Methodology of disaster risk assessment for debris flows in a river basin

Transcription

1 Stoch Environ Res Risk Assess (2015) 29: DOI /s ORIGINAL PAPER Methodology of disaster risk assessment for debris flows in a river basin Ing-Jia Chiou Ching-Ho Chen Wei-Lin Liu Shiao-Mei Huang Yu-Min Chang Published online: 29 July 2014 Ó Springer-Verlag Berlin Heidelberg 2014 Abstract risk assessment is important in the planning of risk management strategies that reduce societal losses. However, governmental agencies in Taiwan generally assess risks that emerge from debris flows without adequately considering risk management and taking a systems approach. This work proposes an approach to thoroughly consider the interactive influence mechanism of debris flow disaster risk. Additionally, a systematic method for assessing disaster risks is developed. This proposed method can be used in the current risk assessment and as a basis for management strategy planning. Based on systems thinking, the components and attributes of a conceptual system of disaster risk management associated with debris flows in a river basin are identified. Subsequently, a conceptual mitigation hazard exposure resistance framework and an indicator system for assessing the debris flow disaster risks in a river basin are identified. The disaster I.-J. Chiou Graduate School of Materials Applied Technology, Taoyuan Innovation Institute of Technology, Jhongli City, Taoyuan County 320, Taiwan, ROC I.-J. Chiou W.-L. Liu Department of Environmental Technology and Management, Taoyuan Innovation Institute of Technology, Jhongli City, Taoyuan County 320, Taiwan, ROC C.-H. Chen (&) S.-M. Huang Department of Social and Regional Development, National Taipei University of Education, No. 134, Sec. 2, Heping E. Rd., Da-an District, Taipei City 106, Taiwan, ROC chchen@tea.ntue.edu.tw; james.chc@msa.hinet.net Y.-M. Chang Institute of Environmental Engineering and Management, National Taipei University of Technology, Taipei City 106, Taiwan risks for each exposed community in each drainage zone can be systematically calculated based on the current status or plans of prevention and evacuation measures using the proposed indicator system. A case study of implementing the proposed methodology that involves the Chishan River Basin is presented, in which disaster risk according to the current status of prevention and evacuation measures is assessed. Drainage zones and communities with a significant debris flow disaster risk are located; this risk is associated with a lack of adequate prevention and evacuation measures that have been planned of government agencies. Analytical results indicate that the proposed methodology can systematically and effectively assess the disaster risks of a river basin. The proposed methodology provides a valuable reference for governmental agencies that must manage disaster risk associated with debris flows. Keywords Debris flow risk assessment Systems thinking Mitigation Hazard Exposure Resistance 1 Introduction As major natural hazards in Taiwan, debris flows inflict tremendous damage to human life and property. From 2006 to 2013, debris flows have caused the deaths of more than 120 individuals in Taiwan and damaged approximately 800 buildings (Soil and Water Conservation Bureau, SWCB 2014). Governmental agencies have strived to develop prevention and evacuation strategies to mitigate the impact of debris flows. However, the performance of disaster risk management has not much improved because these agencies disaster have not thoroughly assess disaster risks of debris flows in a river basin. For example, many

2 776 Stoch Environ Res Risk Assess (2015) 29: Fig. 1 Conceptual framework of disaster risk assessment that incorporates risk management for debris flows Risk Assessment in which Risk Management Is Considered Risk Assessment Mitigating Hazards Risk Management for Debris Flows Current or Planned Prevention Measures Current or Planned Land Uses Risk of Current Status In the Step of Current Status Assessment Land Environment Hazards in s Water Environment Debris Flows Air Environment Risk of Management Alternatives In the Step of Strategy Generation Current or Planned Evacuation Routes and Shelters Resistance to Hazards Current or Planned Communities Exposure to Hazards Legend Influences of debris flows risks of debris flows individuals died in the Typhoon Morakot because some evacuation shelters and routes were located in unsafe area and were not sufficiently strong to be able to resist debris flows (Lo et al. 2012; National Science and Technology Center for Reduction, NCDR 2010). is defined as a serious disruption of the functioning of a community or a society involving widespread human, material, economic or environmental losses and impacts, which exceeds the ability of the affected community or society to cope using its own resources. risk is defined as the potential disaster losses, in lives, health status, livelihoods, assets and services, which could occur to a particular community or a society over some specified future time period (United Nations International Strategy for Reduction, UNISDR 2009). Assessment of disaster risk associated with debris flows in Taiwan is concerned mainly with potential threats to human life. Currently, governmental agencies perform disaster risk assessment and management strategy planning in separated stages. Potential debris flow torrents are identified in the former stage. risks associated with potential debris flow torrents are also assessed in the light of factors for potential occurrence and protection individuals. Prevention strategies and strategies for evacuating potentially affected individuals based on other indicators are devised in the latter stage. The indicators that are used to assess risks are incomplete and inconsistent to develop an effective strategy for protecting potentially affected individuals. risk in the above procedures does not simultaneously consider the mitigation of debris flows by current or planned prevention measures or their possible impacts on current or planned evacuation routes and shelters. The prevention and evacuation strategies that are based on such an assessment of disaster risk may leave possibly affected individuals inadequately protected. Restated, governmental agencies in Taiwan generally assess the risks that arise from debris flows without adequately considering risk management and taking a systems approach. Governmental agencies must therefore develop a more comprehensive method for assessing disaster risk. This work uses systems thinking to develop a method of disaster risk assessment for debris flows by firstly identifying a conceptual framework that incorporates risk management, as shown in Fig. 1. In the figure, green boxes represent environmental factors that potentially cause debris flows. Yellow boxes indicate human activities that may influence debris flows or are affected by them. Blue

3 Stoch Environ Res Risk Assess (2015) 29: solid arrows represent action relationships and red dashed arrows indicate risks that are caused by debris flows. Potential debris flow hazards cause disaster risks. These hazards are determined based on the current land uses and the states of land environment, water environment and air environment. Individuals who live in the potentially affected area are exposed to hazards, from which disaster risks arise. Prevention measures, including hardware (e.g., constructions) and software (e.g., land use regulations and hazard information), can alter the status of the aforementioned factors to mitigate the occurrence or intensity of debris flows (Holub and Fuchs 2009). Additionally, evacuation routes and shelters can protect against debris flow hazards by supporting the evacuation of individuals to safe shelters by proper routes. risks can be reduced by prevention and evacuation measures. Current land uses, the states of land environment, water environment and air environment, prevention measures, and evacuation measures should be incorporated for disaster risk assessment in the step of current status assessment as a basis for management strategy planning. While various management alternatives are generated, the corresponding land uses, the states of land environment, water environment and air environment, prevention measures, and evacuation measures should be estimated and simultaneously applied for disaster risk assessment in the step of strategy generation. Some works have proposed comprehensive procedures and methods for assessing risk associated with debris flows (Kienholz et al. 2004; Tsao et al. 2010; Fuchs et al. 2012, 2013) as a basis for developing management strategies. Although government agencies have tried to develop the prevention and evacuation strategies based on disaster risks, disaster risk assessment is not based on factors have been systematically identified to meet the requirements of risk management. This work assumes that disaster risk assessment should be useful to both the steps of current status assessment and strategy generation. Therefore, this work will review current disaster risk assessment methods, proposed in various studies and by various organizations, to identify factors that should be used in a new assessment method. Hazard of disaster and vulnerability of human society are the most widely considered factors in conventional disaster risk assessment methods (UNDRO 1980; Maskrey 1989; UNDHA 1992; Alexander 2000; TWRA 2010). Other factors have also been considered in related research, including the probability of disaster, disaster-related consequences, exposure of human society, capacity of community for bearing disaster, and capability for emergency response and recovery (Levy and Hall 2005; Sisson et al. 2006; Eroglu et al. 2010; Li et al. 2013; Zhang et al. 2014). In some studies, models based on particulate indicators have been used directly to estimate whether hazards would exist (Mendes and Pericchi 2009; Zhao et al. 2014; Lekina et al. 2013). These studies have proposed various factors that should be considered in assessing disaster risk but they have not systematically identified the purpose and aspects of disaster risk management were or considered them in their proposed methods. Defined as a potentially damaging natural phenomenon or human activity, a hazard is evaluated by its location, intensity, frequency and probability (UNISDR 2004). Various methods have been developed to quantify the impact of hazards on elements at risk, and their forces for debris flow hazards. Some studied have focused on using probability of occurrence to represent a hazard by applying the concept of a design event. Such a design event is defined as an event that occurs statistically once during the period under investigation. Andersson-Skold et al. (2013) supposed that landslide risk is a function of the probability of an event and its consequences. Probability alone can represent how often a disaster occurs, yet fails to represent the potential impact level of debris flow disasters. Huang (2013) suggested that the hourly intensity versus cumulative rainfall (I R) path is potentially useful in facilitating debris flow predictions. Fell et al. (2008) developed guidelines for landslide susceptibility, hazard and risk zoning for land-use planning. Other studies were focused on composite indicators or numerical models to estimate probability (Lin et al. 2002; Calvo and Savi 2009; Jones et al. 2014). As revealed by the conceptual framework in Fig. 1, although capable of identifying some occurrence factors of debris flows, the above methods cannot be used to assess whether individuals were exposed to debris flow hazards or whether they can be evacuated so as not to be impacted by such hazards. Furthermore, these methods do not consider measures to prevent debris flows. Liu et al. (2009) developed a process for estimating areas affected by debris flows and the expected loss caused by them, then, calculated the disaster risk by combining these data. Subsequent studies assessed disaster risk by using indicators which were developed to quantify the intensity and the affected area of the debris flows (Magirl et al. 2010; Yang et al. 2011). Aronica et al. (2012) classified the risk of a disaster hazard area by using total hydrodynamic force per unit width (i.e., impact pressure) as an indicator for event intensity. Yu et al. (2013) proposed a formation model for debris flows, in which topographic, geological, and hydraulic factors are combined. In sum, the above studies identified some factors affecting the intensity and affected area of debris flows, which were important in measuring the impacts caused by debris flow hazards. However, in addition to lacking the ability identify

4 778 Stoch Environ Res Risk Assess (2015) 29: the occurrence frequency of debris flows, the above methods do not consider measures that could reduce the intensity and affected area of debris flows. Besides considering the foresaid hazards, debris flows can cause disaster in a society that is disrupted by them. Vulnerability of a society is consequently an important factor of disaster risk. Vulnerability is the degree of loss determined by physical, social, economic, and environmental factors or processes, owing to the impact of hazards (UNISDR 2004; Aronica et al. 2012; Ge et al. 2013). Many methods have also been proposed to measure the vulnerability of elements exposed to debris flows. Vulnerability may involve the concepts of exposure, sensitivity, and adaptive capacity (Polsky et al. 2007). By using exposurerelated concepts, some studies have evaluated disaster risks (De La Cruz-Reyna 1996). For example, Jakob et al. (2013) estimated risks for individuals and groups who live within a hazard-affected area, including residential homes and a fire hall, based on hazard assessment results. Although some methods have recently become available for assessing the vulnerability of the built environment to debris flows by using empirical data (Fuchs et al. 2007; Totschnig and Fuchs 2013) and results of numerical modeling (Jakob et al. 2013), the assessment of risk to exposed persons is still the major concern. Other studies examined the ability of disaster mitigation, including coping capacity, preparedness, or disaster management (De La Cruz-Reyna 1996; Benouar and Mimi 2001; Wisner 2001; Villagran de Leon 2006). Hazard, exposure, vulnerability, and emergency response and recovery capability were used as the components of natural disaster risks. Emergency response and recovery capability represent the extent to which the affected regions recover to a normal status after disasters (Zhang et al. 2006, 2014). However, while considering the public capability for emergency response and recovery to disasters as the protection ability, the above studies did not consider the resistance ability of the affected objects. Based on the conceptual framework in this work (Fig. 1), the evacuation routes and shelters should also be considered as the resistance ability of the affected objects to reduce disaster risks. In summary, whereas the above works used various indicators to describe various aspects of disaster risk, they did not systematically assess disaster risks with reference to all stages of disaster risk management for debris flows. In particular, despite measuring different aspects of disaster risks associated with debris flows, the above studies did not integrate these various aspects in an adequate procedure. Therefore, this work develops a method for assessing disaster risk associated with debris flows based on systems thinking and the requirement of risk management, which are applied both in the steps of current status assessment and strategy generation. 2 MHER disaster risk assessment framework for debris flows in a river basin Few of the risk assessment methods that were reviewed in the previous section consider the purpose of management and systems approach. Therefore, this work first uses systems thinking to develop a conceptual framework of disaster risk assessment that consider risk management for debris flows. Subsequently, the problem of debris flows and their potential threats are treated as a conceptual system. The disaster risks of the current state can be adequately clarified because the interactive influence mechanism of debris flows are clearly identified using the systems approach. Current and planned measures for prevention and evacuation are simultaneously considered in the assessment framework, as reflected by the indicators used. The above systems approach provides a basis for generating prevention and evacuation strategies to reduce risk. Most related studies focused on directly developing assessment indicators without considering the purpose of management and taking a systems approach. Some indicators may be ignored and so disaster risks may not be comprehensively assessed in those studies. risks assessed by such methods cannot adequately become as a basis for generating risk management strategies. This systems approach, which can be used to overcome the above shortcomings, is one of the major contributions of this work. The major steps of this systems approach are listed as follows; their details are explained in the following sections. (1) Identifying a system based on the interactive influence mechanism of debris flows and the purpose of management, including the components, attributes, and interactions of this system. (2) Identifying a disaster risk assessment framework based on the above system identification and the purpose of management. (3) Developing the disaster risk assessment indicators based on the above assessment framework and the system identification. (4) Developing the methods for calculating assessment values based on the characteristics of the above assessment indicators. (5) Developing a comprehensive disaster risk assessment equation for debris flows in a river basin. 2.1 Conceptual system of disaster risk management for debris flows The purpose of this work is to develop a disaster risk assessment methodology for debris flows in a river basin. The methodology can be simultaneously applied in current

5 Stoch Environ Res Risk Assess (2015) 29: Air Environment (Rainfall) River Basin Runoff Landvover Topography Land Environment Mitigating Land Environment Water Flow Soil and Rock Collapsed Debris Water Environment Hazard of Mitigating Hazard of Human Society Exposure to Drainage Zone 1 Resistance to Prevention Measure Evacuation Route Human Society Exposure to Community Resistance to Evacuation Shelter Land Environment Debris Flow Land Environment Human Society Community Resistance to Drainage Zone 2 Mitigating Prevention Measure Exposure to Evacuation Route Evacuation Shelter Drainage Zone 3 Water Flow Drainage Zone 4 Legend Component of human society Component of land environment Component of climate Influence River Debris flow Water flow Management Fig. 2 Conceptual system of disaster risk management for debris flows in a river basin status assessment and as the basis of management strategy planning. The disaster risks of debris flows are firstly identified based on systems thinking and systems analysis. A conceptual system of disaster risk management for debris flows is then identified, and presented in Fig. 2. A river basin is treated herein as a system and the drainage zones of the river basin are treated as sub-systems. Additionally, systems thinking is used to identify the components, attributes, and interactions of disaster risk management for debris flows in a river basin. According to the system identifications in Fig. 2, the occurrence, transmission, and impact of the debris flows are correlated closely with each other, and significantly influence the disaster risks. Therefore, we recommend that the measures of prevention and evacuation for debris flows should not be generated separately in different aspects. Instead, the disaster risk should be identified and used as the basis of strategies in various steps of risk management. The measures of prevention and evacuation for debris flows should systematically take into account factors that can increase or decrease disaster risks. In this work, systems thinking is applied to develop a disaster risk assessment method for debris flows with reference to the four phases, including disaster mitigation, disaster hazard, exposure to disaster, and resistance to disaster. Debris flows occur owing to the presence of many sources of loose soils and gravels, sufficient water, and steep slopes. Each drainage zone has its own factors of air environment, land environment, water environment, and human society, all of which affect risk management for debris flows. Rainfall of air environment causes runoff in the land environment. Collapsed debris, which is generated from runoff, soil, rock, land cover, and topography, form the hazards of disasters. Prevention measures, implemented by people, can mitigate the hazards of debris flows. Communities located on the transit region of debris flows are exposed to the aforementioned hazards. Evacuation routes and shelters can provide resistance against debris flows. If some evacuation routes and shelters are also exposed to a disaster, individuals can select other evacuation routes and shelters (even in another drainage zone) to reduce the disaster risks. Therefore, the above components, attributes, and interactions of this conceptual system should be simultaneously considered in disaster risk assessment for debris flows. 2.2 risk assessment framework for debris flows According to the conceptual disaster risk management framework (Fig. 1) and the components, attributes, and interactions of the conceptual system (Fig. 2), this work

6 780 Stoch Environ Res Risk Assess (2015) 29: categorizes disaster risk assessment for debris flows in four aspects, which involve mitigation, hazard, exposure, and resistance. Figure 3 shows the mitigation hazard exposure resistance (MHER) disaster risk assessment framework for debris flows in a river basin. First, the current statuses of the four aspects in the river basin are simultaneously assessed in the step of current status assessment to obtain all of the disaster risks of debris flows. This complete disaster risk assessment should provide a basis for developing management objectives and strategies. In the step of strategy generation, various management alternatives change the future statuses of the four aspects. Therefore, the disaster risks of various alternatives should be also assessed and used as a basis for decision making. In addition to its applicability in the step of current status assessment to assess the current disaster risks, the proposed framework is applicable in the step of strategy generation to assess the disaster risks for various management alternatives. The MHER framework is introduced as follows Mitigation (M) According to Fig. 1, various prevention measures can prevent or reduce the possibility of debris flows occurrence or their impacts on communities. The disaster risks of debris flows can consequently be reduced by building more prevention facilities or implementing management systems (including land-use regulations). Analyzing the effectiveness of prevention measures before debris flows occur involves assessing the ability to mitigate the hazard; M is used to denote the risk value. A higher value of M implies a lower disaster risk. In this work, the value of (1 - M) is multiplied by the values of other aspects as the total disaster risk Hazard (H) Based on Fig. 1, the factors of land environment, air environment, and water environment influence the forces exerted by debris flows if debris flows occur. These factors, including rainfall, runoff, soil, rock, land cover, and topography are identified herein. The disaster risks of debris flows increase if the values of the environmental factors lead to a situation in which debris flows may occur. Analyzing hazards in which debris flows affect individuals involves assessing the probability, intensity, and influenced region of debris flows; H is used to denote the risk value. A high value of H implies a high disaster risk. Therefore, this study uses the value of H to multiply the values of other aspects as the total disaster risk. This work identifies that the hazard is composed of probability of debris flow occurrence (HP), intensity of debris flow (HI), and influenced region of debris flows (HR) Exposure (E) According to Fig. 1, the disaster risks of debris flows create if the communities, evacuation routes, and shelters are located on the potential run-out region of the debris flows and may be affected by them. The disaster risks of debris flows consequently increase as communities, evacuation routes, and shelters are located nearer the influenced region of the debris flows. Analyzing exposure during debris flow transit involves assessing the possibility that debris flows may affect humans; E is used to denote the risk value. A high value of E implies a high disaster risk. In this work, the value of E is multiplies by the values of other aspects as the total disaster risk. According to our results, the exposure is measured by the distances between the possibly affected humans and the occurrence positions and the pathways of debris flows Resistance (R) Based on Fig. 1, resistance refers to the ability to protect individuals in order to reduce the impacts from debris flows. Individuals can consequently select proper evacuation routes and shelters to protect themselves from harm by the debris flows. The disaster risk of debris flows can be reduced by the establishment of safer evacuation routes and shelters. Analyzing resistance to debris flows involves assessing the ability that evacuation routes and shelters can be used to protect individuals from being affected by debris flows; R is used to denote the risk value. A higher value of R implies a lower disaster risk. In this work, the value of (1 - R) is multiplies by the values of other aspects as the total disaster risk. The resistance is evaluated using the location, safety and construction of evacuation routes and shelters. Based on the above identification, this work develops a conceptual mathematical function for disaster risk, given by Eq. (1). Risk ¼ f ðm; H; E; RÞ: ð1þ Since the risks that are caused by the four aspects are independent of each other, the total disaster risk can be calculated by the product of the risks caused by the four aspects. The gross equation for assessing the disaster risks is identified as Eq. (2). In addition to identifying various indicators to calculate the risk that is caused by each aspect, this work develops a detailed mathematical equation, as described in the following section. Risk ¼ð1 MÞH E ð1 RÞ: ð2þ

7 Stoch Environ Res Risk Assess (2015) 29: Fig. 3 MHER disaster risk assessment framework for debris flows in a river basin Before Debris Flows Occurred (Prevention Measures by People) Mitigation Impacts form Debris Flows (Resistances by People to Hazards ) Resistance Risk of Debris Flow Hazard Occurrence of Debris Flows (Hazards to People) Exposure Transportation of Debris Flows ( Exposures of People to Hazards) 2.3 Assessment indicator system for disaster risks of debris flows Based on the conceptual framework of disaster risk assessment that incorporates risk management (Fig. 1), the system identification (Fig. 2), and the MHER disaster risk assessment framework (Fig. 3), this work develops an assessment indicator system for disaster risks of debris flows in a river basin, as shown in Fig. 4. The proposed indicator system is conceptually divided into four aspects: mitigation, hazard, exposure, and resistance. Twenty indicators are involved in the four aspects. This work develops three indicators about the prevention constructions to quantify the mitigation aspect based on system identification (Fig. 2), and MHER disaster risk assessment framework (Fig. 3) for evaluating the capabilities that can reduce the hazards associated with debris flows. The indicator of existence of prevention constructions refers to a situation in which the prevention constructions can generate the mitigation capability. Furthermore, the mitigation capability can be evaluated by the indicators of types of prevention constructions and quantities of prevention constructions. Seven indicators about the environmental factors are developed to specify the hazard aspect. The indicators of types of rocks and ratios of potential collapse area represent the influence from debris sources. The indicators of accumulated rainfalls and areas of effective watersheds represent the influence from water. The indicator of average slopes of riverbeds represents the influence from slope. The indicators of volumes of debris flow sediments and areas of deposition regions are used to evaluate the intensity and range of debris flows, respectively. This work develops three indicators to quantify the exposure aspect, including: distances between communities and affected areas, distances between evacuation shelters and affected areas, and ratios of evacuation route lengths in the affected areas. These indicators represent the risks occurred due to the communities, evacuation routes, and shelters that are located nearby the affected areas of debris flows. Seven indicators are developed to specify the resistance aspect. The indicators of architectural types, architectural materials, and used time of architecture represent the structural safety of evacuation shelters. The indicators of evacuation route lengths for walking and evacuation route lengths for driving are used to evaluate the evacuation time. Furthermore, the indicators of numbers of bridges in evacuation route and ratios of road type lengths represent the structural safety of evacuation routes. The above indicators can clarify the disaster risks of debris flows by using quantitative values. Based on the

8 782 Stoch Environ Res Risk Assess (2015) 29: (Me) Existence of prevention constructions Mitigation (M) (Mt) Types of prevention constructions (Mq) Quantities of prevention constructions (Hd1) Types of rocks Risks of Debris Flows in a River Basin Hazard (H) Probability (Hd2) Ratios of potential collapse areas (Hw1) Accumulated rainfalls (Hw2) Areas of effective watersheds (Hs1) Average slopes of riverbeds Intensity (Hi) Volumes of sediments of debris flows Range (Hr) Areas of deposition regions Exposure (E) (Ep) Distances between communities and affected areas (Es) Distances between evacuation shelters and affected areas (Er) Ratios of evacuation route lengths in the affected areas Resistance (R) (Rs) Safety on architectural structure of evacuation shelter (Rl) Length of evacuation route (Rr) Safety on structure of evacuation route (Rs1) Architectural types (Rs2) Architectural materials (Rs3) Used time of architecture (Rl1) Evacuation route lengths for walking (Rl2) Evacuation route lengths for driving (Rr1) Numbers of bridges in evacuation route (Rr2) Ratios of road type lengths Fig. 4 Assessment indicators for disaster risks of debris flows in a river basin concepts of risk management, the disaster risks that are caused by the environmental factors of land, air, and water or are reduced by the human actions and facilities are both assessed. Therefore, governmental agencies can collect the related data of current status or management alternatives. Then, by applying the indicators, the disaster risks are calculated as a basis of strategy planning. 2.4 Methods of calculating assessment values The calculation of the indicators of mitigation aspect is explained as an example. Based on risk management considerations, various constructions can mitigate the hazards of debris flows, including check dams and sands traps. The ability to mitigate debris flow disasters increases

9 Stoch Environ Res Risk Assess (2015) 29: with an increasing number of mitigation constructions. Therefore, this study derives Eq. (3) to estimate the ability to mitigate the debris flow disasters. M ¼ M e w Mt M t þ w Mq M q ; ð3þ M is the indicator value of ability to mitigate the debris flow disasters, M e is the indicator value of existence of prevention constructions, M t is the indicator value of types of prevention constructions, M q is the indicator value of quantity of prevention constructions, w Mt is the indicator weight of types of prevention constructions, and w Mq is the indicator weight of types of prevention constructions. The values of all indicators are normalized to 0 1 and made dimensionless as their original values and dimensions are quite different. The normalized values are determined based on the theoretical or practical data. Indicator of existence of prevention constructions is used as an example to explain how to be defined based on the theoretical data. This indicator refers to a situation in which the prevention constructions can generate the mitigation capability. If no prevention construction exists, the mitigation capability can be theoretically defined as none. The normalized value is consequently defined as 0. If prevention constructions exist, the mitigation capability is theoretically determined by the types and quantities of prevention constructions. The normalized value is consequently defined as 1 to multiply the other two indicator values in the aspect of mitigation. The indicator of quantities of prevention constructions is used as an example and its definition based on the practical data is explained. This work collects and analyzes the practical data of prevention constructions in Taiwan. According to analytical results, the prevention effect is very good if the quantity of prevention constructions for a potential torrent is more than 50. The normalized value is consequently defined as 0.9. Furthermore, the prevention effect is very poor if the quantity of prevention constructions for a potential torrent is less than 10, and the normalized value is consequently defined as 0.1. The quantities between 10 and 50 are classified into three intervals and defined adequate values. Table 1 lists the normalized values of these indicators. 2.5 risk assessment equation for debris flows in a river basin Based on Eq. (2) and the identification of the assessment indicators (Fig. 4), this work develops a detailed mathematical equation, Eq. (4), to assess the disaster risks of debris flows in a river basin. Where i denotes the number of drainage zones, j represents the number of communities, k refers the number of evacuation shelters for community j of drainage zone i, u denotes the number of Table 1 Normalized values of the aspect of mitigation of debris flow disaster risk Existence of prevention constructions (M e ) architecture type of evacuation shelter k for community j of drainage zone i, v represents the number of evacuation routes for evacuation shelter k for community j of drainage zone i. Furthermore, w denotes the weights of the assessment indicators. Risk ¼ XN X Mi X Pij X Qijk 1 Me i w Mt Mt i þ w Mq : i¼1 j¼1 k¼1 u¼1 Mq i ÞŠ ½ðw Hd1 Hd1 i þ w Hd2 Hd2 i Þ ðw Hw1 Hw1 i þ w Hw2 Hw2 i ÞHs1 i 1=4 Hi i Hr i Š 1=5 Es ijku 1 Rs ijku þ XN i¼1 X Mi X Rij j¼1 v¼1 Types of prevention constructions (M t ) 1 Me i w Mt Mt i þ w Mq Mq i ½ðw Hd1 Hd1 i þ w Hd2 Hd2 i Þ ðw Hw1 Hw1 i þ w Hw2 Hw2 i Þ Hs1 i Hi i Hr i Š 1=5 w Ep Ep ij þ w Er Er ijv 1=4 1 w Rl Rl ijv þ w Rr Rr ijv :? Quantity of prevention constructions (M q ) Yes (1) 9 More than 101? More than 51 (0.9) (0.9) (0.7) (0.7) (0.5) (0.5) No (0) (0.3) (0.3) 1 10 (0.1) 1 10 (0.1) ð4þ Based on the MHER framework, disaster risks arise when the related factors simultaneously prevail in the phases of mitigation, hazard, exposure, resistance. The assessment values of the four phases should consequently be multiplied by each other to express the total disaster risk. For example, if the hazard of debris flows is very low, i.e., the assessment value of hazard is close to 0, even if the mitigation capability is low, exposure ratio is high, and resistance capability is low, i.e., the assessment values of these three phases are close to 1, the disaster risk is still close to 0. This work calculates the fourth root of the product of the assessment values of the four phases and the disaster risk can be consequently obtains between 0 and 1. For example, if the assessment values of the four phases are all 0.9, then the product is , whose fourth root is 0.9. If the influence factors in an aspect should simultaneously exist to create disaster risk, the assessment values

10 784 Stoch Environ Res Risk Assess (2015) 29: Table 2 Classes of disaster risk Classes of disaster risk Assessment values of disaster risk 3.2 Calculations of indicator values of disaster risks of debris flows in the Chishan River Basin Very high Risk C 0.7 High 0.7 [ Risk C 0.5 Medium 0.5 [ Risk C 0.3 Low 0.3 [ Risk C 0.2 Very low Risk \ 0.2 of the factors should consequently be multiplied by each other to express the assessment value of the aspect. For example, debris flows occur because of proper conditions of debris sources, water, and slope. The assessment values of the factors should consequently be multiplied by each other to express the assessment value of the aspect. If the influence factors in a phase separately create disaster risk, the assessment values of the factors should consequently be multiplied by their weights and added by each other to express the assessment value of the phase. For example, the risks caused by debris sources are influenced by the geology and ratio of potential collapse area. The assessment values of these two factors should consequently be multiplied by their weights and added by each other to express the risk caused by debris sources. The sum of the weights in a phase is 1. The calculation results of assessment value of each phase can consequently be obtained between 0 and 1. The disaster risk of each community is firstly assessed by using Eq. (4). Since the disaster risks of the communities in a drainage zone differ from each other, this work defines the average value of the disaster risks of all the communities in a drainage zone as the disaster risk of this drainage zone. The disaster risk of a river basin is then identified by using the average value of the disaster risks of all of the drainage zones in this river basin. The disaster risk is identified into five classes, shown in Table 2. 3 Results and discussion 3.1 Case study: Chishan River Basin Chishan River Basin is used as a case study. The current disaster risks of debris flows in this river basin by using the above methodology. Chishan River is an important branch of Kaoping River in southern Taiwan. Chishan River is 104 km in length; the area of this basin is km 2 (Chen et al. 2011, 2012, 2013; Huang 2012). In this study, the Chishan River Basin is conceptually divided into 49 drainage zones in this study, as shown in Fig. 5. Each drainage zone may include one or several streams. Governmental agencies have identified 50 streams as potential torrents of debris flow in this basin. Consistent with the above methodology, this work collects related data and processes it using the ArcGIS software (Environmental Systems Research Institute, Inc., ESRI 2009) to calculate the indicator values of disaster risks. The indicator of ratio of potential collapse area serves as an example to explain the calculation process. The potential collapse area includes the expanding area of a collapsed area and the area whose slope is more than 40. In this study, the potential collapse area is identified using the following method: (1) The expanding area of a collapsed area is identified using the collapsed areas of 2009 and By using the ERDAS software (Intergraph, Inc. 2011), this study analyzes the satellite images of 2009 and 2010 and classifies the land images into 20 types. All of the collapsed areas are then identified based the image types. The collapsed area of 2009 is erased from the collapsed area of 2010 to obtain the expanding area of a collapsed area. (2) The slopes of different drainages are verified by using the digital terrain model data. The area whose slope is more than 40 is identified using the spatial analyst function of ArcGIS and calculated using the intersect function of ArcGIS. Figure 6 summarizes the analytical results of potential collapse areas. The potential collapse area is then divided by the area of the drainage zone to obtain this indicator value. Table 3 lists the indicator values of ratio of potential collapse area of each drainage zone. The indicator of safety on the architectural structure of evacuation shelter in the resistance aspect is used as another example to present the assessment results. Governmental agencies have set up 24 evacuation shelters in the Chishan River Basin. The GIS data of evacuation shelters, evacuation routes, and protected communities are built in this study for calculating the indicator values, as shown in Fig. 7. This study gathers relevant data from various governmental agencies, including the types, materials, and used time of architecture, as shown in Table 4, to calculate the indicator values of these evacuation shelters. First, the architectural types of these evacuation shelters are identified as conservation to assess their current status. These types can be identified either as reinforcement or build if the evacuation shelters are to be improved. This work identifies the normalized values for various materials and used time of architecture of conserved evacuation shelters, as shown in Table 5. If the architectural material is steel reinforced concrete (SRC), the

11 Stoch Environ Res Risk Assess (2015) 29: Fig. 5 Drainage zones of the Chishan River Basin evacuation shelter is identified as the highest value for safety. Reinforced concrete (RC), brick, and wood are identified as the second, third, and fourth values, respectively. If the architecture has been used less than one third of its designed life time, i.e., short used time, the evacuation shelter is identified as the highest value for safety. Medium, long, and over-used are identified as the second, third, and fourth values, respectively. The last column of Table 4 summarizes the assessment results for the current status of safety on architectural structure of evacuation shelter in the Chishan River Basin. 3.3 Assessment results of disaster risks of debris flows in the Chishan River Basin Thirty one communities locate near the potential debris flow torrents which are identified by the Water-Soil Conservation Bureau of Taiwan. The governmental agencies

12 786 Stoch Environ Res Risk Assess (2015) 29: Fig. 6 Potential collapse area in the drainage zones in the Chishan River Basin have set up evacuation shelters and routes for these communities. This study also identifies other possible deposition regions in the Chishan River Basin. Although this study has identified 25 communities near these possible deposition regions, governmental agencies have not yet planned evacuation shelters and routes for these communities. All of the 56 communities are distributed in 21 drainage zones. By using Visual Basic (VB) software (Microsoft, Inc. 1998), this study develops a disaster risk assessment model for debris flows in a river basin based on Eq. (4). This study collects the data of locations where debris flows have occurred in in the Chishan River Basin (SWCB 2014) to adjust the coefficients and normalization values of the proposed model for calibration and verification. risks of the above 56 communities are calculated using the proposed assessment model. Twelve historical debris flow disasters occurred in the Chishan River Basin in Four of these historical disasters (33 %) occurred in two drainage zones (i.e., Nos. 22 and 35) that are assessed as belonging to a high risk level. Eight of these historical disasters (67 %) occurred in four drainage zones (i.e., Nos. 31, 33, 36, 39) that are assessed as belonging to a medium risk level. No disaster occurred in the drainage zones which are assessed as belonging to low or very low risk levels. Therefore, the proposed model can reflect the risks of the drainage zones which debris flow disasters had occurred. Since the disaster risks of the communities in a drainage zone differ from each other, this work defines the average value of the disaster risks of all the communities in a drainage zone as the disaster risk of this drainage zone. risks of the 21 drainage zones involving these communities are then assessed. The disaster risk of a river basin is then identified by using the average value of the disaster risks of all of the drainage zones in this river basin. The complete Chishan River Basin is then assessed. Figure 8 summarizes the assessment results of debris flow disaster risks of the Chishan River Basin. The overall disaster risk for debris flows of the Chishan River Basin is , which is classified as a high risk level. Three drainage zones (i.e., Nos. 17, 19, and 29) are assessed as belonging to a very high risk level. Risk in the hazard aspect is high because these drainage zones have environmental characteristics that may lead an increase of many indicator values. However, governmental agencies installed very few prevention constructions in these drainage zones. Therefore, risk in the mitigation aspect is very high. Risk in the exposure aspect is also high because some communities are located near the possible deposition regions. Moreover, risk in the resistance aspect is very high since governmental agencies have not yet identified evacuation shelters and routes for these drainage zones. Consequently, the complete disaster risks of these drainage zones are very high.

13 Stoch Environ Res Risk Assess (2015) 29: Table 3 Indicator values of ratio of potential collapse area of each drainage zone in the Chishan River Basin Drainage zones Area of drainage zone (km 2 ) Area of slope more than 40 (m 2 ) Expanding area of past collapsed land (m 2 ) Increasing area of new collapsed land (m 2 ) Ratio of potential collapse area (%) Normalized indicator value ,579, ,523, ,706, ,769, , ,919, ,770, , ,189, ,035, , ,644, , , , , , ,312, ,147, , ,518, , , ,284, ,281, , ,678, , , , ,132, , ,638, ,273, ,412, ,317, ,420, , ,467, , , , , , , , , , ,353, , ,522, , , , ,247, , ,374, , , , , , , ,154, , ,093, , , , ,647, , ,715, , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , ,200, ,298, , , , , , , , , , , , , , , , , , , ,

14 788 Stoch Environ Res Risk Assess (2015) 29: Table 3 continued Drainage zones Area of drainage zone (km 2 ) Area of slope more than 40 (m 2 ) Expanding area of past collapsed land (m 2 ) Increasing area of new collapsed land (m 2 ) Ratio of potential collapse area (%) Normalized indicator value , , Fig. 7 Examples of evacuation shelters, evacuation routes, and protected communities in the Chishan River Basin These assessment results indicate that the current governmental system of debris flow disaster management focuses on identifying the potential debris flow torrents first. The prevention and evacuation strategies for potentially affected individuals are devised in the second stage. These three drainage zones are not the most risky ones in the first assessment stage in the current governmental system. The prevention constructions, evacuation routes, and shelters have not been adequately installed yet in these three drainage zones. They consequently belong to the very high risk level due to the incomplete considerations by the current governmental agencies. Seven drainage zones (i.e., Nos. 15, 22, 32, 35, 38, 44, and 47) are assessed as a high risk level. The drainage zones of Nos. 15 and 22 are located in the upstream area of this river basin. Risk in the hazard aspect is high because these drainage zones have environmental characteristics that may lead an increase of many indicator values increasing. Governmental agencies only installed some prevention constructions in these drainage zones, leading to a high risk in the mitigation aspect. Risk in the exposure aspect is high because some communities are located near the possible deposition regions. However, risk in the resistance aspect is medium since governmental agencies have already identified evacuation shelters and routes for these drainage zones. Therefore, the complete disaster risks of these drainage zones are high. The aforementioned seven drainage zones are also not the most risky ones in the first assessment stage in the current governmental system. Evacuation facilities are identified but the prevention constructions have not been adequately installed yet in these drainage zones. The drainage zones of Nos. 32, 35, 38, 44, and 47 are located in the downstream area of this river basin. Risk in

15 Stoch Environ Res Risk Assess (2015) 29: Table 4 Indicator values of safety on architectural structure of evacuation shelter for the current status in the Chishan River Basin ID of evacuation shelter Architectural type Architectural material Used time of architecture 1 Conservation RC Medium Conservation RC Medium Conservation Wood Short Conservation Wood Short Conservation RC Long Conservation RC Medium Conservation Brick Medium Conservation RC Medium Conservation RC Long Conservation RC Medium Conservation RC Medium Conservation Wood Medium Conservation RC Medium Conservation RC Long Conservation RC Medium Conservation RC Medium Conservation RC Long Conservation RC Long Conservation Wood Medium Conservation RC Medium Conservation RC Medium Conservation RC Medium Conservation Wood Medium Conservation Wood Medium 0.6 Normalized indicator value Table 5 Normalized values for various materials and used time of architecture of conserved evacuation shelters Used time SRC RC Brick Wood Short Medium Long Over-used the hazard aspect is medium because the environmental characteristics cause a slight increase in the indicator values. However, governmental agencies have not installed sufficient prevention constructions in these drainage zones. Risk in the mitigation aspect is subsequently high. Additionally, risk in the exposure aspect is high because some communities are located near the possible deposition regions. Furthermore, governmental agencies have not yet identified evacuation shelters and routes for these drainage zones, explaining why risk in the resistance aspect is very high. Consequently, the complete disaster risks of these drainage zones are high. The above five drainage zones are the medium risky ones in the first assessment stage in the current governmental system. The prevention constructions or evacuation facilities have not been adequately installed yet in these drainage zones. Since debris flow disaster risk of the Chishan River Basin is assessed as a high risk level, the current prevention and evacuation measures for drainage zones have not reduced the disaster risks of the river basin effectively. Moreover, six communities are assessed as belonging to a very high risk level, in which no evacuation measure is planned for them. Among the communities with no evacuation measure, only one community is assessed as having a low risk level. The above results demonstrate that a lack of consideration of mitigation, hazard, exposure, and resistance aspects makes it impossible to present the disaster risks of these communities, drainage zones, and overall river basin completely. Therefore, the proposed MHER assessment framework and indicator system can assess systematically and effectively the disaster risks of a river basin.

16 790 Stoch Environ Res Risk Assess (2015) 29: Fig. 8 Assessment results of disaster risks of debris flows in the Chishan River Basin 4 Conclusions This work developed a conceptual MHER framework to assess the disaster risks for debris flows in a river basin. This framework is developed based on risk management thinking and systems approach to thoroughly consider the interactive influence mechanism of debris flow disaster risk. Based on the concepts of systems thinking, this work develops a conceptual framework of disaster risk assessment that incorporates risk management for debris flows. A conceptual system of disaster risk management for debris flows in a river basin is accordingly defined. The MHER framework and indicator system, which are developed based on the above system identifications, can be used to assess the disaster risks of debris flows for both current status and various management alternatives. The limitations of current risk management strategies that are devised in separate stages can be overcome. Based on the proposed MHER framework, this work developed an indicator system for assessing disaster risks of debris flows. Each aspect (including mitigation, hazard, exposure and resistance) involves several indicators to thoroughly examine the disaster risks caused by debris flows. Mathematical equations for calculating indicator values are also defined. The proposed indicator system can be used to calculate systematically the disaster risk for each community in each drainage zone with different prevention or evacuation measures. Moreover, the variations of disaster risks can be calculated while various management alternatives are planned. Although some indicators have been used to present related information for the governmental agencies in Taiwan, these indicators are directly developed without the steps to consider the purpose of management and systems approach at first. Some indicators may be ignored in the current assessment methods of governmental agencies in Taiwan and may not assess the disaster risks thoroughly. The disaster risks that are assessed by such methods cannot form an adequately basis for generating risk management strategies. The proposed MHER framework and assessment indicators are developed based on systems approach to overcome the above defects and become as a basis for generating prevention and evacuation strategies adequately. However, a proper management strategy planning methodology based on this risk assessment method may be further developed. Since such a management strategy planning methodology is expected to be complicated, innovative development in a new work will be required. Chishan River Basin is used herein as a case study. This work used the data from various governmental agencies and ArcGIS and VB software to calculate the indicator values of disaster risks. Data for locations of debris flows in the year in the Chishan River Basin were collected to adjust the coefficients and normalized values in the proposed model for calibration and verification. Subsequently, this model was used to assess the debris flow disaster risk of the Chishan River Basin, which belongs to a high risk level. The drainage zones and communities with significant disaster risks related to debris flows are identified, alone with highly risky streams that have not been defined as the potential debris flow torrents by governmental agencies. Analytical results reveal that the proposed MHER assessment indicator system can systematically and effectively assess the disaster risks of a river basin. The proposed assessment methodology and results are applicable in assessing the disaster risks of the current status and the management alternatives which are newly generated. The proposed methodology provides a valuable reference for government agencies that are involved in debris flow disaster management. Acknowledgments The authors would like to thank the National Science Council of the Republic of China, Taiwan for financially supporting this research under Contracts No. NSC M , NSC M , and NSC M MY2.