Life-loss vulnerability models, which estimate the probability of an individual being killed given landslide impact, are critical inputs for landslide risk assessments focused on life loss. Land-use decisions in western Canadian landslide-prone areas increasingly rely on these assessments, however, there are no published life-loss vulnerability models for individuals in typical Canadian buildings. To address this gap, we developed an empirical life-loss vulnerability curve for people inside wood-frame buildings as a function of impact pressures from flow-type landslides. It is based on a new method which combines estimates of building damage probability given landslide impact and life-loss probability for people in damaged buildings. It is produced from a dataset of 421 wood-frame buildings damaged by flow-type landslides and 242 people in those buildings. The results show that life-loss vulnerability increases with building damage state, and that occupants are much more likely to be killed in buildings that at least partially collapse, likely because occupants have fewer options to avoid harm as building damage increases. The vulnerability model indicates that life-loss vulnerability increases rapidly from impact pressures of 1–32 kPa because, over this range, at least partial collapse of the building increases to the most likely damage state. The resulting vulnerability curve likely overestimates vulnerability at low impact pressures and underestimates vulnerability at high impact pressures. We provide an annotated vulnerability curve to help users account for and communicate these limitations.

Life-loss risk estimates inform decisions that have dramatic impacts on individuals and communities in landslide hazard zones. Literature provides methods for making these estimates through multiplication of site-specific parameters. However, there is no method in the literature for calibrating those estimates, and regional context is needed to make efficient, fair, and affordable decisions. Therefore, we developed methods to quantify risk to individuals and groups based on regional historical fatality and hazard zone population data. The methods can be used to evaluate the accuracy of large sets of site-specific risk estimates, to assign quantitative risk values to buildings and hazard zones at a regional scale, to develop a long-term regional budget for landslide risk management, or to inform selection of a risk tolerance threshold that can inform where limited resources are invested in landslide mitigation. An example of the methods is provided for a case study of residential landslide risk in British Columbia, Canada. It estimates that there are between a few hundred and a few thousand people living with individual risk exceeding 100 micromorts (1 in 10 000) per year, and between 70 and 370 landslide hazard zones with annual probable life loss greater than 1E-3 (1 death in 1000 years), based on a historical fatality rate of 0.4 to 1.4 deaths per year across the entire province.

The Cheekeye Fan, located within the District of Squamish, is prone to debris-flow hazards that pose unacceptable risk to development. This article describes how a debris-flow barrier that would protect existing and proposed development was designed to achieve locally adopted risk tolerance criteria. A risk assessment showed that the barrier should manage debris flows with volumes up to 2.8 Mm3 (1:10 000-year return-period events) to achieve tolerable risk, and that debris flows with volumes below 0.2 Mm3 (10-30-year events) can pass the barrier through an outlet without exceeding risk tolerance thresholds. The local government specifies that tolerable debris-flow risks be reduced "as low as reasonably practicable" (ALARP), defined in this project as the point where the cost of additional mitigation measures is grossly disproportionate to the benefits gained. By estimating the disproportionality ratio for potential auxiliary measures, this study shows that the barrier reduces risk ALARP without additional measures in place. The authors believe that new development approval on Cheekeye fan would not be possible without the risk-informed decision-making process described in this article.

The proposed Cheekeye debris-flow barrier in Squamish, BC, Canada is an open check dam with a very large storage capacity of 2.4 million m3. There are several unique features of the design that deviate from standard international practice, including the very low probability (1 in 10,000 years) design event, the use of a single large structure, the selected construction material, the crest design, and the outlet design. These features were selected for the Cheekeye barrier due to idiosyncrasies of the site specifically, and of debris-flow mitigation in Canada, in general.

We show how a quantitative estimate of debris-flow hazard and risk can be derived simply from the position of infrastructure on the fan relative to the fan apex and the most likely flow path (e.g., active channel). Fan sectors and the spatial probability of impact in each sector are based on a fan-normalized heat map of debris-flow impacts derived from 146 mapped impact areas across 30 fans in southwestern British Columbia, Canada. As a proof-of-concept, we provide an example for annual life loss risk to an individual who occupies a home in various sectors of a debris-flow fan. The results are comparable to broad findings from quantitative risk assessments completed at 10 fans in British Columbia and Alberta, Canada with similar characteristics. The method presented here is a way to obtain a high-level quantitative risk estimate prior to a detailed site-specific assessment.

Risk-taking is an essential part of life. As individuals, we evaluate risks intuitively and often subconsciously by comparing the perceived risks with expected benefits. We do this so commonly that it passes unnoticed, like when we decide to speed home from work or go for a swim. The comparison changes, however, when one entity (such as a government) imposes a risk evaluation on another person. For example, in a quantitative risk management framework, the estimated risk is compared with a tolerable risk threshold to decide if the person is ‘safe enough’. Landslide risk management methods are well established and there is consensus on tolerable life-loss risk thresholds. However, beneath this consensus lie several key details that are explored by this article, along with suggestions for refinement. Specifically, we suggest using the risk unit, micromort (one micromort equals a life loss risk of 1 in 1 million), in describing risk estimates and thresholds, to improve risk communication. For risk estimation, we provide guidance for defining and combining landslide scenarios and for recognizing where unquantified risk from low-probability/high-consequence scenarios ought to inform risk management decisions. For risk tolerance thresholds, we highlight the pitfalls of selecting unachievably low thresholds and suggest that there is no single universal threshold. Additionally, we argue that gross disproportion between costs and benefits of further risk reduction, which is integral to the As Low As Reasonably Practicable (ALARP) principle, is a commonly unachievable and counter-productive condition for risk tolerance, and other conditions centered on proportionality often apply. Finally, we provide several figures that can be used as risk communication tools, to provide context for risk estimates and risk tolerance thresholds when these values are reported to decision makers and the public.

Large boulders can jam in constrictions, both in canyons and man‐made structures (e.g., slit‐dams). Boulder jamming occurs stochastically, and partially governs the outflow rate of debris flow material. Explicit hydro‐mechanical models of boulder‐laden flows are too computationally demanding to study this stochasticity. This study presents a new framework implemented into a numerical program. This program encapsulates basic hydraulic equations and criteria predicting boulder blockage at narrow sections, along with a simple statistical way to compute boulder sizes and numbers. The program applies empirical estimates by experts of the average number of boulders in a deposit to evaluate how equivalent multi‐phase flows would interact with a constriction. The model stochastically generates boulders that can obstruct the constriction(s). The program outputs simple descriptions of the upstream flow level over time, as well as the downstream outlet discharge rate and volume. The model is fast (5–30 s per run), allowing uncertainty propagation analyses of interactions between flows, boulders and constrictions. Interaction between both low‐ and high‐risk flows can be quickly evaluated for different scenarios. For uncertainty propagation, we use possibility theory, which accommodates uncertainties relevant to debris flows. The framework gives practitioners a much‐needed generalized approach for understanding the long‐term behavior of boulder‐laden flows through canyons, or for designing engineered structures. Finally, we apply the model to a field case – the design of a proposed North American slit‐dam – thence elucidating the design requirements for effective flow control. We found that horizontal bars are necessary for dependably controlling outflow from this structure.

Predicting the spatial impact of debris flows on fans is challenging due to complex runout behaviour. Debris flow mobility is highly variable and flows can sporadically avulse the channel. For hazard and risk assessments, practitioners typically base the probability of spatial impact or avulsion on their experience and expert judgement. To support decision‐making with empirical observations, we studied spatial impact distributions on 30 active debris‐flow fans in south‐western British Columbia, Canada. We mapped 146 debris‐flow impact areas over an average observation period of 74 years using orthorectified airphotos, satellite imagery, topographic base maps, LiDAR data, orthophotos, and field observations. We devised a graphical method to convert our geospatial mapping into spatial impact heat maps normalized by fan boundaries, enabling comparison of runout distributions across different fans. About 90% of the mapped debris flows reached beyond the mid‐points of fans, while less than 10% avulsed more than half‐way across the fan relative to the previous flow path. Most avulsions initiated at distances of 20% to 40% of the maximum fan length from the fan apex and upstream of the fan intersection point. Large volume events tend to be more mobile in the down‐fan direction, but the relation between volume and cross‐fan runout (e.g., avulsions) is more complex. Differences in spatial impact distributions can be explained, in part, by the degree of fan incision and whether a fan is truncated at its toe by a river or lake. There were no significant differences in spatial impact distributions based on the geology of the source area, sediment supply condition, or hydrogeomorphic process classification.

According to a Canadian government database, landslides are the most common type of disaster that occurs in the province of British Columbia. Recently there has been a trend in British Columbia toward using quantitative risk assessments to estimate life-loss risk at landslide hazard sites, and to compare these estimates with risk tolerance thresholds to determine the necessity for, and extent of, risk management measures. These risk estimates are most often calibrated by so-called ‘expert judgment’ because historical landslide fatality data are not readily available. This article addresses this gap by summarizing available historical data to better inform expert judgment. It shows that fatalities caused by landslides in British Columbia are rare (approximately one fatality per year in the last decade) and have decreased with time despite rapid population growth. Approximately half of these fatalities in the last decade are related to debris flows and debris floods that impact houses, whereas the other half are related to rockfalls, debris flows, and debris floods that impact highways. A comparison with other hazard types in the Canadian government’s disaster database suggests that, while not particularly deadly, landslides are still important because of the economic damage and service disruptions they cause. Although the data are specific to British Columbia, the methods for identifying and presenting landslide risk trends could be modified and adopted in other world regions where landslide fatality data are collected and quantitative risk management methods are utilized.

When a life-threatening landslide is recognized, the questions most relevant to community leaders are as follows: is our community safe enough, and if not, how much should we spend on protection? A risk evaluation tool that helps answer these questions, which compares landslide risk faced by a group of people to perceptions of tolerable risk, was first proposed in Hong Kong and is now being used widely in Western Canada within a quantitative risk management framework. After more than a decade of sporadic application in Western Canada, challenges of applying this tool are becoming apparent. For example, its use has resulted in landslide mitigation designs that are unaffordable, and it has failed to persuade funding authorities that proposed risk management solutions are a justifiable allocation of governments’ limited resources. This article suggests that the risk evaluation tool designed in Hong Kong should not be universally applied. We propose modifications believed to be more appropriate for Western Canada, including tolerating landslide risk levels similar to other natural hazard types and emphasizing cost-effectiveness of landslide risk reduction options. Although the proposed tools were developed for the sociopolitical context of Western Canada, we hope the discussions included in this article motivate others to modify these risk evaluation tools for other societies and hazard types and that ultimately this will lead to more rational and consistent decisions that, with time, save lives and resources in landslide-prone regions of the world.

CN is one of the largest Class I railways in North America. In recent years, CN has lengthened many existing sidings to accommodate increased rail traffic and train length. In British Columbia, widening the railway grade for additional track often means potentially encroaching on environmentally sensitive areas or land that is not part of the CN right-ofway, filling over unfavourable soils or into riparian areas of rivers, lakes, small streams and wetlands, or constructing high soil or rock cut slopes to traverse sloping ground. Grade widening also requires extension of culverts in streams that are often important fish habitat. Additional construction considerations include remote sites, site access that is often only by rail, distant fill borrow sources, and the difficulty of working next to an active railway. Efforts to minimize the environmental impact, physical footprint, and cost of siding extensions have led to the use of geosynthetic fabric reinforced soils (GRS) to steepen fill slopes, buttress and steepen cut slopes, for retaining walls, and for culvert head walls. While GRS structures have been in use for many years, examples of geotextile (as opposed to geogrid or steel) reinforced structures in a heavy freight railway environment are few. This paper reviews the general approach used for GRS retaining wall design in a railway environment, and uses case histories to illustrate the use of one type of GRS system for retaining walls and culvert head walls. The costs and benefits of the GRS techniques are discussed.

Hazard assessments involving large rock slopes are often problematic, given the influence of geology on failure kinematics and the subsequent influence of the failure kinematics on the rockslide runout. The 2003 Afternoon Creek rockslide in northwest Washington is one such example, where 750 000 m3 of rock slid from a steep ridge harmlessly into Afternoon Creek. However, topographic and structural controls at the source area unexpectedly redirected a small volume (<10%) down the opposite side of the ridge along a much steeper travel path, impacting the highway below. Postfailure investigations indicate that the slope still presents a danger. To address this, a framework was developed that links back analyses with forward modelling of failure initiation and runout. Field mapping and data collection were specifically tailored for these analyses. Advanced numerical modelling was used to assess likely rockslide source areas, volumes, and controlling features. These were then used to guide a series of three-dimensional runout simulations to model rockslide travel path, reach, and velocities. The results show that a rockslide originating from the ridge could occur again and that the topography would again direct a small percentage of material down the backside which would reach the highway below.

A series of mass wasting events occurred above a Washington, USA, highway in the Cascade Mountains in November and December 2003. The largest event was a rockslide involving approximately 750,000 m(3) that occurred on November 9, 2003. The source zone for this event was located at the crest of a ridge. Most of the debris fell to the east of the sharp ridge and was deposited in the relatively shallow sloping Afternoon Creek without causing damage to the highway. Lesser amounts of debris fell to the west of the ridge, sliding 600 m down the steeper Falls Creek and impacting the road. There is an evidence of one or more historical rock avalanches at this location. Displacement of reference points, ground vibration, crack extension, and tilting are being monitored due to concerns that future slope failures or remobilization of debris might again damage or block the highway.

A "Total Slope Analysis" methodology, that combines several numerical techniques, is adopted to investigate an unstable rock slope in Washington State, USA. For this specific study, the distinct-element code UDEC is used to assess the stability and potential failure volume of the rockslide. Once the potential rockslide volume has been estimated and failure mechanism assessed, the runout path, distance and velocity are assessed using the dynamic or rheological flow model DAN3D. Site investigation and data reconnaissance plays an important role for both stages in the "Total Slope Analysis", including outcrop mapping, aerial photograph interpretation, scanline joint surveys and 3-D laser scanning. The results of the "Total Slope Analysis" can be directly applied to assessment and mitigation of the landslide hazard, greatly aiding engineering judgment by providing key qualitative and quantitative insights into the risk analysis.