![My Comments in Green] in General, My Concern Is That If the Council Is Going to Enact Land Usage Restrictions, It Should Be Based on the Best Information Available](http://data.docslib.org/img/8fbc7205acf861352abcaa55fdf8bc4a-1.webp)
From: Andrew Wiser To: Don Easterbrook Cc: Cliff Strong; Council Subject: RE: Mt Baker volcanic hazards Date: Wednesday, February 01, 2017 10:16:03 AM Attachments: Porter_2013_Landslide_Risk_Evaluation.pdf Risk-based landslide safety assessments in Canada 12042016.pdf nhess-12-1277-2012-Meagerlandslide.pdf
Hi Don,
You certainly have identified the technical difficulties associated with crafting regulations for this particular geologic hazard. Staff is aware of, and is incorporating all of the nuanced issues you have mentioned in their presentation to the Council. In staff’s opinion best available science (BAS) would require that an updated hazard analysis be completed, which is presently in production at the USGS. I consider it unacceptable to rely on the 1995 Hazard map when we know updated BAS will be available in the next couple of years. In addition, I would add that the distinction between potential and probability is most appropriately addressed through completion of a Quantitative Risk Assessment (QRA). I’ve attached a few research articles describing QRA’s for your review. I urge you to attend the presentation as Council may wish to address some of your concerns directly if not done so sufficiently during the Staff presentation.
Sincerely, Andy Wiser, L.E.G. Geohazard Specialist, Planner Whatcom County Planning and Development Services [email protected] 360.778.5945
From: Don Easterbrook [mailto:[email protected]] Sent: Tuesday, January 31, 2017 9:42 PM Cc: Cliff Strong; Council Subject: Mt Baker volcanic hazards
Hi Andy, [my comments in green] In general, my concern is that if the Council is going to enact land usage restrictions, it should be based on the best information available. By their own admission, the USGS map is based on their 1995 map that has not been updated in 22 years, does not include any of the new data accumulated since then, and contains flaws that could significantly affect restrictions placed on private property. To make this the basis for restricting land usage without considering the wealth of other data is unduly narrow and does not lead to the best possible decisions. If I understand what you are saying, the Council is only going to use obsolete, flawed USGS information and is not interested in any my of work (which includes 51 publications and a 2016 book with a chapter specifically on lahars). I find this difficult to understand. “The purpose of the presentations to Council is to provide them with information regarding areas that could be potentially affected by future lahars. The role of the USGS folks is to present recent lahar modeling work against a general background of Mt. Baker volcanic hazards so that the Council can better understand the geographic areas that might be affected by future events.” But how can they do this adequately using only obsolete and flawed information? My concern here is that some property owners may end up with serious, unjustified restrictions on their property. “If I understand correctly your concern would be that the USGS is inferring lahar deposits downstream of Nugent’s Corner, where your research has shown none exist? I’m not sure that the USGS would contest that observation.” I think we agree on this. The question then becomes, was the 6,800-year old lahar at Deming viscous enough that it just stopped there, or did a soupy phase extend far downstream? The USGS says they believe it extended all the way to Bellingham Bay, which, if true has serious implications for the populated Nooksack floodplain, including the town of Ferndale. Will severe restrictions be placed on construction in Ferndale and elsewhere on the floodplain according to the guidelines posted online? This would seem to be the case according to the posted guidelines. So this becomes a very important issue. Here is one of the major flaws in the USGS map. I have specific, well-dated, geologic evidence that no lahars have flowed downvalley from Everson in the past 10,000 years, which makes a huge difference in possible restrictive building codes. What about possible future lahars?—I have specific evidence that any future lahars would not likely extend downvalley from Everson. The implications of the differences between the USGS interpretations and mine are very significant. “The 1995 USGS Hazards Map depicts potential hazards from future volcanic activity. It is an interpretation of areas that may be subject to lahar impacts based on a future lahar of a inferred size and behavior. It is not a map of observed lahar deposits at Mt. Baker.” I realize that the 1995 USGS map is intended to be a potential hazard map, not a probability map. Potential hazard maps are, by necessity, subject to a wide range of interpretations. For example, the Nooksack North Fork has no record of ever having had a lahar from Mt. Baker flow down it, the probability is zero, but that doesn’t mean it might not happenin the future, depending on conditions on the flanks of the volcano. The probability of such a lahar is zero (never happened before), but the possibility is not zero (it could happen in the future). To get a more complete picture of volcanic hazards from Mt. Baker, having separate probability and possibility maps would be useful. As we discussed earlier, lahars can be generated without volcanic eruptions, simply by collapse of large rocks masses on the flanks of the volcano. However, these do not usually extend as far downvalley as lahars generated during eruption of lava, which generate large volumes of water from melting of glacial ice. As shown by my mapping and dating of lahars and lava near Baker Pass, the extensive 6800-year-old lahar that travelled as far as Deming was the result of a volcanic eruption. No non-eruptive lahars are known in the Nooksack Valley, which suggests that the most destructive future lahar hazard might be expected during a volcanic eruption and these can be monitored and predicted reasonably well. “Interestingly, the USGS has recently completed updated modeling (LaharZ) of potential lahar impac is based on newly acquired lidar for Mt. Baker.” I’d love to get my hands on this lidar—it is readily available? The results are preliminary in nature, but generally agree with the 1995 Potential Hazards Map, especially in the confined mountain valleys. The larger presumed lahars are shown to extend beyond Nugent’s Corner, but only for a modest distance; I’d sure like to see this.a result that may be corroborated by your field observations. Of course associated lahar impacts such as flooding, sedimentation, and frequent and continued avulsion, likely persisting for decades, would extend far beyond the front of the lahar deposit into the Sumas and lower Nooksack River Valleys. “I had concluded from our previous conversations that you would concur that the potential exists for a lahar to be generated at Mt. Baker that could travel down either the North or Middle Fork Valleys, and that siting large-density developments in these areas, if possible to avoid, would be advisable.” Yes, as we discussed, that seems advisable. The question is, how restrictive should such measures be and where they should apply. For example, since the towns of Glacier, Deming, Everson, Nooksack, Sumas, and Ferndale all lie within the potential lahar zone on the hazard map, what kind of restrictive codes will apply to them? Would a COSTCO or WalMart be prohibited in all of these towns? Would schools, fire halls, hospitals, city halls be restricted? This is the issue where I believe common ground can be found between you and the researchers at USGS, and for the purpose of our presentation to the Council I believe this is the important scientific message to be communicated. We will also be recommending that strides be taken to quantify the risks posed to existing and proposed development as a result of lahar (and other geologic) hazards known to exist in Whatcom County, and that a lahar monitoring and response plan be created. Such a risk assessment would consider all available data and would provide policy makers a more defensible basis for determining an acceptable level of risk to public safety.
As I mentioned earlier, no amount of restrictive zoning can guarantee the safety of residents from volcanic hazards in the Nooksack Valley. The best approach to this is with a combined seismic monitoring program and a well developed evacuation plan. Don
GEOLOGICAL SURVEY OF CANADA OPEN FILE 7312
LANDSLIDE RISK EVALUATION
Canadian Technical Guidelines and Best Practices related to Landslides: a national initiative for loss reduction
M. Porter and N. Morgenstern
2013
GEOLOGICAL SURVEY OF CANADA OPEN FILE 7312
LANDSLIDE RISK EVALUATION
Canadian Technical Guidelines and Best Practices related to Landslides: a national initiative for loss reduction
M. Porter1 and N. Morgenstern2
1BGC Engineering, Suite 800 – 1045 Howe Street, Vancouver, BC V6Z 2A9 2Faculty of Engineering, University of Alberta, Edmonton, AB, T6G 2V4
2013
©Her Majesty the Queen in Right of Canada 2013 doi: 10.4095/292234
This publication is available for free download through GEOSCAN (http://geoscan.ess.nrcan.gc.ca/).
Recommended citation Porter, M., and Morgenstern, N., 2013. Landslide Risk Evaluation – Canadian Technical Guidelines and Best Practices related to Landslides: a national initiative for loss reduction; Geological Survey of Canada, Open File 7312, 21 p. doi:10.4095/292234
Publications in this series have not been edited; they are released as submitted by the author.
Canadian Technical Guidelines and Best Practices related to Landslides: a national initiative for loss reduction
LANDSLIDE RISK EVALUATION
Note to Reader
This is the seventh in a series of Geological Survey of Canada Open Files that will be published over the next several months. The series forms the basis of the Canadian Technical Guidelines and Best Practices related to Landslides: a national initiative for loss reduction. Once all Open Files have been published, they will be compiled, updated and published as a GSC Bulletin. The intent is to have each Open File in the series correspond to a chapter in the Bulletin. Comments on this Open File, or any of the Open Files in this series, should be sent before the end of March 2013 to Dr. P. Bobrowsky, [email protected]
1. INTRODUCTION
Landslide risk evaluation compares landslide risks, as determined from the risk analysis, against risk tolerance or risk acceptance criteria to guide the design and approval of proposed development and to prioritize treatment and monitoring efforts for existing development that is or could be exposed to a landslide (Figure 1, VanDine, 2012, and reproduced below). In situations where consequences are not considered, the process is technically hazard evaluation, but for simplicity, in this paper the term risk evaluation is used throughout. Landslide risk tolerance and risk acceptance criteria are more broadly referred to as landslide safety criteria. The combined process of risk identification, risk analysis and risk evaluation is referred to as risk assessment. The risk evaluation approach used and the landslide safety criteria adopted can vary depending on the risk scenario (sequence of events with an associated likelihood or probability of occurrence and consequences), the applicable legal framework, regulations and standards of practice and, for governments and corporations, factors such as market capitalization and insurance coverage that can influence the level of risk that can be tolerated or accepted. This contribution focuses on general principles and approaches of evaluating different measures of landslide risk. The attention is on the evaluation of landslide risk associated with existing and proposed residential development, because it is here that national guidelines can prove most beneficial. Many of the concepts and techniques applied to landslide risk evaluation for residential development can also be applied to other elements at risk (objects or assets such as human health and safety, property, aspects of the environment and/or financial interests that could be adversely affected by a landslide). Many of the examples provided involve sub-aerial landslides, but the risk evaluation concepts can also be applied to submarine landslides and landslide-generated waves.
Initiation: recognize landslide risk scenario(s); identify stakeholders; establish scope, goals, methods of risk management, and risk management team and responsibilities
Risk identification: confirm landslide risk scenario(s); identify study area and time frame; identify, inventory and characterize landslide(s) and elements at risk; collect and review background information
Risk analysis: for each landslide risk scenario, estimate likelihood or : on an ongoing basis keep all stake- all keep : on an ongoing basis probability, factor of safety, or slope deformation; travel path and distance; consequences; level of risk Risk assessment : on an ongoing basis monitor landslide/slope monitor: landslide/slope onbasis an ongoing Risk evaluation: for each landslide risk scenario compare risk estimates against tolerable risk or acceptable risk criteria; prioritize risk treatment and monitoring
Risk treatment: identify landslide risk treatment options; select performancescenario(s); and risk review management of methods risk
preferred option(s); implement risk treatment; estimate residual risk Review and Monitoring holders knowledgeable on all aspects of landslide risk; a two risk; process way oflandslide aspects holders on all knowledgeable Communication and Consultation and Communication
Figure 1: Landslide Risk Management Process (from VanDine, 2012; adapted from ISO, 2009)
Approving authorities across Canada frequently review the results of landslide risk assessments as part of the submissions for development and/or building permits. In such assessment reports, landslide professionals are often required to use a statement similar to the land may be used safely for the use intended, but rarely has safe been actually defined by the approving authority. Some improvements have been made in this regard, such as those documented in the Association of Professional Engineers and Geoscientists of British Columbia's landslide guidelines (APEGBC, 2010), but provincial and national approaches to risk evaluation and landslide safety criteria are typically lacking. Landslide safety criteria from jurisdictions across Canada and internationally are reviewed and the potential benefits of establishing provincial and/or national criteria are discussed. Considerations for communicating and consulting on landslide safety criteria and the results of risk evaluations are provided herein. Because risk evaluation is a rapidly evolving topic in Canada, this contribution should be treated as a 'work in progress' and updates will likely be required as Canadian approaches and landslide safety criteria evolve.
2. GENERAL PRINCIPLES
2.1 Individual versus Societal Risk
Where rapid landslides are possible, the potential for loss of life typically represents the overriding consequence of concern to authorities charged with approving proposed developments above, on or below landslide prone terrain. Safety criteria based on the risk of loss of life guide the development approval process for landslide prone areas such as in Hong Kong, Australia, and recently in the District of North Vancouver, BC, and, although not specific to landslides, form part of industrial health and safety regulations in the United Kingdom and the Netherlands (AGS (Australian Geomechanics Society), 2000; AGS, 2007; Ale, 2005; Leroi et al., 2005; Whittingham, 2009). Two measures of risk are considered: risks to individuals and risks to groups (or societal risk). Individual risk addresses the safety of individuals who are most at risk in an existing or proposed development. Societal risk addresses the potential societal losses as a whole caused by total potential losses of people in the community from a hazard event. When considering the exposure to a single landslide, risk is calculated according to Equation 1:
R = PH * PS:H * PT:S * V * E [1] where: R = risk; PH = annual probability of the hazard (i.e. landslide) occurring; PS:H = spatial probability that the landslide will reach the individual; PT:S = temporal probability that the individual will be present when the landslide occurs; V = the vulnerability, or probability of loss of life if an individual is impacted; and E = the number of people at risk; equal to 1 for individual risk. Partial risk is the combination of the first two terms, PH * PS:H. Partial risk is also known as encounter probability. Where risk of loss of life criteria are used in countries with a common law (case law or precedent) legal system, the maximum tolerable level of risk for a new development is typically 1:100,000 per annum for the individual most at risk (Leroi et al., 2005). A distinction is often made between new and existing development, with individual risks as high as 1:10,000 per annum sometimes tolerated for existing development. When the area of a potential landslide is small and the density of development is low, approval decisions are typically governed by the estimated individual risk. In contrast, when large groups are exposed to a potential landslide, societal risk analysis is typically used. For societal risk, if the spatial and temporal probabilities and the vulnerability vary across the population exposed to the hazard, the group is subdivided according to uniform levels of exposure with the results then summed to arrive at a total expected number of fatalities from the potential landslide. Societal risk estimates are typically presented on graphs showing the expected frequency of occurrence and cumulative number of fatalities, referred to as F-N curves (Figure 2 is one example). F-N curves were originally developed for nuclear hazards and the aerospace industries (Kendall et al., 1977) to illustrate thresholds that reflect societal aversion to multiple fatalities during a single catastrophic event. The graph is subdivided into four areas: unacceptable risk; tolerable risk that should be reduced further if practicable according to the as low as reasonably practicable (ALARP) principle; broadly acceptable risk; and a region of very low probability but with the potential for >1000 fatalities that require intense scrutiny. From the perspective of potential loss of life from a landslide, development is typically approved if it can be demonstrated that the landslide risk falls in the ALARP or broadly acceptable regions on an F-N curve (Kendall et al., 1977).
1.0E-02
1.0E-03
UNACCEPTABLE
1.0E-04
1.0E-05
ALARP
1.0E-06
1.0E-07
BROADLY ACCEPTABLE INTENSE SCRUTINY 1.0E-08
Frequency (F) of N or more Fatalities per year per moreN Fatalities of or (F) Frequency REGION
1.0E-09 1 10 100 1000 10000 Number (N) of Fatalities
Figure 2. Example F-N Curve for Evaluating Societal Risk.
2.2 Consultation Zone
The geographic area considered for a landslide risk assessment is known as the consultation zone (Geotechnical Engineering Office, 1998): a zone of standard extent that includes the area of proposed development within the maximum credible extent of potential landslides (Hungr and Wong, 2007). In Hong Kong, for example, assessments for potential rock fall, typically corresponds to a 500 m wide strip of land along the base of a slope. Altering the size of the consultation zone can change the estimates of societal risk. The above definition is effective for proposed or existing development in an area that is the responsibility of a single approving authority, but otherwise has its limitations. A more inclusive definition is proposed (Porter et al., 2009). The consultation zone is: a zone that includes existing and proposed development in one or more jurisdictional areas, that contains the largest credible area potentially affected by one or more concurrent landslides. Determining the largest credible area potentially affected by landslides requires an inventory of past landslides, an estimation of landslide volume, area or discharge and frequency, and a landslide runout analysis. In some cases, a preliminary estimate of the consultation zone can be made based on the area of the landform: for example, talus slopes affected by rock falls and creek fans subject to debris flows are typically well defined. Such information may not be known at the outset of a risk assessment unless regional landslide studies have been carried out and the resulting maps prepared.
2.3 Voluntary and Involuntary Risk
Individuals and organizations are typically willing to accept greater voluntary risks, that is, risks that are perceived to be within their control. Examples include an individual's risk of fatality from smoking (1:200 per annum), canoeing (1:500 per annum) and driving (1:10,000 per annum) (Whittingham, 2008). Residential occupants, however, rarely consider landslide risks as voluntary. Such landslide risks are typically considered involuntary, and thus landslide safety criteria values are likely to be less than the values reported earlier. Risks to workers from landslides might be considered voluntary because employees know that benefits (income) are, at least, partial compensation for the perceived risks, provided the risks are adequately understood and communicated. For example, Bunce and Martin (2011) suggest that a risk of fatality of 1:10,000 per annum represents a reasonable target for train crews operating in landslide prone terrain.
2.4 Tolerable versus Acceptable Risk
The following definitions are modified from VanDine (2012): • tolerable risk: risk within a range that society or an individual can live with so as to secure certain net benefits; a range of risk regarded as non-negligible and needing to be kept under review and reduced further if possible (adapted from AGS, 2007); and • acceptable risk: risk that society or an individual is prepared to accept and for which no further risk reduction is required (adapted from AGS, 2007). The use of risk of loss of life criteria originated in the United Kingdom and the Netherlands during the 1970s and 1980s in response to the need to manage risks from major industrial accidents (Ale, 2005). Hong Kong adapted the United Kingdom criteria for the management of landslide risks, and similar approaches have been applied in Australia (AGS 2007). While landslide safety criteria may vary amongst jurisdictions and the criteria for individual and societal risk are different, some common general principles apply (Leroi et al., 2005): • the risk from a landslide to an individual should not be significant when compared to other risks to which a person is exposed in everyday life; • the risk from a landslide should be reduced wherever reasonably practicable; that is, the ALARP principle should apply; • if the potential number of lives lost from a landslide is high, the corresponding likelihood that the landslide will occur should be low; this accounts for society's intolerance to many simultaneous casualties, and is embodied in societal landslide safety criteria; and, • higher risks are likely to be tolerated or accepted for existing developments than for proposed developments. In the United Kingdom, the maximum tolerable risk to an individual from an industrial accident in a new development has been set by the Health and Safety Executive at 1:100,000 per annum. The maximum tolerable risk for workers, based on the assumption that the risk faced by workers is somewhat voluntary, has been set at 1:1,000 per annum (Whittingham, 2008). In the Netherlands, maximum acceptable risk to an individual in a new development is 1:1,000,000 per annum. In practice, however, Ale (2005) has shown that the United Kingdom and Netherlands risk tolerance criteria are very similar as a result of the different legal systems employed by the two countries and mandatory application of the ALARP principle in the U.K.
2.5 Mortality Rates and Risks in Everyday Life
While there is precedent for using F-N curves and maximum tolerable risk criteria for individuals to evaluate landslides risks in Hong Kong and Australia, is it appropriate to apply similar tolerable risk criteria in Canada? A comparison of the Hong Kong landslide risk tolerance criteria against Canadians' level of background risk suggests these criteria may in fact be appropriate. An individual's annual risk of loss of life depends on a number of factors including his/her age, occupation, general state of health and other environmental factors. The Government of Canada (Canada, StatsCan, 2005) reports the average Canadian mortality rates by cause. Between 2000 and 2005, the age-standardized risk of loss of life by all causes was approximately 1:175 per annum, the average risk from accidental causes was about 1:2,500 per annum, and the average risk from automobile accidents was about 1:10,000 per annum. Table 1 compares the increase in the average Canadian's risk of loss of life if exposed to various levels of landslide risk. As discussed earlier, a general principle in establishing landslide safety criteria is that the incremental risk from a landslide should not be significant when compared to other risks in everyday life. Although significant is not defined (Leroi et al., 2005), an analysis of the increase in risk from various levels of landslide exposure suggests that the increase is <0.2% (low) for landslide risk levels less than 1:100,000 per annum.
Table 1. Canadians’ Incremental Risk of Loss of Life (per annum) under various Landslide Risk Levels (after Canada, StatsCan, 2005).
Landslide risk per annum Total Average Risk % Increase (expressed in a number of different ways)
0 0 0 5.637*10-3 0
1:1,000,000 10-6 0.001*10-3 5.638*10-3 0.018
1:100,000 10-5 0.01*10-3 5.647*10-3 0.18
1:10,000 10-4 0.1*10-3 5.737*10-3 1.8
1:1,000 10-3 1*10-3 6.637*10-3 18
2.6 Economic Risk Evaluation
The level of tolerable economic risk from landslides is a function of an individual's or organization's financial ability to tolerate or survive the potential economic loss. Influencing factors can include income or revenue, net worth or market capitalization, access to insurance, societal responsibilities, awareness of the risks, and availability of suitable emergency response plans to help recover from the potential loss. For example, large mining corporations and highway, railway and pipeline operators can often plan for, and recover from, multiple landslide incidents affecting their operations. Most local governments have much less experience and capacity to sustain economic losses caused by landslides. Most individual home owners, who typically do not have access to landslide insurance (see VanDine, 2011) have few options to financially recover from a landslide. Because of these different viewpoints, it is difficult to establish economic risk tolerance criteria for landslides that apply across a range of industries and organizational types and sizes, and individuals.
2.7 Qualitative Risk Evaluation
The potential consequences of landslides are wide ranging, and organizations and individuals have different levels of risk tolerance. Within some organizations there can also be a reluctance to express landslide risk in quantitative terms. In such cases, qualitative methods are useful to communicate and evaluate risks from landslides (and other hazards) and risks to a wide range of potential consequences. Risk management protocols can be assigned to a range of qualitative risk ratings. Order-of-magnitude estimates of landslide likelihood of occurrence and consequence are typically required to assign a qualitative risk rating. Thus, qualitative risk evaluation usually requires some numerical calculations to assist with systematically assigning qualitative risk ratings. For consistency, it is suggested that the qualitative descriptor moderate represent the limit of tolerable risk for an organization or society. Moderate and low risks typically fall in the ALARP risk zone and are tracked for further review and risk reduction where practicable, whereas risks ranked as high or very high are considered intolerable and require risk control. As one example, Table 2 shows the AGS' recommended qualitative terms for individual risk of loss of life from landslides (AGS, 2007). Using these qualitative descriptors, moderate risk represents the limit of tolerance for existing development that was adopted as the landslide risk tolerance criteria for the District of North Vancouver, BC (DNV, 2009).
Table 2. Sample Qualitative Descriptors for Risk of Loss of Life (after AGS, 2007).
Annual Probability of Loss of Life for the Qualitative Descriptor Individual Most at Risk >1:1,000 Very High
1:1,000 to 1:10,000 High
1:10,000 to 1:100,000 Moderate
1:100,000 to 1:1,000,000 Low
<1:1,000,000 Very Low
Figure 3 provides a sample qualitative risk evaluation matrix modified from many sources by BGC Engineering Inc. for application to landslide and other natural hazard risk assessments associated with large infrastructure projects. Likelihood and partial risk categories (the annual probability of a landslide occurring and reaching an element at risk) are shown on the vertical axis of the matrix; consequence categories for a range of potential consequences (safety, environment, social/cultural, and economic losses) are shown on the horizontal axis. Typically the likelihood and partial risk categories, and risk evaluation and response protocol, are kept constant, whereas the consequence descriptors are modified to match the landslide safety criteria established for a specific organization. For example, the economic loss category Catastrophic (risk evaluation and response column 6) would be adjusted to reflect the estimated economic loss that might lead to bankruptcy of the organization.
Multi-hazard Risk Evaluation Matrix (SAMPLE) For the Qualitative Assessment of Natural Hazards
Risk Evaluation and Response
Risk is imminent; short-term risk reduction required; long-term risk reduction plan must be developed and VH Very High implemented
Risk is unnacceptable; long-term risk reduction plan must be developed and implemented in a reasonable H High time frame. Planning should begin immediately
Risk may be tolerable; more detailed review required; reduce risk to As Low As Reasonably Practicable M Moder ate (ALARP)
L Low Risk is tolerable; continue to monitor and reduce risk to As Low As Reasonably Practicable (ALARP)
Partial Risk (annual probability) VL Very Low Risk is broadly acceptable; no further review or risk reduction required
Probability Indices Likelihood Descriptions Range Event typically occurs at least once per year F Almost certain >0.9 MHHVHVHVH
Event typically occurs every few years E Very Likely 0.1 to 0.9 L M H H VH VH
Event expected to occur every 10 to 100 years D Likely 0.01 to 0.1 LLMHHVH
Event expected to occur every 100 to 1,000 years C Possible 0.001 to 0.01 VL L L M H H
Event expected to occur every 1,000 to 10,000 years B Unlikely 0.0001 to 0.001 VL VL L L M H
Event is possible but expected to occur less than once every A Very Unlikely <0.0001 VL VL VL L L M 10,000 years 123456 Indices Incidental Minor Moderate Major Severe Catastrophic No impact Slight impact; recoverable Minor injury or personal Serious injury or personal Fatality or serious Multiple fatalities Health and w ithin days hardship; recoverable hardship; recoverable personal long-term w ithin days or w eeks w ithin w eeks or months hardship Safety
Insignificant Localized short-term Localized long-term Widespread long-term Widespread impact; not Irreparable loss of a impact; recovery w ithin impact; recoverable impact; recoverable recoverable w ithin the species Environment days or w eeks w ithin w eeks or months w ithin months or years lifetime of the project
Negligible impact Slight impact to social & Moderate impact to social Significant impact to Partial loss of social & Complete loss of social & cultural values; & cultural values; social & cultural values; cultural values; not cultural values
(Consequence) Social & recoverable w ithin days recoverable w ithin w eeks recoverable w ithin recoverable w ithin the Cultural or w eeks or months months or years lifetime of the project
Negligible; no business <$10,000 business <$100,000 business <$1M business <$10M business >$10M business interruption interruption loss or interruption loss or interruption loss or interruption loss or interruption loss or Economic damage to public or damage to public or damage to public or damage to public or damage to public or Description of expected negative outcome outcome negative Description of expected private property private property private property private property priviate property
Figure 3. Sample Qualitative Risk Evaluation Matrix.
2.8 Selecting a Method of Evaluating Landslide Safety
A few organizations and approving authorities have formally adopted methods of evaluating landslide hazard and risk; most others have not. In the latter case, a landslide professional should determine which method of evaluating landslide safety is appropriate. This section suggests a process for making this determination for a number of examples.
2.8.1 Limit Equilibrium Slope Stability Analysis and Factor of Safety
Limit equilibrium slope stability analyses can be used to obtain reliable estimates of the factor of safety where the kinematic failure mode of instability are understood and where the basic model input parameters, such as stratigraphy, shear strength, groundwater conditions and external loads, can be determined with reasonable accuracy. Slope stability analysis can be used: • to support the selection of residential setback guidelines from the top of potentially unstable slopes; taking into account the potential for future erosion at the base of the slope where appropriate; • in conjunction with liquefaction susceptibility and lateral spreading or deformation analyses, to assess the level of landslide safety under earthquake loading scenarios; and • to help assess and manage the level of landslide safety where it is determined that development is situated on a pre-existing deep-seated landslide. The observational method, by which predicted ground conditions and slope behavior are made in advance and verified during construction and management of a slope, helps minimize the effects of parameter, model, and human uncertainty (Morgenstern, 1995). When used in conjunction with the observational method, with few exceptions slope stability analyses have been applied successfully to the design and management of "engineered" slopes such as cuts, embankment fills, and retaining walls, and for the design of structures located on or at the crest of potentially unstable slopes.
2.8.2 Partial Risk (Encounter Probability)
Where existing or proposed development is located downslope (not on the slope itself) of a potential landslide, or behind a potential retrogressive landslide, partial risk (also known as encounter probability) can offer a suitable means of evaluating landslide safety. The application of partial risk criteria is best suited where it can be demonstrated that landslides pose a very low risk to an existing or proposed development, or where the probability of a landslide occurring and reaching the development is less than 1:10,000 per annum. Examples include: • sites where Holocene-age landslide deposits are absent and no potential source of large- scale instability is identified up slope; • sites located beyond the influence of the maximum credible landslide, such as outside of the rock fall shadow below a well-defined source area; • sites located behind the potential extent of long-term landslide retrogression as determined through geological mapping, landslide inventory, and the use of ultimate slope angles (e.g., De Lugt et al. 1993); and • sites where displaced material from the maximum credible landslide can be stopped by the design, construction and maintenance of physical barriers such as ditches, berms, catch nets or walls.
2.8.3 Quantitative Evaluation of the Risk of Loss of Life
For other situations it may be more appropriate to conduct a quantitative evaluation of the risk of loss of life and encourage the approving authority, in collaboration with the landslide professional, to compare the results against published landslide safety criteria. These special situations can for instance involve: • sites located at the base of slopes or in the potential landslide runout zone; • sites where it is impractical to demonstrate that the factor of safety for all landslides exceeds common acceptance criteria; and • sites where providing for physical protection against all credible landslide effects is impractical.
2.8.4 Relative Ranking of Likelihood and Consequences
Relative ranking of the likelihood and consequences is typically used by operators of linear infrastructure, such as highways, railways and pipelines who often have to manage their operations across numerous landslides. In this approach an inventory of landslides is compiled and ranked, often using semi-quantitative methods that consider likelihood and consequences. The relative ranking is used to prioritize sites for follow-up inspection and mitigation. Examples include CN Rail's Rock Fall Hazard Risk Assessment program (Abbott et al., 1998); the BC Ministry of Transportation and Infrastructure's rock fall hazard rating system; and geohazard inspection programs managed by several operators of oil and gas pipelines in western Canada. In such circumstances, the number of sites addressed in a given year is often a function of the available capital or operating budget assigned to landslide management which, indirectly, is a reflection of the organization's landslide risk tolerance.
3. PUBLISHED LANDSLIDE SAFETY CRITERIA IN CANADA
Landslide safety criteria for residential development and public infrastructure should reflect societal values. Criteria should be established and adopted by local provincial and/or federal governments. Where such criteria are not available, landslide professionals can advise decision makers as to appropriate criteria based on the risk scenario and criteria adopted elsewhere. The following summarizes landslide safety criteria that have been adopted or are in use in various jurisdictions across Canada. Much of the information is taken from Appendix C of APEGBC (2010). In that document landslide safety criteria are referred to as levels of landslide safety.
3.1 Canada
There are no nationally adopted landslide safety criteria in Canada. The National Building Code of Canada 2005 (NBCC, 2005) only provides the statement, Where a foundation is to rest on, in or near sloping ground, this particular condition shall be provided for in the design. The Canadian Foundation Engineering Manual (CGS, 2006), although it emphasizes foundation engineering, not landslides, contains several references to landslides: • the possibility of landslides should always be considered, and it is best to avoid building in a landslide area or potential landslide area; and • when a potential landslide area is identified, the area should be investigated thoroughly and designs and construction procedures should be adopted to improve the stability. CGS (2006) does not provide landslide safety criteria. It does, however, address limit equilibrium analysis and factors of safety. Although limit states design is now mandatory for foundation design (NBCC, 2005), limit equilibrium analysis and factors of safety remain applicable for landslide analysis. From CGS (2006): • factors of safety represent past experience under similar conditions; • the greater the potential consequences and/or the higher the uncertainty, the higher the design factor of safety should be; and • over time, similar factors of safety have become common to geotechnical design throughout the world. CGS (2006) does not provide a range of factors of safety that address landslides specifically; however, based on data from Terzaghi and Peck (1948 and 1967), that document indicates factors of safety for earthworks (engineered fills) that range from 1.3 to 1.5, and for unsupported excavations (engineered cuts) that range from 1.5 to 2.0. CGS (2006) indicates a lower factor of safety can be acceptable if: • a particularly detailed site investigation has been carried out; • the analysis is supported by well documented local experience; • geotechnical instrumentation to measure pore pressure and movement is provided and monitored at regular intervals to check the slope behaviour; or • where slope failure would only have minor consequences.
CGS (2006) also addresses earthquake loading, and indicates: • the NBCC (2005) has selected ground motions with a probability of exceedance of 2% in 50 years (1:2,475 per annum) for earthquake-resistant design purposes; • the factor of safety of a slope under static conditions must usually be significantly greater than 1.0 to accommodate earthquake loads; and • acceptable factors of safety depend on the uncertainty in the analysis, the soil parameters and the magnitude and duration of seismic excitation, in addition to the potential consequences of slope failure.
3.2 British Columbia
3.2.1 BC Building Code
Until 2010, the BC Building Code (BCBC, 2006) did not mention landslide safety for buildings. It stated only Where a foundation is to rest on, in or near sloping ground, this particular condition shall be provided for in the design. In December 2009, BC Ministerial Order M297 (BC, Province of, 2009) added: The potential for slope instability, and its consequences, such as slope displacement, shall be evaluated based on site-specific material properties and ground motion parameters in Subsection 1.1.3 [of BCBC, 2006] and shall be taken into account in the design of the structures and its foundations.
3.2.2 Seismic Slope Stability
In seismically active areas, earthquakes can trigger liquefaction or destabilize slopes leading to landslides or slope deformation. Section 4 and Appendix E of APEGBC (2010) provides recommendations for both methods of assessment and acceptance criteria. Guidance is based on consideration of earthquake ground motions with a 1:2,475 chance of exceedance, as per the NBCC (2005) and BCBC (2006). Where liquefiable soils may be present, it is recommended that a liquefaction susceptibility analysis be carried out. For other 'engineered' slopes, the following guidelines are provided: • the use of k = PGA with a factor of safety >1.0 in a pseudo-static slope stability analysis is considered as too conservative, and is recommended as only a preliminary screening tool; • methods by Bray and Travasarou (2007) are recommended to estimate median slope displacements for the design earthquake; • the proposed procedure is intended to define the critical slip surface that has an estimated 15 cm of median displacement so that the building can be located behind the critical slip surface; • the tolerable slope displacement of 15 cm is proposed as a guideline, based on experience with residential wood-frame construction. This guideline is not intended to preclude the landslide professional from selecting another value that he/she deems appropriate; and • since the estimated displacements are median estimates with a 50% of exceedance during the design earthquake (with a 1:2,475 return period), the proposed tolerable slope displacements roughly correlate with a partial risk (of structures being subjected to >15 cm of slope displacement) equal to a 1:5,000 chance of exceedance.
Further details can be found in APEGBC (2010).
3.2.3 Ministry of Transportation and Infrastructure
In British Columbia, the Ministry of Transportation and Infrastructure (BC MOTI) is the approving authority for rural subdivision approval outside of municipal boundaries and within those Regional Districts that have not assumed the role of the rural subdivision Approving Authority. In 2009, BC MOTI Approving Officers provided guidance on landslide safety criteria in a document entitled "Subdivision Preliminary Layout Review - Natural Hazard Risk." With respect to landslides, landslide safety criteria, paraphrased from that document, are as follows: • for a building site, unless otherwise specified, an annual probability of occurrence for a damaging landslide of 1:475 (10% probability in 50 years); • for a building site or a large scale development, an annual probability of occurrence of a life-threatening or catastrophic landslide of 1:10,000 (or 0.5% in 50 years); and • large scale developments must also consider total risk and refer to international standards. This guidance document has not yet been published and until the terms 'damaging' and 'life- threatening' are clearly defined, BC MOTI Approving Officers should be contacted for further details. Although the probabilities above are indicated as probabilities of occurrence in APEGBC (2010), this is considered to be incorrect terminology; they should be considered as probabilities of partial risk (VanDine, pers comm).
3.2.4 Fraser Valley Regional District
In the 1990's the Fraser Valley Regional District published landslide safety criteria for that Regional District for various types of natural hazards for a range of residential development (Cave 1992, revised 1993). These criteria, which are current today, were based on: • an interpretation of Mr. Justice Thomas Berger's 1973 decision that a return period of 1:10,000 years for a potentially catastrophic landslide affecting a proposed subdivision was unacceptable (Berger, 1973); • The 200-year return period for provincially sponsored flood-proofing; and • The BC MOTI's guideline of 10% probability in 50 years (BC MOTI, 1993). The criteria are lower (return periods as high as 1:50 per annum) for proposed modifications to existing development, while higher standards apply to new development. Higher landslide return periods (as high as 1:1,000 per annum) are tolerated for small landslides with potential to impact a single new residential structure, while only very low landslide return periods (<1:10,000 per annum) are tolerated for larger landslides with potential to impact a new subdivision. Implicitly, therefore, the criteria are risk-based. Although the probabilities above are indicated as probabilities of occurrence in APEGBC (2010), this is considered to be incorrect terminology; they should be considered as probabilities of partial risk (VanDine, pers comm).
3.2.5 District of North Vancouver
Two scenarios commonly encountered in the District of North Vancouver (DNV) are: • existing or proposed residential developments at the base of steep slopes or on debris- flow fans; most amenable to a risk of loss of life method (Section 2.8.3 above); and. • existing or proposed residential developments and associated retaining structures on or at the crest of slopes; most amenable to a limit equilibrium slope stability analysis and factor of safety method (Section 2.8.1 above). Landslide safety criteria were proposed by DNV staff based on discussion and review with landslide professionals and a task force of community citizens convened specifically to explore this issue. The criteria were adopted by DNV Council in 2009 (DNV, 2009). The DNV criteria were established to help evaluate landslide risk to life associated with both existing and proposed residential developments and for the two common development scenarios described above. They were also established to be compatible with recommended approaches to the landslide risk assessments outlined in APEGBC (2008, a prior version of APEGBC 2010) including use of the landslide assurance statement and the guidelines for seismic slope stability assessment contained in that document. The criteria are summarized in Table 3. These landslide safety criteria are applied at the development and building permit phases of development. Additional details are presented in Porter et al. (2007) and Porter et al. (2009).
Table 3. DNV Landslide Safety Criteria (DNV, 2009).
Application Type Risk <1:10,000 Risk < 1:100,000 FS >1.3 (static); FS >1.5 (static); 1:475 (seismic) 1:2,475 (seismic) Less than 25% increase X X in building footprint Repair or replace X retaining structure New residence, new retaining structure, or X X >25% increase in building footprint
Notes: 1. Risk = annual probability of fatality for individual most at risk 2. FS = limit equilibrium factor of safety for global failure 3. Seismic slope stability criteria based on specified ground motion chance of exceedance and either FS >1.0 or ground deformation <0.15 m in non-liquefiable soils, as per APEGBC (2010) 4. In addition to meeting these criteria, landslide risks must be reduced to ALARP so that the cost of further risk reduction would be grossly disproportionate to any risk reduction benefits gained.
3.3 Alberta
3.3.1 City of Calgary
Factor of safety based landslide safety criteria are utilized to guide residential development in the City of Calgary. Guidance documents on slope stability (Calgary, City of 2008 and 2009) state that: • a geotechnical report, prepared by a qualified geotechnical engineer, is required for all sites where existing or final design slopes exceed 15% or where, in the opinion of the City Engineer, acting reasonably, slope stability is a concern; • no development shall occur if the factor of safety against slope failure is less than 1.5; • lands with a factor of safety equal to or greater than 1.5 will be acceptable for development from a slope stability point of view; • if the factor of safety is less than 1.5, subject to the approval of the appropriate approving authority, the slope may be modified using remedial measures which are to the satisfaction of the City Engineer, to increase the factor of safety to a minimum of 1.5, thus increasing the area of developability; and • the setback limit, based on a minimum factor of safety of 1.5 shall be shown on the final development plan.
3.4 Ontario
The Ontario Ministry of Natural Resources published a guide that describes the province's Natural Hazards Policies (3.1) of the Provincial Policy Statement of the Planning Act (OMNR, 2001). It provides some guidance on landslide safety criteria which are used by municipal and regional approving authorities. Two examples are provided below.
3.4.1 Great Lakes and St Lawrence River Slope Setback Guidelines
Setback guidelines from potentially unstable slopes have been established for some approving authorities along the Great Lakes and St Lawrence River, and other river and stream systems (for example, Cataraqui Region Conservation Authority, 2005). Most of these guidelines call for setbacks that include an allowance for a prediction of 100 years of toe erosion (an erosion allowance) and a stable slope allowance that reflects the long term stability of the existing soil material. Along rivers and streams, an 'erosion access' allowance is also often required to provide access to the site for emergencies, regular maintenance, or unforeseen conditions. If development is proposed within these established limits, a site-specific geotechnical investigation is required.
3.4.2 City of Ottawa
The City of Ottawa has prepared Slope Stability Guidelines for Development Applications (Golder Associates Ltd, 2004). In these guidelines, unstable slopes (referred to as hazard lands) are defined as those that have a factor of safety of less than 1.5 against slope failure (less than 1.1 for seismic loading conditions). Where appropriate, allowances must also be provided for potential extreme retrogression of flow slides in sensitive clays, future toe erosion, and in some cases, an additional allowance for access to future slope failures. Development of permanent structures, including residential development, is typically precluded within hazard lands.
3.5 Other Jurisdictions
The Province of Saskatchewan has developed a relative ranking of landslide hazards and risks to aid in prioritizing and mitigating landslides affecting provincial highways (Kelly et al., 2004). The City of Winnipeg, MB, is developing a relative ranking of landslide hazards and risks affecting public lands within the city (James, 2009). Most other Canadian provinces, territories and municipal approving authorities have policies or guidelines that outline the need for landslide assessments and types of assessments that should be undertaken (for example in the Saguenay-Lac-Saint-Jean region of Quebec, Bilodeau et al. 2005). However, the authors are not aware of any other formally adopted provincial or municipal landslide safety criteria for residential development. Such criteria likely do exist or are in preparation, and as such criteria are brought to the attention of the GSC, it is hoped they will be incorporated into future versions of the Canadian Technical Guidelines and Best Practices related to Landslides.
3.6 Requirements for Provincial and National Landslide Safety Criteria
Most of the published landslide safety criteria described above have been developed by local governments (municipal or regional districts) in the absence of provincial or national standards and in response to the types of landslides and development pressures faced in those jurisdictions. What works well in one municipality or regional district is not necessarily appropriate in another. However, there are considerable benefits to establishing provincial and/or national landslide safety criteria. Such benefits include: • more consistent landslide safety criteria between local governments and provinces; • improved communication between developers, landslide professionals, approving authorities, insurance providers, real estate agencies, and the public; and, • in some cases reduced levels of landslide risk in jurisdictions where criteria have not been established. To be applicable across geographically diverse regions and a wide range of development scenarios, such guidelines likely require reference to a range of landslide risk evaluation and risk assessment methods and recommendations to landslide professionals on which methods are appropriate for given conditions and circumstances. Based on the review of available published guidelines, one or more of the following criteria are suggested as appropriate for proposed new residential development: • <1:10,000 per annum probability for a landslide occurring and reaching the area of proposed development; • <1:100,000 per annum risk of loss of life to individuals most at risk; • group or societal risk of loss of life evaluated on an F-N curve, with the ALARP or broadly acceptable regions as the landslide safety criteria; • tolerable slope deformation under seismic loading = 0.15 m (where it can be demonstrated that soils are not prone to earthquake-triggered liquefaction); and, • where appropriate, an allowance for 100 years of predicted toe erosion along river, lake, ocean, or reservoir shorelines. It is suggested that less stringent criteria, that is, risks up to one order of magnitude higher, may be appropriate for ongoing occupation of, or the approval of minor modifications to, existing residential development. Greater risks may also be tolerable for employees of organizations with infrastructure exposed to known landslides, provided systematic procedures are followed to understand, prioritize and manage the risks. Landslide safety criteria based on factors of safety would also be beneficial and are under review. These will need to take into consideration variables such as soil or rock type, site investigation effort, and the methods of analysis used to estimate the factor of safety. Under special circumstances, less stringent factor of safety criteria may also be appropriate for development on large, stabilized landslides if it can be demonstrated that landslide failure geometry and groundwater conditions are clearly defined through very detailed geotechnical investigation and analysis, and that strengths acting on the landslide shear surfaces are already at residual values.
4. COMMUNICATION AND CONSULTATION
As shown on Figure 1, and as introduced in VanDine (2012), risk communication and consultation are key components of the landslide risk management process and should be carried out during all stages of the risk management process. During the early stages of addressing a landslide hazard or risk, communication typically focuses on describing the potential risk scenario(s) and the process to be followed to characterize hazards and assess risks. Consultation focuses on establishing stakeholder objectives, the types of elements at risk, and the values that the stakeholders place on those elements. Maps, photographs and schematic illustrations are very useful to help convey technical information. Once risk estimates are available, communication focuses on an improved description of the potential landslides causative factors, the associated hazards, the potential range of consequences, and the estimated risk levels. Uncertainties need to be described along with proposed methods of managing uncertainty. Risk levels need to be placed into context through analogy (for example, comparison with other risks that stakeholders encounter in everyday life). If landslide safety criteria have not been established, consultation is needed to address what levels of risk the stakeholders are willing to tolerate and how those compare with what is used in other jurisdictions. Where, through comparison with available landslide safety criteria or through the consultation process, it is determined that landslide risks are unacceptable, the communication process needs to focus on describing the range of options available to reduce risk, the associated costs, the likelihood of success, and ongoing maintenance requirements to treat residual risks. The feasibility and cost of achieving extremely low risk levels needs to be described. Consultation is required to determine stakeholder preferences for risk treatment. During the treatment and monitoring phases, communication can involve use of warning signs, publication of hazard and risk maps and technical reports, testing of emergency response protocols, and making the results of instrument monitoring, slope inspection and updated hazard or risk ratings available to interested stakeholders. Web-based communication of landslide stabilization, monitoring and inspection results is becoming a more feasible and common means of timely dissemination of information to interested stakeholders.
5. ACKNOWLEDGEMENTS
Doug VanDine (VanDine Geological Engineering Limited, Victoria, BC) collaborated on this chapter. Numerous landslide professionals provided invaluable technical and editorial comments. Reviewers (in alphabetical order) included: Peter Bobrowsky, Andrew Corkum, Matthias Jakob, Martin Lawrence, Scott McDougall, Lynden Penner, Robert Powell, Bert Struik and Dave van Zeyl.
6. REFERENCES
Abbott, B., Bruce, I., Savigny, W., Keegan, T., and Oboni, F., 1998. A methodology for the assessment of rock fall hazard and risk along linear transportation corridors; in Proceedings 8th International Congress, International Association for Engineering Geology and the Environment, (ed.) D.P. Moore, and O. Hungr; v. 2, p. 1195-1200.
Ale, B.J.M., 2005. Tolerable or Acceptable: a Comparison of Risk Regulation in the United Kingdom and in the Netherlands; Risk Analysis, v. 25, p. 231-241.
APEGBC (Association of Professional Engineers and Geoscientists of British Columbia), 2010 (prior version 2008). Guidelines for Legislated Landslide Assessments for Proposed Residential Developments in British Columbia; 75 p.
AGS (Australian Geomechanics Society) Sub-committee on Landslide Risk Management, 2000. Landslide Risk Management Concepts and Guidelines; Australian Geomechanics; p. 49-92.
AGS (Australian Geomechanics Society) Sub-committee on Landslide Risk Management, 2007. A National Landslide Risk Management Framework for Australia; Australian Geomechanics; v. 42, p. 1-36.
BCBC (BC Building Code), 2006. Published by the Office of Housing and Construction Standards, BC Ministry of Forests and Range, Victoria, BC.
BC MOTI (Ministry of Transportation and Infrastructure), 1993. Subdivision policy and Procedures Manual; Systems Planning Research Office, Victoria, BC.
BC, Province of, 2009. Ministerial Order M297, Regulation of the Minister of Housing and Social Development, Province of BC, Victoria, BC, December 15, 2009.
Berger, T.B., 1973. Reasons for judgment for the Honourable Mr. Justice Berger on the matter of the Land Registry Act – and on application for approval of a proposed subdivision by Cleveland Holdings Ltd. Supreme Court of British Columbia, Victoria, BC, 26 p.
Bilodeau, C., Genois, A., Demers, D., Potvin, J., Robitaille, D., and Dupuis, C., 2005. Cartographie des zones exposées aux glissements de terrain dans les dépôts meubles : Guide d’utilisation des cartes de zones de contraintes et d’application du cadre normatif. Publication du Gouvernement du Québec, Ministère de la Sécurité publique., 77 p.
Bray, J.D. and Travasarou, T., 2007. Simplified procedure for estimating earthquake-induced deviatoric slope displacements; Journal of Geotechnical and Environmental Engineering, ASCE, v. 153, p. 381-392.
Bunce, C. and Martin, D., 2011. Risk estimation for railways exposed to landslides; in Proceedings of Geohazards 2011, Kelowna, BC, Paper 215, 13p.
CGS (Canadian Geotechnical Society). 2006. Canadian Foundation Engineering Manual; 4th edition, Bi-Tech Publishers, Richmond, BC, 488 p.
Calgary, City of, 2008. Design Guidelines for Development Permits; Development Site Servicing Plans; and Waste & Recycling Services for Commercial/Industrial Applications; City of Calgary, AB, February 2008.
Calgary, City of, 2009. Slope Adaptive Development Policy and Guidelines & Conservation Planning and Design Guidelines; City of Calgary, AB, March 2009.
Canada, StatsCan, 2005. Mortality, Summary List of Causes; Government of Canada, Catalogue no. 84F0209X.
Cataraqui Region Conservation Authority, 2005. CRCA Planning Policy, Cataraqui Region Conservation Authority, ON, Canada.
Cave, P.W. 1992, (revised 1993). Hazard Acceptability Thresholds for Development Approvals by Local Government; in Proceedings of the Geological Hazards Workshop, Victoria, BC, (ed.) Peter Bobrowsky; Geological Survey Branch, Open File 1992-15, p. 15-26.
DNV (District of North Vancouver), 2009. Report to Council: Natural Hazards Risk Tolerance Criteria; District of North Vancouver, BC, November 10, 2009.
Geotechnical Engineering Office, 1998. Landslides and boulder falls from natural terrain: interim risk guidelines; Hong Kong Government, GEO Report no. 75.
Golder Associates Ltd., 2004. Slope Stability Guidelines for Development Applications in the City of Ottawa; report to the City of Ottawa, ON, Canada.
Hungr, O. and Wong, H.N., 2007. Landslide risk acceptability criteria: are F-N plots objective?; Geotechnical News, December 2007, p. 3-6.
ISO (International Organization for Standardization), 2009. Risk management – principles and guidelines; ISO 31000, 24 p.
James, A., 2009. Development of a riverbank asset management system for the City of Winnipeg; Master of Science Thesis, Department of Civil Engineering, University of Manitoba, Winnipeg, MB, Canada.
Kelly, A.J., Antunes, P.J., Clifton, A.W., and Widger, R.A., 2004. Application of a Landslide Risk Management System to the Saskatchewan Highway Network; in Proceedings of the 57th Canadian Geotechnical Conference, Quebec City, QU, paper G13-501, 8 p.
Kendall, H.W., Hubbard, R.B., Minor, G.C. and Bryan, W.M., 1977. The Risks of Nuclear Power Reactors: a Review of the NRC Reactor Safety Study; WASH-1400 (NUREG-75/014), Unioin of Concerned Scientists, Cambridge MA, 210 p.
Leroi, E., Bonnard, C., Fell, R., and McInnes, R., 2005. Risk assessment and management; in Proceedings of International Conference on Landslide Risk Management, Vancouver, Canada, (ed.) O. Hungr, R. Fell, R. Couture, and E. Eberhardt; Balkema, p. 159-198.
Morgenstern, N.R., 1995. Managing Risk in Geotechnical Engineering; in Proceedings of X Panamerican Conference on Soil Mechanics and Foundation Engineering, Guadalajara, Mexico, Sociedad Mexicana de Mecanica de Suelos, A.C., p. 102-126.
NBCC (National Building Code of Canada), 2005. Published by the Natural Research Council of Canada, Ottawa, ON, Canada.
OMNR (Ontario Ministry of Natural Resources), 2001. Understanding Natural Hazards: an introductory guide for public health and safety policies 3.1, provincial policy statement; Ontario Ministry of Natural Resources, Toronto, ON, Canada.
Porter, M., Jakob, M., and Holm, K., 2009. Proposed Landslide Risk Tolerance Criteria. Proceedings, in Proceedings of the Canadian Geotechnical Conference 2009, Halifax, NS, Canada, Paper 75, 9 p.
Porter, M., Jakob, M, Savigny, K.W., Fougere, S., and Morgenstern, N., 2007. Risk Management for Urban Flow Slides in North Vancouver, Canada; in Proceedings of the Canadian Geotechnical Conference 2007, Ottawa, ON, Canada, Paper 95, 9 p.
Terzaghi, K. and Peck, R.P., 1948, 2nd edition 1967. Soil Mechanics in Engineering Practice. John Wiley and Sons, 729 p.
VanDine, D.F., 2011. Professional Practice and Insurance Issues – Canadian Technical Guidelines and Best Practices Related to Landslides; Geological Survey of Canada, Open File 6981, 13 p.
VanDine, D.F., 2012. Risk Management – Canadian Technical Guidelines and Best Practices Related to Landslides; Geological Survey of Canada, Open File 6996, 8 p.
Whittingham, R.B., 2009. Preventing Corporate Accidents, an Ethical Approach; Elsevier, Oxford, UK, 370 p.
Risk-based landslide safety assessments in Canada
Porter, M., [email protected], Jakob, M., [email protected], Holm, K., [email protected] BGC Engineering Inc., Vancouver, BC V6Z 0C8
McDougall, S. [email protected] Department of Earth, Ocean & Atmospheric Sciences, University of British Columbia, Vancouver, BC V6T 1Z4
ABSTRACT: Various methods are applied by landslide professionals in consulting, government and industry to assess and communicate landslide safety. These include factor of safety calculations, designation and delineation of slope setback distances, estimates of hazard probability or encounter probability, and semi-quantitative and fully quantitative estimates of risk of loss of life. Because the sudden and often unexpected occurrence of very rapid to extremely rapid landslides can result in life loss, and because a consensus on appropriate risk tolerance criteria is emerging in western Canada, quantitative risk assessment (QRA) is gaining popularity amongst regulators and landslide professionals to assess and communicate levels of landslide safety where residential development and other infrastructure are exposed to these hazards. Use of QRAs requires development of appropriate methods for landslide inventories, magnitude, frequency, mobility and intensity estimates of past and future landslides as well as vulnerability estimates of people and infrastructure at risk. It also requires access to landslide risk tolerance criteria to contextualize risk estimates and guide communication and risk management decisions.
Association of Professional Engineers and INTRODUCTION Geoscientists of British Columbia's landslide guidelines (APEGBC, 2010), but consistent Approving authorities across Canada review the provincial and national landslide safety criteria do results of landslide assessments as part of the not exist. submissions for development and/or building Despite this limited guidance, several Canadian permits. In such assessment reports, landslide landslide professionals and government agencies are professionals are required to use a statement similar leading advancements in the application of to: the land may be used safely for the use intended, quantitative risk assessment (QRA) for landslide risk but the term ‘safe’ is rarely defined by the approving management. Use of QRA to manage landslide authority. Some improvements have been made in safety originated in Hong Kong in the early 1990s, this regard, such as those documented in the borrowing methods for risk evaluation from the
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United Kingdom Health and Safety Executive (Malone, 2005). The District of North Vancouver, British Columbia, was the first municipality in Canada to formally adopt and apply landslide risk tolerance criteria similar to those developed in Hong Kong (DNV, 2009). This work was completed transparently with public consultation and input (Tappenden, 2014). Quantitative landslide risk assessments have since been carried out by several other local and provincial government agencies in British Columbia and Alberta, including the Town of Canmore, Alberta, who recently formally adopted risk tolerance criteria to manage debris flood and
debris flow risk (Canmore, 2016). Figure 1: Landslide Risk Management Process (from VanDine, This paper summarizes key concepts presented 2012; adapted from ISO, 2009) in two chapters of draft Canadian Technical Guidelines and Best Practices related to Landslides: Risk Management (VanDine, 2012); and Landslide METHODS FOR ASSESSING LANDSLIDE Risk Evaluation (Porter and Morgenstern, 2015). It SAFETY provides an overview of methods used to conduct landslide safety assessments in Canada and In Canada, only a few organizations and approving highlights case histories where QRA has been authorities have adopted a single method of successfully applied. We define risk concepts and evaluating landslide safety. In most other cases, a input parameters for landslide QRAs. Canadian and landslide professional must determine which method international landslide risk tolerance criteria are of evaluating landslide safety is appropriate. This reviewed and compared with criteria commonly used section suggests a process for making such a to guide the management of dam safety in Canada determination. and the United States. Slope stability analysis and factor of safety RISK MANAGEMENT FRAMEWORK Limit equilibrium slope stability analyses can provide estimates of the factor of safety where the Figure 1 summarizes the typical landslide risk source and mechanism of instability is understood management process adapted from ISO (2009). The and where the basic model input parameters, such as process is depicted as five horizontal single outlined stratigraphy, shear strength, groundwater conditions boxes in the center of the figure, with descriptors and external loads, can be estimated with reasonable specific to landslides. The double outlined boxes accuracy. Slope stability analysis can be used: indicate that risk assessment is a combination of risk to support the selection of residential setback identification, risk analysis and risk evaluation. The distances from the crest of potentially right hand vertical box indicates that risk monitoring unstable slopes; taking into account the and review of methodology is a process that should potential for future erosion at the base of the be carried out on an ongoing basis throughout the slope, where appropriate; risk management process. Similarly, the left hand in conjunction with liquefaction vertical box indicates that risk communication and susceptibility and lateral spreading or consultation with all stakeholders (including the deformation analyses, to assess the level of appropriate government agencies) should be carried landslide safety under earthquake loading out throughout the process. scenarios; and to help assess and manage the level of landslide safety where it is determined that
3rd North American Symposium on Landslides, Roanoke, VA, June 4-8, 2017 2
development is situated on a pre-existing Risk of loss of life deep-seated landslide. Where landslides pose a credible threat to an existing The observational method, by which predicted or proposed development, it may be more ground conditions and slope behavior are made in appropriate to conduct a quantitative evaluation of advance and verified during construction and the risk of loss of life and encourage the approving management of a slope, helps minimize the effects of authority, in collaboration with the landslide parameter, model, and human uncertainty professional, to compare the results against (Morgenstern, 1995). When used in conjunction with published landslide safety criteria. These special the observational method, slope stability analyses situations can involve sites located in the potential have been applied successfully to the design and landslide runout zone, where it is impractical to management of engineered slopes such as cuts, fills, demonstrate that the factor of safety for all potential and retaining walls, and for the design of structures landslides exceeds common acceptance criteria, and located on or near the crest of potentially unstable where providing for physical protection against all slopes. credible landslide effects is not feasible.
Encounter Probability LANDSLIDE RISK ESTIMATION Where existing or proposed development is located downslope of a potential landslide, the probability of Risk Equation a landslide occurring and impacting an element at Where rapid landslides are possible, the potential for risk, referred to as the encounter probability (or loss of life typically represents the overriding partial risk), can offer a more suitable means of consequence of concern to authorities charged with evaluating landslide risk than the factor of safety approving proposed developments above, on or methods described above. below slopes. The application of encounter probability is best When considering the exposure to a single suited to demonstrate that landslides do not pose a landslide scenario, risk is calculated according to credible threat to an existing or proposed Equation 1: development, or where the probability of a landslide occurring and reaching the development is less than Risk = P(H) P(S:H) P(T:S) (V) (E) (1) the acceptance criteria set by local approving where: authorities. Examples include: P(H) = annual probability of the landslide sites where Holocene-age landslide deposits occurring; are absent and no potential source of large- P(S:H) = spatial probability that the landslide scale instability is identified upslope; will reach the individual most at risk; sites located beyond the reach of the P(T:S) = temporal probability that the maximum credible landslide, such as outside individual most at risk will be present when of the rock fall shadow below a well-defined the landslide occurs; source area; and V = vulnerability, or probability of loss of life sites where debris from the maximum if the individual is impacted; and credible landslide can be stopped by the E = number of people at risk, which is equal design, construction and maintenance of to 1 for the determination of individual risk. physical barriers such as ditches, berms, catch nets or walls. A range of potential landslide volumes and If it is demonstrated that landslides do pose a credible associated probabilities exist at most site (i.e. threat to an existing or proposed development, then landslides often exhibit a magnitude-frequency formal estimation of risk of loss of life is preferred relationship). In these cases, different probabilities over estimates of encounter probability. can be assigned to the range of possible landslide scenarios originating from a given slope. When
3 Porter, M., Jakob, M., Holm, K., McDougall, S. Risk-based landslide safety assessments in Canada
landslide scenarios ‘i’ through ‘n’ are taken into
consideration, Equation 1 becomes: 1.0E-02
∑ : : (2) 1.0E-03
LANDSLIDE RISK TOLERANCE CRITERIA UNACCEPTABLE 1.0E-04 Where risk of loss of life criteria are used in countries with a common law (case or precedent law) legal 1.0E-05 system, the maximum tolerable level of risk for a ALARP
new development is typically 1:100,000 per annum 1.0E-06 for the individual most at risk (Leroi et al., 2005). A
distinction is often made between new and existing 1.0E-07
development, with individual risks as high as BROADLY ACCEPTABLE INTENSE 1:10,000 per annum usually tolerated for existing SCRUTINY 1.0E-08 development (GEO, 1998, 2014, DNV, 2009, per year (F) of Nmore Frequency or Fatalities REGION Canmore, 2016). 1.0E-09 When the geographic area of a potential 1 10 100 1000 10000 landslide is small and the population density is low, Number (N) of Fatalities approval decisions are typically governed by the estimated individual risk. In contrast, when a large Figure 2. Example F-N Graph for Evaluating Societal Risk population is exposed to a potential landslide, (GEO, 1998). societal risk analysis is typically used. For societal risk, if the spatial and temporal probabilities and the Many international organizations responsible for vulnerability vary across the population exposed to dam safety, including those in Canada and the United the hazard, the group is subdivided according to States, utilize similar individual and societal risk uniform levels of exposure with the results summed tolerance criteria in their dam safety programs (e.g. to arrive at a total expected number of fatalities from USACE, 2014). the potential landslide. Societal risk estimates are presented on F-N GUIDANCE ON RISK COMMUNICATION AND graphs (Figure 2 is one example). Curves plotted on CONSULTATION these graphs showing the expected frequency of occurrence and cumulative number of fatalities are As shown on Figure 1 and introduced in VanDine referred to as F-N curves. F-N graphs were (2012), communication and consultation should be originally developed for nuclear hazards and the carried out during all stages of the risk management aerospace industries (Kendall et al., 1977) to process. illustrate thresholds that reflect societal aversion to During the early stages of addressing a landslide multiple fatalities during a single catastrophic event. hazard or risk, communication typically focuses on The F-N graph is subdivided into four areas: describing the potential risk scenario(s) and the unacceptable risk; tolerable risk that should be process to be followed to characterize hazards and reduced further if possible according to the as low as assess risks (Porter and Morgenstern, 2015). reasonably practicable (ALARP) principle; broadly Consultation, which is coordinated by the owner or acceptable risk; and a region of very low probability regulator, focuses on establishing stakeholder but with the potential for >1,000 fatalities that objectives, the types of elements at risk, and the requires intense scrutiny. From the perspective of values that the stakeholders place on those elements. potential loss of life from a landslide, development is Maps, photographs, animations, videos and typically approved if it can be demonstrated that the schematic illustrations are used to help convey landslide risk falls in the ALARP or broadly technical information. acceptable regions on an F-N graph.
3rd North American Symposium on Landslides, Roanoke, VA, June 4-8, 2017 4
Once risk estimates are available, interested stakeholders (e.g. Porter and Dercole, communication focuses on an improved description 2011). of the potential landslide causative factors, the associated hazards, the potential range of EXAMPLE APPLICATIONS OF QRA FOR consequences, and the estimated risk levels. LANDSLIDE ASSESSMENTS Uncertainties are described along with proposed methods of managing uncertainty. However, it can District of North Vancouver, British Columbia be impractical to quantify uncertainty for each factor In the early morning of January 19, 2005, heavy systematically through the risk analysis without rainfall triggered a landslide at the crest of the introducing excessive complexity, or uncertainty Berkley Escarpment in the District of North bounds so wide that they are impractical for decision Vancouver (DNV), British Columbia (Porter et al., making. 2007; Porter and Dercole, 2011). The landslide It is helpful to compare landslide risks with other destroyed two homes at the base of the slope, risks that stakeholders encounter in everyday life. seriously injuring one person and killing another. A For example, in Canada an individual’s average risk review of previous engineering reports, published of fatality in a motor vehicle accident is around literature, and aerial photographs revealed that five 1:10,000 per annum which provides context for the other landslides had occurred along this two- individual risk tolerance criteria commonly kilometre (1.25 mile) long escarpment since 1972. In employed for existing residential development 2005 approximately 75 homes were present along the (Porter et al., 2009). If landslide safety criteria have base of this slope. Concerns over the potential impact not been established, consultation is needed to of future landslides prompted DNV Municipal address what levels of risk the stakeholders are Council to commission a landslide assessment and willing to tolerate and how those compare with what implement a landslide management program. The is used in other jurisdictions. authors were retained to conduct the landslide Where, through comparison with available assessment. landslide safety criteria or through the consultation At the time, it was common for Canadian process, it is determined that landslide risks are engineers and geoscientists to manage the risks from unacceptable, the communication process should slope stability in urban settings by the use of a factor focus on describing the range of options available to of safety against slope failure. A factor of safety reduce risk, the associated costs, the likelihood of greater than 1.5 was often adopted in an attempt to success, and ongoing maintenance requirements to address uncertainty related to input parameters such treat residual risks. The practicality, cost as soil strength, groundwater conditions, model effectiveness, and fairness of achieving extremely uncertainty, and the consequences of failure. low risk levels should be described. Consultation is The authors found these traditional methods to required to determine stakeholder preferences for be inadequate in cases where residential risk treatment. development was situated on stable ground During the treatment and monitoring phases, downslope of a potential landslide, particularly when communication can involve use of warning signs, the factor of safety on the slope above was known to publication of hazard and risk maps and technical be less than 1.5. The factor of safety does not provide reports, newspaper articles, testing of emergency information on the likelihood that a landslide will response protocols, and making the results of impact a development, nor does it provide instrument monitoring, slope inspection and updated information on the consequence of the landslide to hazard or risk ratings available to interested people and infrastructure within the development. stakeholders in an understandable format. Web- We reviewed Canadian examples where hazard based communication of landslide stabilization, return period had been used to guide approvals for monitoring and inspection results is emerging as a residential development and modification in areas means of timely dissemination of information to potentially subject to landslides and floods (e.g., Cave, 1992). While this approach addressed some of
5 Porter, M., Jakob, M., Holm, K., McDougall, S. Risk-based landslide safety assessments in Canada
the shortcomings of the factor of safety approach, unacceptable risks were identified to address embedded in the tolerable hazard return periods were specific questions and concerns they had. assumptions about the likely consequences of failure Engineering reports, hazard and risk maps, that affect its transparency. and meeting presentation materials were We reviewed national and international made available to the public on the DNV literature on best practices for assessing and website and in public libraries. managing landslide hazard and risk and concluded Notice was put on land title for properties that a risk-based approach offered the best where landslide risk mitigation included a opportunity to identify, characterize and requirement for property owners to maintain transparently communicate the potential hazards and storm and perimeter drain infrastructure. risks to the municipality and local residents so that Slope hazard development permit areas were optimal decisions on how to manage those risks defined, including lands within both the could be made. initiation and runout zones of potential Furthermore, there was a need to put the risk landslides. estimates into context in order to determine if they Procedures were created to monitor were tolerable or not. At the time, the literature precipitation and groundwater levels on the provided examples of such criteria that were being slope, and to trigger visual inspection of the used in other countries such as Australia and Hong slope if threshold values were exceeded. Kong (e.g. AGS, 2000), and these criteria served as A Natural Hazards Task Force, comprising a starting point for discussion with DNV and affected members of the community and District Staff residents. was formed to address issues around risk On the basis of the foregoing, a landslide QRA tolerance criteria, and to determine how best was completed and it was determined that the risks to manage risks from landslides and other to individuals in approximately 50 homes along the natural hazards across the District. base of the escarpment were unacceptable (Porter et This case history has subsequently served as a al., 2007). These were eventually treated through a model for landslide assessment and risk combination of drainage improvements and fill communication at several other locations in British removal along the crest of the escarpment. Homes Columbia and Alberta, and elements are being damaged in the 2005 landslide were acquired by incorporated into the Canadian technical guidelines DNV and the land was turned into a municipal park. on landslides (Porter and Morgenstern, 2015). The following risk communication activities Landslide risk tolerance criteria have since been were undertaken in North Vancouver as part of the formally adopted by DNV for ongoing assessment of landslide risk management program: existing and proposed new residential development Town hall meetings were held to discuss the (Porter and Dercole, 2011). risk assessment process, results, mitigation options and proposed mitigation plan. The Catiline Creek, British Columbia levels of risk reduction that could be achieved Catiline Creek is located within a 4 km2 watershed through the proposed mitigation measures on the north side of Lillooet Lake in the Squamish- were described, along with the residual risks Lillooet Regional District (SLRD), British Columbia following mitigation. (Figure 3). The watershed rises from 500 m at the The methods used to estimate hazards and fan apex to 2130 m at the crest of the watershed. The risks, and the results, were communicated upper basin is extensively gullied and steep, with a transparently. We compared life loss risk Melton Ratio of 0.8, and abundant boulder lobes and estimates with statistical risk of fatality in levees on the fan indicate previous debris-flow motor vehicle accidents to put the results into activity. Areas of distressed slope and evidence of a context. rockslide deposit also exist on the fan, suggesting Smaller group meetings were held with rockslides up to 400,000 m3 could occur. residents and owners of property where
3rd North American Symposium on Landslides, Roanoke, VA, June 4-8, 2017 6
Figure 4. Debris flow levee from the 2013 Catiline Creek event adjacent to a building on the lower fan.
Figure 3. Catiline Creek Watershed and Fan. Most of the We completed a debris flow safety risk lower half of the fan contains residential development assessment for persons within residential dwellings obscured by trees. on Catiline Creek fan and evaluated three different risk control options (BGC 2015). Table 1 The fan was subdivided in the early 1970s and summarizes the representative debris flow scenarios contains 155 residential lots, of which about 114 developed for hazard modeling and risk analysis, have been developed and are currently occupied. based on a frequency-magnitude relationship At least 11 debris flows have reached the fan in developed from previous events, interpretation of the past 66 years, including five debris flows post- historical air photographs, test-trenching, fan surface dating development in 1986, 1987, 2004, 2010 and observations, and dendrochronology. The events up 2013 (BGC 2015). A debris flow in 2010 traveled to 100,000 m3 were considered “conventional” through part of the subdivision, damaging a small debris flows, while larger events were considered to shed, narrowly missing several houses and a boat involve a large bedrock failure in the upper basin. launch, burying a truck, and blocking several subdivision roads. A debris flow in 2013 (Figure 4) swept over the driveway of an A-frame house, pushed the same truck that was buried in 2010 into the lake, and destroyed several boats stored on land.
7 Porter, M., Jakob, M., Holm, K., McDougall, S. Risk-based landslide safety assessments in Canada
Table 1. Summary of modelled debris flow volumes by existing conditions and by up to a factor of 20 for BGC (2015). those lots currently at highest risk. This would reduce individual risk to within the tolerable, but not Representative Return Period Representative Event Return broadly acceptable, range (e.g. less than 1:10,000 but Interval (years)1 Volume (m3)2 Period2 greater than 1:100,000 annual probability of loss of life for an individual). 10-30 10 6,000 Figure 5 shows estimated baseline and residual 30-300 100 40,000 group risk for currently occupied lots. The upper 300-3,000 1,000 100,000 bold line represents our best-estimate of group risk. 3,000+ 10,000 300,000 The dashed lines above and below the best-estimate show group risk based on the upper and lower 1 Return period intervals used in risk analysis, ranging from the smallest and most frequent to the largest event that bounds of vulnerability criteria described in BGC could be credibly estimated with the data available. (2015). Best-estimates were used for the other risk 2 Return period and volume reaching the fan used to variables, and the full range of uncertainty was not represent the return period interval in runout modeling. quantified. Estimated residual risk (lower bold line) plots Numerical modeling of debris flows provided partially within the ALARP zone and is reduced by the basis to estimate spatial impact probabilities and about two orders of magnitude for N=1, compared to corresponding debris-flow intensities. The model existing conditions. This reflects high confidence results were used to generate runout exceedance and that the proposed preliminary design should hazard intensity maps as primary inputs to the risk successfully contain relatively frequent, smaller assessment. We estimated the probability that debris events, assuming channel capacity is maintained in flows will impact residential dwellings and cause perpetuity. However, estimated group risk extends loss of life, and compared the estimates to individual into the Unacceptable range for higher numbers of and group risk tolerance criteria, as described earlier fatalities, largely associated with low frequency, in this paper. Our best-estimate of individual risk high magnitude debris flows which were the most exceeded 1:10,000 risk of fatality per year for 76 of uncertain scenarios in terms of volume and return the 114 occupied, residential-classed lots within the period. study area, and estimated group risk fell entirely into The risk assessment results suggest that the the “Unacceptable” range of the F-N graph. proposed preliminary design meets the risk reduction We evaluated three possible risk reduction target for individual, but not group risk if annual options, including measures to improve channel probabilities of up to 1:10,000 are used in the risk capacity and reduce avulsion potential, construction assessment. Additional methods for further reducing of a diversion and new channel extending away from individual and group risk according to the ALARP the development, and construction of a debris barrier principle may include slope deformation monitoring at the fan apex. Of these, improving the existing and a response plan (e.g., evacuation), as well as channel to accommodate up to a 1,000-year (100,000 structural protection to protect individual buildings 3 m ) design event was selected, with input from the from debris flow impacts. community, as having the most favorable combination of cost, constructability, environmental and social impact, and level of risk reduction. This option was carried forward to preliminary design (Kerr Wood Leidal Associates, 2016). We then completed landslide modeling and residual risk analysis to evaluate the level of risk reduction achieved by the proposed risk control measures (BGC 2016). On average, we estimated that the proposed mitigation measures would reduce individual risk by about a factor of ten compared to
3rd North American Symposium on Landslides, Roanoke, VA, June 4-8, 2017 8
the net sediment volumes transported onto fans. Sediment volumes were estimated from a variety of empirical methods (Jakob et al. 2017). Peak flows were estimated from measured channel cross- sections in bedrock reaches and back-calculated flow velocities. Debris flows and debris floods were then simulated with FLO-2D, a two-dimensional runout model suitable to simulate sediment deposition over fans. Flow velocities and flow depths were combined into an intensity index (Jakob et al. 2011), which served as a proxy for the vulnerability of buildings and informed the risk assessment. Figure 5. F-N curve showing estimated risk for groups. The major steps in the risk assessment included Shown are estimated group risk for existing conditions (BGC an assessment of direct consequences or potential 2015) and estimated residual risk assuming mitigation consequences to buildings and infrastructure due to recommendations are adopted (BGC 2016). impact by different flow and debris-flood scenarios, assessment of risk to life (safety risk) due to impact by different debris-flood scenarios for persons Town of Canmore, Alberta located within buildings, and comparison of The southwestern Alberta mountain front was estimated safety risk to international risk tolerance affected by a high intensity/duration rainstorm criteria. between June 19 and 21, 2013, with a return period We assessed risk associated with up to seven of approximately 400 years for the three-day scenarios representing a range in return period duration. Direct runoff, coupled with meltwater classes from 10-30 years to 1,000-3,000 years in released from rain-on-snow, caused sudden and accordance with the Draft Alberta Guidelines for prolonged high flows in the Bow, High and Ghost Steep Creek Risk Assessments (2015). Elements Rivers and their tributaries originating in the eastern impacted by these scenarios and considered in the slopes of the Rocky Mountains. These flows resulted risk assessment included buildings, roads, utilities, in high rates and volumes of sediment transport, bank critical facilities, and persons within buildings. Of erosion and avulsions on alluvial fans subject to these, the risk analysis focused on estimation of debris flows and debris floods. The majority of direct building damage and safety risk. These were damage occurred through bank erosion and re- selected as the key elements that can be deposition of eroded sediment from the proximal systematically assessed and compared to risk portions of fans and along constructed channels. tolerance criteria. The types of consequences that The authors conducted a forensic study of the were assessed are summarized in Table 2. event, followed by detailed hazard and risk Risks were subsequently evaluated as outlined assessment of nine of the most affected creeks in the in this paper using Canmore’s individual and group Bow Valley upstream of Seebe and downstream of risk tolerance criteria. Economic risks were Banff. The hazard assessments combined the presented for each return period scenario and following methods to decipher the recurrence of annualized. Life loss and economic risks were then debris flows and debris floods: historical accounts communicated to the mayor and council of Canmore and newspaper searches, analysis of LiDAR data, and the adjacent Municipal District (MD) of airphoto interpretation of the entire available th Bighorn. Several public meetings with stakeholder chronosequences combined with late 19 century presence were held in Canmore and the Hamlet of oblique air photographs, dendrochronology, Exshaw to provide ample opportunity for the public radiocarbon dating (standard and AMS methods), to be informed and updated regularly. Both Canmore and tephrochronology. and the MD Bighorn created websites in which the LIDAR change detection analysis based on pre- pertinent information was publicized and regularly and post- June 2013 imagery allowed estimation of 9 Porter, M., Jakob, M., Holm, K., McDougall, S. Risk-based landslide safety assessments in Canada
updated. Technical reports were uploaded to the difficulties associated with characterizing very low websites for public access. Local newspapers were probability landslide scenarios, the limited regularly invited and informed to stream information availability of landslide-specific risk acceptance to the public, who appeared to appreciate Canmore’s criteria for risk evaluation, and the level of effort efforts. The highest risk creeks (Cougar Creek in required to demonstrate that risks can be reduced to Canmore and the Exshaw-Jura creek fan complex) tolerable levels where very stringent risk tolerance have received provincial funding for mitigation that criteria are adopted. Given the uncertainties inherent was under construction at the time of publication of in quantitative risk estimates for landslides (as well this contribution. as in other measures of landslide safety), we recommend that risk estimates be used as one of Table 2. Types of consequences assessed for Canmore, many inputs to guide land use planning and Alberta. mitigation design, and not as the sole-basis for Element at Risk Possible Consequences Assessed decision making. Quantitative estimation of damage to building structure or contents expressed as a proportion of appraised building Acknowledgements Buildings value and direct damage cost We would like to acknowledge Doug VanDine for Quantitative estimation of vulnerability his contributions to the draft Canadian Technical of persons located within buildings to loss of life Guidelines and Best Practices Related to Landslides that are referenced herein (VanDine, 2012; Porter Identification of facilities considered and Morgenstern, 2015). Mr. VanDine also provided critical for function during an “Critical” emergency, where debris impact in a a helpful review of an early draft of this manuscript. Facilities given scenario would likely result in We would also like to acknowledge Fiona some duration of loss of function Dercole (DNV), Ryan Wainright (SLRD) and Andy Estarte (Canmore) for allowing us to publish the results of their landslide studies and for their support OPPORTUNITIES AND CHALLENGES throughout the process.
Quantitative risk assessment offers some significant benefits for the assessment, management and REFERENCES communication of landslide safety over traditional methods such as factor of safety calculations and APEGBC (Association of Professional Engineers estimates of encounter probability. The main and Geoscientists of British Columbia), 2010 advantage is that QRA requires careful consideration (prior version 2008). Guidelines for Legislated of the full sequence of events that could lead to injury Landslide Assessments for Proposed Residential or loss of life. Through the process of conducting a Developments in British Columbia; 75 p. risk assessment it is easier to examine which factors AGS (Australian Geomechanics Society) Sub- have the most uncertainty, and where additional committee on Landslide Risk Management, investigation, analysis, or mitigation effort could be 2000. Landslide Risk Management Concepts and most cost effective. The benefits of monitoring and Guidelines; Australian Geomechanics; p. 49-92. other forms of active and passive mitigation can be BGC Engineering Inc. (BGC), 2015. Catiline Creek quantified and accounted for, including justification Debris-Flow Hazard and Risk Assessment. Final of their cost in terms of the level of risk reduction Report issued to Squamish-Lillooet Regional achieved. District, dated January 22, 2015. All landslide safety assessment methods require BGC Engineering Inc. (BGC), 2016. Catiline Creek specialized skill and judgment to be applied Debris Flow Residual Risk Assessment for effectively, and the results are all subject to Mitigated Conditions Final Report issued to Kerr uncertainty. In the authors’ opinion, the main Wood Leidal Associates, dated October 14, challenges with the application of QRA are the 2016.
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Canmore, Town of, 2016. Municipal Development Malone, A.W. 2005. The story of quantified risk and Plan; Bylaw 2016-03; adopted September 13, its place in slope stability policy in Hong Kong. 2016. Chapter 22 in Landslide Hazard and Risk, ed. Cave, P.W. 1992, (revised 1993). Hazard Glade, T., Anderson, M.G. and Crozier, M.J. Acceptability Thresholds for Development John Wiley & Sons Ltd., pp. 643-674. Approvals by Local Government; in Proceedings Morgenstern, N.R., 1995. Managing Risk in of the Geological Hazards Workshop, Victoria, Geotechnical Engineering; in Proceedings of X BC, (ed.) Peter Bobrowsky; Geological Survey Panamerican Conference on Soil Mechanics and Branch, Open File 1992-15, p. 15-26. Foundation Engineering, Guadalajara, Mexico, DNV (District of North Vancouver), 2009. Report to Sociedad Mexicana de Mecanica de Suelos, Council: Natural Hazards Risk Tolerance A.C., p. 102-126. Criteria; District of North Vancouver, BC, Porter, M. and Morgenstern, N.R., 2015, Landslide November 10, 2009. Risk Evaluation – Canadian Technical Geotechnical Engineering Office (GEO), 1998. Guidelines and Best Practices related to Landslides and boulder falls from natural terrain: Landslides: Geological Survey of Canada interim risk guidelines; Hong Kong Government, Open File 7312. GEO Report no. 75. Porter, M., Jakob, M., and Holm, K., 2009. Proposed Geotechnical Engineering Office, 2014. Civil Landslide Risk Tolerance Criteria; in Engineering and Development Department. GEO Proceedings of the Canadian Geotechnical Technical Guidance Note No. 36 (TGN 36) Guidelines on Enhanced Approach for Natural Conference 2009, Halifax, NS, Canada, Paper Terrain Hazard Studies. 75, 9 p. ISO (International Organization for Porter, M., Jakob, M, Savigny, K.W., Fougere, S., Standardization), 2009. Risk management – and Morgenstern, N., 2007. Risk Management principles and guidelines; ISO 31000, 24 p. for Urban Flow Slides in North Vancouver, Jakob, M., Stein, D. and Ulmi, M. Vulnerability of Canada; in Proceedings of the Canadian buildings to debris-flow impact. 2011. Natural Geotechnical Conference 2007, Ottawa, ON, Hazards. DOI 10.1007/s11069-011-0007-2. Canada, Paper 95, 9 p. Jakob, M, Weatherly, H., Bale, S., Perkins, A., Tappenden, K.M. 2014. The district of North MacDonald, B. 2017. A multifaceted hazard Vancouver’s landslide management strategy: assessment of Cougar Creek, Alberta. role of public involvement for determining Hydrology. In print. tolerable risk and increasing community Kendall, H.W., Hubbard, R.B., Minor, G.C. and Bryan, W.M., 1977. The Risks of Nuclear Power resilience; in Natural Hazards, June 2014, Reactors: a Review of the NRC Reactor Safety Volume 72, Issue 2, pp 481-501. Study; WASH-1400 (NUREG-75/014), Unioin US Army Corps of Engineers. 2014. Regulation no. of Concerned Scientists, Cambridge MA, 210 p. 1110-2-1156, Safety of Dams - Policy and Kerr Wood Leidal Associates, 2016. Preliminary Procedures. Design of Catiline Creek Debris Flow Mitigation VanDine, D.F., 2012. Risk Management – Works. Final Report prepared for Lillooet Lake Canadian Technical Guidelines and Best Estates (LLE) dated October 14, 2016. Practices Related to Landslides; Leroi, E., Bonnard, C., Fell, R., and McInnes, R., Geological Survey of Canada, Open File 2005. Risk assessment and management; in 6996, 8 p. Proceedings of International Conference on Landslide Risk Management, Vancouver, Canada, (ed.) O. Hungr, R. Fell, R. Couture, and E. Eberhardt; Balkema, p. 159-198.
11 Porter, M., Jakob, M., Holm, K., McDougall, S. Risk-based landslide safety assessments in Canada
Nat. Hazards Earth Syst. Sci., 12, 1277–1294, 2012 www.nat-hazards-earth-syst-sci.net/12/1277/2012/ Natural Hazards doi:10.5194/nhess-12-1277-2012 and Earth © Author(s) 2012. CC Attribution 3.0 License. System Sciences
The 6 August 2010 Mount Meager rock slide-debris flow, Coast Mountains, British Columbia: characteristics, dynamics, and implications for hazard and risk assessment
R. H. Guthrie1, P. Friele2, K. Allstadt3, N. Roberts4, S. G. Evans5, K. B. Delaney5, D. Roche6, J. J. Clague4, and M. Jakob7 1MDH Engineered Solutions, SNC-Lavalin Group, Calgary, AB, Canada 2Cordilleran Geoscience, Squamish, BC, Canada 3Earth and Space Sciences, University of Washington, Seattle, WA, USA 4Centre for Natural Hazard Research, Simon Fraser University, Burnaby, BC, Canada 5Landslide Research Programme, University of Waterloo, Waterloo, ON, Canada 6Kerr Wood Leidal Associates Limited, Burnaby, BC, Canada 7BGC Engineering Inc., Vancouver, BC, Canada
Correspondence to: R. H. Guthrie ([email protected]) Received: 14 October 2011 – Revised: 7 February 2012 – Accepted: 3 March 2012 – Published: 4 May 2012
Abstract. A large rock avalanche occurred at 03:27:30 PDT, pre- and post-event topography we estimate the volume of 6 August 2010, in the Mount Meager Volcanic Complex the initial displaced mass from the flank of Mount Mea- southwest British Columbia. The landslide initiated as a rock ger to be 48.5 × 106 m3, the height of the path (H) to be slide in Pleistocene rhyodacitic volcanic rock with the col- 2183 m and the total length of the path (L) to be 12.7 km. lapse of the secondary peak of Mount Meager. The detached This yields H/L = 0.172 and a fahrboschung¨ (travel angle) rock mass impacted the volcano’s weathered and saturated of 9.75◦. The movement was recorded on seismographs in flanks, creating a visible seismic signature on nearby seis- British Columbia and Washington State with the initial im- mographs. Undrained loading of the sloping flank caused the pact, the debris flow travelling through bends in Capricorn immediate and extremely rapid evacuation of the entire flank Creek, and the impact with Meager Creek are all evident with a strong horizontal force, as the rock slide transformed on a number of seismograms. The landslide had a seismic into a debris flow. The disintegrating mass travelled down trace equivalent to a M = 2.6 earthquake. Velocities and dy- Capricorn Creek at an average velocity of 64 m s−1, exhibit- namics of the movement were simulated using DAN-W. The ing dramatic super-elevation in bends to the intersection of 2010 event is the third major landslide in the Capricorn Creek Meager Creek, 7.8 km from the source. At Meager Creek the watershed since 1998 and the fifth large-scale mass flow in debris impacted the south side of Meager valley, causing a the Meager Creek watershed since 1930. No lives were lost runup of 270 m above the valley floor and the deflection of in the event, but despite its relatively remote location direct the landslide debris both upstream (for 3.7 km) and down- costs of the 2010 landslide are estimated to be in the order of stream into the Lillooet River valley (for 4.9 km), where it $10 M CAD. blocked the Lillooet River river for a couple of hours, ap- proximately 10 km from the landslide source. Deposition at the Capricorn–Meager confluence also dammed Meager Creek for about 19 h creating a lake 1.5 km long. The over- 1 Introduction topping of the dam and the predicted outburst flood was the basis for a night time evacuation of 1500 residents in the town At 03:27 local time (PDT), on 6 August 2010, a 48.5 × 106 m3 landslide was initiated on Mount Meager, at of Pemberton, 65 km downstream. High-resolution GeoEye ◦ ◦ satellite imagery obtained on 16 October 2010 was used 50.623 N/123.501 W in the Coast Mountains of British to create a post-event digital elevation model. Comparing Columbia (Fig. 1). The landslide originated with the col- lapse of the southern peak of Mount Meager (2554 m a.s.l.)
Published by Copernicus Publications on behalf of the European Geosciences Union. 1278 R. H. Guthrie et al.: The 6 August 2010 Mount Meager rock slide-debris flow
2 The Mount Meager volcanic complex
2.1 Geology and geomorphology
Mount Meager is located in the Province of British Columbia, 150 km north of Vancouver and approximately 65 km northwest of the town of Pemberton (Fig. 1). It is part of the MMVC, which consists of about 20 km3 of volcanic rocks, ranging in age from Pliocene to Holocene (Read, 1978, 1990). The MMVC lies at the northern limit of the Cascade magmatic arc, which includes other prominent volcanoes in the Pacific Northwest such as Mounts Baker, Rainier, and St. Helens in Washington State, Mount Hood in Oregon, and Mount Shasta in California. Volcanic activity at Fig. 1. Location map of the Mount Meager landslide. Three com- these centers is a product of subduction of the oceanic Juan munities lie down-valley from the Mount Meager complex (at the de Fuca plate beneath continental North America (Hickson, head of the landslide): Pemberton Meadows at about 32 km, Pem- 1994). berton at about 65 km, and Mt. Currie just beyond that. The MMVC consists of several coalesced strato- volcanoes, of which Mount Meager is the youngest. The oldest known eruption dates to approximately 2.2 Ma ago; onto the saturated flank of the mountain where it incorpo- the youngest eruption occurred about 2350 years ago (Clague rated a significant volume of material, forming a highly mo- et al., 1995) near the northeast flank of Plinth Peak, when a bile, very rapid debris flow (Fig. 2). The landslide travelled rhyodacitic welded ash tuff and lava flow blocked Lillooet downslope at high velocity over Capricorn Glacier, and then River (Read, 1990; Stasiuk et al., 1996; Stewart 2002), and down the entire 7 km length of Capricorn Creek. At the produced a pumiceous air fall deposit across part of southern mouth of Capricorn Creek the landslide debris encountered British Columbia and into westernmost Alberta (Nasmith et the opposing wall of the Meager Creek – Capricorn Creek al., 1967; Westgate and Dreimanis, 1967). confluence; causing a dramatic split in the moving debris that The MMVC consists of highly fractured, weak, and hy- travelled both upstream, filling and damming Meager Creek, drothermally altered volcanic rocks (Read, 1990), the lower- and downstream into the wider Lillooet River valley, where most of which were assigned by Read (1978) to the Capri- it briefly dammed the Lillooet River. corn Formation, which comprises 200 m of porphyritic rhy- This landslide is significant in that it is one of the largest to odacite breccia and tuff, and 200 m of porphyritic rhyodacite have occurred worldwide since 1945 and is one of the three flows. Above the Capricorn Formation and forming the steep largest to have occurred in the Canadian Cordillera in histor- summit flank is a porphyritic dacite plug of the Plinth For- ical time (Evans, 2006; Guthrie and Evans, 2007; Lipovsky mation. The extrusive units support inclined flow layering et al., 2008). Though there were no deaths or serious in- dipping 35–65◦ to the east-southeast. juries associated with the event, approximately 1500 people Groundwater daylights in the MMVC as hot springs and were evacuated from their homes as part of the emergency re- bedrock seeps, and the lowermost extrusive rocks of the sponse in anticipation of the outburst of a landslide dam that Capricorn Formation and portions of the intrusive plug have formed in Meager Creek as a result of the landslide. Land- been hydrothermally altered to an ochre colour. slides from the Mount Meager Volcanic Complex (MMVC) All except perhaps the top of Mount Meager was cov- have the potential to impact nearby communities, includ- ered by the Cordilleran ice sheet during the last Pleistocene ing the village of Pemberton, a town of slightly more than glaciation (Clague, 1981). Glacial erosion, followed by 2100 people, approximately 65 km southeast of the MMVC, deglaciation between about 14 000 and 11 000 yr ago, sub- and rural communities along the Lillooet River. sequent advances of alpine glaciers, twentieth-century thin- In this paper we summarize the main characteristics (ini- ning and retreat of glaciers and recent thaw of alpine per- tial failure, subsequent behaviour, and landslide damming) mafrost have all contributed to widespread instability within of the 6 August 2010 landslide. We examine the dynamics the massif (Holm et al., 2004). A small glacier is located of the landslide interpreted from seismic records, and its be- at the head of Capricorn Creek adjacent to the source slope haviour as simulated in a dynamic analysis model (DAN-W) of the 2010 landslide. It was much more extensive during for velocity and extent of motion. This report is based on the Little Ice Age than today, extending across the lower part data acquired during field work both immediately after the of the failed slope and about 2.5 km down Capricorn Creek event and over subsequent months, as well as data from post- (Holm et al., 2004). The glacier scoured and steepened val- event high resolution satellite imagery. ley sides in the upper part of the valley, and an apron of sedi- ments accumulated against the ice beneath the summit flank.
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Fig. 2. The 48.5 × 106 m3 rock slide – debris flow from Mount Meager, August 06, 2010. The outline of the initial failure is shown in (A) as a white dashed line. The landslide is approximately south-facing. The lateral shear surfaces are approximately 70 vertical meters high; a helicopter provides scale (inside the white circle). Note the considerable groundwater emerging through weathered bedrock on the western margin and from several other places including the vertical bedrock backscarp. A remnant of the south peak of Mount Meager remains at the apex of the backscarp, where tension cracks persist (some visible in B). Ongoing rock fall from the remaining peak and the sidewalls continued for several months after the main event. The immediate run-out, viewed from the peak looking down, around the first bend of Capricorn Creek is shown in (B). The full extent of the landslide is very difficult to see in any single picture, but (C) gives a sense of the extent to which the run-out inundated several valleys.
Subsequent thinning and retreat of the glacier debutressed the 0.5 × 106 m3 in volume, have been identified from Mount sidewalls in the upper part of the catchment, inducing insta- Meager (Jordan, 1994; Jakob, 1996). However, such an in- bility (Holm et al., 2004). ventory is unlikely to be complete even in historical times, and a sampling bias is certain to occur. At the same time, it is 2.2 Landslide history likely that several large prehistoric landslides have occurred that have not yet been identified, and thus the prehistoric in- The MMVC is arguably the most landslide-prone region in ventory should not be considered complete. Canada (Friele et al., 2008; Friele and Clague 2009); at least 25 landslides ≥0.5 × 106 m3 are known to have occurred in the last 10 000 yr (Table 1). Numerous landslides, less than www.nat-hazards-earth-syst-sci.net/12/1277/2012/ Nat. Hazards Earth Syst. Sci., 12, 1277–1294, 2012 1280 R. H. Guthrie et al.: The 6 August 2010 Mount Meager rock slide-debris flow
Table 1. The history of landslides ≥0.5 × 106 m3 from the Mount Meager Complex (adapted and updated from Friele et al., 2008).
Event Source Date1 Volume2 Reference (× 106m3) Prehistoric Rock avalanche/debris flow Pylon Pk. −7900 450∗ Friele and Clague (2004) Rock avalanche/debris flow Job Ck. −6250 500∗ Friele et al. (2005) Rock avalanche/debris flow Capricorn Ck. −5250 5∗ McNeely and McCuaig (1991) Rock avalanche/debris Pylon Pk −4400 200 Friele and Clague (2004); flow/hyper concentrated flow Friele et al. (2005) Rock avalanche/debris flow Job Ck. −2600 500∗ Friele et al. (2005); Simpson et al. (2006) Pyroclastic flow Syn-eruptive −2400 440 Stasiuk et al. (1996); Stewart (2002) Rock avalanche/outburst Syn-eruptive −2400 200 Stasiuk et al. (1996); Stewart (2002) flood/debris flow/hyper concentrated flow Rock avalanche Syn- to post-eruptive −2400 44 Stasiuk et al. (1996); Stewart (2002) Debris flow Devastation Ck. −2170 12 McNeely and McCuaig (1991) Debris flow Job Ck. −2240 1 Friele et al. (2008) Debris flow Angel Ck. −1920 0.5∗ McNeely and McCuaig (1991) Debris flow Job Ck. −1860 1 McNeely and McCuaig (1991) Debris flow Job Ck. −870 9∗ Jordan (1994) Debris flow Job Ck. −630 1 Friele et al. (2008) Debris flow No Good Ck. −370 5∗ McNeely and McCuaig (1991) Historic (age AD) Debris flow Capricorn Ck. 1850 1.3 Jakob (1996); McNeely and McCuaig (1991) Debris flow Capricorn Ck. 1903 30 Jakob (1996) Debris flow Devastation Ck. 1931 3 Carter (1932); Decker et al. (1977); Jordan (1994) Rock avalanche Capricorn Ck. 1933 0.5∗ Croft (1983) Rock avalanche Devastation Ck. 1947 3 Evans unpublished data Rock avalanche Devastation Ck. 1975 12 Mokievsky-Zubok (1977); Evans (2001) Rock avalanche Mt. Meager 1986 0.5∗ Evans (1987) Debris flow Capricorn Ck. 1998 1.3 Bovis and Jakob (2000) Debris flow Capricorn Ck. 2009 0.5 Friele (unpublished data) Rock slide/debris flow Capricorn Ck. 2010 48.5 This study
1 Prehistoric dates are radiocarbon ages BP. Cited ages are those that closely constrain an event, however, considerable ranges may be present. For more details see Friele and Clague (2004), or the original sources. 2 Volumes denoted by * are the median volume of a wider range.
3 The 6 August 2010 Mount Meager landslide The landslide had no distinct seismic or meteorologi- cal trigger but was recorded on seismographs in British 3.1 Initial failure Columbia and Washington State. It occurred as a result of several important preconditions. The Mount Meager Vol- The 2010 Mount Meager landslide originated with the col- canic Complex is subject to frequent landslide activity in- lapse of its southern peak (2554 m a.s.l.) onto its failing sat- dicating conditions especially favourable to large-scale slope urated flank where it incorporated a significant volume of movement. These conditions include poor quality or struc- material, forming a highly mobile, very rapid debris flow. turally weak volcanic and volcaniclastic bedrock, a Holocene The debris flow travelled the entire 7 km length of Capricorn history of glacial unloading, recent explosive volcanism, and Creek and inundated both the Meager Creek valley and the Little Ice Age glacial activity (slope loading, unloading, and Lillooet River valley. over-steepening). The overall path height (H) is 2183 m and the total path Evidence indicates that groundwater played a key role in length (L) is 12.7 km. This yields H/L of 0.172 and a the 2010 failure. Prior to failure the flanks of Mount Meager fahrboschung¨ (travel angle) of 9.75◦ (Fig. 3). were subject to high pore pressures indicated by extensive
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Fig. 3. Profile of the Mount Meager landslide.
Mount Meager, beginning at 03:27:30 h (PDT) on the morn- ing of 6 August 2010. The rock falls fell approximately 500 m onto the weak and heavily saturated south flank of Mount Meager which then began to disintegrate as it was subjected to impact-generated undrained loading. On the seismic record (see below) the impact of the collapsed peak was measured less than 45 seconds after the first movements and resulted in the extremely rapid horizontal displacement of the materials composing the flank. Interpretation of seis- mic records (see below) indicates that the material left the initiation zone at a velocity of approximately 87 m s−1, leav- ing behind distinct lateral scarps and a high vertical bedrock head scarp (Fig. 2).
3.2 Landslide volume analysis
Fig. 4. Mount Meager before the 2010 landslide. The sources and A robust volume analysis of the initial failure mass was upper paths of the 2009 and 1995 Mount Meager debris flows are accomplished by comparing pre- and post-event digital el- outlined and coincide roughly with the present-day location of the evation models (DEMs) of the landslide (Fig. 5). Topo- lateral shear. The darker colour of the 2009 event is from groundwa- graphic data representing the pre-event surface was obtained ter saturation, and the orange staining of rock on the lower slopes from a 25 m resolution Terrain Resource Inventory Mapping indicates long-term exposure to high groundwater levels and hy- (TRIM) DEM, the British Columbia provincial standard base drothermally altered volcanic rock (photograph by Dave Southam). map. The post-event surface (DEM) was created using two sets of 5 m resolution GeoEye stereo images obtained 22 Au- gust and 21 September 2010. surface seepage observed throughout the failure surface and Each DEM represents a surface model that, while inter- along the lateral shears following the 2010 event (Fig. 2a). nally consistent, may contain differences due to image off- The largest visible bedrock spring occurred along the west sets, resolution, and ground control points. GeoEye used lateral scarp and was the location of at least two previous only the satellite telemetry for control, and was therefore pro- landslides, occurring in 2009 and 1998 (Fig. 4). Water supply jected in space slightly above the surface represented by the was exacerbated by summer melt of snow and ice, causing TRIM DEM. This was corrected by overlaying the GeoEye even greater saturation of slopes. surface onto the TRIM surface, so that the average difference Based on our interpretation of seismic records (see below), in ground elevations outside the landslide track was zero. initial failure is thought to have begun as a series of major rock falls from the high secondary peak, or gendarme, of www.nat-hazards-earth-syst-sci.net/12/1277/2012/ Nat. Hazards Earth Syst. Sci., 12, 1277–1294, 2012 1282 R. H. Guthrie et al.: The 6 August 2010 Mount Meager rock slide-debris flow
Fig. 5. The difference in depth between the TRIM (before) and GeoEye DEMs used to calculate volume. The strong lateral shears, high backscarp and bulge of the pre-failure flank are all readily interpreted from the differencing map. The landslide outline was accurately delineated based on new imagery (right side of figure). The total calculated volume is 48.5 × 106 m3.
A difference calculation was then performed on the two (4) The TRIM DEM was built in 1987. Several geomorpho- surfaces inside the landslide track, using raster math in a Ge- logical events have occurred since then that could each affect ographic Information System (GIS), and summing the total the valley bottom, but not substantially affect the initiation difference for each pixel in the initiation zone. The resulting zone, resulting in a real-life surface that was different than volume calculated for the initial failure is 48.5 × 106 m3. the DEM. These events include two previous landslides in Field estimates of deposit thickness were made at several Capricorn Creek (1998 and 2009), and ongoing reworking of locations following the landslide, then matched to morpho- material from Meager Creek, Capricorn Creek and Lillooet logical units, and corroborated by comparing cross sections River, since 1987. With these limitations in mind, we con- of the TRIM DEM with the GeoEye DEM. The total vol- sider the deposit volume to be in reasonable agreement with ume of the deposit was calculated at 41 × 106 m3, including the initial volume. bulking. The volume estimate from the field is about 15 % Our estimate of the initial volume of the 2010 Mount Mea- lower than expected, or, if approximately 6 M m3 of bulking ger landslide is thus approximately 48.5 × 106 m3. With this is estimated, then it represents a difference of almost 30 %. volume it is one of the largest landslides to have occurred Several explanations may be considered: (1) the initial worldwide since 1945 (Evans, 2006) and one of the three volume is from a limited area (∼0.4 km2) of exceptionally largest landslides to have occurred in the Canadian Cordillera deep scour. It is not particularly sensitive to a change in in historical time. It is comparable in volume to the 1965 depth of a few meters; however, it is sensitive to even a small Hope Slide (Mathews and McTaggart, 1978). shift in the outline of the initiation zone. Fortunately, this We note that the characteristics of the 6 August Mount error is controllable in the production of an accurate out- Meager event are largely depositional outside of the initiation line of the head scarp (Fig. 5). In contrast, the deposit vol- zone. ume was determined across a broad area and it is therefore about 20× more sensitive to a change in depth estimation. 3.3 Transformation into debris flow and run out (2) The volume estimation of the deposit, involved consider- ably more subjective analysis and expert judgement at each The disintegrated landslide mass left the initiation zone in a point across a landscape that was ∼9 km2. To minimize this, largely horizontal trajectory and traversed Capricorn Glacier, cross-sections of the two surfaces were also used in careful causing almost no damage to the glacier itself (note the corroboration of field estimates. (3) The volume contained drainage features of Capricorn Glacier, perpendicular to the in Capricorn valley, V-shaped in 2009 and now truncated landslide flow, in Fig. 6a), and flowed into the first bend in profile with new deposition in 2010 was underestimated. of the Capricorn Creek valley (Fig. 6b). By this time the
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Fig. 6. Looking down the initial landslide path (helicopter in white circle for scale). Note the ice of Capricorn Glacier, intact with longitudinal melt lines perpendicular to the direction of flow (A). Overspray is evident beyond the path edge, particularly on the snow. The lower portion of the valley was in-filled by deposits from the avalanche, which was subsequently evacuated along the left side of the valley (shown by white dotted lines) in one or more liquid slurries. Looking further down the landslide run-out, through the first bend (B), deposition is evident even on the valley sidewalls. Images (C) and (D) show valley floor deposition as viewed from below and (E) looks back up at the initiation zone. White dotted lines in (C) correspond to those in A and the white circle in (D) shows a person for scale. Ice persists in the valley floor at the base of the deposit (A) and remains largely undamaged, though directly over-ridden (indicated by white arrow). landslide was typical of a debris flow as the initial failure exposure to hydrothermally derived groundwater for years mass had completely disintegrated and had become fluidised prior to the event; (3) the sudden transfer of energy from the by mixing with the pore water contained in the failed mass. collapse of the mountain peak; and (4) a rapid increase in hy- We interpret the rapid transformation to a debris flow to be drostatic pressure as a result of the impact (undrained loading a consequence of: (1) elevated groundwater conditions lead- of the slope). ing up to the event; (2) poor rock quality caused by prolonged www.nat-hazards-earth-syst-sci.net/12/1277/2012/ Nat. Hazards Earth Syst. Sci., 12, 1277–1294, 2012 1284 R. H. Guthrie et al.: The 6 August 2010 Mount Meager rock slide-debris flow
3.4 Analysis and interpretation of seismic signature complete collapse of the secondary peak of Mount Meager onto the footslope. The Meager Creek landslide generated long-period seismic Phases of landslide activity were inferred over the next waves that were visible at stations from southern Califor- 200 s based on analysis of seismic wave type, amplitude, and nia to northern Alaska, up to 2800 km away. A sample of relative timing and corroborated by field observations. The vertical-component broadband seismograms of this event is pulses, their source description, and time from the signal on- displayed on Fig. 7. No known earthquake was associated set are described in Table 2, with locations shown on Fig. 9. with the failure, but the event itself was assigned an equiv- Identified pulses are nearly identical at other broadband sta- ialent local magnitude of 2.6 by the Canadian Seismic Net- tions, with some variation due to the radiation pattern, show- work. Landslides are distinguished from seismic signals by ing that these are source effects, not site effects or path ef- a characteristically high ratio of long period to short pe- fects. riod energy waves (Weichert et al., 1994; McSaveney and The most well-defined of the long-period source events are Downes, 2002). The long-period surface waves generated by A, B, and F. Event A is interpreted to be initial failures from the Mount Meager event were much stronger than would be the upper slopes onto the flanks below. The dominant pe- expected from an earthquake of the same magnitude. riod of these waves is about 17 to 20 s, indicating a source Landslides do not always generate identifiable seismic sig- process of about an equivalent length (Kanamori and Given natures due in part to their slower source process and poor 1982). This was confirmed by preliminary waveform mod- ground coupling as compared to earthquakes. Seismic en- eling. Event B is higher in amplitude than Event A and is ergy conversion rates for similar events are estimated to be composed primarily of Rayleigh waves, suggesting a domi- as low as 0.01 % of the kinetic energy and 1 % of the po- nantly vertical force. This is interpreted as the collapse of the tential energy released by the slide (Berrocal et al., 1978). secondary peak onto the flanks below, mobilizing and disag- It requires an extremely energetic source to generate waves gregating the material below into a high-velocity debris flow. high enough in amplitude to be visible for thousands of kilo- The period of these Rayleigh waves was 10 s, indicating a meters. The seismogenic nature of this landslide is a result shorter source process than the initial failures (A), consistent of the large volume of material involved and the extremely with the expected signature of a collapse of the secondary rapid velocities. peak of Mount Meager. These initial massive failures (A and The seismic signal recorded from the Mount Meager land- B) are roughly equivalent to a single force source mecha- slide can be examined in two frequency bands to separate nism in the opposite direction of slide movement as a block the waves coming from different sources. Figure 8 is a accelerates, and in the same direction as slope movement as a plot of the vertical component seismogram and envelope for block decelerates (Kanamori and Given, 1982; Dahlen, 1993; two frequency bands at the closest station, WSLR, a three- Julian et al., 1998). The polarization of the first arrivals of component broadband station. This station is operated by the Rayleigh waves and the variability in the amplitudes of the Canadian National Seismograph Network, located 70 km both types of surface waves with azimuth indicate the correct southeast of Mount Meager. The signal is filtered to separate radiation pattern for a single force. Preliminary waveform the short period and long period waves, and also plotted as an modeling supports the single force mechanism and suggests envelope to show the overall shape of a signal by including forces on the order of 109 N for event A and 109–1010 N for only the amplitude information from the seismogram. This is event B. helpful in picking arrivals and separating sources that cannot Events C through E can roughly be correlated with turns in easily be seen in the seismogram. The long-period portion, the path of the debris flow, a product of seismic waves gen- filtered to include only frequencies below 0.2 Hz (>5 s pe- erated by rotational accelerations. These events also occur riod), is generated by large and slower sources such as the when the short period energy is at its highest, suggesting that acceleration of the slide mass, and turns and large bumps they coincide with peak debris flow velocities. in the path of the debris flow (Kanamori and Given 1982; Event F is defined by a pulse nearly as high in amplitude Dahlen 1993; Julian et al., 1998; Deparis et al., 2008). The as Event B, indicating another massive impact force. Site short-period signals, higher than 1 Hz (<1 s period) are gen- evidence showed that the debris flow burst out of Capricorn erated by smaller scale source processes such as chaotic flow Creek valley into the facing wall of the Meager Creek val- within the debris flow and at the base of the debris flow as it ley, impacting a steep slope on the opposite side and running flows over smaller features (Brodsky et al., 2003; Huang et up 270 vertical meters before splitting up and down Meager al., 2007). The short period (>1 Hz) motions had an emer- Creek valley. This impact on the valley wall is the interpreted gent onset, spindle-shaped signal, and low amplitudes in rela- source for Event F. tion to their duration, typical of rock slides, debris avalanches Using these well-defined time markers and calculating and debris flows (Norris, 1994). The short period portion of path distance, we can make velocity estimates of the debris the signal emerges about 25 s after the onset of the long pe- flow at various stages along the path. We calculate an aver- riod signal, but before the main long period impact signal, age debris flow velocity of 67 m s−1. This is measured from suggesting that disintegration of the mass began before the the secondary peak failure (B) to the impact of the debris
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Fig. 7. Raw seismic signals from Mount Meager landslide recorded across southwestern Canada and northern Washington State (CNSN and PNSN data). Station and landslide location indicated on inset map.
flow at a right angle to Meager Creek (F), covering a distance There were no precursory seismic events recorded prior to of about 7.3 km in 109 s. Rough velocity estimates between the main landslide. However, the seismic stations are 70 km each individual point are given on Table 2, including error away or farther, so small precursors might have escaped our margins on distance and timing. The error margins are due to observation. uncertainties in defining points, selecting distances between points because these are not point sources, uncertainties in 3.5 Simulation using DAN-W picking arrival times for these long period pulses, and differ- ences in surface waves velocities at different periods. It is A first-order dynamic back analysis of the Mount Meager more difficult to accurately define the end of the debris flow, rock avalanche was completed using the two-dimensional as the energy fades gradually on the seismic record. numerical simulation model DAN-W (Hungr, 1995; Hungr A second long-period seismic pulse occurs about 7 min and McDougall, 2009). DAN-W has been used to simulate after the main collapse began. The waveform of Event H the behavior (i.e. run-out distance, velocity along path, and is similar to event A, but smaller in amplitude, and is inter- deposition) of a number of catastrophic landslides including preted as a subsequent smaller volume rock fall or rock slide rock avalanches, debris avalanches, and debris flows (e.g. from the now over-steepened face of Mount Meager, an “af- Hungr and Evans, 1996; Evans et al., 2001, 2007; Hungr tershock” of sorts. Field work over the next month revealed and Evans, 2004; Sosio et al., 2008; among others). that smaller rock falls were frequent and ongoing from the A Voellmy rheology (Hungr, 1995) was selected to model source zone. the basal resistance of the moving mass. The Voellmy basal resistance model includes frictional (f ) and turbulence (ξ) www.nat-hazards-earth-syst-sci.net/12/1277/2012/ Nat. Hazards Earth Syst. Sci., 12, 1277–1294, 2012 1286 R. H. Guthrie et al.: The 6 August 2010 Mount Meager rock slide-debris flow
Fig. 8. WSLR broadband recording of the landslide single-pass highpass filtered (>1 Hz top) and lowpass filtered (<0.2 Hz, bottom) and plotted with envelopes. Dashed vertical lines are labeled with letters that correspond to Fig. 9 and discussion in text.
Fig. 9. The complete outline of the Mount Meager landslide showing the initiation zone (A–B), the two major bends (C and E), the facing wall of Meager Creek (F), and the bifurcated flow that travelled up Meager Creek, and across the Lillooet River (G). The image was taken following the breach of the Meager Creek dam, and fluvial reworking and considerable incision of the dam itself are evident.
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Table 2. Time markers along landslide path according to the seismic signature at stations in the Pacific Northwest (see Figs. 7–9 for path and station locations and seismic trace associated with each marker).
Source Description Time from Error Distance from Range of distances Average Velocity event slide onset (s) previous point (error margin) velocity1 range1 (s) (km) (km) (m s−1) (m s−1) A Initial failures from high on 0 ±1 – – – – slopes B Secondary peak collapses and 45.3 ±2 0 – – – mobilizes flanks below into de- bris flow C Debris flow turns first corner 64.9 ±2 1.7 1.5–2.5 87 70–142 D Debris flow flows over bump 96.5 ±4 1.7 1.7–3 54 54–109 and slight turn in path E Debris flow turns last corner in 119.3 ±5 1.4 0.8–1.9 61 29–107 Capricorn Creek F Front of debris flow strikes wall 154.1 ±0.5 2.2 1.8–2.5 63 51–73 across Meager Creek G Seismic signal drops below 324.3 ±10 3 3–3.5 18 17–22 noise level, apparent end of de- bris flow H Second smaller rockslide re- 424.1 ±0.5 – – – – leased from Mount Meager
1 From previous event location to current event.
Path width from the GeoEye image (Fig. 9) was entered directly into DAN-W along with topography and model pa- rameters. Field evidence of post-failure behaviour indicated limited path-erosion and therefore entrainment was not con- sidered in this simulation. Four surface materials were defined for four segments along the path of the landslide. The values of friction and tur- bulence parameters for each of these segments that best sim- Fig. 10. Cross-sectional profiles from the upper bend (C) and the ulated the landslide behavior are listed in Table 3. We note lower bend (E) used for velocity calculations in Fig. 9. that the values of the Voellmy parameters for the surface ma- terials in the four path segments fall within the range used in parameters, which are estimated on the basis of field con- previous successful simulations of similar movements (e.g. ditions to produce a simulation that best fits observed be- Hungr and Evans, 1996; Ayotte and Hungr, 2000; Evans et haviour al., 2009) and generally reflect the transformation of an ini- tial fragmenting rockslide into a fast-moving flow. 2 τzx = σzf +pgv /ξ (1) Simulations of velocity verus distance and velocity versus time relationships were compared to estimation of velocity where τzx is the basal friction model, σz is the bed normal from superelevation of debris in the two major bends (C and stress at the base of the flow, f is the frictional coefficient, ρ E on Fig. 10), run-up velocity at the Capricorn Creek – Mea- is the material density, g is the gravitational constant, v is the ger Creek confluence, and velocity measured in the seismic averaged depth flow velocity, and ξ is the turbulence term. analysis. The reader is referred to Hungr (1995, 2008) for details Superelevation velocities were measured using about the input parameters. Output parameters include path Vmin = ghr/w (2) velocity and time. −1 The BC TRIM DEM was used for pre-event surface to- where Vmin = the minimum velocity in m s , g = the grav- pography and the GeoEye DEM was used to generate a post- itational constant, h = the run-up height, r = is the radius of event surface (see Sect. 3.2 for details). These two datasets curvature in a bend, and w = the width of the path. defined the source region and initial volume (48.5 Mm3) of the landslide. www.nat-hazards-earth-syst-sci.net/12/1277/2012/ Nat. Hazards Earth Syst. Sci., 12, 1277–1294, 2012 1288 R. H. Guthrie et al.: The 6 August 2010 Mount Meager rock slide-debris flow
Table 3. Friction and turbulence parameters along path segments used in DAN-W.
Segment Path distance (m) Friction Turbulance Start End (f )(ξ) 1 0 1407 0.06 750 2 1408 6452 0.1 900 3 6453 7793 0.02 1250 4 7792 12 453 0.02 550
Run-up velocity was also measured at the Capricorn Creek – Meager Creek confluence using
0.5 Vmin = (2gh) (3) Fig. 11. The plot of frontal velocity versus path distance (solid where V = the minimum velocity in m s−1, g = the gravi- min line) and path width versus path distance (dashed line) for the 2010 = tational constant, and h the run-up height. Mount Meager landslide. Velocities are compared to estimates from Results of the superelevation analysis indicate minimum superelevation and run-up data (black dots) and seismic analysis −1 −1 flow velocities of 62 m s in bend 1 and 73 m s in bend 2. (open diamonds). The black square is actual field run-out and filled − − These compare to 84.3 m s 1 and 81.2 m s 1 for the respec- black diamonds denote the segments corresponding to the four ma- tive bends in the DAN-W simulation, and 87 m s−1 and terials in Table 3. 61 m s−1, respectively, on the seismic trace. The simulated velocities are higher than the velocities calculated from the profile alone, but match the seismic trace quite closely for the first bend. It is likely that a bedrock wall at the beginning of bend 1 limited the superelevation, making the profile ve- locity sensitive to where in the curve the measurement was taken. We had difficulty matching DAN-W simulated veloc- ity values with the inferred field velocities based on debris behavior, whilst at the same time matching the path geom- etry requirement of run-out to the Lillooet River. Velocities approximating field estimates in bends in Capricorn Creek did not produce the observed long run out and higher-than- field-estimate velocities were necessary to achieve approxi- mate run-out distance in the simulation. Figures 11 and 12 show the results of the DAN-W analysis. The geometry of the run-up at the confluence of Capricorn and Meager creeks suggests a minimum velocity of 63 m s−1 compared to a DAN-W simulated estimate of 53.3 m s−1 (Fig. 11). Fig. 12. DAN-W simulation plot of velocity versus time. Velocities Overall, the simulation gave a mean velocity to the con- are compared to estimates from superelevation in bends and run-up fluence of Capricorn and Meager creeks of 68.4 m s−1 and a geometry (black dots) and seismic analysis (diamonds). peak velocity of 91.2 m s−1. This compares favorably to the mean velocity of 67 m s−1 derived from the seismic traces. Velocities following impact with the facing wall of Meager fan into the Lillooet River valley. The sudden deceleration is Creek were reduced as the debris bifucated and decelerated evident in Figs. 11 and 12. to the landslide’s distal limit at Lillooet River. DAN-W indi- With reference to velocity versus time relationships, there cates a mean velocity of 9.5 m s−1 between the landslide dam is broad correspondence between the results of DAN-W and and the distal limit. We propose that this comparatively low the seismic trace for the initial part of the path up to the velocity results from sudden deceleration following (1) the run-up at the confluence of Capricorn and Meager creeks. impact with the south side of Meager Creek at the Capricorn- We note, however, that in Fig. 12, a large discrepancy is Meager confluence, and (2) the sudden increase in the width evident in run-out times between the seismic (324 s) and of the path as the debris spread out over the Meager Creek DAN-W analyses (621 s). This may be explained by the low
Nat. Hazards Earth Syst. Sci., 12, 1277–1294, 2012 www.nat-hazards-earth-syst-sci.net/12/1277/2012/ R. H. Guthrie et al.: The 6 August 2010 Mount Meager rock slide-debris flow 1289
deposit; (5) the downstream flow in Lillooet River valley in- cluding; (6) a thick deposition plug; and (7) late-stage debris flow zone forming the south edge of the deposit (after flow sector). The spray zone consists of wind-thrown trees and thin de- position of debris ejected laterally as the debris front burst from the confining source valley into Meager Creek valley. The spray zone is about 500–600 m long on both sides of Capricorn Creek mouth on the north wall of Meager Creek valley. The trim zone lies between the thick debris deposit and the forest edge. Here, in the area eroded by the passing of the frontal lobe, the pre-landslide surface was scoured of vege- tation and covered with debris, thin on the side slopes, and thick in the valley bottom where the old V-shaped gulley was in-filled. The deposit in Meager Creek upstream of the mouth Fig. 13. The Meager Creek barrier after the dam outburst flood at of Capricorn Creek (the upstream flow) consists mainly of the confluence of Capricorn and Meager creeks. mixed sediment and debris filling an incised channel 5–15 m deep, 100–200 m wide, and extending 3250 m upstream from the Meager Creek barrier. sensitivity threshold of the seismic recordings. We hypoth- The barrier consists of compressed ridges formed of mixed − esise that once the moving mass slowed to below 10 m s 1, debris and large grey blocks; the debris directly below the it was no longer detectable by the seismographs above back- south valley wall, however, is hummocky rather than ridged, ground seismic noise. The DAN-W analysis however, con- having run up the slope and and fallen back. The Meager − tinues the movement until the velocities reach 0 m s 1, sig- Creek barrier was substantially altered by ongoing debris nificantly extending the overall run-out time. flows incising the deep valley floor deposits and the edge of the barrier, and then ultimately by the dam outburst flood de- 3.6 Deposit morphology scribed below. The main deposit downstream along Meager Creek and Capricorn Creek was predominantly V-shaped prior to the Lillooet River was largely eroded by the outburst flood; its landslide in 2010. Following the 2010 event, the V-shaped initial morphology was reconstructed from field observations valley floor was modified to form a flat-bottomed narrow and oblique air photographs from 6 August 2010. The land- trapezoid as material was deposited along the path. Depo- slide partially blocked the Lillooet River, causing a 10–15 m sition began immediately below the initiation zone of the rise in water levels on the upstream side that destroyed a Mount Meager landslide, in-filling the valley bottom and bridge at the landslide margin. As it spread across the Lil- draping the valley sidewalls otherwise stripped bare of vege- looet River valley, the flow was divided and slowed by an tation during the event (Fig. 6a–e). Debris incision along the elevated rise in the valley floor from previous landslide de- valley floor occurred in later stages of the event, eroded by posits. The former deposits were overwhelmed by debris a liquid slurry as the debris flow reduced in size (inside the from 2010 event, and were buried under a deep plug of de- white dotted lines of Fig. 6a and c). bris. The flow crossed Lillooet River valley, narrowly missed At the confluence of Capricorn and Meager creeks, the a forest recreation campsite on the other side and isolated landslide encountered the opposing rock wall of the Meager campers who required rescue. Creek valley causing a vertical run up of 270 m and leaving a The debris flow spread across the Lillooet River valley, roughly triangular landslide dam (the Meager Creek barrier) leaving behind abundant ponded surface water, hummocks, approximately 700 m × 500 m at the base, with a thickness blocks, and compression ridges. Ongoing debris flows sub- of 30–50 m (Figs. 13 and 14). The landslide dam completely sequent to the main event in-filled the south side of the valley, blocked Meager Creek for 19 hours, causing the formation of and reworking of the deposit began almost immediately fol- a landslide dam lake, and ultimately failing into the Lillooet lowing the landslide dam failure. River valley. The morphology and architecture of the deposit is typi- The path of the landslide along Meager Creek and Lil- cal of large volcanic landslides, with hummocky topogra- looet River was divided into six zones, each with a dis- phy, longitudinal flow banding and areas of transverse com- tinct geomorphic expression (Figs. 14 and 15): (1) spray pression ridges (Siebert, 1984; Glicken, 1991; Dufresne and zone; (2) trim zone; (3) Meager Creek deposit upstream of Davies, 2009). The low relief areas are underlain by matrix- Capricorn Creek (upstream flow); (4) Meager Creek barrier supported, poorly-sorted debris with grain sizes ranging from www.nat-hazards-earth-syst-sci.net/12/1277/2012/ Nat. Hazards Earth Syst. Sci., 12, 1277–1294, 2012 1290 R. H. Guthrie et al.: The 6 August 2010 Mount Meager rock slide-debris flow
Fig. 14. Map of the landslide deposit in the valleys of lowermost Meager Creek and Lillooet River (see text for description). clay to boulder-size. Individual hummocks stand up to sev- and 2.7 × 106 m3 of new water over the next 20 h (Roche et eral meters above the landslide surface, and are cored by al., 2011). blocks of the original rock mass. By 20:30 h on 6 August, it was clear that overtopping of the dam was imminent and approximatley 1500 residents in 3.7 Failure of the Meager Creek landslide dam and the lower Lillooet Valley were asked to evacuate. outburst flood We estimate a total floodwave volume of 2.9 × 106 m3 following overtopping of the Meager Creek barrier. This The landslide completely blocked Meager Creek and par- amount includes introduced water (stored in the dam or in tially blocked the Lillooet River at its confluence with Mea- sediments along the path) and sediment content in the vol- ger Creek. The interruption in the flow of both rivers was ume calculation. recorded at Water Survey of Canada (WSC) hydrometric sta- The lake overtopped the landslide dam at around 23:30 h tion 08MG005, located on Lillooet River, upstream of Pem- on 6 August and reached a peak discharge of several hun- berton, 61.3 km downstream from the confluence of the two dred m3 s−1 at about 01:00 h on 7 August. The floodwave rivers. The WSC gauge recorded the hydrometric response travelled downstream toward Pemberton with an average ve- of both dams, and provides a unique opportunity to analyze locity of approximately 2.5 m s−1. The peak passed the Hur- the impacts of the landslide on the river system. Figure 16 ley River Forest Service Road bridge (38 km downstream) at shows the Lillooet River hydrograph for 6–7 August 2010. about 04:30 h and the Water Survey of Canada gauge (61 km The flow of Meager Creek was determined by compar- downstream) at 07:45 h. Flows attenuated in velocity and ing the flow at the Lillooet River WSC station following the volume with distance; however, high flows persisted and event, to the estimated contribution of Meager Creek by wa- discharge at Pemberton, almost 7 h after the peak remained − tershed area. Meager Creek flow was thereby estimated at about 200 m3 s 1 above the pre-wave condition. 36–38 m3 s−1. The landslide dam impounded between 2.6
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Fig. 15. Deposition features in lower Meager Creek. PL = plug; DF = downstream flow; UF = upstream flow; T = trim zone; S = spray zone. Image (A) is at the Capricorn–Meager confluence (photo by B. Makarewicz); (B) shows the downstream flow against the east wall of the Lillooet River valley; (C) and (D) are upstream of the Meager Barrier (photo C by D. B. Steers); and (E) is the downstream flow and plug.
3.8 Socio-economic and environmental impacts multiplied by the average log market value for August 2010 (Province of British Columbia, 2011). The total poten- tial loss based on the markets at the time of the event was Despite the remote location of the 2010 Mount Meager land- $8.7 M CAD. In addition, two forest service bridges and road slide, the event had considerable socio-economic impact. construction equipment were destroyed, along with several Approximately 110 000 m3 of wood was stripped away from kilometers of roads including almost 6 km of the Meager the slopes of Capricorn Creek and Meager Creek and the Lil- Creek forest service road. looet River valley bottom and either pulverized into fine or- The landslide and the subsequent threat of a dam outburst ganic material, or transported as large woody debris into the flood on Meager Creek caused the evacuation of approxi- river system. The wood was a mixture of western hemlock, mately 1500 residents in the lower Lillooet River valley for amabilis and subalpine fir, western red cedar, and to a lesser one night, and rescue efforts for several campers and workers extent lodgepole pine and balsam poplar. Wood species and in the vicinity of Mount Meager. volume were obtained from recent forest cover maps, and www.nat-hazards-earth-syst-sci.net/12/1277/2012/ Nat. Hazards Earth Syst. Sci., 12, 1277–1294, 2012 1292 R. H. Guthrie et al.: The 6 August 2010 Mount Meager rock slide-debris flow
headed for higher ground. Chaos ensued for the next few hours before daylight as they encountered debris flows, mud, falling trees and other hazards at the edge of the landslide deposit in the Lillooet River valley. The most alarming part of their story occurred as they came face to face with a 1.5- m high wall of mud and water that appeared to them like a “turbulent bubbling wedge of black oil” (P. Smith, personal communication, 2010). They turned their truck around, but not before they were overtaken by the hyper-concentrated flow. Accelerating rapidly, they nonetheless escaped, in what P. Smith described as most resembling “the Millennium Fal- con escaping the Death Star explosion at the end of Return of the Jedi” (personal communication, 2010). The account corroborates both the approximate time of the event, the violent nature and noise level of a massive land- Fig. 16. WSC gauge 08MG005, Lillooet River near Pemberton. The gauge records the response of the Lillooet River at Pemberton slide even at the distal margins, and ongoing activity on the to the Mount Meager landslide and subsequent damming events. landslide long after the main event occurred. We propose Delay time to the station is approximately 6 h and 45 min. 1, sta- that the two cracks (loud explosive noises) are the subjective ble base flow (curve is diurnal fluxuation in stream flow); 2, initial memory of the impact of the failing secondary peak. drop in flow due to the complete blockage of Meager Creek, and partial blockage of Lillooet at 10:15; flow dropped to 182 m3 s−1; 3, rapid recovery of flow beginning at 13:15 associated with the in- 4 Conclusions cision of Lillooet River through the landslide deposit; 4, continued gradual recovery of Lillooet River flow, minus the Meager Creek In this paper we have documented a large catastrophic land- portion; 5, arrival of the Meager dam breach floodwave at 05:15; 6, slide that occurred in August 2010 on the southern flank arrival of the flood peak at 07:45; 7, re-establishment of stream flow of Mount Meager, a dissected Quaternary volcanic centre conditions. The blue line represents the modelled dam breach and within the Mount Meager Volcanic Complex (MMVC) of corroborates the observed stage discharge at Pemberton. southwestern British Columbia. The initial failure mass had a volume of 48.5 × 106 m3 and involved a slope consist- Sediment moving downstream puts pressure on the Pem- ing of poor quality volcanic rock. The failed rock mass berton district dyking system, raising the effective flood lev- rapidly transformed into a high-velocity debris flow that els, and the obliteration of Capricorn Creek and inundation of swept 7.8 km down Capricorn Creek to dam Meager Creek. Meager Creek will have long-lasting environmental effects. At Meager Creek, the debris flow split and traveled 3.7 km In total, direct costs associated with the event are estimated upstream, and 4.9 km downstream across the Lillooet River, at $10 M CAD. If gravel removal or dike elevation changes creating landslide dams on both streams. Our analysis of become necessary as a result of the increased sediment load, superelevation in the bends of Capricorn Creek and the dra- the total long-term costs of this landslide could exceed the matic run-up at the confluence of Capricorn Creek and Mea- −1 direct costs. ger Creek suggest average velocities of 62–73 m s and 63 m s−1, respectively. 3.9 Eyewitness account The 2010 Mount Meager landslide is significant for seven reasons; (1) it is the tenth mass flow involving volumes in Eyewitness accounts, while arguably less scientific, provide excess of 0.5 × 106 m3 to have occurred in the MMVC since important corroboration and context to rarely seen catas- 1850 and the sixth largest mass flow identified in the MMVC trophic events. In the case of the Mount Meager landslide, over the Holocene; (2) it is one of the largest landslides to no lives were lost; however, four witnesses to the event were have occurred worldwide since 1945; (3) the landslide exhib- in extreme jeopardy on several occasions during and imme- ited dramatic transformation from an initial rock slope failure diately following the landslide. K. Kraliz, J. Duffy, J. Tilley, to a rapidly moving debris flow as a result of the high degree and P. Smith, along with their dog, arrived at upper Lillooet of fragmentation of the initial failure mass; (4) the forma- forest campsite at 3:25 a.m. and began unloading their gear. tion of a significant landslide dam illustrated the importance They were surprised by “two large cracks” (loud explosive of landslide damming as a secondary landslide process and noises, not physical cracks in the ground) occurring in quick as a hazard to distant downstream communities; (5) the seis- succession, followed by a rumbling that initially sounded like mic trace of the landslide initiation and motion allowed de- a train or a forest fire, but that grew to a deafening volume in tailed reconstruction of the stages and velocity of movement; about 20 s (P. Smith, personal communication, 2010). All (6) the event demonstrates the role of rapid post-Little Ice four campers and their dog got back into their truck and Age deglaciation in destabilising slopes adjacent to modern
Nat. Hazards Earth Syst. Sci., 12, 1277–1294, 2012 www.nat-hazards-earth-syst-sci.net/12/1277/2012/ R. H. Guthrie et al.: The 6 August 2010 Mount Meager rock slide-debris flow 1293 glaciers; and (7) it represents a unique opportunity to con- Dufresne, A. and Davies, T. R.: Longitudinal ridges in mass move- sider hazard and risk from catastrophic failures to mountain ment deposits, Geomorphology, 105, 171–181, 2009. communities. Evans, S. G.: A rock avalanche from the peak of Mount Meager, The event illustrates the extreme landslide hazard of British Columbia, Geol. Surv. Can. Pap. 87-1A, 929–934, 1987. glacier-clad dissected Quaternary volcanic centres, which re- Evans, S. G.: Landslides, in: A synthesis of Geological Hazards in sults from the existence of steep slopes in poor quality rock Canada, Geol. Surv. Can. Bull., 548, 43–80, 2001. Evans, S. G.: Single event landslides resulting from massive rock (reflecting such factors as hydrothermal alteration and het- slope failure: Characterising their frequency and impact on soci- erogeneity of volcanic products). In this geological envi- ety, in: Landslides from Massive Rock Slope Failure, edited by: ronment, landslides are a major process of denudation and, Evans, S. G., Scarascia-Mugnozza, G., Strom, A. L., and Her- through transformation into debris flows, deliver large vol- manns, R. L., NATO Sci. Ser. IV, Earth Environ. Sci., Springer, umes of debris to river systems. Dordrecht, 49, 53–73, 2006. Evans, S. G., Hungr, O., and Clague, J. J.: Dynamics of the 1984 Acknowledgements. The authors gratefully acknowledge logistical rock avalanche and associated debris flow on Mount Cayley, support provided by the province of British Columbia, the Centre British Columbia, Canada; Implications for landslide hazard as- for Natural Hazard Research at the Simon Fraser University, and sessment on dissected volcanoes, Eng. Geol., 61, 29–51, 2001. the Landslide Research Programme at the University of Waterloo. Evans, S. G., Guthrie, R. H., Roberts, N. J., and Bishop, N. F.: The disastrous 17 February 2006 rockslide-debris avalanche Edited by: D. Keefer on Leyte Island, Philippines: a catastrophic landslide in tropi- Reviewed by: E. Harp and another anonymous referee cal mountain terrain, Nat. Hazards Earth Syst. Sci., 7, 89–101, doi:10.5194/nhess-7-89-2007, 2007. Evans, S. G., Roberts, N. J., Ischuck, A., Delaney, K. B., Morozova, References G. S., and Tutubalina, O.: Landslides triggered by the 1949 Khait earthquake, Tajikistan, and associated loss of life, Eng. Geol., Ayotte, D. and Hungr, O.: Calibration of a runout prediction model 109, 195–212, 2009. for debris flows and avalanches, in: Proceedings of the 2nd Inter- Friele, P. and Clague, J. J.: Large Holocene landsldies from Py- national Conference on Debris Flows, Taipei, edited by: Wiec- lon Peak, southwestern British Columbia, Can. J. Earth Sci., 41, zorek, G. F. and Naeser, N. D., Balkema, Rotterdam, 505–514, 165–182, 2004. 2000. Friele, P. and Clague, J.J.: Paraglacial geomorphology of Qua- Berrocal, J., Espinosa, A. F., and Galdos, J.: Seismological and ternary volcanic landscapes in the southern Coast Mountains, geological aspects of the Mantaro landslide in Peru, Nature, 275, British Columbia, in: Periglacial and Paraglacial Processes and 532–536, 1978. Environments, edited by: Knight, J. and Harrison, S., Geol. Soc. Bovis, M. J. and Jakob, M.: The July 29, 1998, debris flow and land- Spec. Publ. London, 320, 219–233, 2009. slide dam at Capricorn Creek, Mount Meager Volcanic Complex, Friele, P., Clague, J. J., Simpson, K., and Stasiuk, M.: Impact of southern Coast Mountains, British Columbia, Can. J. Earth Sci., a Quaternary volcano on Holocene sedimentation in the Lillooet 37, 1321–1334, 2000. River valley, British Columbia, Sediment. Geol., 176, 305–322, Brodsky, E. E, Gordeev, E., and Kanamori, H.: Landslide basal 2005. friction as measured by seismic waves, Geophys. Res. Lett., 30, Friele, P., Jakob, M., and Clague, J. J.: Hazard and risk from 2236 I-1–2236 I-5, 2003. large landslides from Mount Meager volcano, British Columbia, Carter, N. M.: Exploration in the Lillooet River watershed, Can. Canada, Georisk, 2, 48–64, 2008. Alpine J., 21, 8–18, 1932. Glicken, H.: Sedimentary architecture of large volcanic-debris Clague, J. J.: Late Quaternary Geology and Geochronology avalanches, in: Sedimentation in volcanic settings, edited by: of British Columbia Part 2: Summary and Discussion of Smith, G. A. and Fisher, R. V., SEPM, Tulsa, Special Publica- Radiocarbon-dated Quaternary History, Geol. Surv. Can. Pap., tion, 45, 99–106, 1991. 80–35, 41 pp., 1981. Guthrie, R. H. and Evans, S. G.: Work, persistence and formative Clague, J. J., Evans, S. G., Rampton, V. N., and Woodsworth, G. J.: events: The geomorphic impact of landslides, Geomorphology, Improved age estimates for the White River and Bridge River 88, 266–275, 2007. tephras, western Canada, Can. J. Earth Sci., 32, 1172–1179, Hickson, C. J.: Character of volcanism, volcanic hazards, and risk, 1995. northern end of the Cascade magmatic arc, British Colubmia and Croft, S. A. S.: Stability Assessment of the Capricorn Creek Valley, Washington State, in: Geology and Geological Hazards of the British Columbia, B.Sc. thesis, University of British Columbia, Vancouver Region, Southwestern British Columbia, edited by: Vancouver, BC, 1983. Monger, J. W. H., Geol. Surv. Can. Bull., 481, 231–250, 1994. Dahlen, F. H.: Single-force representation of shallow landslide Holm, K., Bovis, M., and Jakob, M.: The landslide response of sources, Bull. Seismol. Soc. Am., 83, 130–143, 1993. alpine basins to post-Little Ice Age glacial thinning and retreat in Decker, F., Fougberg, M., and Ronayne, M.: Pemberton: The His- southwestern British Columbia, Geomorphology, 57, 201–216, tory of a Settlement, Hemlock Printing Ltd., Burnaby, BC, 1977. 2004. Deparis, J., Jongmans, D., Cotton, F., Baillet, L., Thouvenot, F., and Huang, C.-J., Yin, H.-Y, Chen, C.-Y., Yeh, C.-H., and Wang, C.-L.: Hantz, D.: Analysis of rock-fall and rock-fall avalanche seismo- Ground vibrations produced by rock motions and debris flows, J. grams in the French Alps, Bull. Seismol. Soc. Am., 98, 1781– Geophys. Res., 112, F02014, doi:10.1029/2005JF000437, 2007. 1796, 2008. www.nat-hazards-earth-syst-sci.net/12/1277/2012/ Nat. Hazards Earth Syst. Sci., 12, 1277–1294, 2012 1294 R. H. Guthrie et al.: The 6 August 2010 Mount Meager rock slide-debris flow
Hungr, O.: A model for the runout analysis of rapid flow slides, de- Nasmith, H., Mathews, W. H., and Rouse, G. E.: Bridge River ash bris flows, and avalanches, Can. Geotech. J., 32, 610–623, 1995. and some other Recent ash beds in British Columbia, Can. J. Hungr, O.: Simplified models of spreading flow of dry granular Earth Sci., 4, 163–170, 1967. material, Can. Geotech. J., 45, 1156–1168, 2008. Norris, R. D.: Seismicity of rockfalls and avalanches at three Cas- Hungr, O. and Evans, S. G.: Rock avalanche runout prediction using cade Range volcanoes: Implications for seismic detection of haz- a dynamic model, in: Proceedings of the 7th International Sym- ardous mass movements, Bull. Seismol. Soc. Am., 84, 1925– posium on Landslides, Trondheim, Norway, 1, 233–238, 1996. 1939, 1994. Hungr, O. and Evans, S. G.: Entrainment of debris in rock Province of British Columbia: Coast selling price system, average avalanches: An analysis of a long run-out mechanism, Geol. Soc. log prices for the 1 month period ending 2010-08-31, Province of Am. Bull., 116, 1240–1252, 2004. British Columbia, Ministry of Forests, Mines and Lands online Hungr, O. and McDougall, S.: Two numerical models for landslide resource, available at: http://www.for.gov.bc.ca/hva/logreports dynamic analysis, Comp. Geosci., 35, 978–992, 2009. coast.htm, last access: 4 March 2011. Jakob, M.: Morphometric and Geotechnical Controls on De- Read, P. B.: Geology, Meager Creek Geothermal Area, British bris Flow Frequency and Magnitude in Southwestern British Columbia, Geol. Surv. Can. Open File 603, 1978. Columbia. Ph.D. thesis, University of British Columbia, Vancou- Read, P. B.: Mount Meager complex, Garibaldi Belt, southwestern ver, BC, 1996. British Columbia, Geosci. Can., 17, 167–170, 1990. Jordan, P.: Debris Flows in the Southern Coast Mountains, British Roche, A. D., Guthrie, R. H., Roberts, N. J., Ellis, E., and Friele, Columbia: Dynamic behaviour and physical properties, Ph.D. P.: Once more into the breach: A forensic analysis of the Au- thesis, University of British Columbia, Vancouver, BC, 1994. gust 2010 landslide dam outburst flood at Meager Creek, British Julian, B. R., Miller, A. D., and Foulger, G. R.: Non-double-couple Columbia, in: Proceedings of the 5th Canadian Conference on Eearthquakes, 1. Theory, Rev. Geophys., 36, 525–549, 1998. Geotechnique and Natural Hazards, Canadian Geotechnical So- Kanamori, H. and Given, J. W.: Analysis of long-period seismic ciety, May 15–17, 2011, Kelowna, BC, 10 pp., 2011. waves excited by the May 18, 1980 eruption of Mount St. He- Siebert, L.: Large volcanic debris avalanches: Characteristics of lens – A terrestrial monopole?, J. Geophys. Res., 87, 5422–5432, source areas, deposits and associated eruptions, J. Volc. Geo. 1982. Res., 22, 163–197, 1984. Lipovsky, P. S., Evans, S. G., Clague, J. J., Hopkinson, C., Couture, Simpson, K. A., Stasiuk, M., Shimamura, K., Clague, J. J., and R., Bobrowsky, P., Ekstrom,¨ G., Demuth, M. N., Delaney, K. B., Friele, P. A.: Evidence for catastrophic volcanic debris flows in Roberts, N. J., Clarke, G., and Schaeffer, A.: The July 2007 rock Pemberton Valley, British Columbia, Can. J. Earth Sci., 43, 679– and ice avalanches at Mount Steele, St. Elias Mountains, Yukon, 689, 2006. Canada, Landslides, 5, 445–455, doi:10.1007/s10346-008-0133- Sosio, R., Crosta, G. B., and Hungr, O.: Complete dynamic model- 4, 2008. ing calibration for the Thurwieser rock avalanche (Italian Central Mathews, W. H. and McTaggart, K. C.: Hope rockslides, British Alps), Eng. Geol., 100, 11–26, 2008. Columbia Canada, in: Rockslides and Avalanches, Natural Phe- Stasiuk, M. V., Russell, J. K., and Hickson, C. J.: Distribution, nomona, edited by: Voight, B., Elsevier, New York, 259–275, nature, and origins of the 2400 BP eruption products of Mount 1978. Meager, British Columbia, Linkages between magma chemistry McNeely, R. and McCuaig, S.: Geological Survey of Canada Ra- and eruption behaviour, Geol. Surv. Can. Bull., 486, 1–27 1996. diocarbon Dates XXIX, Geol. Surv. Can. Pap. 89–9, 1991. Stewart, M.: Dacite Block and Ash Avalanche Hazards in Moun- McSaveney, M. J. and Downes, G.: Application of landslide seis- tainous Terrain: 2360 yr BP Eruption of Mount Meager, British mology to some New Zealand rock avalanches, in: Landslides, Columbia, M.Sc. thesis, University of British Columbia, 2002. edited by: Rybar, J., Stemberk, J., and Wagner, P., Balkema, Weichert, D., Horner, R. B., and Evans, S. G.: Seismic signatures of Lisse, 649–654, 2002. landslides: The 1990 Brenda Mine collapse and the 1965 Hope Mokievsky-Zubok, O.: Glacier caused slide near Pylon Peak, rockslides, Bull. Seismol. Soc. Am., 84, 1523–1532, 1994. British Columbia, Can. J. Earth Sci., 15, 1039–1052, 1977. Westgate, J. A. and Dreimanis, A.: Volcanic ash layers of Recent age at Banff National Park, Alberta, Canada, Can. J. Earth Sci., 4, 155–161, 1967.
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