 0.10 0.00 0.10 0.11 0.10 0.27 0.21 0.30 0.00 0.15 0.00 0.00 0.00 0.00 0.00 0.09 0.13 0.06 0.24 0.17 0.24 0.25 0.14 0.00 0.352.5 0.20 0.24 0.17 0.06 0.13 0.12 0.19 0.05 0.09 0.23 0.12 0.15 0.54 0.20 0.27 0.27 0.07 0.07 0.13 0.27 0.40 0.32 0.24 0.11 0.12 0.572.0 0.21 0.23 0.28 0.00 0.11 0.13 0.16 0.39 0.39 0.00 0.10 0.09 0.61 0.22 0.18 0.31 0.08 0.09 0.13 0.22 0.15 0.39 0.07 0.30 0.00 (knots)
0.81 0.53 0.41 0.40 0.08 0.11 0.10 0.12 0.32 0.40 0.37 0.33 0.30 1.5 0.98 0.55 0.61 0.49 0.08 0.12 0.14 0.23 0.36 0.41 0.41 0.41 0.43
1.55Velocity 0.53 0.75 0.77 0.08 0.12 0.15 0.27 0.34 0.42 0.42 0.55 0.60
1.271.0 0.51 0.65 0.63Beam 0.07 0.14 0.15 0.35 0.36 0.42Beam 0.41 0.56 0.61
1.04Flow 0.52 0.56 0.52 0.09 0.15 0.16 0.38 0.35 0.36 0.38 0.47 0.34 0.930.5 0.54 0.52 0.46 0.08 0.16 0.14 0.40 0.34 0.35 0.37 0.32 0.27 0.99 0.54 0.49 0.49 0.07 0.16 0.10 0.26 0.33 0.34 0.35 0.28 0.23 0.70 0.30 0.37 0.35 0.15 0.11 0.09 0.29 0.27 0.20 0.32 0.24 0.20 0.000.0 0.24 0.30 0.00 0.16 0.10 0.00 0.25 0.21 0.12 0.29 0.23 0.19 0.360 0.17 20 0.00 40 60 0.18 80 100 0.09 120 0.10 140 160 0.09 180 0.29 200 220 0.24 240 2600.15 280 0.28 300 320 0.24 340 0.16 360 0.76 0.20 0.39 0.38 0.10 0.09 0.08Heading (°) 0.31 0.14 0.13 0.25 0.00 0.16 Figure 5‐8: Comparison of tsunami‐induced flow velocities and OCIMF cases adopted in Berth 1 mooring analysis
The OCIMF cases studied in the mooring analysis can be summarized as follows:
60‐knot constant wind from any direction simultaneously with either: • 3 knots current at 0 deg or 180 deg; • 2 knots current at 10 deg or 170 deg; and, • 0.75 knots current from the direction of maximum beam current loading.
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In the mooring analysis, these cases passed verification in terms of mooring system capacity by having mooring line tensions and fender compression within allowable limits.
The results for Berth 1, 2 and 3 summarized in Figure 5‐8 to Figure 5‐10 show that tsunami‐ induced flow velocities are generally well within the performance envelope required by OCIMF. Additionally, the OCIMF cases are analyzed for 60‐knot winds, which in the present comparison equates to a factor of safety because of the high wind speed combined with the considerable windage area of the moored vessels analyzed.
The only case where tsunami‐induced flow velocities approach or exceed the envelope of analyzed OCIMF cases, is for the extreme case of Landslide Scenario 1, which assumes that the entire landslide mass fails as one uniform block, which is considered improbable until deemed plausible through further geological study. The results for this scenario are therefore graded in yellow to white in order to de‐emphasize the results.
0.29 0.78 0.65 0.15 0.10 0.12 0.09 0.21 0.20 0.15 0.26 0.00 0.05 0.96 0.97 0.83Scenario 0.48 1A 0.13Scenario 0.12 2 0.09Scenario 0.31 4 0.19Scenario 0.17 5 0.24OCIMF Envelope 0.28 0.21 1.38 0.66 0.79 0.69 0.12 0.14 0.11 0.39 0.28 0.18 0.30 0.41 0.27 Bow Stern Bow 1.56 0.66 0.81 0.78 0.11 0.17 0.13 0.39 0.39 0.24 0.55 0.55 0.47 3.0 1.65 0.62 0.81 0.82 0.11 0.16 0.16 0.38 0.45 0.40 0.56 0.59 0.57 1.77 0.58 0.77 0.89 0.10 0.13 0.16 0.49 0.54 0.50 0.63 0.79 0.68 1.662.5 0.69 0.77 0.83 0.10 0.12 0.15 0.52 0.68 0.70 0.60 0.85 0.89 1.96 0.73 0.92 0.98 0.10 0.13 0.15 0.53 0.59 0.63 0.40 0.78 0.84 1.892.0 0.72 0.93 0.94 0.11 0.13 0.15 0.56 0.66 0.64 0.49 0.71 0.77 1.78 0.74 0.94 0.89 0.11 0.12 0.15 0.55 0.64 0.49 0.47 0.69 0.72 (knots)
1.02 0.87 0.74 0.51 0.11 0.12 0.15 0.54 0.53 0.39 0.49 0.69 0.69 1.5 0.74 0.47 0.51 0.37 0.10 0.11 0.15 0.53 0.43 0.34 0.52 0.68 0.64 Velocity
1.0 Beam Beam Flow
0.5
0.0 0 20 40 60 80 100 120 140 160 180 200 220 240 260 280 300 320 340 360 Heading (°) Figure 5‐9: Comparison of tsunami‐induced flow velocities and OCIMF cases adopted in Berth 2 mooring analysis
It can therefore be concluded that vessels moored at Berths 1, 2 and 3 can be expected to perform well if exposed to tsunami‐induced currents, and the potential for breakaway events due to parted mooring lines is considered to be low.
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Scenario 1A Scenario 2 Scenario 4 Scenario 5 OCIMF Envelope
Bow Stern Bow 3.0
2.5
2.0 (knots)
1.5 Velocity
1.0 Beam Beam Flow
0.5
0.0 0 20 40 60 80 100 120 140 160 180 200 220 240 260 280 300 320 340 360 Heading (°) Figure 5‐10: Comparison of tsunami‐induced flow velocities and OCIMF cases adopted in Berth 3 mooring analysis
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6. SUMMARY AND CONCLUSIONS
Findings of the study and conclusions derived thereof are summarized and discussed in the following.
6.1 DISCUSSION OF LANDSLIDE‐GENERATED TSUNAMI HAZARD
There are numerous records worldwide of tsunamigenic events, these occurring mostly in connection with seismic events, and a smaller portion attributed to landslide activity only. For the larger landslide‐tsunami events, these are commonly (but not always) triggered by earthquakes. As general guidance, seismically induced tsunami activity is noted mostly for earthquakes with magnitudes of 7 and larger. Although evidence of numerous landslides are on record worldwide, the details of the events are rarely known. Therefore, most research has focused on relating tsunami particulars to seismic magnitude, and only a limited number of cases have been studied in detail sufficient to establish the landslide volume, water depth, speed of impact, and other parameters. Only a handful of incidents have been captured as they happened and even in these cases, key parameters such as landslide thickness and initial wave height are not known or well established. Much of the data available is also distorted by the fact that there is a large uncertainty associated with the tsunami wave height, which varies substantially with distance from the location of impact, and what’s commonly measured are runup elevations along shorelines, which can vary greatly. What is known from the cases that have been studied in detail, is that each landslide tsunami event has its own particular circumstances and tsunami parameters are not easily generalized.
In most cases, the energy of landslide‐generated tsunamis amounts to around 1 to 5% of the kinetic energy of the landslide, and as the tsunami wave disperses over the water, additional wave attenuation may be as much as 95% depending on the topographic conditions. The problem thus lies in capturing the dynamics of the impact over two to three orders of magnitude of difference relative to the initial kinetic impact energy.
What is also known from the historical record is that the larger the tsunami‐landslide event, the more limited the number of realistic cases become. Thus, the truly large‐scale (mega) tsunami events are usually attributed to flank collapse associated with volcanic activity. Several historical cases are known such as Krakatau (Indonesia), Santorini (Greece), Kamchatka (Russia), Hokkaido (Japan), Stromboli (Italy), and several others, many of which a repeated in the historical record. Such events are geographically limited to the site of volcanic activity. Similar considerations can be made with respect to landslides not attributed to volcanic activity, but commonly triggered by seismic events. Probably the most notable case in recent history is Lituya Bay, Alaska (1958) where a seismically triggered landslide resulted in a tsunami wave and runup of around 524 meters on the opposite shore. Other notable subaerial landslides and associated
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tsunamis have occurred at Vajont (Italy), at Loen and Tafjord (Norway), Sanriku (Japan) and at Fatu Hiva (French Polynesia). Again, the cases of large landslide masses are commonly repeat offenders and Lituya Bay, Alaska, and sites in Norway have spawned several landslide‐tsunami events over the historical record, again probably because these sites have deposits of unstable rock on the mountain slopes.
Additionally, for a given landslide scenario, the chance that a large landslide occurs as a coherent block is less than a segregated slide. In order words, in order for a slide mass to fail as a single large block requires very specific characteristics of the local geology, joints sets of the rock mass, and a progressive weakening of the rock that’s conducive to failure of the rock mass limited to the sliding plane. Whether such conditions exist in the field is not part of the current study, but should be addressed by a geologist with local knowledge of the region.
Known tsunami events in BC in recent history attributed to seismic activity and landslide activity are summarized in Table 6‐1.
Table 6‐1: Tsunamigenic events in British Columbia in Recent History Max. Water Level Year Event Type Magnitude Location (m) 2012 Earthquake 7.7 Haida Gwaii 13.0 Earthquake and 1946 7.3 Vancouver Island 9.0 Landslide 1963 Unknown ‐ Graham Island 5.5 ‐ 1975 Submarine Landslide Kitimat Inlet 4.1 ‐ 1974 Submarine Landslide Kitimat Inlet 2.8 ‐ 2007 Subaerial Landslide Chehalis Lake 2.0 1929 Earthquake 7.0 Lake Ontario 1.5 Earthquake and Queen Charlotte 1949 8.1 0.6 Landslide Islands 2004 Earthquake 6.6 Vancouver Island 0.1
For perspective, Figure 6‐1 compares the slide scenarios investigated in the present study (yellow bars) with historical landslide events recorded worldwide (blue bars). The vertical scale is representative of the landslide volume in cubic meters. It can be seen that landslide scenarios 2 and 6 rank among the smaller ones, whereas scenario 4 and 5 can be categorized as intermediate. Landslide scenario 1 ranks as a truly large event, exceeding that of Lituya Bay.
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4.00E+07 3.50E+07 3.00E+07 2.50E+07 2.00E+07 1.50E+07 1.00E+07 5.00E+06 0.00E+00 Aegion Nice Scenario Eikesdahl Trondheim Sokkelvik Scenario Tafjord Fatu Loenvann Ravnefjell Ravnefjell Skagway Scenario Izmit Loenvann Rammerfjell Songevann Stegane Scenario Nordset Follero Kitimat Tjelle Scenario Lituya Vajont Scenario Papua
Hiva
Bay New
4 5 2 6 3 1
Guinea
Figure 6‐1: Comparison of slide scenarios investigated with slides recorded worldwide
Given that past landslide events occurring in BC have been limited and in most cases tied to significant seismic events, the association between tsunami risk and seismic hazards are discussed in the following.
Based on NBC values of peak ground acceleration for the Indian Arm region, the associated earthquake magnitude is estimated in Table 6‐2. The relationship between peak ground acceleration and magnitude is based on the relationship by Hasegava et al. (1981) for Western Canada:
1.02 . . 100
Where is the peak ground acceleration (g), is the earthquake (Richter) magnitude, and is the hypocentral distance.
The recurrence interval of seismic events of around magnitude 7 and greater is therefore estimated to be approximately 475 years or longer. The associated risk of occurrence within a project design life of 50 years is estimated to be 10% or lower, which is within the standards required in the NBC.
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Table 6‐2: Earthquake Risk, Recurrence, Peak Ground Acceleration, and Magnitude Risk of Exceedance Recurrence Magnitude PGA (g) in 50 years Interval (years) (est.) 2% 2,475 0.418 7.8 5% 1,000 0.299 7.5 10% 475 0.220 7.3 40% 100 0.104 6.7
6.1.1 Evidence of Past Tsunami Events
A substantial amount of research has been devoted to establishing whether the Vancouver, BC area has been exposed to tsunami hazards in the past. Sampling of sediments in the Fraser River Delta (1977, 1981, 1994, 1995, 2005) on the southwest side of Vancouver have not identified sediment deposits that can be attributed to tsunami activity over the past 4,000 years of the geological record. Therefore, the tsunami threat to the region can be categorized as very small.
Regarding potential landslides in Burrard Inlet and Indian Arm, investigations (Figure 2‐4) have not identified evidence of large landslides in the geological record evidenced by blocks, boulders, and runout deposits on the seabed. Additionally, the region has been exposed to the shaking associated with the 1700 Cascadia earthquake. This earthquake had a 1,000 km fault rupture and an average slip of 20 m. The magnitude of the earthquake has been estimated to 8.7‐9.2, and the recurrence interval around 570 to 590 years. It can be argued that potential deposits of unstable rock that did not slip during this event would be unlikely to fail during an earthquake of lesser magnitude, given that the Cascadia event can be considered to be fairly recent on a geological time scale.
6.2 CONCLUSIONS
Based on the scenarios investigated, the following conclusions can be made.
6.2.1 Landslide Scenarios
The landslide scenarios investigated cover the entire range of hazard exposure from no tsunami generation, to an event of extreme severity. The present analysis focuses on hydrodynamic aspects of tsunami waves at the project site.
A geological or geotechnical assessment could establish to what extent the investigated landslide scenarios are credible. At present, the extreme condition of landslide scenario 1 is
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comparable (actually slightly greater) than mega‐landslides in the historical record evidenced in Lituya Bay, Alaska. The current findings point to the fact that landslides of such magnitude tend to be localized by particular geological conditions conducive to formation of large deposits of unstable rock. It is recommended that a geologist be consulted to establish whether that is the case in Indian Arm.
Because no present findings point to large landslides in Indian Arm and Burrard Inlet in the past, and because the area has had (geologically) recent exposure to the 1700 Cascadia fault rupture, which was approximately magnitude 9.0, landslide scenario 1 is at present deemed implausible.
If landslide parameters are revisited and revised downwards, the result would be a substantial reduction in the magnitude of tsunami wave height, and corresponding flow velocities.
6.2.2 Tsunami Wave Formation and Propagation
Regarding the formation of tsunami waves as a result of landslide activity, it can be noted that while the initial wave height produced at the point of landslide impact can be substantial, features of Indian Arm tend to limit the maximum wave heights somewhat.
Factors affecting initial tsunami wave heights include the water depth and bathymetry, and the fact that the fjord is quite narrow and elongate with the primary landslide momentum directed fjord the sound.
Aspects of the bathymetry, such as the change from deep water in the central portion of Indian Arm to quite shallow water at the southern end down in Burrard Inlet work to impede tsunami wave propagation. The opening of the inlet to the east and west at the southern end of Indian Arm works to disperse tsunami wave energy.
6.2.3 Impacts to Berthed Vessels
The record of simulated water level variations and tsunami‐induced currents at the project site shows that tsunami‐induced water level variations are within the range of typical tides occurring in Burrard Inlet. While tsunami wave propagation would unfold over a matter of minutes as opposed to hours for tidal variations, the water level changes are believed to be slow enough that vessel moorings would be able to accommodate the change.
Likewise, it is found that tsunami‐induced currents are within the range of the OCIMF cases investigated in the mooring analysis (M&N, 2014), and moored vessels would therefore not be particularly prone to tsunami‐related impacts.
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