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The application of quantitative risk assessments is increasing for decision-making in many industries and contexts, with the evaluation of risks against some adopted criteria. In this article, we review risk criteria developed and used for landslide management, in particular criteria associated with risk to life. We show that while this natural hazard is encountered worldwide, the social and regulatory contexts under which evaluation criteria are developed can vary significantly. Thus, the applicability of developed criteria to any specific situation should be assessed before adopting them elsewhere. We describe selected considerations for developing risk evaluation criteria, propose a framework for defining these criteria in Canada, and assess the applicability of previously proposed criteria. Examples of risk criteria development and adoption for new and existing residential developments and for railway employees are presented to illustrate some of these concepts. Keywords: Risk evaluation criteria, Risk assessment, Decision making, Risk governance, Review Introduction (33 fatalities in 1908), Burnaby and New Westminster in Landslide occurrences are ubiquitous across Canada. British Columbia (22 fatalities in 1909), Cooper mine in Brit- They occur in the thousands each year, and landslide ish Columbia (56 fatalities in 1915), Britannia Beach in Brit- types include rock falls, slides and debris flows in the ish Columbia (37 fatalities in 1921), Saint-Jean-Vianney in Cordillera, glaciomarine earthflows in the St. Lawrence Quebec (31 fatalities in 1971) (Agrawal 2018). Lowlands, translational rock slides in the prairies and Increasingly, quantitative risk assessments are being earthflows associated with permafrost in the northern- used broadly; from examining new industrial facilities to most regions (Couture et al. 2013). Fortunately, most managing existing geohazards (Khan et al. 2015; Mor- landslides do not impact populated areas or infrastruc- genstern 1997). Regulatory agencies are requiring the ture however a subset affect communication lines, popu- use of quantitative methods in their published regula- lated areas and other infrastructure. Clague and tions and guidelines and clients are aiming to improve Bobrowsky (2010) reported an estimated $200 million in the economic efficiency of their risk management annual costs from landslide effects, and Evans (1999)esti- decision-making. In landslide management, examples mated that over 600 people lost their lives in landslides in can be found in the implementation of quantitative risk the last two centuries for an average of 4 lives lost per year criteria in Hong Kong (ERM-Hong Kong Ltd 1998), and (Couture et al. 2013). The occurrences with the largest num- the initiative by the Australian Geomechanics Society to ber of fatalities (over 10) include: the Frank Slide in Alberta develop risk management guidelines that emphasize (70 fatalities in 1903), Spences Bridge in British Columbia quantitative methodologies, in response to local author- (15 fatalities in 1905), Notre-Dame-de-la-Salette in Quebec ities’ increasing demands for risk assessments of new urban development (Australian Geomechanics Society * Correspondence: email@example.com (AGS) 2007). In Canada, The province of British Colum- David and Joan Lynch School of Engineering Safety and Risk Management, bia’s Guidelines for Legislated Landslide Assessments for Department of Civil and Environmental Engineering, University of Alberta, Proposed Residential Developments in BC (Association 12-324 Donadeo Innovation Centre for Engineering, Edmonton, AB T6G 1H9, Canada of Professional Engineers and Geoscientists of British Full list of author information is available at the end of the article © The Author(s). 2018 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Macciotta and Lefsrud Geoenvironmental Disasters (2018) 5:10 Page 2 of 14 Columbia (APEGBC) 2010) also considers the application most widely used are the criteria developed by: the Health of quantitative assessments for land use planning, and and Safety Executive (Health and Safety Executive, UK quantitative landslide risk evaluations have been success- (HSE) 2001) in the United Kingdom for land use planning fully used for decision-making regarding new and existing around industries, the Hong Kong Special Administrative developments in the provinces of British Columbia and Region Government for land development in landslide Alberta (Clague et al. 2015; Hungr et al. 2016). Recently, prone areas (ERM-Hong Kong Ltd 1998), the Australian the North American railway industry is developing com- National Committee on Large Dams (Australian National prehensive risk management strategies, which are consist- Committee on Large Dams (ANCOLD) 2003) and Austra- ently leaning towards more detailed quantitative analyses lian Geomechanics Society for landslides (Australian Geo- of track sections, considered critical given their high land- mechanics Society (AGS) 2007), and the U.S. Bureau of slide hazard levels (Macciotta et al. 2016). Canadian guide- Reclamation (2003) for people exposed to potential dam lines related to landslides also include discussions about failures. These criteria have been discussed in detail by measures of landslide risk, risk evaluation and adoption of others (Finlay and Fell 1997; ERM-HongKongLtd 1998; criteria (Porter and Morgenstern 2013). Leroi et al. 2005;Ale 2005;Porter et al. 2009; Scarlett et Criteria in these studies focus on the risk to the lives of al. 2011). The acceptable frequency of incidents resulting the population exposed. When risk levels are measured in in lives lost, across such criteria, varies by several orders terms of the probability of lives lost, the development of of magnitude (Frank and Jones 2010). These variances evaluation criteria becomes an increasingly difficult task. arise from differences in their originating context: the Establishing acceptable limits to lives lost is not only a sci- types of companies or organizations proposing the pro- entific fact-based matter, but also involves the consider- jects (i.e., government agencies versus corporations), the ation of value-based legal, political, social and economic nature of the hazards and associated environmental im- issues (Aven 2016;Fell 1994;Hoetal. 2000), all influen- pacts, the social/political context which affects the nature cing landslide risk perception as shown in Prasad et al. of risk perceptions, and the philosophical nature of risk (2016) and Heitz and Shimabuku (2017). The final evalu- evaluation principles embedded in the regulatory struc- ation of risk often lies with the project owner and regula- tures (Skjong 2002;Vrijling et al. 1998). tor. Yet, by being informed about the precedents, details, The HSE criteria started with a focus on the risks and limitations of the analyses, risk analysts can become posed by nuclear power plants, whereby low probability valuable advisors in the development and use of such risk accidents have high consequences, with immediate lives evaluation criteria (Leroi et al. 2005). lost, and also chronic effects and significantly shorter life Given the difficulties outlined above, policy- and expectancy. In this context, consequences are regional, decision-makers may be tempted to adopt others’ risk long lasting, and newly imposed (Health and Safety Ex- evaluation criteria, such as the United Kingdom’s (UK) ecutive, UK (HSE) 1992). In contrast, slope failures in ALARP (As Low As Reasonably Practicable). However, developments within landslide prone areas, such as for the contexts under which these criteria were developed Hong Kong landslides (ERM-Hong Kong Ltd 1998), the can be quite different and their applicability to any spe- consequences are localized, immediate, and not long cific situation should be assessed before adopting them. term. Hong Kong is also densely populated and charac- In this regard, our article describes the primary consid- terized by its hilly terrain. As such, the development of erations for developing risk evaluation criteria based on areas unaffected by landslides is not always an option. In an extensive literature review, proposes a framework for such a context, even when there is a long history of defining landslide risk criteria, and assesses the applic- landsliding (ERM-Hong Kong Ltd 1998), too conserva- ability of previously proposed criteria to a specific con- tive approaches towards development regulations would text. Two simplified examples discussing some aspects lead to significant economic losses related to limitations of development of proposed risk evaluation thresholds in land use. As a result, there is a sense of higher toler- are also discussed. The paper focuses on the risk to life ance towards these risks (Fell 1994; Finlay and Fell in the Canadian Context in terms of individual risk –the 1997). In sum, the Hong Kong criteria would be less ap- increase of risk to a particular individual or the average plicable in regions where population density is low and individual associated with the existence of a hazard (Fell more area is available for new development. However, et al. 2005)- and societal risk –risk of large number of the Hong Kong criteria has been used successfully as losses such that the consequence would provoke a basis for landslide prone areas in Canada (District of socio/political response (Fell et al. 2005). North Vancouver and the Cheekye debris fan in British Columbia, and the Cougar Creek debris fan in Canmore, Existing risk evaluation criteria Alberta - Clague et al. 2015; Hungr et al. 2016). There are several risk evaluation criteria that are becom- The variability in these contexts suggests that the de- ing commonly used for evaluating landslide risks. The velopment of risk evaluation criteria is best suited when Macciotta and Lefsrud Geoenvironmental Disasters (2018) 5:10 Page 3 of 14 defined at regional, industry, client and even case spe- posed by nature and development; 2) risk thresholds to cific scales. Existing criteria can be helpful as starting aid in evaluating the real urgency for mitigation strat- point for the development of risk evaluation thresholds, egies, risks deemed as not tolerable would require miti- given the context of the system matter of analyses is gation to assure a minimum quality of life; and 3) the similar. In any case, however, the applicability of previ- uncertainties inherent to a system that must be consid- ously proposed criteria should be assessed before it is ered during assessment and management, including pre- adopted. cautionary approaches when required. Several principles are embedded within this (Aven 2016; Vanem 2012). Framework to develop risk evaluation criteria The development and adoption of risk acceptability – Absolute risk criterion: The level of risk itself is studied thresholds will be influenced by the organizational, en- and the risk criterion is formulated as a maximum level vironmental, and social/political context of the project of risk that should not be exceeded, without regard to (Rowe 1977; Fell 1994; Fell et al. 2005; Leroi et al. 2005). the cost and benefit associated with it. As shown in Fig. 1, any framework for the development – Precautionary principle: If risks are irreversible, of risk evaluation criteria needs to consider three main scientific uncertainty is not a justification for aspects in an integrative manner (Aven 2016; Renn postponing cost-effective mitigation measures. If the 2008). The first aspect, the characteristics of the system uncertainty and consequences are too great, then it being analysed – including the project owner, project, may be best not to assume the risks at all. and its environment - is technical in nature. It considers – The principle of equivalency: Risk should be compared the hazard characteristics, such as extent and type of with known levels of risks from similar activities or consequences, frequencies of occurrence, and the associ- systems that are widely regarded as acceptable or ated uncertainties. The second aspect is the social, polit- tolerable, to require that an equal level of risk be ical, economic and cultural context. This aspect is obtained. Similarly, comparisons can be against historic fundamentally of social nature, which affects how risks data, natural disasters, and life expectancy. are perceived depending upon if they are natural or – The holistic principle: Decisions regarding safety on manmade, existing or newly imposed, voluntary or invol- behalf of the public should be based on a holistic untary and, as a result, how risk responses become amp- consideration of all risks and apply across the lified or attenuated. The third aspect, the principles for complete range of hazards. Only when the total risk developing risk evaluation criteria, has a philosophical the public is exposed to is properly assessed, can the nature as dependent upon the technical and social con- proposed risk reduction measures be evaluated and texts and the principles of regulation. We draw from risk criteria established. Given the difficulties and these fundamental principles to develop risk criteria effort this would require, the principle is applied at (Pandey and Nathwani 2004; Skjong et al. 2005; Vanem the scale of the system being analyzed and requires 2012; Morrison 2014). simplification (such as apportioning and scaling). – The ALARP principle: Risks should be managed to Principles for developing evaluation criteria be As Low As Reasonably Practicable (ALARP). Selecting the principles that would lead to a clear frame- Both risk levels and the cost associated with work for risk criteria development should address three mitigating the risk are considered, and all risk important considerations: 1) the need to regulate risks reduction measures should be implemented as long as the cost of implementing them is reasonably practicable according to cost effectiveness considerations. This requires the weighing of risks versus benefits/expectations, with a maximization of benefit and minimization of harm. – The accountability principle: Transparent and clearly defined criteria, which should be quantitative rather than qualitative and based on objective assessments (as far as possible) rather than subjective interpretation of risk. The formulation of the criteria should be explicit, rendering little room for different interpretations of the evaluation criteria themselves. Fig. 1 Aspects (ellipses) to be considered within a framework for The framework should be consistent with these princi- developing proposed risk to life evaluation criteria ples and with common practice in other industries. As Macciotta and Lefsrud Geoenvironmental Disasters (2018) 5:10 Page 4 of 14 such, it considers proposing threshold values for accept- gain, 2) Users, who are those exposed to the hazard in ex- able and tolerable risks. Risks above the tolerable thresh- change of a gain or to cover a need, and 3) Public, who are old are considered not tolerable and risk mitigation is people exposed to the hazard not being aware of any direct mandatory. An ALARP region is placed below the toler- benefit in exchange. This grouping is fundamentally about able threshold. If the owner / regulator decides on defining voluntary versus involuntary risk levels and risks adopting an acceptability threshold (risks below this versus benefit trade-offs, which is then used for characteriz- threshold need no further reduction), the ALARP region ing the type of risk. As previously discussed, the framework lower boundary is determined by it. Elsewise, the proposes to assess the risks in terms of individual and soci- ALARP concept is applied to all risks lying below the etal risks. However, there can be systems where societal cri- tolerable threshold. teria are not necessary, such as systems where the The framework also considers risks to be assessed in maximum number of people exposed is small (railway terms of the individual risk (for the individual estimated freight crew members usually travel in groups of 2). to be at highest risk) and societal risk (through F-N The second stage is carried out by risk analysts given plots). F-N plots consist of log-log plots with the num- their acquired knowledge of the entire system. The steps ber of fatalities on the horizontal axis (N) and the prob- in this stage need to be followed for each population ability of N or more fatalities on the vertical axis (F). type and for individual and societal risks, as applicable. The proposed thresholds for risk acceptability and toler- The steps, assumptions, and principles behind the devel- ance are drawn as lines with negative slopes to show risk opment of the proposed criterion need to be clearly aversion towards accidents involving large number of fa- stated, as they will be reviewed in the later stage by the talities. F-N plots are common practice in industries owner / regulator and, ideally, the population exposed. such as nuclear-power generation, land use planning When possible, more than one option for the proposed around industries, dam operations, maritime industries, thresholds should be presented. In that case, the differ- and land development in landslide prone areas (Health ences in the fundamental thinking behind each proposed and Safety Executive, UK (HSE) 1992; Morgenstern threshold should also be stated. The second stage starts 1995; Ho et al. 2000; Health and Safety Executive, UK with a characterization of the risks according to 1) is it a (HSE) 2001; Leroi et al. 2005; Skjong et al. 2005; Porter new or existing slope, facility or system. This includes et al. 2009, Scarlett et al. 2011). A point in the F-N plot differentiating between man-made structures (road cuts (anchor point) and a slope of a line are required to and reservoirs) and natural slopes, and 2) is the risk vol- building a risk threshold line. The anchor point is a untary or involuntary, which follows the population clas- threshold cumulative probability for a certain number of sification in the first stage. The remainder steps (5 fatalities, typically N = 1. The slope of the line is typically through 8 in Fig. 2) develop the proposed risk thresh- defined between − 1 and − 2, being − 1 mostly adopted. olds, the ALARP principle evaluation criterion, and how A slope of − 1 in the F-N curve corresponds to reducing to apportion and scale the proposed criterion within the the tolerable threshold (cumulative frequency, F value in system, when applicable. the vertical axis) by one order of magnitude for each in- The last stage is the responsibility of the owner and/or crease in the number of fatalities (N value in the hori- regulator. It consists of making final decisions and adjust- zontal axis) by one order of magnitude. Others (Skjong ments to the developed criteria considering the social, 2002; Ale 2005; Skjong et al. 2005; Center for Chemical economic, political and cultural context. The risk analyst Process Safety (CCPS) 2009; Vanem 2012) discuss the acts only as an advisor and it is strongly recommended for implications of the slope chosen. the public to be involved in the decision-making. This im- plies simple and clear explanations of the risk analysis Proposed framework for developing risk evaluation process, its limitations, and the development of the criter- criteria ion as well as the principles behind it. It will require a The proposed framework is presented in Fig. 2.It has good risk communication strategy and proper risk/benefit been divided in three major stages according to the par- distribution among those exposed. ties that lead the analyses. The first stage is carried out The risk criterion should be reviewed and periodically through a coordinated effort between the risk analyst updated in light of changes in both the hazard and the owner / regulator and must to be consistent (stabilization, changes in technology, new measure- with the risk analysis scope of work. The three steps ments) and the elements at risk (changes in public ex- proposed should be thought of while defining the output pectations, exposure). of the analysis (or how is risk to be measured), as the criterion and estimated risks need to be compatible. Methods for developing proposed risk thresholds Those affected include: 1) Employees, who are those ex- There are several methods for developing proposed risk posed to the hazard in exchange of economic or professional thresholds, which are subject to continuous debate in Macciotta and Lefsrud Geoenvironmental Disasters (2018) 5:10 Page 5 of 14 Fig. 2 Proposed framework for the development of risk to life evaluation criteria the literature. Methods available are (after Morgenstern 2. Comparison with natural hazards 1995; Skjong 2002; Center for Chemical Process Safety (CCPS) 2009): This method consists of comparing the system to sta- tistics on lives lost due to natural events. It implies that 1. Comparison with statistics within the industry or the risks we impose on ourselves should be a small por- for similar industries and activities tion of what can be blamed on nature. For example; the annual risk to the public imposed by the slope cut This method implies that, if the industry or activity should be equal or less than the risk associated with chosen for comparison is currently taking place, the thunderstorms. risks associated with it are considered in balance with the benefits gained. The activity chosen needs to reflect 3. Comparison with common risks the same risk characteristics as the system being assessed (voluntary or involuntary). For example; the This implies the imposed risks by the system analyzed railway passenger industry should be as safe as the air should not be greater than risks from common activities passenger transportation industry. such as swimming and driving. Example: The risks Macciotta and Lefsrud Geoenvironmental Disasters (2018) 5:10 Page 6 of 14 associated with landsliding along a section of the high- A preferred method is the use of a cost-effectiveness way should be less than those associated with crossing analysis (Skjong 2002; Skjong et al. 2005). This method the street or driving the highway. calculates a ratio of the cost of implementing risk reduc- tion measures to the reduction in risk, thus avoiding put- 4. Comparison with previous decisions ting an economic value to life. According to Leroi et al. (2005) two commonly used variants of the method are the In this method, owners, regulators or court decisions adjusted cost-to-save-a-statistical-life (ACSSL) and the un- on cases where the risks involved can be estimated are adjusted cost-to-save-a-statistical-life (UCSSL). These are taken as indicative of society’s tolerance of a particular analogous to the gross-cost-of-averting-a-fatality (GCAF) activity or industry. and the net-cost-of-averting-a-fatality (NCAF) respect- ively, as presented in Skjong (2002) and Skjong et al. 5. Comparison with existing criteria (2005). These can be defined as (Leroi et al. 2005,p.167): C −ðÞ E½ bef −E½ aft −ðÞ O½ bef −O½ aft In this method, previously proposed criteria for sys- ACSSL ¼ ð1Þ L½ bef −L½ aft tems reflecting risk characteristics similar to the ones being assessed are used for comparison or validation. UCSSL ¼ ð2Þ Example: Tolerable risks from landsliding for land devel- L½ bef −L½ aft opment areas should be similar to those proposed in Hong Kong. Where: When developing proposed landslide risk thresholds it C is the annualized cost of implementing the risk re- is important to keep in mind the considerations pre- duction measure in dollars per year, E[bef] and E[aft] are sented by IUGS (1997). These considerations are con- the economic risks (failure probability times monetary sistent with the principles presented here and can be loss) in dollars per year before and after implementing summarized as follows: the risk reduction measures, O[bef] and O[aft] are the annual operational costs before and after implementing – Incremental risk associated with the system analyzed the risk reduction measures, and; L[bef] and L[aft] are should not be significant when compared to the the estimated risk to life in lives per year before and risks associated with everyday life, and whenever after implementing the risk reduction measures. possible the ALARP principle be applied, When implementing this method for evaluation of in- – Events with the potential to cause a large number of dividual risk, the value of L is the value of risk (annual fatalities should have low occurrence probabilities. probability of the individual being killed). When evaluat- This accounts for society’s lower risk tolerance of ing societal risk, the value of L can be estimated as the large numbers of fatalities, total risk, ∑f . N where f and N are the annual probabil- i i i i – Some populations tolerate higher risks than others ity (or frequency) and the correspondent number of lives in relation to the required efforts to mitigate those lost, respectively. risks, as they benefit from their existence or the This method is also useful for comparison of different activity realized (workers in the mining industry as risk mitigation options. More expensive options give lar- opposed to the public), and; ger CSSL (cost-to-save-a-statistical-life, either adjusted – Tolerable risks are higher for natural hazards than or unadjusted) values, while more effective options will for those engineered or controlled (i.e., natural give lower CSSL values (larger reductions in risk). How- slopes versus slope cuts, earth fill dams, or even for ever, assessing if the ALARP principle has been satisfied natural slopes that are known to be monitored). requires a criterion regarding the CSSL values consid- ered cost-effective. Deciding what is considered to be cost effective CSSL values is not a simple matter. Risk Methods for evaluation of the ALARP principle reduction will have a direct economic impact on the One method to evaluate if the ALARP principle is being owner, which could ultimately lead to an activity not be- met is through conventional cost-benefit analyses. Here, ing profitable. As a consequence, the workers might also the costs of implementing risk reduction measures are be economically impacted (loss of jobs or income reduc- compared against the reduced risks in monetary units, tion). Where the regulator is also responsible for safety, thus assigning an economic value to life. Some estimates thus having to pay for the risk reduction measures, the of the economic value of life appear in the literature, public is economically affected (their taxes pay for the mainly considering people as a resource in an economic reduction of risk). activity, however this approach conflicts with ethical tra- Given these complexities, it is suggested that for esti- ditions (Skjong 2002). mated risks falling within the ALARP region, the risk Macciotta and Lefsrud Geoenvironmental Disasters (2018) 5:10 Page 7 of 14 analyst estimate the CSSL values for the risk mitigation Weighted or even apportioning should reflect the exist- measures proposed as part of the risk management ing understanding of the situation and the adequacy of process and final decisions should be left for the owners apportioning criteria based on even distribution of risk and regulators. When presenting the information, not or weighted distributions needs to be assessed for each only the CSSL ratios should be included, but also the ac- particular case. Apportioning criteria should be reviewed tual increment in cost associated with the mitigation and periodically updated together with the risk estima- strategy and the estimated risks after their implementa- tions and in light of risk analyses of hazards not previ- tion. These absolute values are as useful as the CSSL for ously assessed. the decision-making process. The participation of the public through surveys or public meetings should be en- The issue of low probability – High consequence events couraged, as they are part of the exposed population and When assessing societal risks, the issue of high conse- are likely to be impacted economically by the decisions. quence events (large number of fatalities) having ex- tremely low, but existent, occurrence probabilities, Methods for apportioning and scaling criteria arises. These risks are difficult to quantify, and are often Risk criteria are often defined for a certain scale of the sys- the most worrisome as well (Oldenburg and Budnitz tem. These criteria need to be adjusted to reflect the scale 2016). Although some methods have been used to ana- of the particular system evaluated or have to be appor- lyse these risks (Oldenburg and Budnitz 2016)and to tioned throughout the sub-systems for their individual quantify them (Cadini et al. 2017; Mignan et al. 2014), evaluation (for example risks along a particular segment tolerable thresholds are difficult to define for systems of road as opposed to the entire transportation corridor). where large populations are exposed (densely populated If the criteria are not scaled or properly apportioned, developments where hundreds to thousands of people higher risks than desired could end up being tolerated. A can potentially be affected – such as communities down- detailed discussion is presented in CCPS ( 2009). stream of dam facilities). It can be argued that on these An example is used to illustrate the concept. Typical circumstances the precautionary principle be applied. systems analyzed are new developments within landslide This principle states that where there are threats of ser- prone areas or existing/proposed alignments of high- ious consequences, all cost-effective measures to prevent ways. Risk criteria can then be defined for the system them should be applied. In our context, this would imply (either the new development or the highway). The risk that where the consequences are unknown but may be value to be evaluated against the selected criterion is the judged by some to be of catastrophic magnitude (large integration of the risks associated with all hazards and number of fatalities), it may be better to implement all sectors within the system (all slopes, all potential failure known risk control measures or even to abandon the volumes, every mile along the highway, and consider- project rather than to accept the uncertain but potentially ations of all other ground hazards). In practice, however, high risk (Skjong 2002). In this context, risk aversion it is unlikely that all hazards and sectors be considered would play an important role in the decision-making due to time constrains, budget limitations, or scenarios process for systems that could be associated with low deemed negligible. To account for the different sectors probability but high consequence events (Cha and Elling- (i.e. mileage along the highway), the criterion needs to wood 2012). An example of the higher risk aversion to- be apportioned. If the apportioning is based only on the wards large number of fatalities is the adoption of a linear or aerial extent of the system, (same risk thresh- vertical cut-off for tolerable risk thresholds when assessing olds for each mile along the highway or for each slope the risk to developments in landslide prone areas in Hong within the proposed development), it is an even appor- Kong (Leroi et al. 2005). It is noted, however, that the pre- tioning or distribution. It is often the case, however, that cautionary principle needs to be considered with care, as a number of sectors will be more hazardous than others. avoiding a risk is typically associated with incurring in In cases where the analyst is aware of this variability in other risks (i.e. economic stagnation, crisis in the housing hazard levels, an even apportioning could lead to criter- market if new developments are stopped, lack of employ- ion that is too strict at some locations and not a reflec- ment, insufficient power, water or transportation services). tion of the defined overall criterion and perception of risk. Apportioning the criterion with considerations of Results and discussion on risk evaluation criteria the relative hazard levels or the exposure of the popula- for residential areas in mountainous terrain tion would be a weighted apportioning or distribution. This section presents a discussion on validating the The criteria for defining the weights for apportioning adoption of existing risk evaluation criteria for landslide will reflect the perceived variation in hazard levels phenomena through statistical databases and precedent through the entire system and for all potential hazards cases, particularly for defining acceptability and toler- in light of the available knowledge and information. ability thresholds. Macciotta and Lefsrud Geoenvironmental Disasters (2018) 5:10 Page 8 of 14 Figure 3 presents a schematic illustration of existing followed by the HSE (1992 and 2001). Definition of the and proposed residential areas in mountainous terrain. Hong Kong criterion (similar to the HSE criteria) mainly For simplicity, only three dangers are highlighted in this relied on assessing how common risks to the exposed figure: Natural slope instabilities (deep seated slides, population are related to the population’s background shallow slides, snow avalanches, rock falls, rock ava- risk (age standardized death probability by all causes). lanches), river flooding, and periodic debris flows that The Hong Kong criterion is then compared to risks shape the observed depositional fan. posed by activities common for the Canadian population Porter et al. (2009) describe a case in the District of in order to assess its applicability to the Canadian con- North Vancouver (DNV) where risk to life evaluation text. The tolerable risk threshold for new situations ap- criterion was adopted as part of a quantitative risk pear to be in the order of those risks imposed by air assessment for developments in landslide prone areas. travel and drowning, and about an order of magnitude The Hong Kong evaluation criterion (ERM-Hong Kong lower than for motor vehicle accidents. It is considered Ltd 1998) was selected on the basis of having a similar the population is willing to tolerate these risks related to legal system (Common Law Legal System inherited from transportation (for the case of air travel and motor vehi- the United Kingdom) and to be developed for a similar cles) and their interaction with water bodies (for recre- context. The criterion developed in Hong Kong was ation purposes in pools and lakes). Risk tolerance for based on previous studies for diverse industries within new situations in Canada can then be proposed to be and outside the region (Dam management, transporta- about the same order of magnitude, which supports the tion, nuclear power plants). Details on the definition of adoption of the Hong Kong criterion. The threshold is the criterion are presented in ERM (1998). The decision further supported by the estimation of Porter et al. of adopting the Hong Kong criterion was supported by (2009) that this risk value (1E-5) corresponds to less consultant’s recommendations and informal feedback than 0.2% incremental risk, which can be considered from the public. Porter et al. (2009) postulated that the low. For existing situations, a proposed threshold value Hong Kong tolerance criterion might be appropriate for one order of magnitude higher would be about the risk application in Canada. These criteria were also used as of death due to motor vehicle accidents and lower than basis for decision making in other Canadian cases, de- the risk of death due to all accidents. velopment in Cougar Creek and Cheekye fans (Clague et It is believed the most important step in developing al. 2015; Hungr et al. 2016). the risk criteria is its validation by the public exposed. In the case of the DNV described in Porter et al. (2009), a Individual risk criterion public task force convened by the DNV supported the Figure 4 presents the Hong Kong tolerable individual adoption of the Hong Kong criterion based on a number risk thresholds. The criterion allows for risks associated of public meetings and public surveys. with existing situations to be one order of magnitude Regarding acceptability thresholds, the HSE adopts a higher than for new situations. Figure 4 also shows the value of 1E-6, which for the Canadian context is about thresholds adopted by other organizations (HSE, the same order of magnitude than events considered ex- ANCOLD and AGS). It is not surprising the risk thresh- tremely rare (such as death by lightning in Fig. 4). It also olds are common between different organizations, which represents an increase of less than 0.02% in the stan- typically adopt the considerations and the methodology dardized risk of death for the population. However, Fig. 3 Schematic illustration of existing and proposed residential areas in mountainous terrain highlighting some potential ground hazards Macciotta and Lefsrud Geoenvironmental Disasters (2018) 5:10 Page 9 of 14 Fig. 4 Individual risk evaluation criteria (as annual probability) commonly cited in the geotechnical literature and common risks in North America. AGS (2007); Statistics Canada (2010); Baecher and Christian (2003); Porter et al. (2009); ANCOLD (2003); HSE (2001); ERM (1998) deciding if an acceptable threshold is to be adopted and Morgenstern 2012). Both estimated risks are plotted in below which the ALARP principle is not mandatory, is Fig. 5 and lie above the tolerable threshold line. responsibility of owners and regulator and should be The Thredbo landslide in Australia (Mostyn and Sullivan done in consultation with the exposed population. 2002) is also plotted in this figure given the similar social and economic context. This suggests that the Hong Kong Societal risk criterion tolerable threshold might be applicable in the Canadian Figure 5 presents the Hong Kong societal risk criterion context for new developments. It also suggests that in- (a) and criteria adopted by other organizations (U.S. Bur- creasing the threshold one order of magnitude might not eau of Reclamation 2003; Australian National Commit- be applicable for existing developments, although more tee on Large Dams (ANCOLD) 2003; and Health and published cases should be analyzed. This is further vali- Safety Executive, UK (HSE) 1992 and 2001) (b). Unlike dated by its recent application in some Canadian cases. the individual criteria, the societal criteria vary among The cut-off values adopted in Hong Kong for the area these organizations. This corresponds to differences in of intense scrutiny (between 1000 and 5000 fatalities) the type and scale of the hazards being evaluated and corresponds to a local policy, and its adoption needs to the number of people exposed. The Hong Kong criterion be based on political and social considerations. Again, was chosen given the similar hazard context for which it here the involvement of regulator, consultant and the was proposed (development is landslide prone areas). public is of critical importance. The same applies to the Two court decisions published in the geotechnical litera- acceptability threshold. ture where chosen to assess the applicability of the Hong Kong criterion. Both decisions implied the risks to the pub- Apportioning the risk criterion lic were considered intolerable. The risk values were esti- The Hong Kong criterion was defined for a given areal ex- mated after the decisions were made. The first case tent, or “Consultation Zone” (ERM-Hong Kong Ltd 1998). corresponds to rock fall hazards along a highway (Bunce et The Consultation Zone in ERM (1998) was defined as “… al. 1997) where a rock fall impacted a vehicle and killed the area of natural terrain that must be considered..” in one person. The second case corresponds to a proposed the analysis (ERM-Hong Kong Ltd 1998), and consisted of development in the path of a potential debris flow (Porter the summit of the slope, the slope itself and up to a Macciotta and Lefsrud Geoenvironmental Disasters (2018) 5:10 Page 10 of 14 a b Fig. 5 Societal risk evaluation criteria. a Criterion proposed in Hong Kong for developments in landslide prone areas and cases where risks were found to be intolerable. b Criteria commonly cited in the literature distance of 150 m from the toe of the slope; and for a Application of the ALARP principle length of 500 m. Porter et al. (2009) proposed a definition Once a risk analysis is performed and the calculated of the consultation zone for the Canadian context as: “The risks are evaluated against the criteria defined for this Consultation Zone shall include all proposed and existing context, risks within tolerable thresholds need to be development in a zone defined by the approving authority evaluated against the ALARP principle. As previously that contains the largest credible area affected by land- discussed, this would require evaluating the reduction of slides, and where fatalities arising from one or more con- risk given potential mitigation strategies to further re- current landslides would be viewed as a single duce risk, and the costs of these strategies (capital and catastrophic loss”. This implies that the criterion is applic- operational costs). The feasibility for further risk reduc- able to the area of influence of the hazard, and doesn’t tion would become dependent of the available oper- need to be scaled for the size of the Consultation Zone. ational budget and the population’s perception of risk. However, evaluation thresholds are applicable when all Balanced decisions will render minimum residual risks risks posed are integrated. If other hazards are consid- within tolerable thresholds, without overstressing avail- ered negligible when compared to landslide hazards, the able resources, and optimized through evaluation of dif- criterion described can be readily applicable to evaluate ferent options through ACSSL or UCSSL comparisons. landslide related risks in the area. If the risk analysis is comprehensive regarding all hazards, the overall risk can Results and discussion of individual risk also be readily assessed against the criterion. It is com- thresholds for railway employees exposed to mon for risk analyses to focus on one or a few particular landslides based on accident statistics and hazards. The existence of other potential hazards needs comparison to other criteria to be considered when adopting threshold values for risk This section presents a brief example on developing pro- evaluation. Qualitative or relative risk assessments can posed acceptable and tolerable individual risk thresholds shown to be useful to apportion the risk thresholds based on statistical data and validation against other cri- among hazards. If this information is lacking, a conser- teria in similar contexts. vative approach can be to scale the thresholds to one According to WorkSafeBC (2009), transportation and re- order of magnitude lower, as long as no other hazard is lated services is one of the high-risk economic activities in deemed to pose higher risks than the one being the province of British Columbia. Within the railway indus- evaluated. try, train crew members and maintenance-of-way (MOW) Macciotta and Lefsrud Geoenvironmental Disasters (2018) 5:10 Page 11 of 14 personnel are the most exposed to operation hazards, Table 1 shows the distributed accident-related risks for which include a variety of ground-hazards such as rock the average employee following these assumptions. Table 1 falls, embankment settlements, river erosion, and rock and suggests that accident-related risks during working hours soil slides. are about 5.7E-5 per year. Conservatively, an annual indi- vidual risk threshold of 1E-5 can be proposed. This is consistent with the Hong Kong criteria (ERM-Hong Kong Acceptable risk threshold Ltd 1998) for new developments, however is one order of Proposing a threshold for risk acceptability involves an- magnitude higher than the HSE ( 2001) broadly acceptable swering the question of how much risk increment we criteria. are willing to accept. In this example an acceptability policy of zero risk increase is adopted. As long as the ac- Tolerable risk threshold cidental risks during working hours do not exceed the Proposing tolerable risk thresholds requires understand- average accidental risks for other causes, it can be con- ing the increase in risk the workers are willing to toler- sidered that the activity does not impose an increase on ate in exchange for the benefits of the activity. It will be the individual’s risk. Average risk to life statistics can argued in this example that average fatality statistics for then be used for calculating these thresholds. particular activities reflect risk levels the average worker It is assumed the employee’s age is between 20 and is willing to tolerate, and will be taken as lower than 49 years. Figure 6 shows the annual life loss probability maximum tolerable risk thresholds. This can then be for the average Canadian resident within that same age proposed as maximum starting point for regulators and group. owners to decide if these are deemed adequate or fur- For this age group, the average risk increment caused ther decrease is necessary. by work and non-work accidents is about 1.9E-4. Of Work-related accident statistics can aid in quantifying course, this accident-related increment in risk is distrib- activity-associated risks. These statistics for the province uted throughout the day. In order to estimate the incre- of British Columbia (WorkSafeBC 2009) were used to mental risk corresponding to the period a crew member estimate the workers annual probability of death for four spends working, the following assumptions were made: high-risk sub sectors of the economy. Table 2 presents the analysis. Also shown is the ana- – Three periods of time where distinguished: working, lysis for a low-risk sector as a reference. From these sta- sleeping and other (i.e. recreational), tistics, and considering that the estimates include – The average employee spends about 30% of the time workers exposed to variable levels of risk; a tolerable in- working each year, dividual risk threshold of 1E-3 can be proposed. This is – The average employee spends 30% of the time consistent with HSE individual risk tolerable threshold sleeping (7 to 8 h a day in average), for workers (Health and Safety Executive, UK (HSE) – Accident-related increase in risk while sleeping is 2001), considered as being developed for a similar socio- perceived by the population to be acceptable, and economic context. therefore it is used as a benchmark for proposing risk acceptance thresholds, and; – All accidents during the working period are Application of the ALARP principle considered work-related. Similar to the previous example, estimated risks within tolerable thresholds need to comply with the ALARP principle. In the case of railway operations, options for further risk reduction would require comparison of dif- ferent options through their ACSSL and UCSSL values, and balanced decisions will render minimum residual risks, with optimal allocation of available resources, at costs that maintain profitable operations. Table 1 Accident-related risks for railway crew members and MOW personnel distributed throughout the day Activity All Working Sleeping Other % Time 30% 30% 40% 100% Fig. 6 Annual life loss probability per age group based on the Canadian population mortality rates 2015 (Statistics Canada 2018) Accident-related risk 5.7E-5 5.7E-5 7.6E-5 1.9E-4 Macciotta and Lefsrud Geoenvironmental Disasters (2018) 5:10 Page 12 of 14 Table 2 Employee annual death probability by sub sector estimated from work-related accident statistics by WorkSafeBC (2009) a b c d Sub Sector Fatalities per year Claims per year Fatalities per Claims Injury Rate Annual Probability of death Transportation and related services 26.6 4098 0.65% 5.8% 3.77E-4 Construction 33.6 8759 0.38% 5.9% 2.24E-4 Forestry 13.8 720 1.9% 6% 1.14E-3 Oil and gas or mineral resources 7.4 388 1.9% 2% 3.80E-4 Business services 1.2 711 0.17% 0% < 8.50E-6 (less than 0.5%) Annual probability of death is estimated as c) x d) obtaining average number of fatalities per number of workers per year. Assumed to be an approximate measure of the likelihood of work related death of an average worker in 1 year Average number of accepted fatal claims per year between 2005 and 2009 Average number of accepted claims per year (short-term, long-term and fatal claims) for 2008 and 2009 c a b ratio of respect to Average number of claims (short-term, long-term and fatal claims) per 100 workers employed all year (per 100 person-years of employment) Conclusions evaluation criteria should be an integral part of the risk Quantitative risk assessments are becoming common management framework, where not only the risk analyst practice for projects with high risks associated with takes part, but the regulators, clients and exposed popu- them. A critical step in the risk assessment process is lations participate in establishing the criteria. the adoption of risk evaluation criteria. Because of the diverse contexts for which previously defined criteria Methods were proposed, different regions should derive their own Given the importance of risk criteria that is adequate for criterion or perform an assessment of the applicability of the context of the potential geoenvironmental hazard, any criteria to be adopted. As such, development of this article describes the primary considerations for de- these criteria becomes necessary at a regional, industry, veloping risk evaluation criteria based on an extensive client, and even case specific scales. literature review. Based on this review, the authors fur- The paper proposes a framework for the development ther propose a framework for defining landslide risk cri- of risk-to-life evaluation criteria using landslide hazards teria and assess the applicability of previously proposed as an example, which could be extrapolated to other ac- criteria to a specific context. Two simplified examples tivities. The framework is developed considering the discussing some aspects of development of proposed risk main aspects involved (system characteristics, the evaluation thresholds are discussed for illustration socio-economic, cultural and political context) and the purposes. principles for developing the criteria. The framework is Abbreviations linked to the risk management process at its initial steps, ACSSL: Adjusted Cost-to-Save-a-Statistical-Life; AGS: Australian Geomechanics ensuring the estimated risks and the defined thresholds Society; ALARP: As Low As Reasonable Practicable; ANCOLD: Australian National Committee on Large Dams; APEGBC: Association of Professional are compatible. Engineers and Geoscientists of British Columbia; CCPS: Centre for Chemical The proposed framework basically consists of defining Process Safety; DNV: District of North Vancouver; ERM: Environmental absolute risk thresholds and making a decision on the Resources Management; GCAF: Gross-Cost- of-Averting-a-Fatality; HSE: Health and Safety Executive; IUGS: International Union of Geological Sciences; application of the ALARP principle. This structure was MOW: Maintenance of Way; NCAF: Net-Cost- of-Averting-a-Fatality; adopted to keep consistency with common practices, UCSSL: Unadjusted Cost-to-Save-a-Statistical-Life; UK: United Kingdom and because it has shown its adequacy for risk commu- nication. Methods for the development of risk accept- Acknowledgments This research was made possible by the (Canadian) Railway Ground Hazard ability and tolerance thresholds are also presented. Research Program, which is funded by the Natural Sciences and Engineering A discussion is presented on risk evaluation criteria Research Council of Canada, Canadian Pacific Railway, Canadian National for residential areas in mountainous terrain. An example Railway, and Transport Canada; and the Canadian Rail Research Laboratory (CaRRL) (www.carrl.ca). Special thanks to Dr. Derek Martin, Dr. David Cruden of individual risk thresholds for railway employees ex- and Dr. Norbert Morgenstern for their valuable insights. None of the authors posed to landslides is also presented. This is based on have any competing interests in the manuscript. accident statistics and comparison to other criteria. Funding These aim to illustrate and discuss some of the issues This work was made possible through the School of Engineering Safety and presented, and highlight that establishing risk evaluation Risk Management and the Department of Civil and Environmental criteria is not an easy task; however, tools are available Engineering at the University of Alberta. to propose starting values for stakeholder consultation. Availability of data and materials The defined criteria, as shown by the examples, are All information is included in this manuscript. Statistical information used for highly dependent on the system being analysed and its the examples is publicly available through the references (Statistics Canada, context. This implies that the development of the risk WorkSafeBC). Macciotta and Lefsrud Geoenvironmental Disasters (2018) 5:10 Page 13 of 14 Authors’ contributions Finlay, P.J., and R. Fell. 1997. Landslides: Risk perception and acceptance. 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