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A review of landslide acceptable risk and tolerable risk

A review of landslide acceptable risk and tolerable risk Landslides are one of the most important geohazards. In 2004–2016, more than 55,000 people lost their lives to landslides and this does not include deaths caused by seismically triggered landslides. Overall losses were estimated to be at USD 20 billion annually. The lives of many could be saved if more had been known regarding forecasting and mitigation. Studies have shown an increasing trend in landslides occurrence and fatalities. Over recent years, landslide risk assessment has been carried out extensively by geo-scientists worldwide. This review concentrates on the societal risks posed by landslides in various countries and the risk criteria used by various parts of the world in assessing landslide risks. The landslide risk tolerance criteria are strongly governed by utilitarian concerns i.e. financial power and the need for development. In developing countries, surprisingly high levels of tolerance are proposed for landslides. The risk criteria of Hong Kong and that of the Australian Geomechanics Society are widely employed in many coun- tries. Although various risk tolerance levels have been proposed by various nations, many of them are still not being applied in their real-life scenarios. The procedures for setting risk criteria call for a wide agreement between geo- scientists, government decision makers, and the community. Risk criteria should be developed locally with historical landslide inventory, public perception, and engineering aspects being considered. Keywords: Landslide, Tolerable risk, Acceptable risk, Societal risk, Frequency-number of fatalities diagram Introduction result in casualties to both humans and animals but dis- Landslides are one of the major devastating geohazards rupt the water quality of streams and rivers as well as and claim thousands of lives and create acute economic the destruction of structural and infrastructural devel- losses related to property damage every year (Schus- opments. Of the total loss of life resulting from natural ter and Highland 2001; Dilley et  al. 2005; Petley 2012). hazards worldwide, 5% of it comes from highly devel- Centuries prior, many countries worldwide have suf- oped nations. The remaining 95% of total deaths are from fered deaths and economic losses due to landslides and medium and developing countries (Lacasse et  al. 2010; the impact is still on the rise. The first documented land - Lacasse and Nadim 2014). Klose et  al. (2015) reported slide was an earthquake induced landslide dam in Honan that the economic impact of landslides is likely the great- Province of China in the year 1767 B.C. (Schuster 1996). est on transportation infrastructures. This is especially International Disaster Database (EM-DAT) of the Centre true in rural regions where the transportation network for Research on the Epidemiology of Disasters reported is scattered, and the availability of alternate routes is few that landslides caused 17% of the deaths associated with to none. As a result, a minor landslide will bring a great natural hazards worldwide annually (Lacasse and Nadim impact on the economic sector over an extensive region 2014; Aristizábal and Sánchez 2020). Landslides not only (Winter et  al. 2018). According to Dowling and Santi (2014), landslides are catastrophes resulting from social vulnerability. *Correspondence: minlee.lee@nottingham.edu.my Socioeconomic impacts of landslides are always under- Department of Civil Engineering, Faculty of Science and Engineering, estimated because landslides in many countries are University of Nottingham Malaysia, 43500 Semenyih, Selangor, Malaysia © The Author(s) 2022. Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http:// creat iveco mmons. org/ licen ses/ by/4. 0/. Sim et al. Geoenvironmental Disasters (2022) 9:3 Page 2 of 17 always taken as a consequence arising from other trigger- on the tourism industry, as well as other economic activi- ing processes such as extreme precipitation, typhoons, ties. However, estimations of these long-term losses are volcano eruptions or earthquakes. The damage costs challenging as they are typically widely spread out both from landslides could surpass all other costs from the socially and geographically. This underestimation brings overall multiple-hazard catastrophes (Froude and Pet- about a lower awareness and appreciation regarding ley 2018; Sultana 2020). Studies have indicated that the landslide risks among regulators, authorities, and the EM-DAT database often underestimates the number general public. of fatal landslides by 1400% (Kirschbaum et  al. 2015) or In the new millennium, landslides have claimed tens 2000% (Petley 2012), and the number of deaths by 331% of thousands of lives and an estimated annual average (Kirschbaum et  al. 2015) or 430% (Petley 2012). The of economic losses of $20 billion, which is 17% of the under-reporting is caused by the perception of landslides total ($121 billion) yearly mean global disaster losses as a secondary hazard, in which the cause of death is nor- from 1980–2013 (Klose et  al. 2016). The average eco - mally reported in relation to the primary hazards (e.g. an nomic losses of various nations caused by landslides earthquake rather than a seismically triggered landslide) worldwide are reviewed in the present study and sum- instead of the actual cause of the loss (Froude and Petley marized in Table  1. To determine the economic losses, 2018). Furthermore, conducting a quantitative economic government authorities and land-use planners often dis- analysis of landslides is often handicapped by an incom- tinguish between direct and indirect costs. Direct costs plete landslide database (Winter et al. 2018). As stated by comprise of damages directly related to the destruction Winter et  al. (2019), landslides disrupt access to remote resulting from the landslide as well as costs for investiga- rural regions where the economic activities are typically tion, monitoring, and remedial works to lower the risk. transport-dependent. Extensive areal vulnerability can be All other costs are indirect and are generally not avail- attributed to the transport network instead of the event able. However, those that have been estimated in studies itself. For example, a landslide event that occurred at an and reports, their values are included in the total costs. access road in the Blue Mountains of Jamaica completely It is clear from Table  1 that Japan and Italy suffered the cut off the nearest route linking local coffee manufac - greatest economic loss caused by landslides among the turers with the international market (Winter et  al. 2018, developed countries. The economic costs in these moun - 2019). A long-term impact following that event would be tainous regions are attributed to their high population Table 1 Estimated average annual cost of landslide of different countries Country Average Annual Direct Costs Average Annual Total Costs (USD) Comments (USD) Canada – $70 million–1.4 billion Japan $1.5 billion $4 billion Korea $60 million – Based on poor record Italy – $2.6–5 billion Rough Estimate Sweden $10–20 million – Spain $0.2 billion – Former USSR $0.5 billion – New Zealand $26.3 million 90% of costs are sustained in rural areas Belgium $0.85 million $4.48 million Germany $0.3 billion – India $1.3 billion China $0.5 billion – Costs based on valuations in 1989 Nepal $19.6 million – Includes flood damage, but likely incomplete Brazil $45 million $0.35 billion Worldwide $ 20 billion The rest of the data were extracted from Sidle and Ochiai (2006) Cruden et al. (1989), Schuster and Highland (2001) and Sidle and Ochiai (2006) Vranken et al. (2013) Klose et al. (2015) Batista et al. (2019) Sim  et al. Geoenvironmental Disasters (2022) 9:3 Page 3 of 17 densities. Considering the lower property values in devel- climate change which have resulted in an increase in pre- oping countries such as China, India, and the Himalayas, cipitation, variations in the hydrological cycle, extreme the economic costs in these mountainous regions are sig- weather, short-duration heavy downpours (storms), nificantly lower. The majority of the landslides in nations melting of ice in the Alps, and other meteorological such as Nepal, New Zealand, and Canada have their events, and coastal erosion due to sea storms (Lacasse landslide damage occurring in rural areas; and hence the and Nadim 2014). An increasing trend was observed in associated costs are comparably lower. Landslide costs in fatalities and occurrence of fatal landslides worldwide the Scandinavian regions are trivial compared to moun- in numerous studies published (Petley 2012; Aristizábal tainous topography in the rest of the European continent. and Sánchez 2020; Görüm and Fidan 2021). For exam- The cumulative landslide costs in Europe are lower than ple, a rising trend in death toll by landslide disasters was in the United States. reported in China with the increase of average annual From 2004 to 2016, 4862 landslide events have been death toll by about 78% from 1950 to 1999 (Lin and reported with fatalities exceeding 55,000 (Froude and Wang 2018). The numbers of deaths owing to landslides Petley 2018). Studies have also identified a number of reported on the African continent, however, remain low, landslide clusters, most notably in the tropical regions i.e. 13 in 2007 (Kjekstad and Highland 2009) and 43 in such as the Caribbean, Central, and South America, and 2020 (European Commission 2021). This implies a likely parts of South and Southeast Asia (Petley 2012; Froude severe underreporting as the number of people exposed and Petley 2018). Tropical climate countries suffered to landslide hazards there is tremendously high (Kjekstad the greatest loss of lives caused by landslides due to the and Highland 2009). The severe underreporting is mainly destabilizing effects of rising groundwater table and loss caused by the remoteness of the regions and the scarcity of suction under extreme downpours (Turner 2018). For of systematic records (Monsieurs et  al. 2017). As such, example, rainfall-induced landslides in Brazil have killed the actual total losses caused by landslides worldwide an excess of 2700 people in 1966 and 1967 (Jones 1974). could be far exceeding the reported statistics. Additionally, multiple debris flows were driven by several The emergence of landslide hazard and risk issues, days of torrential downpours that claimed 30,000 lives in the obligation to look after the society and properties, Caraballeda, the north coast of Venezuela in December expected climate change which will further aggravate the 1999. Figure  1 classifies the landslide triggering factors landslide situation, and the harsh reality for the public to worldwide. It was further stated by Lacasse and Nadim live with landslide risk have led to requirements for pro- (2014) that heavy rainfall is the main trigger for mud- fessionals to carry out Quantitative Risk Assessments flows, the most lethal and destructive of all types of land - (QRA). Unless risks are reduced, landslide disasters slides. Rainfall-induced landslides were reported to have will continue to rise (Dilley et  al. 2005). The Geotechni - resulted in nearly 90% of deaths worldwide (Haque et al. cal Engineering Office (GEO) of the Civil Engineering 2016; Sultana 2020). and Development Department (CEDD), Hong Kong are It is no surprise that landslide disasters will increase among the pioneers in implementing the risk-informed significantly as time pass due to global warming and approach in landslide assessment. Since the establish- ment of this approach by the GEO, the overall landslide risk related to man-made slopes in Hong Kong has been lowered significantly by 75% (Chiu 2015). Since then, the Slope geometry approach had been adopted by several developed west- change ern countries such as the UK (Winter and Wong 2020), 3% the Netherlands (Hungr et al. 2016), Australia (Leventhal Water level and Withycombe 2009), and Canada (Hungr 2016; Porter change et al. 2017; Strouth and McDougall 2020). Lin and Wang 35% (2018) reported that China has invested heavily in geo- logical disaster control and prevention in recent years, i.e. an increase in allocation from Chinese RMB 330 mil- lion in 2000 to 17.6 billion in 2015. As the result, a lower death toll (by about 10%) by landslides was reported in Rainfall the 2010s. The United Nations Sendai Framework for 58% Loading change Disaster Risk Reduction (2015–2030) has also outlined 5% seven clear targets and four priorities for action to pre- vent new and reduce existing disaster risks (UNISDR Fig. 1 Distribution of landslide triggering factors around the world (Ng 2012) 2015). These efforts showed that landslides have received Sim et al. Geoenvironmental Disasters (2022) 9:3 Page 4 of 17 increasing attention globally owing to their high fre- fatalities (N) plotted against the cumulative frequency (F) quency and severity of occurrences in recent decades. of N on a log–log scale (Dai et al. 2002; Song et al. 2007; In recent decades, QRA has been studied extensively Hungr et  al. 2016). To be made simpler, the F–N Curve and numerous guidelines have been published (Geo- compares the number of fatalities (N) of a landslide event technical Engineering Office 1999; AGS 2000, 2007; with the probability (or frequency) of that event (F). Wong et  al. 2006; Lee and Jones 2014). The flow chart Numerous review studies on quantitative risk assess- for QRA from Hungr (2016) is shown in Fig. 2. The QRA ment of landslides can be found from the currently consists of two stages: Stage 1, Hazard assessment, and available literature. However, review studies on land- Stage 2, Risk assessment. Hazard assessment comprises slide acceptable risk and tolerable risk as well as F–N geo-scientific works such as investigation and analysis to Curves are scarce despite substantial works having been determine and quantitatively define the probable land - reported by researchers from different parts of the world. slide hazards. Hazard is defined as the potential occur - This paper aims to review the F–N curves reported from rence of a natural or human-induced physical event that various countries. The F–N curves collected deal exclu- may cause loss of life, injury, or other health impacts, as sively with risks to life which is normally the governing well as damage and loss to property, infrastructure, live- factor of landslide disasters (Hungr et al. 2016). Landslide lihoods, service provision, and environmental resources risk acceptance criteria from various nations will also be (Bobrowsky and Couture 2014). A hazard should be discussed and reviewed. The findings reported herein can identified and characterized in the first place before its serve as a useful reference for developing the landslide features i.e. magnitude, probability, regional extent, and acceptable risk and tolerable risk for a nation. intensity can be determined. The estimated intensity and its probability of occurrence can then be quantified and F–N curve for landslide mapped into the region of study. Introduction to F–N curve Risk assessment starts with determination of elements An F–N curve consists of a series of data points that at risk which could be humans, infrastructures, buildings denote each scenario analyzed. It is a combination of or environmental values. They could be existing ones or scenarios into a single curve that defines the probabil - those in planning for future construction in the area of ity (or frequency) of “N or more” fatalities of the com- study. Estimation of risk comes from an intersection of plete dataset (Strouth and McDougall 2021). It should hazard intensity map with the map of elements at risk, be noted that the label (N) can also be represented by in space and time considering vulnerabilities. The Stage other quantities of a consequence such as monetary 2 is completed with the determination of acceptable risk loss. Such curves could be utilized to demonstrate soci- and designation of mitigation measures. However, the etal risk (Vrijling and van Gelder 1997; Saw et  al. 2009; determination of acceptable risks proves to be one of the Strouth and McDougall 2021) and to define the safety most challenging aspects of QRA. To mitigate this chal- levels of the region of interest. It is crucial to be clear that lenge, it is recommended to use the F–N curve for the the F–N curves only provide statistical readings and not region of interest which is a plot showing the number of the threshold of risk acceptance and tolerance. Figure  3 Fig. 2 Flow chart of landslide risk assessment (Modified from Hungr 2016) Sim  et al. Geoenvironmental Disasters (2022) 9:3 Page 5 of 17 Fig. 3 F–N curves demonstrating the number of fatalities against the annual frequency of occurrence for natural and man-made hazards (Modified from Van Westen et al. 2011) shows the typical F–N-curves for various hazards. It intensity scenarios (Guzzetti et  al. 2002). As stated by can be seen that man-made hazards have a tendency to Song et al. (2007), many organizations still do not possess demonstrate a steeper curve than natural hazards (Van sufficient database to progress very far in this direction Westen et al. 2011). with few exceptions such as Hong Kong. Nevertheless, the Japanese and Chinese F–N curves stand higher than Landslide F–N curves for various countries the curves computed for other countries, even for the Figure 4 presents the family of F–N curves for landslides shortest time intervals which denotes that the afore- compiled from various geographical locations. It should mentioned nations faced a high landslide risk, with a be clarified that some of the F–N curves do not repre- great number of cataclysmic events that caused severe sent an entire country because of the unavailability of fatalities. data. There are mixtures of F–N curves extracted from It can be seen that the F–N curves of both Hong Kong different countries and local municipalities. It is appar - and Canada share a very similar slope and trend and they ent that the F–N curve is unique to a country/location. converge at N = 20. From there, the trend becomes differ - From Fig. 4, the individual risk (1 death) for Guatemala is ent with Canada showing higher probabilities of casual- significantly lower than those of other nations. However, ties until they converge again at N = 70. Interestingly, the this highlights a bias in the Guatemala catalogue, which maximum N value of Hong Kong is lower than 100 even lacks comparable coverage. Another thing to note about though it has a high population density living in steep Guatemala is that the F–N curve only covers probabili- hillsides. According to Duzgun and Lacasse (2005), the ties of single deaths and no more than that which reflects Hong Kong curve represents the performance of engi- on the severe lack of data. It can be seen that the F–N neered slopes (risk imposed upon the community by curve for China only begins for landslides that result in designed slopes). At N = 10, the city of Rio de Janeiro has a minimum of 100 deaths while the Japanese F–N curve almost the same F-value as China which is not surpris- only starts from 10 which signifies the incompleteness of ing considering both Rio and China are heavily populated the landslide catalogue of the former countries for low region and country, respectively. Canada, Nilgiri Hills, Sim et al. Geoenvironmental Disasters (2022) 9:3 Page 6 of 17 1.00E+01 Hong Kong Norway 1.00E+00 Japan China 1.00E-01 Nepal 1.00E-02 Canada Italy 1.00E-03 Colombia Alps 1.00E-04 Portugal 1.00E-05 Nilgiri Hills, India Taiwan 1.00E-06 A83 Rest and Be Thankful, Scotland Rio de Janeiro 1.00E-07 Coal Cliff, Australia 1.00E-08 Matata, New Zealand 110 100 1000 10000 Guatemala N, Number of Deaths Fig. 4 Global comparison of frequency (F) versus number of deaths (N) curves (Guzzetti et al. 2002; Pacheco et al. 2002; Duzgun and Lacasse 2005; Hsi and Fell 2005; Jaiswal et al. 2011; Tsao et al. 2011; Whakatane District Council 2015; Pereira et al. 2017; Laporte 2018; Winter and Wong 2020) India, and Hong Kong were observed to share approxi- respectively which show that a higher risk level is associ- mately similar curves. The curves for Nepal and Rio de ated with the slopes in Japan than in Italy. Comparisons Janeiro both converged at N = 141. However, for events of F–N curves between developed and developing worlds associated with lower casualties (i.e. N < 100), the prob- and between different continents are shown in Fig.  5. It ability of landslide fatality in Rio de Janeiro is signifi - should be made clear that the curves of more discrete cantly higher than that in Nepal. This is clearly related to regions such as Nilgiri Hills, India and Matata, New Zea- the much higher population density in the city of Rio de land were included in Fig.  5 as well. Overall, despite the Janeiro than Nepal as a country. curves being inconsistent, a general observation indi- The population density as well as the hazard spatial cated that the F–N curves of developing countries are distribution should be included to compare the F–N mostly plotted higher than those of developed countries. curves of various nations (Duzgun and Lacasse 2005). In terms of continents, the Asian continent generally has Population density, F-values, and slope of the best-fit a higher frequency of deaths followed by Latin America. curve are tabulated in Table  2. The probability of occur - This is because countries in Asia such as Japan are situ - rence, F for a specific N is governed by the slope of the ated in a geographical location where natural disasters curve. A steeper slope signifies a higher risk aversion are frequent. Furthermore, countries like China and or in other words, a lower probability of occurrence for Nepal have many of their population living on hazardous a specific number of fatality or lower risk. For example, slopes which have further increased the risk of landslide the slope of Japan’s curve is − 0.53 while Italy’s curve has fatalities. a slope of − 1.99. The probabilities of occurrence, F for It was stated by Holcombe et  al. (2016) that N = 100 fatalities for Japan and Italy are 0.07 and 0.00005, rapid unplanned development is causing a rise in F, Annual Probability of Occurence Sim  et al. Geoenvironmental Disasters (2022) 9:3 Page 7 of 17 Table 2 Population density ( World Bank 2018), F-values, and slope of F–N curves for various geographical locations Locations Population density/ Slope of F–N curve F-values km N = 1 N = 10 N = 100 Japan 347 − 0.53 – 0.8 0.07 China 148 − 0.61 – – 0.06 Rio de Janeiro, Brazil – − 3.3 1 0.52 0.0011 Italy 203 − 1.99 0.9 0.008 0.00005 Portugal 112 − 2.35 0.63 0.04 – Hong Kong 7096 − 0.79 0.6 0.01 – Canada 4 − 0.92 0.4 0.07 – Alps 215 − 0.538 0.29 0.16 0.003 Nilgiri Hills, India – − 0.275 0.21 0.05 0.006 Nepal 196 − 1.31 0.08 0.004 – Colombia 45 − 1.63 0.04 0.001 – Norway 15 − 0.72 0.006 0.001 – Matata, New Zealand 19 − 0.73 0.0057 – – A83 Rest and be Thankful site, – − 1.924 0.002 0.00005 – Scotland Taiwan 637 − 0.89 0.00128 0.000157 – Coal Cliff Australia – − 2.375 0.00125 0.0000206 – Guatemala 153 – 0.000134 – – (b) (a) Asia North America Developing worldDeveloped world Europe Latin America Oceania 1.00E+01 1.00E+01 1.00E+00 1.00E+00 1.00E-01 1.00E-01 1.00E-02 1.00E-02 1.00E-03 1.00E-03 1.00E-04 1.00E-04 1.00E-05 1.00E-05 1.00E-06 1.00E-06 1.00E-07 1.00E-07 1.00E-08 1.00E-08 1 100 10000 1100 10000 N, Number of Deaths N, Number of Deaths Fig. 5 F–N curves grouped based on a developed and developing worlds, b geographical continents precipitation-triggered landslide risk in low-income lacking infrastructure almost reaches the 1 billion areas in tropical developing nations. The number of mark exposing them to the dangers of both small and people inhabiting overcrowded impoverished houses large scales catastrophes. In Latin America, Gross F, Annual Probability of Occurence F, Annual Probability of Occurence Sim et al. Geoenvironmental Disasters (2022) 9:3 Page 8 of 17 National Income (GNI) and value of life are generally Concept of acceptable risk and tolerable risk low (Daniell et  al. 2015; World Bank 2020) with many It is crucial to differentiate between acceptable risks of their populations living in slums constructed on which the public aims to determine, mainly for new hazardous slopes with no compliance to safety stand- developments, and tolerable risks which they will live ards. The majority of these slums are constructed on with, albeit lower risks still prevail as their main prefer- lands on which significant landslide hazards persist, ence (AGS 2007). This applies to both property and loss i.e. unstable slopes (see Fig.  6) without any design of life. People in a society whose lives might be impacted or assessment for the factor of safety. This had fur- by landslides, and authorities in charge of development ther propagated the landslide hazard due to localised approval must determine the acceptable and tolerable changes in drainage, vegetation, loadings, and topog- risks for property loss and damage (Song et al. 2007; Lev- raphy, and hence increased the associated risk to the enthal and Withycombe 2009). Practically, the authorities population. Disasters of these types serve as a warning will be the ones obligated to ascertain the risk levels given sign of a bigger issue stemming from poverty as well as its obligation to control hazards at the local municipal a poorly-organized development (Hungr et  al. 2016). level. In most cases, that is the National or State author- Unfortunately, there are no known F–N curves avail- ity or Local Government Area Council. Following are the able for countries in the African continent. brief definitions of acceptable risk and tolerable risk: It is crucial to note that the F–N curves only pro- vide statistical readings and not the threshold of risk I. Acceptable Risk—A risk that the public is inclined acceptance and tolerance. Nevertheless, the F–N to accept without regard to its management for life curves are useful for evaluating the current risk level and work purposes. Further expenditure in lower- of a region/country. This information is useful for ing such risks is normally not taken under consid- developing the risk acceptance and tolerance levels for eration by the public. a country. As can be seen from Figs. 4 and 5, the land- II. Tolerable Risk—A risk that the general public is slide risk level varies considerably from a country to inclined to live with in order to safeguard certain another, and hence different standards of acceptance net benefits having faith that the particular risk is and tolerance should be adopted with considerations being properly contained. The risk level is periodi - of various factors (Duzgun and Lacasse 2005), which cally reviewed and lowered further whenever feasi- will be reviewed in further details in the following ble. Also, the criteria where the ALARP principle, section. which stands for “As Low As Reasonably Practica- ble”, may be applied so that the risk is mitigated to a marginally and practically tolerable level since the Fig. 6 a Archetypal unplanned housing on unstable slopes in Castries, Saint Lucia, Eastern Caribbean; b landslide on unplanned development (Holcombe et al. 2016) Sim  et al. Geoenvironmental Disasters (2022) 9:3 Page 9 of 17 1.E+00 1.E-01 1.E-02 Slope, α = 1 1.E-03 1.E-04 Unacceptable 1.E-05 Tolerable (ALARP) 1.E-06 1.E-07 Detailed 1.E-08 study Acceptable required 1.E-09 Fig. 7 Illustration of “As Low As Reasonably Practicable” ALARP principle for risk evaluation (Campbell et al. 2016) 1.E-10 110 100 1000 10000 Number of Fatalities Fig. 8 Risk criterion proposed for natural slopes in Hong Kong. reduction to further acceptable levels is not feasible Modified from Geotechnical Engineering Office (1999), Lacasse et al. (2010), Lacasse (2016) and Strouth and McDougall (2020) with regards to the cost to the individual or public. Figure  7 shows the significance of ALARP (Camp - bell et al. 2016). the risk criteria is such that the frequency of events that According to Winter and Bromhead (2012), society will cause 100 deaths or greater must be 100 times lower than be more tolerant towards landslides that occur for natu- the frequency of events that cause 10 or more deaths ral slopes than for engineered slopes, although the effects (Roy and Kshirsagar 2020). The tolerable/ALARP region of climate change may cause recurring natural slope fail- reflects the risk to be "As Low As Reasonably Practicable" ures in a small area defeating this greater tolerance. Tol- which is illustrated in Fig. 8 (Campbell et al. 2016). erable risks differ between nations subjected to historic A study by Winter and Bromhead (2012) summarized exposure to landslides, control and ownership of slopes, issues that govern landslide risk acceptance such as plan- and natural landslide hazards. Relevant regulators may ning and regulations, event footprint against vulnerabil- choose to apply “acceptable risk” criteria for high-risk ity shadow, budgetary issue, vulnerability of vehicles all cases, i.e. hospitals, schools, as well as emergency ser- fit into the influences of social and economic factors on vices taking into account their significance, and also as a the risk acceptance. Figure  9 shows a conceptual ternary means of mitigating societal risk concerns. For involun- willingness diagram of various landslide incidents in dif- tary individual risk, generally, the acceptable risk values ferent countries. The approaches of various nations in −5 −6 will be in the frequency range of 10 to 10 per year handling landslides are qualitatively compared to provide −3 while for voluntary risk, it will be typically between 10 a better understanding of the factors involved in land- −4 to 10 per annum (Fell 1994). slide risk mitigation. Figure  8 shows a provisional risk criterion proposed It is clear that the risk level that exists in a particular for natural slopes in Hong Kong (Geotechnical Engi- area and the willingness to pay or financial power will, neering Office 1999; Lacasse et  al. 2010; Lacasse 2016). have a significant impact on the risk acceptance. Never - The degree of aversion is represented by the slope of the theless, risk acceptance and financial power/willingness F–N lines. Lines with a slope gradient of 1 are indicated to pay are factors that are considered as forcing agents as equi-risk, in which the points along the line have the in the discourse on worldwide landslide risk mitiga- same risk level. The F–N curves may be conveyed by the tion. As stated by Turner (2018), industrialized nations equation: have procedures for landslide mitigation planning and implementation that call for accurate knowledge of land- F · N = k(1) (1) slide hazards and risks. The landslide risk assessments are gradually becoming quantitative. Mitigation meas- where k-value is 0.001, α equals unity (2). The greater ures include structural and geotechnical procedures, the F–N slope, the higher the risk aversion (Anand 2015; in addition to political, legal, as well as administrative Roy and Kshirsagar 2020). For example, for a slope of 2, Annual Probability Sim et al. Geoenvironmental Disasters (2022) 9:3 Page 10 of 17 and collaboration from developed nations are needed (Kjekstad and Highland 2009; Turner 2018). From the foregoing, it can be concluded that if accepta- ble and tolerable societal risk levels are to be established, each country should select different levels governed by factors such as socioeconomic standing, Gross Domestic Product (GDP), statistical value of lives, as well as finan - cial prowess of the country (Leroi et  al. 2005; Roy and Kshirsagar 2020). A review of risk tolerance criteria for various countries Hong Kong In the late 90s, the Hong Kong Government provisional criterion for “potentially hazardous installations” for “landslides and boulder falls from natural terrain” was Fig. 9 Willingness diagram by Winter and Bromhead (2012) implemented by the Hong Kong Geotechnical Engineer- demonstrating various approaches to landslide risk in different ing Office (GEO) (Geotechnical Engineering Office 1999) countries (see Fig. 2). According to Wong (2005), the criterion was occasionally adopted in the country to determine land- slide risk tolerance related to developments of structures procedures that impact the lifestyle and behaviour of the and infrastructures, etc. in near proximity to modified or endangered society. For example, Zentoku, Japan leans natural slopes. Hungr and Wong (2007) described that in to the bottom left region of the Willingness diagram in order to scale the F–N chart in proportion to its original Fig.  9 signifying a very high willingness to pay and alter reference frame (the perimeter of the industrial plant), the environment to mitigate landslide risk in the region. it is used on a consultation area of around 500  m long This implies that Japan has a tremendously low tolerance section of a boundary between hazardous slopes and of landslide risk as well as the financial ability to pay to buildings. mitigate the risk. The saga of the founding of the Hong Kong landslide In contrast, Jamaica’s landslide mitigation approach in risk management system was explained by Malone the Blue Mountains puts them at the top of the triangle, (2012). It was decided by the Hong Kong GEO that the showing that there is a very high willingness to accept methodology of the United Kingdom Health and Safety risk and a very low willingness to pay. Jamaica is a coun- Executive (HSE) to create the risk tolerance criterion try with limited economic resources and that limited with a few adjustments could help in decision making budget forces them to accept the risk (Winter and Brom- procedures in Hong Kong in regards to landslides. The head 2012). Similar scenarios were reported by Yifru et al. Hong Kong GEO conducted QRA on the basis of avail- (2015), where there was no mitigation carried out on the able records, which restricts the timeline to around 50 landslides that occurred along the road corridors of two to 100 years. This also means that high magnitude infre - Caribbean islands, i.e. Saint Lucia and Dominica (both quent landslides do not appear in the inventory and, the known to be deprived islands). Focuses given to the two probabilities and levels of the infrequent landslides were Caribbean islands were only for road repairs and debris then determined through extrapolation. This limited removal. This observation further supported the will - period over which landslide inventories exist, and the ingness diagram of Winter and Bromhead (2012) which associated lack of data on high magnitude/low frequency demonstrates that a limited economic resource leads to a landslides is a challenge with which all regions of the low willingness to pay, and hence leads to a high accept- world must grapple. ance of landslide risk. Most developing nations do not Other than the F–N diagram for societal risk, it is possess adequate resources, capacity, and technical skills also recommended by GEO that individual risk toler- to carry out landslide risk reduction measures to the ance criteria of 1 in 10,000 be applied for current resi- required speed and level. Rather than lowering vulner- dential areas and 1 in 100,000 for new developments. ability, their central emphasis on handling landslides has The ALARP method was employed quantitatively to been to respond actively to emergencies and catastrophes Hong Kong scenarios to establish the feasibility of land- which again results in higher risk acceptance. Support slide remedial works, i.e. the Pat Heung landslide, North Sim  et al. Geoenvironmental Disasters (2022) 9:3 Page 11 of 17 Lantau Expressway, Lei Yue Mun, and Shanti Heights, etc Table 3 Proposed tolerable risks by AGS (2000) as seen in Lacasse (2016) (Wong 2005). Slope types Tolerable risk for loss of life −4 China Engineered slopes 10 /year for person most at risk −5 Following a landslide that results in 79 casualties at 10 /year for average person at risk −5 Wulong, China on labour day of 2001, new legislation on New engineered slopes 10 /year for person most at risk −6 landslide hazard and risk assessments were incited. How- 10 /year for average person at risk ever, no landslide individual and societal risk criteria have been recognized by the Chinese government (Song et al. 2007) though numerous criteria have been developed It is then further stated by AGS (2007) that no risk tol- and employed for other projects such as dams. Neverthe- erance criteria of Australia are legally binding lest the less, the verdict on acceptable and tolerable risks lies in owner or regulator acknowledge it. Nevertheless, the the decision of the client, owner, regulator, and persons landslide tolerance criteria developed in Hong Kong for at risk and not the risk analysts (Fell 1994). Societal and Individual risk are endorsed in documented The Chinese landslide societal risk criteria proposed studies regarding landslide risk management practices in by Song et  al. (2007) is comparable to the Hong Kong Australia (Fell et al. 2005). acceptable and tolerable landslide risk criterion found in Fell (1994), Geotechnical Engineering Office (1999), Canada and Dai et  al. (2002). The proposed landslide risk crite - A review by Evans et  al. (2005) concluded that Canada ria are about 3 times higher than the projected risk cri- has adequate comprehension of landslide hazards that terion for manmade structures such as buildings. More it could deliver a regional quantitative risk assessment recently, landslide risk assessment has been implemented (QRA) as well as the design of tough risk-reduction pro- extensively in China, with real-life landslide case studies cedures for majority areas that are heavily predisposed to employed in various regions i.e. Western Hubei (Fu et al. landslides. North Vancouver was the first municipality in 2020), eastern Jiangxi (Zhang et  al. 2020), and Mayang Canada to employ the Hong Kong individual risk accept- county (Sui et  al. 2020). It is further stated by Wu et  al. ance criteria after the landslide in North Vancouver that (2020) that the Chinese government invented a system occurred in 2005 due to heavy downpour which resulted called ‘Public Participation Monitoring and Warning’ in the destruction of two homes, one seriously injured (PPMW) which aims to bring down mortality rate with resident, and one fatality (Hungr et al. 2016; Porter et al. the least cost by grouping residents to evacuate ahead of 2017). Restrictions were enforced by the legislation on a disaster. It works by captivating the public in the disas- the development of residential areas, as well as redevel- ter risk management operation. opment and rezoning in regions where the Personal Indi- vidual Risk (PIR) exceeds 1 to 10,000 for existing houses Australia or 1 to 100,000 for new development. Extensive public In 1997, 18 deaths occurred as a result of the Thredbo engagement is of paramount importance in the legislative landslide in Australia. This resulted in the development operation (District of North Vancouver 2009; Tappenden of a framework for risk-based landslide management by 2014). the Australian Geomechanics Society (AGS) (AGS 2007). During an extreme rainstorm between June 19 and 21 It is stated in the framework that “risk evaluation is to be 2013 that brought about flood, debris flow, and debris conducted by comparing estimated risks to levels of tol- flood in Canmore, Alberta, numerous buildings and erable or acceptable risk, in order to assess priorities and infrastructure suffered severe damage (Town of Canmore options” and the risk tolerance criteria are to be deter- 2015). As a result, the municipality engaged consultants mined by the “client/owner/regulator with advice from a conducted thorough quantitative risk assessments (QRA) technical specialist”. on debris flow and debris flood risk on the Cougar Creek For individual risk, it is recommended by AGS (2000) fan, on which a part of Canmore is built. Return periods that the tolerable individual risk criteria employed for between 30 and 3000  years were adopted for the hazard Potentially Hazardous Industries, Australian National circumstances (Hungr et al. 2016). The main focus of the Committee on Large Dams (ANCOLD), as tabulated in QRA was on direct building damage, grievances, and Table 3, could be reasonably applied to engineered slopes casualties by utilizing the Hong Kong risk tolerance crite- as well (Lacasse 2016). Moreover, it is also being pro- rion. A total of 190 areas in Canmore were found to have posed that acceptable risks may be taken as one order of unacceptable individual and societal risks. It was not magnitude lower than tolerable risks. feasible to relocate the public residing there, hence four Sim et al. Geoenvironmental Disasters (2022) 9:3 Page 12 of 17 proposed options for mitigation were studied extensively, mapping method to distinguish unstable rock slopes dis- resulting in the construction of a debris flood contain - posed to calamitous failures, so that future events could ment structure (approximately height 30  m and width be predicted and the population at risk could become 100  m storage capacity 650,000 m ) to reduce the unac- accustomed to the hazard. The vulnerable regions are ceptable risk as much as possible to the ALARP/tolerable continuously monitored and an early warning system zone (Town of Canmore 2015). Problems regarding prac- together with an evacuation plan is at hand (Hermanns ticability, impartiality as well as affordability were taken et  al. 2013). In 2015, procedures were established by the into account. Since then, QRA has also been conducted Direktoratet for Byggequalitet to specify development by other local and provincial government regulators in restrictions in regions vulnerable to landslides or land- British Columbia and Alberta (Porter et al. 2017), which slide-generated waves (Clague et  al. 2015). The restric - is a crucial move to obtain supports of stakeholders. tions take into account single and multi-family structures Since 2005, following a certain amount of public con- of fewer than 10 households, and construction is only sultation, it is widely recognized that Canadian’s insights allowed if the probability of landslide impact falls below and values in regards to landslide risk are similar to those the 1000-year return period event. Facilities and tall of Hong Kong (Strouth and McDougall 2020). While buildings housing numerous occupants i.e. schools, hos- there is no universal acceptance, F–N curves have influ - pitals must be situated in zones with a probability of land enced land-use decision making processes exceeding 50 sliding impact under the 5000-year return period event. landslides and steep-creek flood hazard regions. Since There are exceptions such as when “the consequences 2020, the tolerable and acceptable risk criteria have been of building restrictions are serious, and the construc- heavily referenced with discussions of integrating them tion has a significant impact on the society.” In addition, into municipal and provincial guidelines, regulations, as exceptions are also allowed for scenarios where a warn- well as design codes (Strouth and McDougall 2020). ing could be given out three days prior to the incident. As of the present, 523 unstable rock slopes were detected, of which 110 were categorized as hazardous and risky. Out New Zealand of the 110 hazardous and risky slopes, hazard zones were In New Zealand, local governments were delegated with found in 48 slopes resulting in impediments on building risk tolerance criteria by the New Zealand Resource projects. 7 sites were labelled as high risk and they are Management Act. According to Enright (2015), the Hong under constant monitoring (Hermanns et al. 2020). Kong risk tolerance criteria were employed by the City of Christchurch following the 2010 and 2011 earthquakes. GNS science proposed a tolerable risk threshold for nat- −4 Switzerland ural hazards of 1 in 10,000 (10 ) which was deemed to Bründl et  al. (2009) briefly explained that the Swiss be too high and not acceptable as it doubles the number National Platform for Natural Hazards (PLANAT) rec- of “annual” casualties of the 2011 Christchurch earth- ommended an individual landslide risk “goal” of 1 in quake. The study proposed that the annual tolerable risks −5 −6 100,000 per year for residential areas in the year 2005. threshold for existing risks to be 10 and 10 for new However, the government of Switzerland apparently has risks. However, the application of these criteria is still not approved the usage of risk tolerance criteria for natu- undergoing tests in the local courts and there is a pos- ral disasters. The present practice in avalanche warning sibility that they will be used by other local authorities. demonstrates that educational courses are one of the As of present, there are no restrictions placed on land use crucial components in introducing new ways and skills development for debris flow deposits adjoining the bor - to natural hazard experts. It is one of the means of com- ders of active volcanoes which could only be a century municating risk and there is a high possibility that edu- old. cational courses will serve as the main part of integral risk management in Switzerland (Bründl et al. 2009). Fur- Norway thermore, frequent training and educational courses will Throughout history, the Norwegian Fjords have been result in an increased public awareness towards potential impacted by large rock slope failures triggering tsuna- consequences, as well as improvement to the compliance mis that cause many deaths (Hermanns et  al. 2013). A rate of the warning systems. Although showing promis- trio of the most epic of natural disasters transpired in ing results during the Preonzo rockfall event (zero injury the twentieth century where tsunamis caused by mas- and everybody evacuated successfully), further stud- sive rockslides into fjords or lakes (Loen in 1905 and ies are still required to evaluate the effectiveness of the 1936 and Tafjord in 1934), claiming an excess of 170 lives system on the basis of an integrated risk management (Eidsvig et  al. 2011). Over the past few years, the Geo- approach (Sättele et al. 2016a, b, c). logical Survey of Norway has employed a systematic Sim  et al. Geoenvironmental Disasters (2022) 9:3 Page 13 of 17 Malaysia It was stated by the Swiss Ministry of the Environ- Based on data obtained from the Global Landslide Cat- ment that “while industrial risks can be governed by alog (GLC) of the United States National Aeronauti- legislations using risk tolerance criteria, the same does cal Space Administration (NASA), Malaysia is the 10th not apply to natural hazard risks”. This is an arguable highest ranked country for the highest number of slope statement that refuted by Hungr et al. (2016). The risk failures, with 171 major slope failures between 2007 and tolerance criteria have been well applied to natural March 2016 (Abd Majid and Rainis 2019). The majority hazards in different parts of the world such as Hong of the landslides occurred on cut slopes or embankments Kong (Wong 2005; Wong and Ko 2005; Chiu 2015), the alongside roads and highways in mountainous regions UK (Winter and Wong 2020), the Netherlands (Hungr with a few occurring close to high-rise apartments and et  al. 2016), Australia (Leventhal and Withycombe residential areas, resulting in numerous causalities. 2009), Italy (Rossi et  al. 2019) and Canada (Town of Over the years, there have been numerous studies con- Canmore 2015; Strouth and McDougall 2020). ducted on Malaysian landslide hazards using various A documented discourse by PLANAT (National Plat- methods such as GIS (Mukhlisin et al. 2010; Althuwaynee form for Natural Hazards) (2014) regarding tolerable and Pradhan 2017; Abd Majid and Rainis 2019), statisti- landslide risk criteria stated that “The average risk of cal logistic regression method (Lee and Pradhan 2007; death for human beings is not significantly augmented Pradhan and Lee 2010) and deterministic safety factor by natural hazards. The yearly risk of death resulting method (Ng 2012; Ismail and Yaacob 2018). However, from natural hazards is significantly lower than the research on acceptable and tolerable risk is still scarce in average probability of death for the age group with the Malaysia, with a few researchers (Ahmad et al. 2017; Ros- lowest mortality rate in Switzerland.” It can be con- lee 2019) having a more thorough understanding of it. It cluded that their local regulators would prefer to use was proposed by Ahmad et al. (2017) that the interim risk the landslide hazard probability assessments described criteria for Malaysia should be higher (higher acceptance in Lateltin et  al. (2005). According to Lateltin et  al. of risk, one fatal landslide in every 50  years) than Hong (2005), it was made mandatory by the new Federal Kong (one fatal landslide every 1000  years) while Ros- Ordinances on Flood and Forest Protection for the lee (2019) proposed a risk tolerance criterion similar to cantons to develop hazard maps for incorporation into that found in Hong Kong (Song et al. 2007; Lacasse et al. the regional and local development plans. This pro- 2010). It can be concluded that QRA and landslide risk gram was subsidized by the federal authorities up to assessments are still a developing art in Malaysia and 70%. The three steps of constructing the hazard maps much surveys and studies are still required. were outlined by Raetzo et al. (2002) as follows: (i) Hazard identification step which comprises making an inventory of past slope failures. (ii) Hazard assessment of the magnitude or intensity Comparison of risk tolerance criteria of landslides with time. Hazards are mapped into Figure  10 presents comparisons between societal risk one of four hazard classes based on the probability criteria adopted by different countries. While there are of the land sliding hazard: high danger (probability: variations, the risk levels of Denmark, New South Wales, 82–100%; red zone), moderate danger (probabil- China, and ANCOLD/AGS generally converge at the −4 ity: 40–82%; blue zone), low danger (probability: anchor point of F = 1 × 10 /year (1 in 10,000) for N = 10 −2 15–40%; yellow zone) and no danger (probabil- casualties. Most countries adopt F between 1 × 10 /year −3 ity: < 15%; white zone); to 1 × 10 /year for N = 1 of individual risk. The Neth - (iii) Risk management and land-use planning erlands, the Czech Republic, and European Commis- sion (EC) have the most risk-averse slope; the steepest It was made mandatory by the federal government that compared to those of other nations and entities. The risk the maps must follow a standard colour coding: red criterion of Malaysia proposed by Ahmad et  al. (2017) where construction is prohibited, blue where construc- is liberal and has the highest risk tolerance (N = 1000 −4 tion is permitted provided certain safety requirements occurring at a frequency of 1 × 10 /year). The Chinese are fulfilled, and yellow where construction can be risk criterion proposed by Song et al. (2007) is exactly the carried out without any restriction. Hazard maps are same as the one of the Australian National Committee on deemed invaluable for planning protective measures Large Dams (ANCOLD)/Australian Geomechanics Soci- such as warning systems and emergency plans (Raetzo ety (AGS). It can be seen that the French criterion is not et al. 2002; Lateltin et al. 2005). governed by the number of fatalities, N. This will lead to a lower level of safety at high N values and at the same time result in extremely stringent and uneconomical safety Sim et al. Geoenvironmental Disasters (2022) 9:3 Page 14 of 17 employ risk criteria similar to that of developed countries Hong Kong Denmark (China’s being exact same criterion as that of Australia’s, Belgium ANCOLD/AGS −3 while Sri Lanka uses the same slope and F = 1 × 10 for New South Wales United Kingdom N = 1 as that of Hong Kong’s). It can be assumed that risk Netherlands European Commision Taiwan China assessment is still new in many developing countries. Malaysia Western Canada Many of them employ the criteria of developed countries Sri Lanka Czech rather than making their own. As stated by Duzgun and France Lacasse (2005), every nation should develop its own risk 1.E+00 criterion as they all possess different F–N curves. More 1.E-01 works should be devoted to developing risk tolerance 1.E-02 criteria particularly for less developed and mountainous 1.E-03 countries. 1.E-04 1.E-05 Conclusion 1.E-06 It is apparent that every nation has its own priorities and approaches towards landslide risk management. While 1.E-07 it is interesting to compare the broad range of policies 1.E-08 and discussions among various nations worldwide, this 1.E-09 review by far does not feature coverage on the countries 1.E-10 of the African continent. There is practically no or very 110 100 1000 10000 limited data available on the QRA of landslides in the Number of Fatalities African continent. It is also clear from the comparison Fig. 10 Risk criteria of different nations ( Trbojevic 2005; Song et al. of F–N curves that the Asian world has the highest fre- 2007; Tsao et al. 2011; Lacasse 2016; Ahmad et al. 2017; De Silva et al. 2017; Strouth and McDougall 2020) quency of landslide deaths followed by Latin America. Generally, the developed countries impose stringent risk “acceptance” criteria for death, ranging from 1 in procedures at low N values. Remarkably, lowland nations 10,000 to 1 in 100,000 annual individual risks of loss of such as Belgium and the Netherlands employ stricter, life. Although it is easy to state that the landslide risk more risk-averse criteria than mountainous nations such criterion should have a minimum level of tolerance as Hong Kong and the UK as seen in the steeper slope −3 for landslides, i.e. F = 1 × 10 for N = 1, the accept- and a lower F value for N = 1. As events and fatalities ance of tolerance for landslides in every country will be are rare in any case in lowland nations, it is relatively governed by its financial standing. While a developing straightforward to establish and work successfully with nation might face similar matters in risk governance and stricter risk-averse criteria. decision-making as the developed nations, the trade- As seen in Fig.  10, the risk tolerance criteria for most off between economy and safety might differ (Roy and countries fall below the Hong Kong criteria with the Kshirsagar 2020). Developments that occur in developing exception of Malaysia and the UK. The risk criteria of nations will amass bigger benefits than their developed Denmark and New South Wales (NSW) are higher than counterpart. For example, the construction of railways, Hong Kong’s at lower fatalities until they converge at roads may contribute to job opportunities, easier access N = 10. At higher N levels, the Fs of NSW and Denmark to certain areas, and goods transportation which will fall below Hong Kong’s as evident in its steeper slope, help to increase turnover and create wealth to boost which signifies a more stringent risk aversion and lower the economy (European Maritime Safety Agency 2015). tolerance of risk for higher fatalities (N > 10). The risk cri - However, against these potential advantages, the devel- teria of many countries lie below Hong Kong’s suggesting opment of roads may inflict higher risk to the users i.e. a more stringent approach should be employed when it landslides that occur along roads in Scotland, Jamaica, comes to risk assessments. However, the Hong Kong cri- and Colorado as well as many others mentioned by Win- terion is still widely adopted and referred to in other soci- ter and Bromhead (2012). Nevertheless, under an agreed eties (Fell et al. 2005; Strouth and McDougall 2020). level of risk, there are potentially higher net benefits The criterion proposed by Malaysia, a developing coun - from the developments of a developing nation than for try, is above the criteria of other nations. It has the high- a developed one. Thus, should a variety of risk accept - est risk tolerance compared to other countries which is ance and tolerance criteria be employed in the develop- unduly liberal and calls for improvements to be made. ing nations in regards to their developed counterparts? Other developing countries such as China and Sri Lanka An unduly liberal risk acceptance criteria will bring about Annual Probability Sim  et al. Geoenvironmental Disasters (2022) 9:3 Page 15 of 17 References more development but at the expense of higher societal Abd Majid N, Rainis R (2019) Aplikasi Sistem Maklumat Geografi (GIS) dan risk. 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A review of landslide acceptable risk and tolerable risk

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Abstract

Landslides are one of the most important geohazards. In 2004–2016, more than 55,000 people lost their lives to landslides and this does not include deaths caused by seismically triggered landslides. Overall losses were estimated to be at USD 20 billion annually. The lives of many could be saved if more had been known regarding forecasting and mitigation. Studies have shown an increasing trend in landslides occurrence and fatalities. Over recent years, landslide risk assessment has been carried out extensively by geo-scientists worldwide. This review concentrates on the societal risks posed by landslides in various countries and the risk criteria used by various parts of the world in assessing landslide risks. The landslide risk tolerance criteria are strongly governed by utilitarian concerns i.e. financial power and the need for development. In developing countries, surprisingly high levels of tolerance are proposed for landslides. The risk criteria of Hong Kong and that of the Australian Geomechanics Society are widely employed in many coun- tries. Although various risk tolerance levels have been proposed by various nations, many of them are still not being applied in their real-life scenarios. The procedures for setting risk criteria call for a wide agreement between geo- scientists, government decision makers, and the community. Risk criteria should be developed locally with historical landslide inventory, public perception, and engineering aspects being considered. Keywords: Landslide, Tolerable risk, Acceptable risk, Societal risk, Frequency-number of fatalities diagram Introduction result in casualties to both humans and animals but dis- Landslides are one of the major devastating geohazards rupt the water quality of streams and rivers as well as and claim thousands of lives and create acute economic the destruction of structural and infrastructural devel- losses related to property damage every year (Schus- opments. Of the total loss of life resulting from natural ter and Highland 2001; Dilley et  al. 2005; Petley 2012). hazards worldwide, 5% of it comes from highly devel- Centuries prior, many countries worldwide have suf- oped nations. The remaining 95% of total deaths are from fered deaths and economic losses due to landslides and medium and developing countries (Lacasse et  al. 2010; the impact is still on the rise. The first documented land - Lacasse and Nadim 2014). Klose et  al. (2015) reported slide was an earthquake induced landslide dam in Honan that the economic impact of landslides is likely the great- Province of China in the year 1767 B.C. (Schuster 1996). est on transportation infrastructures. This is especially International Disaster Database (EM-DAT) of the Centre true in rural regions where the transportation network for Research on the Epidemiology of Disasters reported is scattered, and the availability of alternate routes is few that landslides caused 17% of the deaths associated with to none. As a result, a minor landslide will bring a great natural hazards worldwide annually (Lacasse and Nadim impact on the economic sector over an extensive region 2014; Aristizábal and Sánchez 2020). Landslides not only (Winter et  al. 2018). According to Dowling and Santi (2014), landslides are catastrophes resulting from social vulnerability. *Correspondence: minlee.lee@nottingham.edu.my Socioeconomic impacts of landslides are always under- Department of Civil Engineering, Faculty of Science and Engineering, estimated because landslides in many countries are University of Nottingham Malaysia, 43500 Semenyih, Selangor, Malaysia © The Author(s) 2022. Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http:// creat iveco mmons. org/ licen ses/ by/4. 0/. Sim et al. Geoenvironmental Disasters (2022) 9:3 Page 2 of 17 always taken as a consequence arising from other trigger- on the tourism industry, as well as other economic activi- ing processes such as extreme precipitation, typhoons, ties. However, estimations of these long-term losses are volcano eruptions or earthquakes. The damage costs challenging as they are typically widely spread out both from landslides could surpass all other costs from the socially and geographically. This underestimation brings overall multiple-hazard catastrophes (Froude and Pet- about a lower awareness and appreciation regarding ley 2018; Sultana 2020). Studies have indicated that the landslide risks among regulators, authorities, and the EM-DAT database often underestimates the number general public. of fatal landslides by 1400% (Kirschbaum et  al. 2015) or In the new millennium, landslides have claimed tens 2000% (Petley 2012), and the number of deaths by 331% of thousands of lives and an estimated annual average (Kirschbaum et  al. 2015) or 430% (Petley 2012). The of economic losses of $20 billion, which is 17% of the under-reporting is caused by the perception of landslides total ($121 billion) yearly mean global disaster losses as a secondary hazard, in which the cause of death is nor- from 1980–2013 (Klose et  al. 2016). The average eco - mally reported in relation to the primary hazards (e.g. an nomic losses of various nations caused by landslides earthquake rather than a seismically triggered landslide) worldwide are reviewed in the present study and sum- instead of the actual cause of the loss (Froude and Petley marized in Table  1. To determine the economic losses, 2018). Furthermore, conducting a quantitative economic government authorities and land-use planners often dis- analysis of landslides is often handicapped by an incom- tinguish between direct and indirect costs. Direct costs plete landslide database (Winter et al. 2018). As stated by comprise of damages directly related to the destruction Winter et  al. (2019), landslides disrupt access to remote resulting from the landslide as well as costs for investiga- rural regions where the economic activities are typically tion, monitoring, and remedial works to lower the risk. transport-dependent. Extensive areal vulnerability can be All other costs are indirect and are generally not avail- attributed to the transport network instead of the event able. However, those that have been estimated in studies itself. For example, a landslide event that occurred at an and reports, their values are included in the total costs. access road in the Blue Mountains of Jamaica completely It is clear from Table  1 that Japan and Italy suffered the cut off the nearest route linking local coffee manufac - greatest economic loss caused by landslides among the turers with the international market (Winter et  al. 2018, developed countries. The economic costs in these moun - 2019). A long-term impact following that event would be tainous regions are attributed to their high population Table 1 Estimated average annual cost of landslide of different countries Country Average Annual Direct Costs Average Annual Total Costs (USD) Comments (USD) Canada – $70 million–1.4 billion Japan $1.5 billion $4 billion Korea $60 million – Based on poor record Italy – $2.6–5 billion Rough Estimate Sweden $10–20 million – Spain $0.2 billion – Former USSR $0.5 billion – New Zealand $26.3 million 90% of costs are sustained in rural areas Belgium $0.85 million $4.48 million Germany $0.3 billion – India $1.3 billion China $0.5 billion – Costs based on valuations in 1989 Nepal $19.6 million – Includes flood damage, but likely incomplete Brazil $45 million $0.35 billion Worldwide $ 20 billion The rest of the data were extracted from Sidle and Ochiai (2006) Cruden et al. (1989), Schuster and Highland (2001) and Sidle and Ochiai (2006) Vranken et al. (2013) Klose et al. (2015) Batista et al. (2019) Sim  et al. Geoenvironmental Disasters (2022) 9:3 Page 3 of 17 densities. Considering the lower property values in devel- climate change which have resulted in an increase in pre- oping countries such as China, India, and the Himalayas, cipitation, variations in the hydrological cycle, extreme the economic costs in these mountainous regions are sig- weather, short-duration heavy downpours (storms), nificantly lower. The majority of the landslides in nations melting of ice in the Alps, and other meteorological such as Nepal, New Zealand, and Canada have their events, and coastal erosion due to sea storms (Lacasse landslide damage occurring in rural areas; and hence the and Nadim 2014). An increasing trend was observed in associated costs are comparably lower. Landslide costs in fatalities and occurrence of fatal landslides worldwide the Scandinavian regions are trivial compared to moun- in numerous studies published (Petley 2012; Aristizábal tainous topography in the rest of the European continent. and Sánchez 2020; Görüm and Fidan 2021). For exam- The cumulative landslide costs in Europe are lower than ple, a rising trend in death toll by landslide disasters was in the United States. reported in China with the increase of average annual From 2004 to 2016, 4862 landslide events have been death toll by about 78% from 1950 to 1999 (Lin and reported with fatalities exceeding 55,000 (Froude and Wang 2018). The numbers of deaths owing to landslides Petley 2018). Studies have also identified a number of reported on the African continent, however, remain low, landslide clusters, most notably in the tropical regions i.e. 13 in 2007 (Kjekstad and Highland 2009) and 43 in such as the Caribbean, Central, and South America, and 2020 (European Commission 2021). This implies a likely parts of South and Southeast Asia (Petley 2012; Froude severe underreporting as the number of people exposed and Petley 2018). Tropical climate countries suffered to landslide hazards there is tremendously high (Kjekstad the greatest loss of lives caused by landslides due to the and Highland 2009). The severe underreporting is mainly destabilizing effects of rising groundwater table and loss caused by the remoteness of the regions and the scarcity of suction under extreme downpours (Turner 2018). For of systematic records (Monsieurs et  al. 2017). As such, example, rainfall-induced landslides in Brazil have killed the actual total losses caused by landslides worldwide an excess of 2700 people in 1966 and 1967 (Jones 1974). could be far exceeding the reported statistics. Additionally, multiple debris flows were driven by several The emergence of landslide hazard and risk issues, days of torrential downpours that claimed 30,000 lives in the obligation to look after the society and properties, Caraballeda, the north coast of Venezuela in December expected climate change which will further aggravate the 1999. Figure  1 classifies the landslide triggering factors landslide situation, and the harsh reality for the public to worldwide. It was further stated by Lacasse and Nadim live with landslide risk have led to requirements for pro- (2014) that heavy rainfall is the main trigger for mud- fessionals to carry out Quantitative Risk Assessments flows, the most lethal and destructive of all types of land - (QRA). Unless risks are reduced, landslide disasters slides. Rainfall-induced landslides were reported to have will continue to rise (Dilley et  al. 2005). The Geotechni - resulted in nearly 90% of deaths worldwide (Haque et al. cal Engineering Office (GEO) of the Civil Engineering 2016; Sultana 2020). and Development Department (CEDD), Hong Kong are It is no surprise that landslide disasters will increase among the pioneers in implementing the risk-informed significantly as time pass due to global warming and approach in landslide assessment. Since the establish- ment of this approach by the GEO, the overall landslide risk related to man-made slopes in Hong Kong has been lowered significantly by 75% (Chiu 2015). Since then, the Slope geometry approach had been adopted by several developed west- change ern countries such as the UK (Winter and Wong 2020), 3% the Netherlands (Hungr et al. 2016), Australia (Leventhal Water level and Withycombe 2009), and Canada (Hungr 2016; Porter change et al. 2017; Strouth and McDougall 2020). Lin and Wang 35% (2018) reported that China has invested heavily in geo- logical disaster control and prevention in recent years, i.e. an increase in allocation from Chinese RMB 330 mil- lion in 2000 to 17.6 billion in 2015. As the result, a lower death toll (by about 10%) by landslides was reported in Rainfall the 2010s. The United Nations Sendai Framework for 58% Loading change Disaster Risk Reduction (2015–2030) has also outlined 5% seven clear targets and four priorities for action to pre- vent new and reduce existing disaster risks (UNISDR Fig. 1 Distribution of landslide triggering factors around the world (Ng 2012) 2015). These efforts showed that landslides have received Sim et al. Geoenvironmental Disasters (2022) 9:3 Page 4 of 17 increasing attention globally owing to their high fre- fatalities (N) plotted against the cumulative frequency (F) quency and severity of occurrences in recent decades. of N on a log–log scale (Dai et al. 2002; Song et al. 2007; In recent decades, QRA has been studied extensively Hungr et  al. 2016). To be made simpler, the F–N Curve and numerous guidelines have been published (Geo- compares the number of fatalities (N) of a landslide event technical Engineering Office 1999; AGS 2000, 2007; with the probability (or frequency) of that event (F). Wong et  al. 2006; Lee and Jones 2014). The flow chart Numerous review studies on quantitative risk assess- for QRA from Hungr (2016) is shown in Fig. 2. The QRA ment of landslides can be found from the currently consists of two stages: Stage 1, Hazard assessment, and available literature. However, review studies on land- Stage 2, Risk assessment. Hazard assessment comprises slide acceptable risk and tolerable risk as well as F–N geo-scientific works such as investigation and analysis to Curves are scarce despite substantial works having been determine and quantitatively define the probable land - reported by researchers from different parts of the world. slide hazards. Hazard is defined as the potential occur - This paper aims to review the F–N curves reported from rence of a natural or human-induced physical event that various countries. The F–N curves collected deal exclu- may cause loss of life, injury, or other health impacts, as sively with risks to life which is normally the governing well as damage and loss to property, infrastructure, live- factor of landslide disasters (Hungr et al. 2016). Landslide lihoods, service provision, and environmental resources risk acceptance criteria from various nations will also be (Bobrowsky and Couture 2014). A hazard should be discussed and reviewed. The findings reported herein can identified and characterized in the first place before its serve as a useful reference for developing the landslide features i.e. magnitude, probability, regional extent, and acceptable risk and tolerable risk for a nation. intensity can be determined. The estimated intensity and its probability of occurrence can then be quantified and F–N curve for landslide mapped into the region of study. Introduction to F–N curve Risk assessment starts with determination of elements An F–N curve consists of a series of data points that at risk which could be humans, infrastructures, buildings denote each scenario analyzed. It is a combination of or environmental values. They could be existing ones or scenarios into a single curve that defines the probabil - those in planning for future construction in the area of ity (or frequency) of “N or more” fatalities of the com- study. Estimation of risk comes from an intersection of plete dataset (Strouth and McDougall 2021). It should hazard intensity map with the map of elements at risk, be noted that the label (N) can also be represented by in space and time considering vulnerabilities. The Stage other quantities of a consequence such as monetary 2 is completed with the determination of acceptable risk loss. Such curves could be utilized to demonstrate soci- and designation of mitigation measures. However, the etal risk (Vrijling and van Gelder 1997; Saw et  al. 2009; determination of acceptable risks proves to be one of the Strouth and McDougall 2021) and to define the safety most challenging aspects of QRA. To mitigate this chal- levels of the region of interest. It is crucial to be clear that lenge, it is recommended to use the F–N curve for the the F–N curves only provide statistical readings and not region of interest which is a plot showing the number of the threshold of risk acceptance and tolerance. Figure  3 Fig. 2 Flow chart of landslide risk assessment (Modified from Hungr 2016) Sim  et al. Geoenvironmental Disasters (2022) 9:3 Page 5 of 17 Fig. 3 F–N curves demonstrating the number of fatalities against the annual frequency of occurrence for natural and man-made hazards (Modified from Van Westen et al. 2011) shows the typical F–N-curves for various hazards. It intensity scenarios (Guzzetti et  al. 2002). As stated by can be seen that man-made hazards have a tendency to Song et al. (2007), many organizations still do not possess demonstrate a steeper curve than natural hazards (Van sufficient database to progress very far in this direction Westen et al. 2011). with few exceptions such as Hong Kong. Nevertheless, the Japanese and Chinese F–N curves stand higher than Landslide F–N curves for various countries the curves computed for other countries, even for the Figure 4 presents the family of F–N curves for landslides shortest time intervals which denotes that the afore- compiled from various geographical locations. It should mentioned nations faced a high landslide risk, with a be clarified that some of the F–N curves do not repre- great number of cataclysmic events that caused severe sent an entire country because of the unavailability of fatalities. data. There are mixtures of F–N curves extracted from It can be seen that the F–N curves of both Hong Kong different countries and local municipalities. It is appar - and Canada share a very similar slope and trend and they ent that the F–N curve is unique to a country/location. converge at N = 20. From there, the trend becomes differ - From Fig. 4, the individual risk (1 death) for Guatemala is ent with Canada showing higher probabilities of casual- significantly lower than those of other nations. However, ties until they converge again at N = 70. Interestingly, the this highlights a bias in the Guatemala catalogue, which maximum N value of Hong Kong is lower than 100 even lacks comparable coverage. Another thing to note about though it has a high population density living in steep Guatemala is that the F–N curve only covers probabili- hillsides. According to Duzgun and Lacasse (2005), the ties of single deaths and no more than that which reflects Hong Kong curve represents the performance of engi- on the severe lack of data. It can be seen that the F–N neered slopes (risk imposed upon the community by curve for China only begins for landslides that result in designed slopes). At N = 10, the city of Rio de Janeiro has a minimum of 100 deaths while the Japanese F–N curve almost the same F-value as China which is not surpris- only starts from 10 which signifies the incompleteness of ing considering both Rio and China are heavily populated the landslide catalogue of the former countries for low region and country, respectively. Canada, Nilgiri Hills, Sim et al. Geoenvironmental Disasters (2022) 9:3 Page 6 of 17 1.00E+01 Hong Kong Norway 1.00E+00 Japan China 1.00E-01 Nepal 1.00E-02 Canada Italy 1.00E-03 Colombia Alps 1.00E-04 Portugal 1.00E-05 Nilgiri Hills, India Taiwan 1.00E-06 A83 Rest and Be Thankful, Scotland Rio de Janeiro 1.00E-07 Coal Cliff, Australia 1.00E-08 Matata, New Zealand 110 100 1000 10000 Guatemala N, Number of Deaths Fig. 4 Global comparison of frequency (F) versus number of deaths (N) curves (Guzzetti et al. 2002; Pacheco et al. 2002; Duzgun and Lacasse 2005; Hsi and Fell 2005; Jaiswal et al. 2011; Tsao et al. 2011; Whakatane District Council 2015; Pereira et al. 2017; Laporte 2018; Winter and Wong 2020) India, and Hong Kong were observed to share approxi- respectively which show that a higher risk level is associ- mately similar curves. The curves for Nepal and Rio de ated with the slopes in Japan than in Italy. Comparisons Janeiro both converged at N = 141. However, for events of F–N curves between developed and developing worlds associated with lower casualties (i.e. N < 100), the prob- and between different continents are shown in Fig.  5. It ability of landslide fatality in Rio de Janeiro is signifi - should be made clear that the curves of more discrete cantly higher than that in Nepal. This is clearly related to regions such as Nilgiri Hills, India and Matata, New Zea- the much higher population density in the city of Rio de land were included in Fig.  5 as well. Overall, despite the Janeiro than Nepal as a country. curves being inconsistent, a general observation indi- The population density as well as the hazard spatial cated that the F–N curves of developing countries are distribution should be included to compare the F–N mostly plotted higher than those of developed countries. curves of various nations (Duzgun and Lacasse 2005). In terms of continents, the Asian continent generally has Population density, F-values, and slope of the best-fit a higher frequency of deaths followed by Latin America. curve are tabulated in Table  2. The probability of occur - This is because countries in Asia such as Japan are situ - rence, F for a specific N is governed by the slope of the ated in a geographical location where natural disasters curve. A steeper slope signifies a higher risk aversion are frequent. Furthermore, countries like China and or in other words, a lower probability of occurrence for Nepal have many of their population living on hazardous a specific number of fatality or lower risk. For example, slopes which have further increased the risk of landslide the slope of Japan’s curve is − 0.53 while Italy’s curve has fatalities. a slope of − 1.99. The probabilities of occurrence, F for It was stated by Holcombe et  al. (2016) that N = 100 fatalities for Japan and Italy are 0.07 and 0.00005, rapid unplanned development is causing a rise in F, Annual Probability of Occurence Sim  et al. Geoenvironmental Disasters (2022) 9:3 Page 7 of 17 Table 2 Population density ( World Bank 2018), F-values, and slope of F–N curves for various geographical locations Locations Population density/ Slope of F–N curve F-values km N = 1 N = 10 N = 100 Japan 347 − 0.53 – 0.8 0.07 China 148 − 0.61 – – 0.06 Rio de Janeiro, Brazil – − 3.3 1 0.52 0.0011 Italy 203 − 1.99 0.9 0.008 0.00005 Portugal 112 − 2.35 0.63 0.04 – Hong Kong 7096 − 0.79 0.6 0.01 – Canada 4 − 0.92 0.4 0.07 – Alps 215 − 0.538 0.29 0.16 0.003 Nilgiri Hills, India – − 0.275 0.21 0.05 0.006 Nepal 196 − 1.31 0.08 0.004 – Colombia 45 − 1.63 0.04 0.001 – Norway 15 − 0.72 0.006 0.001 – Matata, New Zealand 19 − 0.73 0.0057 – – A83 Rest and be Thankful site, – − 1.924 0.002 0.00005 – Scotland Taiwan 637 − 0.89 0.00128 0.000157 – Coal Cliff Australia – − 2.375 0.00125 0.0000206 – Guatemala 153 – 0.000134 – – (b) (a) Asia North America Developing worldDeveloped world Europe Latin America Oceania 1.00E+01 1.00E+01 1.00E+00 1.00E+00 1.00E-01 1.00E-01 1.00E-02 1.00E-02 1.00E-03 1.00E-03 1.00E-04 1.00E-04 1.00E-05 1.00E-05 1.00E-06 1.00E-06 1.00E-07 1.00E-07 1.00E-08 1.00E-08 1 100 10000 1100 10000 N, Number of Deaths N, Number of Deaths Fig. 5 F–N curves grouped based on a developed and developing worlds, b geographical continents precipitation-triggered landslide risk in low-income lacking infrastructure almost reaches the 1 billion areas in tropical developing nations. The number of mark exposing them to the dangers of both small and people inhabiting overcrowded impoverished houses large scales catastrophes. In Latin America, Gross F, Annual Probability of Occurence F, Annual Probability of Occurence Sim et al. Geoenvironmental Disasters (2022) 9:3 Page 8 of 17 National Income (GNI) and value of life are generally Concept of acceptable risk and tolerable risk low (Daniell et  al. 2015; World Bank 2020) with many It is crucial to differentiate between acceptable risks of their populations living in slums constructed on which the public aims to determine, mainly for new hazardous slopes with no compliance to safety stand- developments, and tolerable risks which they will live ards. The majority of these slums are constructed on with, albeit lower risks still prevail as their main prefer- lands on which significant landslide hazards persist, ence (AGS 2007). This applies to both property and loss i.e. unstable slopes (see Fig.  6) without any design of life. People in a society whose lives might be impacted or assessment for the factor of safety. This had fur- by landslides, and authorities in charge of development ther propagated the landslide hazard due to localised approval must determine the acceptable and tolerable changes in drainage, vegetation, loadings, and topog- risks for property loss and damage (Song et al. 2007; Lev- raphy, and hence increased the associated risk to the enthal and Withycombe 2009). Practically, the authorities population. Disasters of these types serve as a warning will be the ones obligated to ascertain the risk levels given sign of a bigger issue stemming from poverty as well as its obligation to control hazards at the local municipal a poorly-organized development (Hungr et  al. 2016). level. In most cases, that is the National or State author- Unfortunately, there are no known F–N curves avail- ity or Local Government Area Council. Following are the able for countries in the African continent. brief definitions of acceptable risk and tolerable risk: It is crucial to note that the F–N curves only pro- vide statistical readings and not the threshold of risk I. Acceptable Risk—A risk that the public is inclined acceptance and tolerance. Nevertheless, the F–N to accept without regard to its management for life curves are useful for evaluating the current risk level and work purposes. Further expenditure in lower- of a region/country. This information is useful for ing such risks is normally not taken under consid- developing the risk acceptance and tolerance levels for eration by the public. a country. As can be seen from Figs. 4 and 5, the land- II. Tolerable Risk—A risk that the general public is slide risk level varies considerably from a country to inclined to live with in order to safeguard certain another, and hence different standards of acceptance net benefits having faith that the particular risk is and tolerance should be adopted with considerations being properly contained. The risk level is periodi - of various factors (Duzgun and Lacasse 2005), which cally reviewed and lowered further whenever feasi- will be reviewed in further details in the following ble. Also, the criteria where the ALARP principle, section. which stands for “As Low As Reasonably Practica- ble”, may be applied so that the risk is mitigated to a marginally and practically tolerable level since the Fig. 6 a Archetypal unplanned housing on unstable slopes in Castries, Saint Lucia, Eastern Caribbean; b landslide on unplanned development (Holcombe et al. 2016) Sim  et al. Geoenvironmental Disasters (2022) 9:3 Page 9 of 17 1.E+00 1.E-01 1.E-02 Slope, α = 1 1.E-03 1.E-04 Unacceptable 1.E-05 Tolerable (ALARP) 1.E-06 1.E-07 Detailed 1.E-08 study Acceptable required 1.E-09 Fig. 7 Illustration of “As Low As Reasonably Practicable” ALARP principle for risk evaluation (Campbell et al. 2016) 1.E-10 110 100 1000 10000 Number of Fatalities Fig. 8 Risk criterion proposed for natural slopes in Hong Kong. reduction to further acceptable levels is not feasible Modified from Geotechnical Engineering Office (1999), Lacasse et al. (2010), Lacasse (2016) and Strouth and McDougall (2020) with regards to the cost to the individual or public. Figure  7 shows the significance of ALARP (Camp - bell et al. 2016). the risk criteria is such that the frequency of events that According to Winter and Bromhead (2012), society will cause 100 deaths or greater must be 100 times lower than be more tolerant towards landslides that occur for natu- the frequency of events that cause 10 or more deaths ral slopes than for engineered slopes, although the effects (Roy and Kshirsagar 2020). The tolerable/ALARP region of climate change may cause recurring natural slope fail- reflects the risk to be "As Low As Reasonably Practicable" ures in a small area defeating this greater tolerance. Tol- which is illustrated in Fig. 8 (Campbell et al. 2016). erable risks differ between nations subjected to historic A study by Winter and Bromhead (2012) summarized exposure to landslides, control and ownership of slopes, issues that govern landslide risk acceptance such as plan- and natural landslide hazards. Relevant regulators may ning and regulations, event footprint against vulnerabil- choose to apply “acceptable risk” criteria for high-risk ity shadow, budgetary issue, vulnerability of vehicles all cases, i.e. hospitals, schools, as well as emergency ser- fit into the influences of social and economic factors on vices taking into account their significance, and also as a the risk acceptance. Figure  9 shows a conceptual ternary means of mitigating societal risk concerns. For involun- willingness diagram of various landslide incidents in dif- tary individual risk, generally, the acceptable risk values ferent countries. The approaches of various nations in −5 −6 will be in the frequency range of 10 to 10 per year handling landslides are qualitatively compared to provide −3 while for voluntary risk, it will be typically between 10 a better understanding of the factors involved in land- −4 to 10 per annum (Fell 1994). slide risk mitigation. Figure  8 shows a provisional risk criterion proposed It is clear that the risk level that exists in a particular for natural slopes in Hong Kong (Geotechnical Engi- area and the willingness to pay or financial power will, neering Office 1999; Lacasse et  al. 2010; Lacasse 2016). have a significant impact on the risk acceptance. Never - The degree of aversion is represented by the slope of the theless, risk acceptance and financial power/willingness F–N lines. Lines with a slope gradient of 1 are indicated to pay are factors that are considered as forcing agents as equi-risk, in which the points along the line have the in the discourse on worldwide landslide risk mitiga- same risk level. The F–N curves may be conveyed by the tion. As stated by Turner (2018), industrialized nations equation: have procedures for landslide mitigation planning and implementation that call for accurate knowledge of land- F · N = k(1) (1) slide hazards and risks. The landslide risk assessments are gradually becoming quantitative. Mitigation meas- where k-value is 0.001, α equals unity (2). The greater ures include structural and geotechnical procedures, the F–N slope, the higher the risk aversion (Anand 2015; in addition to political, legal, as well as administrative Roy and Kshirsagar 2020). For example, for a slope of 2, Annual Probability Sim et al. Geoenvironmental Disasters (2022) 9:3 Page 10 of 17 and collaboration from developed nations are needed (Kjekstad and Highland 2009; Turner 2018). From the foregoing, it can be concluded that if accepta- ble and tolerable societal risk levels are to be established, each country should select different levels governed by factors such as socioeconomic standing, Gross Domestic Product (GDP), statistical value of lives, as well as finan - cial prowess of the country (Leroi et  al. 2005; Roy and Kshirsagar 2020). A review of risk tolerance criteria for various countries Hong Kong In the late 90s, the Hong Kong Government provisional criterion for “potentially hazardous installations” for “landslides and boulder falls from natural terrain” was Fig. 9 Willingness diagram by Winter and Bromhead (2012) implemented by the Hong Kong Geotechnical Engineer- demonstrating various approaches to landslide risk in different ing Office (GEO) (Geotechnical Engineering Office 1999) countries (see Fig. 2). According to Wong (2005), the criterion was occasionally adopted in the country to determine land- slide risk tolerance related to developments of structures procedures that impact the lifestyle and behaviour of the and infrastructures, etc. in near proximity to modified or endangered society. For example, Zentoku, Japan leans natural slopes. Hungr and Wong (2007) described that in to the bottom left region of the Willingness diagram in order to scale the F–N chart in proportion to its original Fig.  9 signifying a very high willingness to pay and alter reference frame (the perimeter of the industrial plant), the environment to mitigate landslide risk in the region. it is used on a consultation area of around 500  m long This implies that Japan has a tremendously low tolerance section of a boundary between hazardous slopes and of landslide risk as well as the financial ability to pay to buildings. mitigate the risk. The saga of the founding of the Hong Kong landslide In contrast, Jamaica’s landslide mitigation approach in risk management system was explained by Malone the Blue Mountains puts them at the top of the triangle, (2012). It was decided by the Hong Kong GEO that the showing that there is a very high willingness to accept methodology of the United Kingdom Health and Safety risk and a very low willingness to pay. Jamaica is a coun- Executive (HSE) to create the risk tolerance criterion try with limited economic resources and that limited with a few adjustments could help in decision making budget forces them to accept the risk (Winter and Brom- procedures in Hong Kong in regards to landslides. The head 2012). Similar scenarios were reported by Yifru et al. Hong Kong GEO conducted QRA on the basis of avail- (2015), where there was no mitigation carried out on the able records, which restricts the timeline to around 50 landslides that occurred along the road corridors of two to 100 years. This also means that high magnitude infre - Caribbean islands, i.e. Saint Lucia and Dominica (both quent landslides do not appear in the inventory and, the known to be deprived islands). Focuses given to the two probabilities and levels of the infrequent landslides were Caribbean islands were only for road repairs and debris then determined through extrapolation. This limited removal. This observation further supported the will - period over which landslide inventories exist, and the ingness diagram of Winter and Bromhead (2012) which associated lack of data on high magnitude/low frequency demonstrates that a limited economic resource leads to a landslides is a challenge with which all regions of the low willingness to pay, and hence leads to a high accept- world must grapple. ance of landslide risk. Most developing nations do not Other than the F–N diagram for societal risk, it is possess adequate resources, capacity, and technical skills also recommended by GEO that individual risk toler- to carry out landslide risk reduction measures to the ance criteria of 1 in 10,000 be applied for current resi- required speed and level. Rather than lowering vulner- dential areas and 1 in 100,000 for new developments. ability, their central emphasis on handling landslides has The ALARP method was employed quantitatively to been to respond actively to emergencies and catastrophes Hong Kong scenarios to establish the feasibility of land- which again results in higher risk acceptance. Support slide remedial works, i.e. the Pat Heung landslide, North Sim  et al. Geoenvironmental Disasters (2022) 9:3 Page 11 of 17 Lantau Expressway, Lei Yue Mun, and Shanti Heights, etc Table 3 Proposed tolerable risks by AGS (2000) as seen in Lacasse (2016) (Wong 2005). Slope types Tolerable risk for loss of life −4 China Engineered slopes 10 /year for person most at risk −5 Following a landslide that results in 79 casualties at 10 /year for average person at risk −5 Wulong, China on labour day of 2001, new legislation on New engineered slopes 10 /year for person most at risk −6 landslide hazard and risk assessments were incited. How- 10 /year for average person at risk ever, no landslide individual and societal risk criteria have been recognized by the Chinese government (Song et al. 2007) though numerous criteria have been developed It is then further stated by AGS (2007) that no risk tol- and employed for other projects such as dams. Neverthe- erance criteria of Australia are legally binding lest the less, the verdict on acceptable and tolerable risks lies in owner or regulator acknowledge it. Nevertheless, the the decision of the client, owner, regulator, and persons landslide tolerance criteria developed in Hong Kong for at risk and not the risk analysts (Fell 1994). Societal and Individual risk are endorsed in documented The Chinese landslide societal risk criteria proposed studies regarding landslide risk management practices in by Song et  al. (2007) is comparable to the Hong Kong Australia (Fell et al. 2005). acceptable and tolerable landslide risk criterion found in Fell (1994), Geotechnical Engineering Office (1999), Canada and Dai et  al. (2002). The proposed landslide risk crite - A review by Evans et  al. (2005) concluded that Canada ria are about 3 times higher than the projected risk cri- has adequate comprehension of landslide hazards that terion for manmade structures such as buildings. More it could deliver a regional quantitative risk assessment recently, landslide risk assessment has been implemented (QRA) as well as the design of tough risk-reduction pro- extensively in China, with real-life landslide case studies cedures for majority areas that are heavily predisposed to employed in various regions i.e. Western Hubei (Fu et al. landslides. North Vancouver was the first municipality in 2020), eastern Jiangxi (Zhang et  al. 2020), and Mayang Canada to employ the Hong Kong individual risk accept- county (Sui et  al. 2020). It is further stated by Wu et  al. ance criteria after the landslide in North Vancouver that (2020) that the Chinese government invented a system occurred in 2005 due to heavy downpour which resulted called ‘Public Participation Monitoring and Warning’ in the destruction of two homes, one seriously injured (PPMW) which aims to bring down mortality rate with resident, and one fatality (Hungr et al. 2016; Porter et al. the least cost by grouping residents to evacuate ahead of 2017). Restrictions were enforced by the legislation on a disaster. It works by captivating the public in the disas- the development of residential areas, as well as redevel- ter risk management operation. opment and rezoning in regions where the Personal Indi- vidual Risk (PIR) exceeds 1 to 10,000 for existing houses Australia or 1 to 100,000 for new development. Extensive public In 1997, 18 deaths occurred as a result of the Thredbo engagement is of paramount importance in the legislative landslide in Australia. This resulted in the development operation (District of North Vancouver 2009; Tappenden of a framework for risk-based landslide management by 2014). the Australian Geomechanics Society (AGS) (AGS 2007). During an extreme rainstorm between June 19 and 21 It is stated in the framework that “risk evaluation is to be 2013 that brought about flood, debris flow, and debris conducted by comparing estimated risks to levels of tol- flood in Canmore, Alberta, numerous buildings and erable or acceptable risk, in order to assess priorities and infrastructure suffered severe damage (Town of Canmore options” and the risk tolerance criteria are to be deter- 2015). As a result, the municipality engaged consultants mined by the “client/owner/regulator with advice from a conducted thorough quantitative risk assessments (QRA) technical specialist”. on debris flow and debris flood risk on the Cougar Creek For individual risk, it is recommended by AGS (2000) fan, on which a part of Canmore is built. Return periods that the tolerable individual risk criteria employed for between 30 and 3000  years were adopted for the hazard Potentially Hazardous Industries, Australian National circumstances (Hungr et al. 2016). The main focus of the Committee on Large Dams (ANCOLD), as tabulated in QRA was on direct building damage, grievances, and Table 3, could be reasonably applied to engineered slopes casualties by utilizing the Hong Kong risk tolerance crite- as well (Lacasse 2016). Moreover, it is also being pro- rion. A total of 190 areas in Canmore were found to have posed that acceptable risks may be taken as one order of unacceptable individual and societal risks. It was not magnitude lower than tolerable risks. feasible to relocate the public residing there, hence four Sim et al. Geoenvironmental Disasters (2022) 9:3 Page 12 of 17 proposed options for mitigation were studied extensively, mapping method to distinguish unstable rock slopes dis- resulting in the construction of a debris flood contain - posed to calamitous failures, so that future events could ment structure (approximately height 30  m and width be predicted and the population at risk could become 100  m storage capacity 650,000 m ) to reduce the unac- accustomed to the hazard. The vulnerable regions are ceptable risk as much as possible to the ALARP/tolerable continuously monitored and an early warning system zone (Town of Canmore 2015). Problems regarding prac- together with an evacuation plan is at hand (Hermanns ticability, impartiality as well as affordability were taken et  al. 2013). In 2015, procedures were established by the into account. Since then, QRA has also been conducted Direktoratet for Byggequalitet to specify development by other local and provincial government regulators in restrictions in regions vulnerable to landslides or land- British Columbia and Alberta (Porter et al. 2017), which slide-generated waves (Clague et  al. 2015). The restric - is a crucial move to obtain supports of stakeholders. tions take into account single and multi-family structures Since 2005, following a certain amount of public con- of fewer than 10 households, and construction is only sultation, it is widely recognized that Canadian’s insights allowed if the probability of landslide impact falls below and values in regards to landslide risk are similar to those the 1000-year return period event. Facilities and tall of Hong Kong (Strouth and McDougall 2020). While buildings housing numerous occupants i.e. schools, hos- there is no universal acceptance, F–N curves have influ - pitals must be situated in zones with a probability of land enced land-use decision making processes exceeding 50 sliding impact under the 5000-year return period event. landslides and steep-creek flood hazard regions. Since There are exceptions such as when “the consequences 2020, the tolerable and acceptable risk criteria have been of building restrictions are serious, and the construc- heavily referenced with discussions of integrating them tion has a significant impact on the society.” In addition, into municipal and provincial guidelines, regulations, as exceptions are also allowed for scenarios where a warn- well as design codes (Strouth and McDougall 2020). ing could be given out three days prior to the incident. As of the present, 523 unstable rock slopes were detected, of which 110 were categorized as hazardous and risky. Out New Zealand of the 110 hazardous and risky slopes, hazard zones were In New Zealand, local governments were delegated with found in 48 slopes resulting in impediments on building risk tolerance criteria by the New Zealand Resource projects. 7 sites were labelled as high risk and they are Management Act. According to Enright (2015), the Hong under constant monitoring (Hermanns et al. 2020). Kong risk tolerance criteria were employed by the City of Christchurch following the 2010 and 2011 earthquakes. GNS science proposed a tolerable risk threshold for nat- −4 Switzerland ural hazards of 1 in 10,000 (10 ) which was deemed to Bründl et  al. (2009) briefly explained that the Swiss be too high and not acceptable as it doubles the number National Platform for Natural Hazards (PLANAT) rec- of “annual” casualties of the 2011 Christchurch earth- ommended an individual landslide risk “goal” of 1 in quake. The study proposed that the annual tolerable risks −5 −6 100,000 per year for residential areas in the year 2005. threshold for existing risks to be 10 and 10 for new However, the government of Switzerland apparently has risks. However, the application of these criteria is still not approved the usage of risk tolerance criteria for natu- undergoing tests in the local courts and there is a pos- ral disasters. The present practice in avalanche warning sibility that they will be used by other local authorities. demonstrates that educational courses are one of the As of present, there are no restrictions placed on land use crucial components in introducing new ways and skills development for debris flow deposits adjoining the bor - to natural hazard experts. It is one of the means of com- ders of active volcanoes which could only be a century municating risk and there is a high possibility that edu- old. cational courses will serve as the main part of integral risk management in Switzerland (Bründl et al. 2009). Fur- Norway thermore, frequent training and educational courses will Throughout history, the Norwegian Fjords have been result in an increased public awareness towards potential impacted by large rock slope failures triggering tsuna- consequences, as well as improvement to the compliance mis that cause many deaths (Hermanns et  al. 2013). A rate of the warning systems. Although showing promis- trio of the most epic of natural disasters transpired in ing results during the Preonzo rockfall event (zero injury the twentieth century where tsunamis caused by mas- and everybody evacuated successfully), further stud- sive rockslides into fjords or lakes (Loen in 1905 and ies are still required to evaluate the effectiveness of the 1936 and Tafjord in 1934), claiming an excess of 170 lives system on the basis of an integrated risk management (Eidsvig et  al. 2011). Over the past few years, the Geo- approach (Sättele et al. 2016a, b, c). logical Survey of Norway has employed a systematic Sim  et al. Geoenvironmental Disasters (2022) 9:3 Page 13 of 17 Malaysia It was stated by the Swiss Ministry of the Environ- Based on data obtained from the Global Landslide Cat- ment that “while industrial risks can be governed by alog (GLC) of the United States National Aeronauti- legislations using risk tolerance criteria, the same does cal Space Administration (NASA), Malaysia is the 10th not apply to natural hazard risks”. This is an arguable highest ranked country for the highest number of slope statement that refuted by Hungr et al. (2016). The risk failures, with 171 major slope failures between 2007 and tolerance criteria have been well applied to natural March 2016 (Abd Majid and Rainis 2019). The majority hazards in different parts of the world such as Hong of the landslides occurred on cut slopes or embankments Kong (Wong 2005; Wong and Ko 2005; Chiu 2015), the alongside roads and highways in mountainous regions UK (Winter and Wong 2020), the Netherlands (Hungr with a few occurring close to high-rise apartments and et  al. 2016), Australia (Leventhal and Withycombe residential areas, resulting in numerous causalities. 2009), Italy (Rossi et  al. 2019) and Canada (Town of Over the years, there have been numerous studies con- Canmore 2015; Strouth and McDougall 2020). ducted on Malaysian landslide hazards using various A documented discourse by PLANAT (National Plat- methods such as GIS (Mukhlisin et al. 2010; Althuwaynee form for Natural Hazards) (2014) regarding tolerable and Pradhan 2017; Abd Majid and Rainis 2019), statisti- landslide risk criteria stated that “The average risk of cal logistic regression method (Lee and Pradhan 2007; death for human beings is not significantly augmented Pradhan and Lee 2010) and deterministic safety factor by natural hazards. The yearly risk of death resulting method (Ng 2012; Ismail and Yaacob 2018). However, from natural hazards is significantly lower than the research on acceptable and tolerable risk is still scarce in average probability of death for the age group with the Malaysia, with a few researchers (Ahmad et al. 2017; Ros- lowest mortality rate in Switzerland.” It can be con- lee 2019) having a more thorough understanding of it. It cluded that their local regulators would prefer to use was proposed by Ahmad et al. (2017) that the interim risk the landslide hazard probability assessments described criteria for Malaysia should be higher (higher acceptance in Lateltin et  al. (2005). According to Lateltin et  al. of risk, one fatal landslide in every 50  years) than Hong (2005), it was made mandatory by the new Federal Kong (one fatal landslide every 1000  years) while Ros- Ordinances on Flood and Forest Protection for the lee (2019) proposed a risk tolerance criterion similar to cantons to develop hazard maps for incorporation into that found in Hong Kong (Song et al. 2007; Lacasse et al. the regional and local development plans. This pro- 2010). It can be concluded that QRA and landslide risk gram was subsidized by the federal authorities up to assessments are still a developing art in Malaysia and 70%. The three steps of constructing the hazard maps much surveys and studies are still required. were outlined by Raetzo et al. (2002) as follows: (i) Hazard identification step which comprises making an inventory of past slope failures. (ii) Hazard assessment of the magnitude or intensity Comparison of risk tolerance criteria of landslides with time. Hazards are mapped into Figure  10 presents comparisons between societal risk one of four hazard classes based on the probability criteria adopted by different countries. While there are of the land sliding hazard: high danger (probability: variations, the risk levels of Denmark, New South Wales, 82–100%; red zone), moderate danger (probabil- China, and ANCOLD/AGS generally converge at the −4 ity: 40–82%; blue zone), low danger (probability: anchor point of F = 1 × 10 /year (1 in 10,000) for N = 10 −2 15–40%; yellow zone) and no danger (probabil- casualties. Most countries adopt F between 1 × 10 /year −3 ity: < 15%; white zone); to 1 × 10 /year for N = 1 of individual risk. The Neth - (iii) Risk management and land-use planning erlands, the Czech Republic, and European Commis- sion (EC) have the most risk-averse slope; the steepest It was made mandatory by the federal government that compared to those of other nations and entities. The risk the maps must follow a standard colour coding: red criterion of Malaysia proposed by Ahmad et  al. (2017) where construction is prohibited, blue where construc- is liberal and has the highest risk tolerance (N = 1000 −4 tion is permitted provided certain safety requirements occurring at a frequency of 1 × 10 /year). The Chinese are fulfilled, and yellow where construction can be risk criterion proposed by Song et al. (2007) is exactly the carried out without any restriction. Hazard maps are same as the one of the Australian National Committee on deemed invaluable for planning protective measures Large Dams (ANCOLD)/Australian Geomechanics Soci- such as warning systems and emergency plans (Raetzo ety (AGS). It can be seen that the French criterion is not et al. 2002; Lateltin et al. 2005). governed by the number of fatalities, N. This will lead to a lower level of safety at high N values and at the same time result in extremely stringent and uneconomical safety Sim et al. Geoenvironmental Disasters (2022) 9:3 Page 14 of 17 employ risk criteria similar to that of developed countries Hong Kong Denmark (China’s being exact same criterion as that of Australia’s, Belgium ANCOLD/AGS −3 while Sri Lanka uses the same slope and F = 1 × 10 for New South Wales United Kingdom N = 1 as that of Hong Kong’s). It can be assumed that risk Netherlands European Commision Taiwan China assessment is still new in many developing countries. Malaysia Western Canada Many of them employ the criteria of developed countries Sri Lanka Czech rather than making their own. As stated by Duzgun and France Lacasse (2005), every nation should develop its own risk 1.E+00 criterion as they all possess different F–N curves. More 1.E-01 works should be devoted to developing risk tolerance 1.E-02 criteria particularly for less developed and mountainous 1.E-03 countries. 1.E-04 1.E-05 Conclusion 1.E-06 It is apparent that every nation has its own priorities and approaches towards landslide risk management. While 1.E-07 it is interesting to compare the broad range of policies 1.E-08 and discussions among various nations worldwide, this 1.E-09 review by far does not feature coverage on the countries 1.E-10 of the African continent. There is practically no or very 110 100 1000 10000 limited data available on the QRA of landslides in the Number of Fatalities African continent. It is also clear from the comparison Fig. 10 Risk criteria of different nations ( Trbojevic 2005; Song et al. of F–N curves that the Asian world has the highest fre- 2007; Tsao et al. 2011; Lacasse 2016; Ahmad et al. 2017; De Silva et al. 2017; Strouth and McDougall 2020) quency of landslide deaths followed by Latin America. Generally, the developed countries impose stringent risk “acceptance” criteria for death, ranging from 1 in procedures at low N values. Remarkably, lowland nations 10,000 to 1 in 100,000 annual individual risks of loss of such as Belgium and the Netherlands employ stricter, life. Although it is easy to state that the landslide risk more risk-averse criteria than mountainous nations such criterion should have a minimum level of tolerance as Hong Kong and the UK as seen in the steeper slope −3 for landslides, i.e. F = 1 × 10 for N = 1, the accept- and a lower F value for N = 1. As events and fatalities ance of tolerance for landslides in every country will be are rare in any case in lowland nations, it is relatively governed by its financial standing. While a developing straightforward to establish and work successfully with nation might face similar matters in risk governance and stricter risk-averse criteria. decision-making as the developed nations, the trade- As seen in Fig.  10, the risk tolerance criteria for most off between economy and safety might differ (Roy and countries fall below the Hong Kong criteria with the Kshirsagar 2020). Developments that occur in developing exception of Malaysia and the UK. The risk criteria of nations will amass bigger benefits than their developed Denmark and New South Wales (NSW) are higher than counterpart. For example, the construction of railways, Hong Kong’s at lower fatalities until they converge at roads may contribute to job opportunities, easier access N = 10. At higher N levels, the Fs of NSW and Denmark to certain areas, and goods transportation which will fall below Hong Kong’s as evident in its steeper slope, help to increase turnover and create wealth to boost which signifies a more stringent risk aversion and lower the economy (European Maritime Safety Agency 2015). tolerance of risk for higher fatalities (N > 10). The risk cri - However, against these potential advantages, the devel- teria of many countries lie below Hong Kong’s suggesting opment of roads may inflict higher risk to the users i.e. a more stringent approach should be employed when it landslides that occur along roads in Scotland, Jamaica, comes to risk assessments. However, the Hong Kong cri- and Colorado as well as many others mentioned by Win- terion is still widely adopted and referred to in other soci- ter and Bromhead (2012). Nevertheless, under an agreed eties (Fell et al. 2005; Strouth and McDougall 2020). level of risk, there are potentially higher net benefits The criterion proposed by Malaysia, a developing coun - from the developments of a developing nation than for try, is above the criteria of other nations. It has the high- a developed one. Thus, should a variety of risk accept - est risk tolerance compared to other countries which is ance and tolerance criteria be employed in the develop- unduly liberal and calls for improvements to be made. ing nations in regards to their developed counterparts? Other developing countries such as China and Sri Lanka An unduly liberal risk acceptance criteria will bring about Annual Probability Sim  et al. Geoenvironmental Disasters (2022) 9:3 Page 15 of 17 References more development but at the expense of higher societal Abd Majid N, Rainis R (2019) Aplikasi Sistem Maklumat Geografi (GIS) dan risk. 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Journal

Geoenvironmental DisastersSpringer Journals

Published: Jan 25, 2022

Keywords: Landslide; Tolerable risk; Acceptable risk; Societal risk; Frequency-number of fatalities diagram

References