RISK ASSESSMENT AND MANAGEMENT FOR TSUNAMI HAZARD

Transcription

1 Sri Lanka Empowered Lives. Resilient Nations. RISK ASSESSMENT AND MANAGEMENT FOR TSUNAMI HAZARD Case Study of the Port City of Galle

2 Dr. S.S.L. Hettiarachchi, University of Moratuwa, Sri Lanka Dr. S.P. Samarawickrama, University of Moratuwa, Sri Lanka Dr. N. Wijeratne, University of Ruhuna, Sri Lanka Risk Assessment and Management for Tsunami Hazard Case Study of the Port City of Galle The analysis, opinions and policy recommendations contained in this publication do not necessarily reflect the views of UNDP. United Nations Development Programme Asia-Pacific Regional Centre UN Service Building Rajdamnern Nok Avenue Bangkok Thailand sia- acific.undp.org Copyright UNDP 2011 Cover photo: WHO / Armando Waak Design and Layout: Inís Communication

3 Galle Risk Assessment and Management for Tsunami Hazard Case Study of the Port City of Galle Published by United Nations Development Programme, Asia-Pacific Regional Centre in partnership with ICG/IOTWS Working Group on Risk Assessment under the UNESCO/IOC framework 2011

4 Key terms Definitions Hazard Vulnerability Risk Capacity Capacity development A natural or human-induced threat to people and their welfare Exposure and susceptibility to losses in terms of deaths, injuries, property, livelihoods, disruption of economic activity or environmental impacts The probability of hazard occurrence, associated with people s vulnerability to potential damage and losses. The means by which available resources, abilities and knowledge are utilized; the ability of individuals, organizations and societies to perform functions, solve problems, and set and achieve objectives The process through which the abilities to do so are obtained, strengthened, adapted and maintained over time Acronyms and abbreviations ANUGA APRC AVI-NAMI EWS ICAM IO IOC IOT IOTWS ISDR UN UNDP UNESCO UNISDR USAID WAPMERR WG-RA This refers to a specific mathematical model for tsunami simulation. Asia-Pacific Regional Centre This refers to a specific mathematical model for tsunami simulation. Early Warning System Integrated Coastal Area Management Indian Ocean Intergovernmental Oceanographic Commission Indian Ocean Tsunami Indian Ocean Tsunami Warning System International Strategy for Disaster Reduction United Nations United Nations Development Programme United Nations Educational, Scientific and Cultural Organization United Nations International Strategy for Disaster Reduction United States Agency for International Development World Agency of Planetary Monitoring and Earthquake Risk Reduction Working Group on Risk Assessment ii Risk Assessment and Management for Tsunami Hazard

5 Contents Foreword 1 1. Risk Components of risk and its assessment Risk Hazards Vulnerability Capacity, preparedness and improvement of community resilience Tools and methods for tsunami risk assessment 5 2. City of Galle damage and mechanics of tsunami wave impact 6 3. Tsunami hazard analysis Approaches to tsunami hazard analysis Post-tsunami field investigations Deterministic tsunami hazard modelling Probabilistic tsunami hazard modelling Hazard analysis for risk assessment Vulnerability From a single dimension of susceptibility to a three-dimensional sector approach Simplified approach to vulnerability Risk assessment Managing Risk Classification and planning risk management measures Classification of risk management measures Planning risk management measures via policy and management options Classification of physical interventions (artificial and natural) Development of guidelines for tsunami-resistant buildings Mitigation and its integration with development projects Preparation of disaster management maps Coastal community resilience Coasts at risk Establishing the resilience of coastal communities Concluding remarks 28 References 30 Risk Assessment and Management for Tsunami Hazard iii

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7 Foreword IN THE AFTERMATH OF THE INDIAN OCEAN TSUNAMI, the Indian Ocean States decided to establish a tsunami warning system under the Intergovernmental Oceanographic Commission of the United Nations Educational, Scientific and Cultural Organization (UNESCO/IOC). Activities in this direction were initiated in March The member States decided that the Indian Ocean Tsunami Warning System (IOTWS) would be a coordinated network of country systems in which each country has the responsibility of identifying hazards, assessing risk and issuing warnings to its own population. In this respect they will be assisted by regional tsunami warning centres to be established in some of the Indian Ocean countries. India, Indonesia and Australia are establishing centres for this purpose. In order to assist the establishment of the IOTWS, six working groups were formed, one of which was dedicated to risk assessment within a multi-hazard framework. The Working Group on Risk Assessment (WG-RA) conducted a survey of the needs of its membership and it was evident that there was a demand for a clear understanding of tsunami hazard in order to develop uniform guidelines on tsunami risk assessment. It was suggested that the guidelines be developed in full consultation with the member States and considering the existing knowledge base. In addition it was requested that training programmes be offered to member States for capacity building in tsunami risk assessment and mitigation. The WG-RA adopted a three-pronged approach to its tasks: 1 The Indian Ocean tsunami hazard map was prepared by Geoscience Australia based on probabilistic tsunami hazard modelling, with funding from AusAID, and in full consultation with regional and external experts. Geoscience Australia provided leadership for this activity. 2 A tsunami risk assessment guide was prepared and published as UNESCO Manual and Guide No. 52. This guide was prepared over a two-year period after several workshops with regional and external experts. The UNDP Asia-Pacific Regional Centre (APRC) in Bangkok, the World Agency of Planetary Monitoring and Earthquake Risk Reduction (WAPMERR) in Dubai and IOTWS provided funding for these activities. UNDP supported the development of the guideline. In doing so, priority was given to understanding the existing risk assessment framework in Indian Ocean countries and to ensuring that the guidelines captured the existing knowledge base of Indian Ocean States. 3 Several seminars and workshops were held during the preparation of the tsunami hazard map and the risk assessment guide. After the successful preparation of these two documents, a seminar and workshop were conducted in Bangkok in November 2009 to provide training to member States on the use of the tsunami hazard map and the risk assessment guide. The UNDP APRC in Bangkok, the Indian Ocean (IO) Coast Map Project and IOTWS provided funding for this activity. UNESCO/IOC has also undertaken a strong initiative within the IO-COAST MAP Project to assist nations in the acquisition of nearshore bathymetric data and development of shallow water maps which are essential for modelling of both episodic and chronic hazards and also for coastal development activities. The WG-RA and UNDP APRC in Bangkok worked in collaboration with the IO-COAST MAP project to organize a successful Risk Assessment and Management for Tsunami Hazard 1

8 training workshop in Bangkok in August 2009 on coastal mapping, modelling and risk assessment. UNDP also sponsored two more national workshops in Sri Lanka and Indonesia in 2010 to provide training and develop national capacity in tsunami risk assessment. At the first global meeting of the tsunami warning systems, held in Paris in March 2009, the WG-RA proposed that case studies of tsunami risk assessment be conducted in cities and regions covered by tsunami warning systems in the Indian Ocean basin and other ocean basins. This report presents such a case study of tsunami risk assessment and management in the Port City of Galle in Sri Lanka. 2 Risk Assessment and Management for Tsunami Hazard

9 1. Risk Components nts of risk and its assessment ent Coastal communities all over the world are under severe pressure resulting from planned and unplanned nne development, population growth and human-induced vulnerability. Coastal al hazards are increasing in magnitude, with greater frequency ency of extreme weather events and other impacts of global climate change. These unprecedented changes are placing communities at increasing risk from coastal hazards such as severe storms, tsunamis leading to coastal erosion, flooding and environmental degradation. Planning and implementation of post-tsunami rehabilitation and conservation of coastlines should ideally be undertaken within a multi-hazard coastal risk assessment framework giving due consideration to all coastal hazards. Even when risk assessments are undertaken specifically in relation to tsunami hazard, it is important to conduct such studies on a platform that can accommodate other coastal hazards. 1.1 Risk Risk is primarily a function of hazard and vulnerability. It is usually expressed by the notation Risk = Hazard x Vulnerability. Hazard is understood as a potential natural or human-induced threat to people and their welfare, while vulnerability is their exposure and susceptibility to losses in terms of deaths, injuries, property, livelihoods, disruption of economic activity or environmental impacts. Therefore, in this equation, risk can be quantified as hazard associated with the probability of occurrence, and with people s vulnerability to potential damage and losses. Another popular expression for risk incorporates capacity and is expressed by the notation Risk = (Hazard x Vulnerability) / Capacity. Capacity represents the means by which the community utilizes available resources, abilities and their knowledge base to confront adverse conditions that could lead to a disaster. In this respect community preparedness is considered the pivotal factor. The strengthening of coping capacities usually builds resilience to withstand the impacts of hazards, both natural and human-induced. Capacities thus focus on group measures that are in place to cope with an event. Prior to the Indian Ocean Tsunami (IOT), Sri Lanka had not adopted a planned approach towards preparedness and response in relation to mega-disasters, an aspect which is considered vital in saving lives. Hence it seemed more appropriate to focus on the importance of preparedness in relation to capacity. In this notation risk is expressed as Risk = Hazard x Vulnerability x Deficiencies in Preparedness. The additional term represents certain measures and tasks, the absence of which could increase the loss of human lives and property in the specific interval of time during which the event is taking place. This term is also commonly identified as the inverse of capacity. Risk Assessment and Management for Tsunami Hazard 3

10 For a detailed assessment of risk it is necessary to quantify the three main components of risk, which is a challenging task. When risk is expressed in the form Risk = Hazard x Vulnerability. it is possible to quantify risk in terms of loss. However with the introduction of capacity or preparedness it is difficult to adopt direct quantification methods. There is no standard technique for such assessment of risk and a number of methods have been used by researchers including quantitative and qualitative methods. Quantification based on qualitative description (ranking methods) and quantification based on detailed analysis of respective parameters have both been successfully adopted. Although studies relating to risk will be able to capture the significance of all the three components, there are limitations in the assessment process. However, it is important that risk assessment studies are conducted within the framework defined by the above formula. This aspect has to be kept in mind when reviewing the outputs from studies on risk assessment. 1.2 Hazards A hazard can be defined as a potentially damaging physical event, phenomenon or human activity that may cause the loss of life or injury, property damage, social and economic disruption or environmental degradation. Hazards are either of natural origin or induced by human activities. Hazards also include latent conditions which may represent future threats. Each hazard is characterized by its location, intensity, frequency of occurrence and associated probability. Therefore, hazards represent the possibility of occurrence of a natural or human-induced event of a probable magnitude or intensity over a specific geographic area. Exposure refers to the geographic area, human life, ecosystems and infrastructure potentially affected by the hazard. The Indian Ocean Tsunami (IOT) focused global attention on the severe impacts of tsunamis. It was also recognized that coastal communities are increasingly at risk from a number of hazards, broadly classified as episodic and chronic hazards. These hazards, which may arise from natural phenomena or human-induced events, have severe impacts on coastal communities, ecosystems and infrastructure. Episodic hazards include severe storms, earthquakes, tsunamis and oil spills, all of which have limited predictability and may result in major disasters. The communities should be made aware of these hazards, their vulnerability and risks, and should be educated on the importance of preparedness in responding to potential disasters which usually require long-term post-event recovery efforts. Chronic conditions include shoreline erosion, flooding, sedimentation, sea-level rise and coastal environmental and resource degradation. These conditions, which may result or increase from disasters arising from episodic hazards, relate to processes which can be measured and monitored. They require long-term planning measures and restoration efforts to reduce risks. 1.3 Vulnerability Vulnerability represents conditions determined by physical, social, economic and environmental factors or processes which increase the susceptibility of a community to the impact of hazards. Vulnerability can be broadly classified into several components, including human, physical, socio-economic, environmental, functional and administrative. Hence vulnerability is dependent on several factors relating to these 4 Risk Assessment and Management for Tsunami Hazard

11 components. These include, among others, population density, building density and status, distance from the shoreline, elevation and evacuation time. Assessment of vulnerability is complex and can be implemented at different levels, commencing from very basic to highly sophisticated databases. Several models of vulnerability are available. 1.4 Capacity, preparedness and improvement of community resilience The assessment of risk is an important element of coastal community resilience. In this respect, coastal community resilience is identified as the capacity to absorb and withstand impacts of hazards, emerge from disaster events and adapt efficiently to changing conditions. Economic and social development pressure in coastal areas, increasing population density and distribution, and other human-induced vulnerabilities, together with increasing frequency and duration of storms, long-term sea-level rise and other hazards, have created conditions for disasters of high severity to occur more frequently. Under such circumstances, communities have restricted capacity and reduced time to recover. Global observations also reveal that the period of time between disasters and recovery is becoming smaller. Therefore some communities are continuously facing disasters, event after event, depriving them of time to plan and achieve long-term recovery. In effect, they lead a life of continuous response to varying disasters. Such communities should be identified as high-risk areas and special area risk assessment and management studies should be undertaken. While vulnerability represents the extent to which a community is prone to be affected by the hazard, deficiencies in preparedness represent the absence of measures and tasks which could reduce the loss of human lives and property during disaster. In this respect, communities must be made aware of the hazards, their exposure, vulnerabilities and capacity. The key areas requiring attention are awareness and education, preparedness, early warning, response, evacuation, safe places and evacuation structures, and hazardresilient infrastructure. Enhanced coastal community resilience enables populations at risk to live with risk, thus facing a wide range of coastal hazards with a greater degree of confidence. The United States Agency for International Development in Asia (USAID/ASIA), in support of the IOTWS, promoted a strategic approach to strengthening coastal community resilience to tsunamis and other coastal hazards. This approach, which has been developed via a consultative process with national stakeholders, provides an effective way of assessing coastal community resilience and is recommended for application. It reflects community understanding of critical risk issues and therefore is linked to capacity. The approach primarily deals with eight generic elements of coastal community resilience which are considered of pivotal importance in reducing risk from coastal hazards, efficient recovery and adaptation to change. Resilience has to be built up in each of these areas to ensure a well-balanced approach to strengthening community resilience. 1.5 Tools and methods for tsunami risk assessment Although a considerable amount of work has been carried out relating to tsunami risk assessment of whole countries, regions and cities, there is no uniform approach to the definition of risk assessment and to methods of risk assessment. The capabilities of the respective countries vary considerably with respect to the availability of expertise, tools for analysis and quality of data used for such analysis. Risk Assessment and Management for Tsunami Hazard 5

12 Having recognized the existing knowledge base, the WG-RA of the Intergovernmental Coordination Group for the establishment of the IOTWS produced two important guidelines with the support of member states and donor agencies. Probabilistic tsunami hazard assessment of the Indian Ocean nations (Burbidge et al, 2009) Tsunami risk assessment and mitigation for the Indian Ocean (UNESCO, 2009a) The latter was published as a UNESCO/IOC Guide. In addition UNESCO/IOC also published a broad guideline on coastal hazards, entitled Hazard Awareness and Risk Mitigation in Integrated Coastal Area Management (ICAM) (UNESCO, 2009b). The three guidelines above provide a broader understanding of coastal hazards in the context of ICAM, probabilistic tsunami hazard analysis, and tsunami risk assessment and mitigation for the Indian Ocean states. They provide useful guidance to implementing tsunami risk assessment and mitigation studies within the broader framework of integrated coastal area management. The guidelines provide sufficient flexibility in implementing such studies, giving priority to critical variables as applicable to the region or city under consideration. The case study presented in this document for the Port City of Galle has made use of these guidelines, focusing attention on critical parameters and the development of cost- effective risk assessment studies with the active participation of stakeholders. 2. City of Galle damage and mechanics of tsunami wave impact Many coastal cities of Sri Lanka, particularly l in the east and the south of the island, were severely er affected by the Indian Ocean tsunami, due to their exposure to the hazard. One of the principal coastal cities devastated was the historic Port City of Galle located in the southern province of Sri Lanka. Incidentally, the first recorded tsunami to have affected Sri Lanka was on 27th August 1883, arising from the eruption of the volcanic island of Krakatoa. On this occasion too, unusually high water levels followed by receding water were observed in Galle around 1.30 pm. The water level fluctuations were not severe and there was no inundation. The reported time of the occurrence corresponds well with the travel time for tsunami waves that would have been generated by the largest eruption of the volcano earlier in the morning. However, unlike this previous occurrence, on 26th December 2004 Galle received the severe impact of tsunami waves, their magnitude having increased due to nearshore transformation processes. Five hundred persons were killed, 90 others disappeared, 1000 were injured and 8120 people were affected in total. Houses and other buildings damaged numbered 1600 and 1300 respectively. Galle is one of many coastal cities around the world remaining heavily exposed to tsunami hazard. The presence of poorly constructed buildings and inadequate drainage contributed towards increased vulnerability. The tsunami waves which reached the offshore waters of Galle were primarily diffracted waves, diffraction having taken place around the southern coast of Sri Lanka. In the context of tsunami hazard, the location of Galle is heavily exposed. It lies beside a wide bay and a natural headland on which is located the historic Galle Fort with very reflective vertical non-porous walls on all sides. Furthermore, there is the Dutch Canal 6 Risk Assessment and Management for Tsunami Hazard

13 west of the headland, conveying water through the city centre. The waves in the vicinity of Galle, which were increasing in height due to reduced water depths, were further subjected to a series of nearshore processes which increased wave heights even further. The canal was a facilitator in conveying the massive wave and associated flow towards the city centre. In the vicinity of the headland on which the Galle Fort is located, the wave energy concentrated due to refraction. These waves then reflected from the vertical solid walls of the fort and moved around the headland. The walls reflected almost all the incident wave energy with very high wave heights at the wall itself. There was hardly any dissipation. On the west of the headland the waves moved ferociously into the Dutch Canal. On the east they moved along the bay. The wide bay in Galle further contributed to the increase in wave heights by modifying the shoaling process via reduced wave crest width to accommodate the bay shape. The combined effect of this phenomenon and the wave coming around the eastern side of the fort caused a massive wave of destruction along the Marine Drive (see Figure 1). It is certainly not surprising that many survivors referred to a large black moving wall similar to that of the Galle Fort. The City of Galle is therefore not only exposed to tsunami waves which diffract around the southern part of Sri Lanka, it is even more exposed to nearshore coastal processes which further increase wave heights. This aspect is identified as increased exposure within the risk assessment framework. Figure 1: Galle Bay and headland Bay increase of speed, height and circulation Headland concentration of energy and spreading around the headland Risk Assessment and Management for Tsunami Hazard 7

14 3. Tsunami hazard analysis 3.1 Approaches to tsunami hazard analysis Tsunami hazard analysis focuses on three areas: the tsunami hazard sources, exposure and the potential impact on land. Tools and methods available to study the hazard include field investigations, image analysis and mathematical modelling. The latter includes both deterministic and probabilistic tsunami hazard modelling. With respect to tsunami hazard sources, attention is focused on previous events (their location, magnitude and sequence), seismic gaps and the identification of credible scenarios. When examining the exposure at a given location, due attention must be focused on submarine geological features, the regional location that will identify the influence of key wave transformation processes, and the location with respect to continental shelf and shoreline geometry. Depending on these aspects, the amplitude of the tsunami wave may be enhanced, as was observed in the City of Galle. Tsunami impact on land can be studied by measurements from field instruments, post- event field observations and satellite images. In the aftermath of the IOT, attention has also focused on paleo-tsunami research work. 3.2 Post-tsunami field investigations Several studies were undertaken to measure the impact of the tsunami. The universities of Ruhuna and Moratuwa undertook studies to assess the inundation, flow directions and damage to buildings and infrastructure. The assessment inundation was carried out in an organized manner by dividing the area under study into 250m x 250m grids. At least one location was selected for each grid and a total of 138 points were selected for the study. People living within the respective areas were interviewed for all grids. Information on inundation depths and flow directions was obtained together with associated parameters of the hydraulic regime. The collected data were used to identify: The inundation profile comprising depth, length, run-up and spatial distribution Inundation contours with wave direction and information that could help to estimate the flow speed Distance from the sea along the tsunami flow path Relevant information for risk assessment. This data generated a tsunami hazard map for the City of Galle based on field measurements. Separate studies were undertaken to study the damage to buildings and infrastructure, and the specific flow regimes generated by the location and spacing of buildings. Those who survived the tsunami were able to describe devastating impacts of flow regimes, such as jetting effects which occurred along the streets running between rows of buildings. 8 Risk Assessment and Management for Tsunami Hazard

15 The results of the study were also useful in identifying the following features for the Indian Ocean Tsunami scenario: Evacuation routes and refuge areas Safe areas and safe buildings Proposed locations for fixing signboards on evacuation routes. Figure 2 illustrates the data collection points and Figure 3 illustrates the inundation depth and the direction of incoming wave. Wave directions and inundation heights were established after the interviews with the people. Figure 4 illustrates the inundation contours established from field investigations. This represents the tsunami hazard map for the Indian Ocean tsunami prepared from extensive field measurement. Figure 2: Data collection locations Figure 3: Inundation depths and wave directions Risk Assessment and Management for Tsunami Hazard 9

16 Figure 4: Inundation contours Above 3.5 m m m m m 3.3 Deterministic tsunami hazard modelling Deterministic tsunami hazard modelling comprising deep water, nearshore and inundation modelling was carried out with three objectives: To study overall exposure of the island to a given hazard source To simulate the IOT and compare with field measurements on the height, inundation length and run-up To simulate potential tsunamis based on credible scenarios from geologic and seismic studies. The results from modelling of different credible scenarios provide a database of the key parameters relating to inundation and the flow regime. Inundation height, length, distribution, run-up and velocity are some of those parameters. These parameters can then be used for the development of critical hazard scenarios by relating them to threshold values for the security of people and infrastructure. The modelling studies generated a valuable database which can be used for multi scenario-based risk assessment. It was evident that the IOT represented a worst-case scenario. The numerical modelling procedure is described below for a few alternative scenarios. Numerical modelling of tsunami phenomena was carried out to obtain information on the coastal region of Sri Lanka that could be affected by potential tsunamis. General coarse-grid modelling was carried out for the coastal region in the southern parts of the island and detailed fine-grid modelling, including tsunami run-up and inundation, was carried out for the City of Galle. The results of this exercise were used for the preparation of hazard maps for the City of Galle for different scenarios based on mathematical modelling. Generation and deepwater propagation of the tsunami waves were modelled using the AVI-NAMI model. The module for co-seismic tsunami generation of AVI-NAMI uses the method developed by Okada (1985) and the module for tsunami propagation solves Non-linear Shallow Water Equations. The ANUGA fluid dynamics model based on a finite-volume method for solving Shallow Water Wave Equations was used for the inundation modelling. In the ANUGA model the study area is represented by a mesh of triangular cells. 10 Risk Assessment and Management for Tsunami Hazard

17 The model has the flexibility to change the resolution of the mesh according to the area of importance. A major capability of the model is that it can simulate the process of wetting and drying as water enters and leaves an area, and is therefore suitable for simulating water flow onto a beach or dry land and around structures such as buildings. High-resolution nearshore bathymetric data obtained for the new Galle Port Development (2007) and high-resolution topographic data obtained after the 2004 tsunami were used for study (LIDA Surveys, 2005). Broad-scale deepwater propagation modelling was carried out for a number of source scenarios selected from the Sunda/Java Trench. The results of four selected scenarios are presented here. A fault length of 500 km, a width of 150 km, a dip angle of 8, a slip angle of 110 and a displacement of 40 m was used for the study. Table 1 gives the source details and the maximum and minimum wave amplitudes from the propagation modelling. Figure 5 provides snapshots of tsunami propagation for these four scenarios 180 minutes after the earthquake. Figure 6 illustrates the distribution of computed maximum tsunami heights over the Indian Ocean. Table 1: Source details and the maximum and minimum wave amplitudes from the propagation modeling Longitude Latitude Strike angle Max. amplitude (m) Min. amplitude (m) Scenario E 8.52 N Scenario E 3.09 N Scenario E 2.07 N Scenario E N Figure 5: Snapshots of tsunami propagation in four scenarios 180 minutes after the earthquake (a) Scenario 1 (b) Scenario 2 (c) Scenario 3 (d) Scenario 4 Risk Assessment and Management for Tsunami Hazard 11

18 Figure 6: Distribution of computed maximum tsunami heights over the Indian Ocean (a) Scenario 1 (b) Scenario 2 (c) Scenario 3 (d) Scenario 4 Based on the results of the deep water model, inundation modelling was carried out using the ANUGA model. Modelling results give valuable information about the coastline of Galle that could be affected by potential tsunamis. The model results are very useful for the preparation of hazard maps. Figure 7 gives the inundation modelling results in four scenarios. 12 Risk Assessment and Management for Tsunami Hazard

19 Figure 7: Deepwater propagation and inundation modelling for Galle (a) 2m_20min tsunami wave (b) 3m_20min tsunami wave (c) 4m_20min tsunami wave (d) 5m_20min tsunami wave Inundation depths in metres Probabilistic tsunami hazard modelling Probabilistic tsunami hazard modelling seeks to assess the probabilities of certain wave heights being exceeded due to the arrival of a tsunami at locations studied. These probabilities are expressed in terms of expected return periods. A probabilistic tsunami hazard assessment of Indian Ocean nations was carried out under the leadership of Geoscience Australia (Burbidge et al, 2009). Two views quickly emerged in discussions among the panel of developers. The hazard assessment should, on the one hand, avoid over-estimating the hazard by considering only those sources for which there is solid evidence for generation of large tsunamis. On the other hand, the assessment should be careful not to miss source zones that may generate large tsunamis even if they have not done so historically as was the case for the 2004 Indian Ocean Tsunami (IOT). The panel decided these two views could best be accommodated by developing two assessments, referred to here as low-hazard and high- hazard end-member assessments. This Risk Assessment and Management for Tsunami Hazard 13

20 affords a clear expression of uncertainty in the degree of hazard, shown as the difference between the two end-members. It was hoped that any blurring caused by the application of the two assessments to mitigation would be manageable. The geographic pattern of the low-hazard assessment is broadly reflective of the impact of the IOT. The high-hazard assessment, on the other hand, highlights areas potentially threatened by local tsunamis, such as the western Makran and southern Java coasts, which are the areas of highest uncertainty in the hazard assessment. These studies led to the development of a range of maps providing the information described below. Hazard curves the relationship between the return period and the maximum tsunami amplitude for a particular model output point. Maximum amplitude maps the maximum tsunami amplitude that will be exceeded at a given return period for every model output point in a region. Probability of exceedance maps for a given amplitude, the annual probability of that amplitude being exceeded at each model output point in a region. Disaggregated hazard maps the relative contribution of different source zones to the hazard at a single location. National weighted disaggregated hazard maps indication of the source of the hazard to a nation or region as a whole. For Sri Lanka, the low-hazard and high-hazard maps are very similar in character, with maximum hazard along the east coast, and the high-hazard case deviating from the low-hazard case by about 30 per cent. Since both high-hazard and low-hazard cases for Sri Lanka are dominated by the events in North Sumatra and the Nicobar Islands, it seems that the IOT was a worst-case scenario. 3.5 Hazard analysis for risk assessment Hazard analysis for risk assessment can be undertaken either via a multi-scenario-based or an event-based approach. For risk assessment against the tsunami hazard it is important to assess scientifically and establish the basis and criteria on which such an exercise is carried out. In developing hazard maps for risk assessment, it is necessary to develop hazard levels for which many approaches are available. In a multi-scenario-based approach, the superimposition of the impact of scenarios will clearly indicate areas which have a greater likelihood of being affected. The probability of inundation and its magnitude can be assessed by several methods. In an event-based approach, attention is focused on individual events, for example, on a worst-case event with specific frequencies of occurrences and impacts. After a mega-tsunami such as the IOT, it is customary to focus attention on planning strategically for such events, for example, considering the location of hospitals, power stations, water treatment plants and other critical facilities. Even if planning is based on observations arising from a single extreme event, it is important to analyse the impacts of credible scenarios and operate within a framework of multi-scenario-based approaches. This provides a justification for the use of an eventbased approach. 14 Risk Assessment and Management for Tsunami Hazard

21 For the City of Galle, it was decided to develop an initial risk assessment based on the IOT, which represents the worst-case hazard scenario. Results from both field investigations and deterministic tsunami hazard modelling were available, and a good comparison existed between the two. These results could be used for long-term strategic planning and location of important buildings, and for the identification of evacuation routes and safe places or buildings, preparation of evacuation plans and placement of signposts for the benefit of the community at risk. Hazard levels were developed based on inundation and flow speeds. The four hazard levels were classified as given below. High Inundation level above 0.5m with high flow/current speeds (>1.5 m/sec) Medium Inundation level between 1m and 2m with low flow speeds Low Inundation level less than 1m and low flow speeds Very low (zero) Very small or no inundation impacting humans Results from investigations conducted by the Ports and Airports Research Institute, Japan, on the resilience of both men and women against tsunami currents, were considered in developing these hazard levels. Figure 8 shows the hazard map developed from this analysis. Figure 8: Hazard map Hazard level High hazard Medium hazard Low hazard Zero hazard 1 km Risk Assessment and Management for Tsunami Hazard 15

22 4. Vulnerability 4.1 From a single dimension of susceptibility ity to a three-dimensional e io sector approach The assessment of vulnerability remains a complex area, in view of the widely varying parameters associated with a detailed analysis, and also due to the difficulties in defining and quantifying certain parameters. The basic components of vulnerability can be broadly classified as human, physical, socioeconomic, environmental, functional and administrative. This approach is primarily a one-dimensional approach. Reviews of recent posttsunami vulnerability studies indicate that the greater focus has been on potential loss of life and damage to houses and dwellings. Only a few studies have considered other aspects in detail. Villagran de Leon proposed a framework to break down vulnerability assessment into components by analysing how disasters can impact the different sectors of society (Villagran de Leon, 2008). The sector approach identifies dimensions of vulnerability in three areas, namely dimensions of susceptibility, sectors and scale. In effect, the dimensions of sectors and scale are added to the existing dimension of susceptibility. Typical sectors identified are housing, communications, education, health, energy, government, industry, commerce, finance, transportation, public infrastructure, environment, tourism etc. Figure 9 illustrates this concept. The framework proposes differentiation within each sector in terms of several areas relating to susceptibility, namely human, physical, socio-economic, environmental, functional and administrative factors. These areas relate to factors identified by the United Nations International Strategy for Disaster Reduction (UNISDR) as Figure 9: The three-dimensional sector approach showing dimensions of vulnerability Geographic level dimension National State or province District or municipal Local or community Single unit or house Housing Basic lifelines Health Education Agriculture Energy Infrastructure Commerce Industry Telecommunications Finance Physical Functional Economic Human Administrative Environmental Dimension of components Dimension of sectors 16 Risk Assessment and Management for Tsunami Hazard

23 increasing the susceptibility of communities to the impact of a hazard. The third dimension of scale expands consideration from household to national level, including city, district and provincial levels. The advantage of this approach, in particular from a policy point of view, is that it promotes the effective assignation of responsibilities relating to the reduction of vulnerabilities. This approach was applied in detail to a benchmark project for the City of Galle and its advantages are clearly seen. In particular, it is easy to understand the factors that maintain, reduce or increase vulnerability levels. The application of this method requires a great deal of data and time. However, on the strength of this study, it is possible to adopt a simplified approach with reduced but critical parameters, and then compare the output of both single-dimension and threedimensional studies. 4.2 Simplified approach to vulnerability A simplified approach will focus attention on critical parameters of interest identified in consultation with all stakeholders. The critical parameters applicable to the City of Galle were: population and its distribution buildings, infrastructure and their status exposure to the hazard distance from the sea elevation capacity to evacuate (within the broader framework of awareness, preparedness, early warning, response and safe evacuation) impact on livelihoods. In addition, attention was focused on the profile of the occupants, their sources of income, household economic levels, conditions of buildings and infrastructure, and community knowledge. Based on the above, it is possible to develop four levels of vulnerability, namely high, medium, low and very low (zero). According to this approach, a level of high vulnerability can be classified as the presence of a large population within short distances of the seafront, at low elevations with direct exposure to the sea, within an environment of easily damaged infrastructure, with easily disrupted livelihoods and without the capacity to evacuate quickly. Similarly it is possible to identify other levels of vulnerability. Figure 10 presents the vulnerability map developed from this analysis. A comparison between this study and the detailed study using the sector approach indicated that, provided the critical parameters are duly identified and evaluated, the simplified approach provided an effective method for vulnerability analysis, based on which risk assessment can be undertaken with confidence. The selection of critical parameters, through a process of scientific analysis and stakeholder consultation, has to be given high priority in the simplified approach. Risk Assessment and Management for Tsunami Hazard 17

24 Figure 10: Vulnerability map Vulnerability level High vulnerability Medium vulnerability Low vulnerability Zero vulnerability 1 km 5. Risk assessment sessm The risk map can be prepared ed by superimposing the hazard and vulnerability maps. In doing so, the method adopted must be clearly identified so as to recognize potential limitations i in applications. In this study, the hazard and vulnerability maps, each comprising four levels of classification high, medium, low and very low (zero) have been prepared. For superimposing them, it is necessary to establish the criteria for risk levels. A high level of hazard superimposed on high, medium or low levels of vulnerability (or vice versa) is classified as a high level of risk. A medium level of hazard with a medium level of vulnerability is also rated as a high level of risk. A medium level of hazard with a low level of vulnerability is rated as a medium level of risk or vice versa. Finally a low level of hazard with low-level vulnerability is rated as a low level of risk. If either the hazard or vulnerability is very low (zero), then risk is considered to be zero. Figure 11 shows the risk map developed using the above analysis. It is emphasized that other criteria can also be used for this type of analysis. It is noted that sufficient information is available to conduct a multi-scenario-based hazard analysis and levels of risk could have been developed using such an analysis. This study focused on event-based hazard analysis, which can be used for strategic planning of buildings and infrastructure, and preparedness for safe evacuation. The IOT certainly represents a worst-case scenario but other scenarios can be used to identify the potential impacts from events with a higher possibility of occurrence. 18 Risk Assessment and Management for Tsunami Hazard

25 Figure 11: Risk map Risk level High risk Medium risk Low risk Zero risk 1 km 6. Managing Risk Classification and planning risk management measures 6.1 Classification of risk management measures There are many measures that could be adopted for risk management in coastal zone management when planning for a tsunami and other coastal hazards that accompany high waves and high inundation. These include early warning systems, regulatory interventions in the form of extending existing setback defence lines, and physical interventions such as the protection of structures and utilizing the full potential of coastal ecosystems. These should be supplemented with public awareness of disaster preparedness and efficient evacuation procedures incorporating planned evacuation routes. Measures for risk management can be broadly classified into three categories, namely, those that mitigate the impact of the hazard, those that mitigate exposure and vulnerability to the hazard, and those that promote successful evacuation Measures that mitigate the impact of tsunami hazard 1. The implementation of artificial measures for protection, including tsunami breakwaters, dikes and revetments 2. The effective use of natural coastal ecosystems including coral reefs, sand dunes and coastal vegetation (mangrove forests) 3. Hybrid systems of artificial and/or natural systems Measures that mitigate exposure and vulnerability to the tsunami hazard 1. Land-use planning 2. Regulatory interventions such as setback of defence lines 3. Hazard-resilient buildings and infrastructure Risk Assessment and Management for Tsunami Hazard 19

26 6.1.3 Measures that promote successful evacuation from tsunami hazard 1. Early warning systems (local and regional) 2. Public warning systems 3. Evacuation routes and structures 4. Community education, including community maps and other measures for community preparedness Hazard, vulnerability and risk maps play a vital role in risk management. In view of the benefits of risk management measures, it is important to upgrade these maps regularly. These maps can be used for the production of disaster preparedness or management maps. 6.2 Planning risk management measures via policy and management options Post-disaster planning should be undertaken in the context of overall coastal hazards, including tsunamis. Although the chances of an extreme event such as that of 26th December 2004 taking place are low, the impacts of such events set the agenda for strategic planning and location of key infrastructure. It is recognized that a coastal hazard protection plan for the city should be an integral part of an overall coastal zone management plan based on various policy and management options. These options should reflect a strategic approach to achieving long-term stability, in particular for sustaining multiple uses of the coastal zone and giving due consideration to the threats and risks of hazards. Policy and management options should be formulated on a sound scientific basis, preferably within the prevailing legal and institutional frameworks. However, if the need arises, institutional improvements should be effected and new laws should be imposed. In this process, a high priority should be given to stakeholder participation. Extreme care has to be exercised when obtaining the active participation of stakeholders who have witnessed disaster and suffered heavily in loss of lives, property and livelihoods from one of the most severe natural disasters to have affected the region. Most of them require a long period to recover completely from their traumatic experiences. Policy options identify possible courses of action on shoreline, such as: Maintenance of the existing defence line Setback of the defence line Retreat Advance In order to implement the policy options, various management options are proposed that may be appropriate for the coastal area. They are summarized as: Do nothing Reinstate previous conditions Modify the existing design Develop new design Once the risk assessment study is completed, mitigation options should be developed within the framework of policy and management options, giving due consideration to stakeholder consultations. 20 Risk Assessment and Management for Tsunami Hazard

27 6.3 Classification of physical interventions (artificial and natural) In light of the discussion in Sections and 6.2, mitigation by physical intervention is classified into three types, depending on location and function in protecting the coast. These interventions may be achieved by artificial methods via coastal engineering design, and also by harnessing the full potential of natural coastal ecosystems. The types of interventions and typical examples for each category are listed below. 1. Reduce the impacts of tsunami waves prior to reaching the shoreline e.g. tsunami breakwaters, coral reefs 2. Protect the coastal zone by preventing the inland movement of tsunami waves e.g. tsunami dikes, sand dunes 3. Mitigate the severe impacts of tsunami waves on entry to the shoreline e.g. tsunami dikes, revetments, mangrove forests On many occasions, both artificial and natural methods can be adopted in parallel to achieve well-integrated hybrid solutions satisfying environmental concerns. 6.4 Development of guidelines for tsunami-resistant buildings The coast is an area of high economic activity and it is not possible to transfer all activities to areas that are completely free from potential tsunami hazards. For some areas of the coast, safe evacuation areas may be too far away for citizens to reach on foot, thus necessitating vertical evacuation structures. Therefore there is a need to develop design guidelines and construction manuals for tsunami-resistant housing and infrastructure for the benefit of the public. Given the background of discussion in Sections and 6.2, two types of guidelines are required: Overall design guidelines providing advice on location, layout, orientation, structural configuration, geotechnical considerations and other considerations relating to good design practice. Detailed design guidelines on hydraulic and structural loads, geo-technical issues and detailed design information. The overall design guidelines could be developed from the experience gained by damage assessment in different parts of the country. Such assessment should be analysed in the context of the hydraulic regime generated by the tsunami at that location. In particular, local effects which can enhance the impact of tsunamis have to be taken into consideration. Relevant information from other countries affected by tsunamis will be very useful for this exercise. It is important that damage assessment should cover a variety of infrastructure that was destroyed or damaged, or that survived with fewer impacts. 7. Mitigation tion and its integration with development elop projects In 2000, Japanese Port Consultants s (JPC) developed a master plan for the development of the Port of Galle. le. In view of environmental issues, s, they recognized that development elopment should be restricted to a two-berth medium-sized harbour. In order to maintain healthy tidal flows for the well-being of the coral reef system located on the eastern side of the bay, JPC in consultation with environmental specialists incorporated an offshore detached breakwater, which coincidentally has all the characteristics of an effective tsunami breakwater (see Figure 12). It must be admitted tsunamis were not part of the agenda of the engineering and environmental terms at that stage. Risk Assessment and Management for Tsunami Hazard 21