The rationale for flooding
1.1.1 The rise in flood hazards
Flood incidents lead to significant losses and damages globally, with a review by Doocy et al (2013) revealing that flooding caused 539,811 deaths and impacted 2.8 billion people between 1980 and 2009 Classified as the most common natural disaster, floods are defined as events that overwhelm local resources, prompting requests for national or international assistance, and often result in extensive damage and human suffering (Guha-Sapir, 2016) Since 2000, the frequency of flood events has surpassed that of other disaster types.
Annual economic losses from floods are projected to rise significantly, from $6 billion in 2005 to $52 billion by 2050, largely due to climate change factors such as sea level rise and intensified tropical cyclones (Nicholls, 2011; Nicholls et al., 2014) The shifting causes and accelerating impacts of floods, driven by demographic growth and urbanization, will be felt most severely in coastal cities (Lamond et al., 2012; Gornitz, 1990) Recent events, like Hurricane Harvey in Houston and Hurricane Irma along the American coastline in 2017, highlight the urgent need to address these escalating risks.
Coastal cities are urban areas situated on the low elevation coastal zone (LECZ), typically defined as land less than 10 meters above sea level The number of coastal mega-cities has risen significantly, increasing from 13 in 1990 to 20 in 2010, highlighting their vulnerability to rising sea levels This trend is part of a broader pattern of urban areas becoming more susceptible to disasters, particularly extreme floods, due to suburban expansion into environmentally hazardous zones In Asia, floods have been occurring with alarming frequency, as noted by Dutta (2011), who documented natural disasters over a 27-year period The impact is severe, with deadly floods in South Asia affecting 16 million people across countries like Nepal, Bangladesh, and India in 2017.
3 by Hanson et al (2011) shows that the majority of the top 20 cities with a population exposed to climate change impacts are located in Asia (Figure 1.2) a) b)
Since the 2000s, the frequency of flood-related natural disasters has surged compared to other events like earthquakes, storms, and droughts Notably, Asia has experienced a significant rise in flood occurrences, highlighting a concerning trend in flood distribution across continents.
1.1.2 Increasing flood vulnerability in emerging coastal cities
Flooding poses a significant threat to urban transportation and is categorized into various types, including fluvial flooding, which occurs when river water levels exceed their banks, and pluvial flooding, resulting from excessive rainfall on impermeable surfaces Additionally, flood vulnerability refers to the human impact of flood hazards, indicating the potential extent of flood effects, both currently and in the future.
As such, coastal cities are being increasingly considered to be more vulnerable to flooding, due to a combination of climate change and rapid urbanisation
As urban populations continue to grow, particularly in coastal cities, these areas are becoming increasingly vulnerable to flooding Low-lying coastal land, which constitutes only 2% of the Earth's surface, now accommodates 13% of the global urban population, with projections indicating that 75% of people will live in urban settings by 2050 This rapid urbanization is leading to larger settlements that face significant flood risks, with an estimated 46 million individuals at risk from storm surges annually Economic incentives, such as the allure of waterfront living, are driving people and assets into flood-prone regions, particularly along major water bodies Moreover, unchecked urban development is encroaching on vital natural defenses like wetlands and mangroves, which have historically mitigated flooding Research indicates a concerning trend of coastal wetland loss due to urban expansion in developing countries, exacerbating the vulnerability of these coastal communities.
Emerging coastal cities in Southeast Asia are experiencing significant development challenges, particularly in low-income countries that lack the financial resources to prepare for urban flooding This situation poses a serious threat to both development and the livelihoods of residents in these rapidly expanding urban areas.
According to the World Bank's 2010 synthesis report, "Climate Risks and Adaptation in Asian Coastal Megacities," Bangkok, Manila, and Ho Chi Minh City (HCMC) are identified as vulnerable hotspots facing climate change impacts Both Manila and Bangkok have suffered significant flooding events in 2009 and 2011, respectively, while HCMC has experienced smaller-scale floods, presenting an opportunity for proactive mitigation measures HCMC is recognized as one of the three coastal cities at risk from climate change effects, such as rising sea levels, and ranks among the top 20 cities with populations exposed to these threats There is growing concern that HCMC could face an extreme flooding event similar to that of Bangkok in 2011, especially given the city's lack of comprehensive long-term urban development plans.
Figure 1 2 Top 20 cities (yellow circle) with a population exposed to climate change impacts by
2070 Adapted from Hanson et al (2011)
1.1.3 Flood resilience: an appropriate strategy for transport development shifting from existing resistance as cities becoming vulnerable to flooding
Many cities worldwide have constructed traditional flood defenses like dams and levees to protect urban areas from flooding While these structures have been effective in reinforcing riverbanks against tidal floods, their design limitations based on historical data mean they can only protect against certain flood levels For instance, the Kuala Lumpur Smart Tunnel, built in 2007, initially succeeded in managing urban flooding but was deemed insufficient during an extreme flood in 2011 due to unexpected rainfall and uncontrolled urbanization As cities face increasing climatic uncertainties, reliance on these engineering solutions is being questioned, especially since they cannot account for all future flood threats This issue is particularly pressing in Southeast Asia, where rapid urban growth in vulnerable areas outpaces the development of adequate flood protection systems Therefore, there is a pressing need to explore alternative strategies to address these vulnerabilities effectively.
Given the increasing challenge of designing effective resilient systems in the light of climate change, urban resilience has become a prominent concept in many growing cities (Stern,
Increased vulnerability due to urban growth can be mitigated through resilience strategies, which offer a sustainable approach to reducing flood risks (Balica et al., 2012; Berkes, 2007; Turner et al., 2003) Resilience is defined as the capacity of social or ecological systems to absorb disturbances while maintaining their fundamental structure and function, as well as their ability to adapt to changes (IPCC, 2007) This concept, often referred to as a social-ecological system, emphasizes the role of actors in managing resilience (Walker and Meyers, 2004; Berkes et al., 2007) Resilience theory has evolved across various interdisciplinary fields, focusing on engineering and ecological perspectives (Holling, 1996; Liao, 2012) In urban planning, resilience has developed as a conceptual framework that informs practical planning and enhances urban physical infrastructures, including transportation systems (Zevenbergen et al., 2008; Liao, 2012; Dezousa and Flanery, 2013; Tierney and Bruneau, 2007; Rogers et al., 2012; Xenidis and Tamvakis, 2012).
Transportation is a crucial urban element that drives productivity, economic growth, and quality of life, making flood disruptions to transport networks a significant concern As an essential part of urban infrastructure, the transport system facilitates the movement of people, goods, and services, supporting social activities and community development Additionally, it plays a vital role in enhancing urban resilience against extreme flood events, particularly in coastal cities However, the challenge of engineering flood-resistant infrastructure is growing, and the expansion of urban areas, especially in low-lying regions, directly increases the vulnerability of transport routes to flooding.
Urban transport systems in areas like Ho Chi Minh City are increasingly vulnerable to hydro-meteorological changes, including high tides and extreme rainfall To maintain efficient operations during flood events, it is crucial to enhance adaptive capacity This improvement is vital not only for managing serious floods, which require effective aid and evacuation measures, but also for minimizing daily disruptions and preventing broader impacts across various sectors.
This thesis argues for enhancing flood resilience in urban transportation systems as a more effective strategy than merely investing in flood defenses By prioritizing robustness and redundancy—key traits of resilient infrastructure—urban transport can better mitigate losses and damages This approach increases travel options through alternative routes and appropriate transport modes Dezousa and Flanery (2013) propose that urban resilience should be developed through a comprehensive framework encompassing the planning, design, and management of urban components, including transportation, although some areas require further refinement.
Engineering flood resilience focuses on maintaining a stable state in systems like urban transport by implementing enhanced structures to manage moderate floods, such as alternative roads for network continuity In contrast, ecological resilience emphasizes the survival of the system, prioritizing transport accessibility during emergencies, exemplified by high elevated roads for evacuation While numerous studies advocate for the ecological approach for sustainable urban development, the engineering perspective remains crucial for effective transport development.
9 demand for urban physical infrastructure development Hence, while these two aspects are normally distinguished for understanding in general theory; they are usefully coordinated together in practice
However, resilient interpretations are still in their infancy (Carpenter and Brock, 2008; Folke,
The theory of resilience requires further contextual applications tailored to the unique conditions of various cities It is essential to adapt this theory to specific fields, particularly urban planning, by incorporating the expertise of professionals like planners and architects However, the challenge lies in effectively translating resilience objectives into practical urban settings.
Study area
1.2.1 Urban development: economic prosperity and transport development
Ho Chi Minh City (HCMC), situated 50 km inland from the sea in southern Vietnam, covers an area of 2,095 km² and has a registered population of 8.2 million, though the actual population is estimated to be around 10 million This number is projected to rise to 12 million by 2025, according to the World Bank.
Between 2010 and 2016, Vietnam's largest coastal city experienced a remarkable growth rate of approximately 9.6% per year, contributing over 20% to the national GDP (Investment and Trade Promotion Centre of HCMC, 2015) Notably, it ranks second only to Delhi among the top 30 Asian cities for projected gross domestic growth through 2021 (Oxford Economics, 2016).
The larger HCMC region, encompassing Binh Duong, Binh Phuoc, Tay Ninh, Long An, Dong Nai, Ba Ria-Vung Tau, and Tien Giang, significantly contributes to the economy, driving immigration to the area In response to the increasing demand for housing, new development districts have swiftly emerged, aligning with the urban spatial development plan.
From 2020 to 2025, as outlined in decree 03/CP/1997 by the central government, urbanization has rapidly expanded from the historic central areas on elevated land, now referred to as the core of zone 1, to the developing suburbs located in zones 2 and 3.
New urban developments are primarily located on the flood-prone eastern side of the Saigon River, increasing the exposure of people and assets to extreme flooding (ADB, 2010) Although several transport investments have been made to connect these areas with the city center, particularly towards the East, the focus remains on horizontal expansion through ground-level infrastructure However, the threat of flooding has limited these investments due to high construction costs stemming from poor planning for a ground-based network in low-lying, flood-vulnerable regions.
Figure 1 3 Location of HCMC in Ho Chi Minh Region (HCMR), and in Vietnam
The region in the middle
Mekong River Delta in Vietnam
Zone 1 (central districts); Zone 2 (new development districts); Zone 3 (rural districts) Old town/ City centre; New city centre (Thu Thiem); 3 districts: 2,9,Thu Duc
Figure 1 4 Urban area division in different zones
Ho Chi Minh City (HCMC) is predominantly situated on a low-lying coastal plain, with approximately 40-45% of its territory lying above +1.0 meters ASL Most of these low-elevation areas are concentrated in the eastern and southern parts of the city, characterized by wetlands and a complex network of channels typical of urban hydro-meteorology in the Mekong Delta The Saigon River's annual high water levels have significantly increased, rising from +1.32 meters ASL in 1997 to +1.71 meters ASL in 2017, posing challenges for urban planning, especially in transport systems The proliferation of housing projects in these vulnerable areas has frequently disrupted transport accessibility, impacting daily commuting and overall urban activities.
3 new districts unprotected Recently expanded areas Sluices
14 indication to increasing flood vulnerability in this city, which needs more effective urban planning for long-term development
1.2.2 The plans for transport development in relation to flooding
In response to flooding challenges, building codes have been updated to require higher elevations for road developments in low-lying urban areas, raising them from +1.5 to +2.0 meters above sea level While this approach may temporarily alleviate flooding on certain local roads, it can adversely affect accessibility to housing and simply relocate inundation issues elsewhere For instance, Nguyen Van Huong Road in Thao Dien was initially designed to mitigate flooding, but has faced re-flooding due to rapid development in Thu Thiem, forcing residents to repeatedly elevate their properties, which disrupts their living environment This situation highlights the need for local governments to pursue more sustainable planning strategies that address long-term flood risks.
In line with the planning framework in Vietnam (Figure 1.5), several plans for the urban development of HCMC were completed during the period 2008 – 2010 (with some updates to
2013) Following the spatial plans according to a decision numbered 24/QĐ-TTg by the then Prime Minister, a plan for transport development was approved in 2013 (see Figure 1.6:
In 2013, TEDI-South highlighted the need for a revision of the urban development plan, including transportation, to be completed by 2020 This upcoming revision presents a valuable opportunity for significant improvements in transport development.
This research aims to integrate flood resilience concepts into the transport system, driving the need for adjustments in the revision process Initially, these changes will be reflected in the general plan (2A, figure 1.5) and later in the transport development plan (2B, figure 1.5).
The transport plan, a crucial component of infrastructure within general or district plans, emphasizes the interaction between various planning levels and the potential integration of the Flexible Road Transport System (FRTS) Figure 1 illustrates how FRTS can be effectively incorporated into transport plans within Vietnam's planning framework.
(including transportation and water discharge) (3A)
(inlcuding transportation and water discharge) (3B)
National/ Regional (HCMC + 7 surrounding-cities) (1)
Water resource and flooding management
Inner urban land; Ring road (existing); High-elevated road
Residential land; Ring road (planned); Other main road
Residence development Highway (planned); Existed rail
Land for army; National road; Planned rail
Green land; Provincial road; Monorail (planned) Wetland; Urban artery; Existed sea-port
Industrial land; Planned sea-port
Figure 1 6 Integrated map of transport plan for developments in HCMC by 2020
1.2.3 Increasing flood impacts and the inefficiency of resistant solutions
Historically, flooding was not a significant issue in Ho Chi Minh City (HCMC), with rare occurrences until the 1960s (Hong, 2011) However, due to rapid urban expansion, over 50% of HCMC's urban area now regularly experiences flooding (ADB, 2010) This increase in flooding has severely disrupted urban transportation, making the city's transport system more critical than ever Between 2003 and 2009, 680 road sections were impacted by floods (Phi, 2013), highlighting the growing challenge faced by the urban flood control initiatives in HCMC (SCFC, 2010).
Between 2010 and 2016, the frequency of inundations nearly doubled, reaching approximately 1,250 incidents This increase in flooding has significantly impacted urban transport systems, particularly affecting roads, as illustrated in Figure 1.7 The disruptions caused by flooding can lead to substantial economic losses for the city.
Despite significant investments in flood management solutions in Ho Chi Minh City (HCMC), including river embankments and sluices costing over USD 1.2 billion, the long-term effectiveness of these measures remains questionable Originally outlined in master plans approved in 2001 and 2008, these defensive structures were intended to protect the central area and parts of zone 2 along the Saigon River However, the city continues to face severe flooding challenges, as evidenced by large-scale flood events in 2015 and 2016, indicating that the flood problem persists and requires a reevaluation of current strategies The anticipated additional investment of USD 4.3 billion for future improvements underscores the ongoing struggle to effectively manage flooding in HCMC.
Recent data shows that there were 66 and 50 road locations experiencing significant inundations, highlighting the inefficiency of current investments and the inadequacy of existing development plans.
Flooding significantly impacts transportation, as evidenced by various scenarios For instance, the powerful floodwaters pose dangers to travelers on Nguyen Van Huong Road in District 2, while traffic jams are prevalent on Huynh Tan Phat Street in District 7 In response to these challenges, road developments have increased in elevation, such as on Luong Dinh Cua Street in Thu Thiem, District 2 However, some developments have created constraints at the entrances of private houses, and high-elevated roads have been constructed to alleviate traffic congestion without adequately considering flood resilience These examples highlight the need for strategic planning in urban infrastructure to address the dual challenges of flooding and transportation efficiency.
1.2.4 Research into flood vulnerability in HCMC
Research aims
In light of the ongoing flood challenges and the growing vulnerability of Ho Chi Minh City's transport system, there is an urgent need for effective long-term development plans to enhance resilience against extreme weather events This study seeks to build on previous research by analyzing flood risks in the city and creating a conceptual model for a Flood Resilient Transport System.
The Flood Resilient Transport System (FRTS) is derived from resilience theory in transportation, addressing the urgent need for a comprehensive flood vulnerability assessment in Ho Chi Minh City (HCMC) due to escalating flood risks This assessment will serve as a foundational application test case for HCMC, aligning with current urban planning frameworks Key objectives include: highlighting the factors contributing to increased flood vulnerability from rapid urbanization in flood-prone areas; identifying best practices in urban planning and the critical role of transportation in enhancing flood resilience in coastal cities; developing a conceptual model of FRTS tailored for HCMC to mitigate vulnerability; and proposing strategic adjustments to transport planning that align with Vietnam's urban development framework.
Thesis structure
The thesis is structured into eight chapters and two appendices, with chapters 4 to 6 published as three papers included in appendix B Additionally, appendix A provides anecdotal evidence of increasing flood severity observed during field trips.
Chapter 1 has introduced the global context of flood hazards to coastal cities, especially in developing countries in Southeast Asia A brief summary of contemporary approaches to flood vulnerability in relation to the emerging resilience theory is also introduced The main case study city, HCMC, is also outlined and highlights how its new economic prosperity has become a driver for revising urban plans under urban planning framework in Vietnam
Finally, this chapter also introduces the current flood situation and the inadequacy of on-going plans for transport development as the rational for the aims of this research
Chapter 2 reviews the background theories of flood vulnerability and resilience, particularly with respect to transportation Using an approach to investigate increasing flood vulnerability using three indicators in an urban context, the role of resilience is discussed to address the need of planning for transport development using the properties of a city level resilient system Flood thresholds related to urban floodable areas is also developed in theory in order to look for potential coordination to classification of the elevations of the transport network, as a basis for vulnerability assessment These have become the fundamental principles to conceptualise a model of flood resilient transport system
Chapter 3 presents the research design and methods Based upon the research aims and informed by the literature reviews, this research uses different methods, combining hydrological modeling and GIS analysis This requires a combined methods approach and the context for this is set via introducing the concept of flood vulnerability assessment for resilience development in this chapter The methodological review is also integrated in this chapter
Chapter 4 Using the case of HCMC, this chapter addresses the potential of increasing flood vulnerability, especially in new development districts where the rapid development of accommodation on floodplains is proving problematic The inaccessibility of the main transport routes between the old city centre and three of these new emerging-districts (on the eastern side of the Saigon River) is analysed as an increasing disincentive to urban resilience This chapter also introduces a vision for urban compactness development which is relevant to
24 the role of resilient transport in connections between different urban high-density areas and perhaps appropriate for minimising new residential developments on floodplains
Chapter 5 compares three flooding incidents in New Orleans - USA, Manila – Philippines and Bangkok – Thailand, to synthesise generic lessons of urban resilience to flooding with the essence of transport accessibility when dealing with flood catastrophes and the implications for HCMC The chapter also highlights why HCMC and other emerging-coastal cities in Southeast Asia still have opportunities for city-wide resilient improvement, particularly with respect to potential revision of transport plans for development
Chapter 6 presents a conceptual model to develop the flood resilience of urban transport systems, regarding to the fundamental principles highlighted in the literature review (chapter
Chapter 4 highlights the growing flood vulnerability in Ho Chi Minh City (HCMC) and identifies specific local issues, while Chapter 5 emphasizes the importance of enhancing flood resilience through improvements in the transport system The current state of transport development in HCMC is also examined Additionally, this chapter outlines a flood vulnerability assessment process aimed at pinpointing urban areas and transportation segments at risk of flooding Simulation tests are presented through two scenarios to demonstrate the potential applications and benefits of the Flood Resilience Transport System (FRTS).
Chapter 7 presents the implications for inadequacy of current plans for transport development, in relevance to the actual causes and factors to flooding in HCMC For potential adjustments to the current plans for transport development, roadmap and blue-prints are proposed for potential application of the FRTS model in HCMC
Chapter 8 concludes the main findings and contributions for HCMC, and also informed to other coastal cities in Southeast Asian Countries (SAC) It also mentions the critiques, as well as suggestions for further work required
Appendix A : Collection of supplementary anecdotal evidence during the field campaigns Appendix B : Papers arising from this thesis
4 Increasing Vulnerability to Floods in
Paper 1 (published in International Journal of Climate Change Strategies and Management)
5 Urban Resilience to Floods in
Paper 2 (published in Journal of Urban Planning and Development)
Reduce Urban Vulnerability to Floods
Paper 3 (published in Journal of Travel Behavior and Society)
Urban growth: urbanisation and transport development in relation to flooding
2.1.1 Trends in developing countries in Southeast Asia
Driven by economic growth, cities all over the world are continuing to expand In 2010, 167 cities had over 750,000 inhabitants, a 10-20 fold increase in population since 1960 (UN,
As of 2014, over 50% of the global population resides in urban areas, leading to rapid urbanization that heightens environmental risks associated with climate change, including rising sea levels This trend is especially pronounced in major cities, which draw workers from nearby regions, resulting in a growing demand for housing due to urban agglomeration (Camagni et al.).
Cities are pivotal for technological innovation, economic growth, and environmental initiatives, which fuel urban population growth and urbanization For instance, urbanization in many American cities surged from 39% in 1890 to 53% by 1940, and then escalated to 75% over the next 50 years, with larger cities showing no signs of decline (Glaeser, 1998; Eaton and others).
Eckstein, 1995; Dobkins and Ioannides, 1996) The concentration of people in the 10 largest cities has fallen slightly since 1970 (from 23% to 21%), but has continued to increase in metropolitan areas (from 41% to 48.1%) (Glaeser, 1998)
Urban growth, originating in the 19th century due to industrialization, has persisted into modern times, often near major water bodies that facilitate transport Historical towns were typically established in areas with favorable conditions for settlement, such as access to clean water and elevated land to mitigate flood risks While development was planned with population limits, surrounding less favorable lands remained underutilized As urban populations rise, the demand for affordable housing drives property development, particularly in low-elevation coastal and river basin areas However, urbanization in these wetlands reduces permeable surfaces and water storage capacity, increasing the risk of flooding.
(2007) stated that a growing imbalance between the natural and human environment has led to a higher risk of urban flooding
Emerging coastal cities in developing countries are witnessing accelerated urbanization, reflecting global trends A study by Angel et al (2005) forecasts that built-up areas will grow from 200,000 km² in 2010 to 600,000 km² by 2030, accompanied by a doubling of the population.
Developing countries face significant challenges as urban populations grow, particularly in coastal cities where climate change impacts are severe These cities often lack the financial resources and urban planning expertise necessary to mitigate risks, leading to increased vulnerability Research indicates that many coastal cities in Asia, such as Bangkok and Manila, are already experiencing severe flooding, with Ho Chi Minh City projected to face even greater risks in the future This situation underscores the urgent need for improved urban management and climate resilience strategies in Southeast Asia.
The emergence of new suburbs, driven by urbanization, significantly influences transport development, expanding networks from city centers to surrounding areas This trend, which dominated the 20th century, will continue to shape the 21st century, especially in developing nations, leading to increased concentration of people and assets in specific locations Consequently, urban mobility issues arise, particularly in major cities where diverse resident demands, lifestyles, and consumption levels coexist Additionally, changes in land use, as highlighted by Wang (2015), are closely linked to transport systems, stemming from decisions made by both residents and policymakers.
Transportation serves as a crucial connection for derived demand, with its network often enhanced alongside the development of physical infrastructure As populations expand into more affordable, peripheral areas, the demand for transport accessibility has surged, reflected in the increased length and frequency of vehicle trips.
The rising demand for transportation in developing countries is overwhelming existing transport systems, as these nations have seen a rapid increase in private vehicle ownership and public transportation needs that far exceed revenue growth for infrastructure Compared to developed countries, urban road allocation in these regions is significantly lower, necessitating key infrastructure investments such as highways, bridges, and metro lines to connect city centers with expanding urban areas To address the challenges of financial investment and increasing transport capacity, effective planning is essential, not only to alleviate congestion but also to mitigate environmental issues like flooding This integration of transport planning with urban development presents an opportunity to enhance resilience in these emerging cities.
Transport development poses significant challenges to planning and management, particularly in coastal cities where it interacts with existing river systems Historically, waterways have served as crucial transport routes in cities located on river basins since their inception Consequently, the intersection of transport networks with urban watercourses is unavoidable Additionally, the expansion of urban areas typically results in horizontal growth, further complicating this interaction.
The development of transport networks, including the number of routes and road widths, is essential for meeting increasing demand; however, long-term efficiency also hinges on addressing environmental challenges like flooding Hart (1993) emphasized that research should focus on the spatial effectiveness of existing roads instead of solely investing in new road construction or widening projects.
Coastal cities and those near river basins often exhibit diverse river networks due to the horizontal expansion of transport systems Hydrology is a fundamental constraint in land use, as highlighted by Rodrigue et al (2016) The growth of transport networks typically results in increased routes that either follow or intersect major water bodies Climate change is causing river levels to rise due to hydro-meteorological shifts, leading to a heightened risk of tidal flooding This poses significant threats to transport systems, especially on critical routes that facilitate urban activity but are situated near flood-prone areas.
Urban growth and the expansion of built-up areas have heightened the demand for improved transport systems, necessitating the extension of networks from older urban regions to newly developed areas While this expansion may increase the risk of flooding due to the intersection of transport structures and river networks, it also presents a significant opportunity to enhance flood vulnerability reduction plans Achieving a balance between horizontal and vertical development in urban transport systems can aid cities in managing flood risks and improve investment efficiency Furthermore, ensuring continuity through alternative links and nodes within the transport network is crucial for effective planning.
31 in emerging coastal cities in Southeast Asia if they can maintain economic prosperity as a driving factor for new investments in urban infrastructure.
Flood vulnerability
2.2.1 Definition, indications and relation with resilience to flooding
Climate change increases risks to people, properties, and ecosystems due to the interplay of hazards, vulnerability, and exposure Urban areas are particularly affected, as their activities are more susceptible to the adverse effects of climate change, highlighting the significant degree of vulnerability related to exposure.
Vulnerability refers to the potential for harm, which can be evaluated through indicators of exposure, susceptibility, and resilience This concept has been explored by various researchers, including Turner et al (2003), Berkes (2007), Balica and Wright (2009), Hufschmidt (2011), Scheuer et al (2010), and Willroth et al (2010), as well as Fuchs et al.
In 2011, Van-Beek proposed a systematic approach to assessing coastal flood vulnerability in large urban areas situated within deltas, emphasizing the need to consider various components, including natural systems, socio-economic factors, and administrative institutions This approach highlights that a comprehensive understanding of diverse issues is essential for effectively evaluating flood vulnerability.
- River networks as a part of the hydro-ecology system related to water levels, especially high tides as a flood factor;
- Presence of citizens and their daily activities (e.g urban commuting potentially exposed to flooding); and
- Authority for planning and implementation in relevance to the urban plans for deployment in practice
Along this, Balica et al (2012) developed an index to assess flood vulnerability consisting of three indicators, which are readily extendable to transport systems, particularly in transportation as follows:
FVI (Flood Vulnerability Index) = E (exposure) x S (Susceptibility) / R (Resilience)
Exposure in urban areas refers to the rising number of people and assets situated in flood-prone locations This concept also extends to transportation networks, where an increasing percentage of routes and links are anticipated to be impacted by flooding.
- Susceptibility: this considers the factors influencing the degree of flood impact arising from changes in urban hydrometeorology, which can have the potential to exacerbate the impacts
Resilience is the capacity to reorganize transportation networks by establishing alternative routes for daily travel and emergency evacuation, thereby maintaining essential transportation levels and adapting to varying flood conditions to mitigate impacts.
Transport links can be assessed for flood vulnerability if they are situated across flood plains, necessitating the use of flood risk maps for the city in question These maps can be developed through hydrological modeling, which allows for the incorporation of probabilistic data regarding flood depths By concentrating on critical nodes and links that are susceptible to flooding, such as river water levels predicted by hydrological models, a comprehensive analysis of flood vulnerability can be achieved.
The model's sophistication increases with the incorporation of detailed elevations and attribute data, such as localized flood defenses While this data may not always align with the resolution of existing digital elevation models, combining it with observations or manually collected information can enhance its potential applications.
This approach allows for the examination of current resistance and the potential effects of planned developments on transportation and infrastructure It enables an exploration of the resilience of the broader network For instance, by combining data from experimental studies, safety literature, expert insights, and anecdotal video observations, Pregnolato et al contribute valuable findings.
(2017) identified a 30cm deep flood as being passable for cars travelling on flooded roads a) b)
Figure 2 1 Clarification of flood vulnerability assessment in transportation a) Flood vulnerability: urban areas/transport segment b) Difference between “affected” and “vulnerable” level
Subways linked to main routes
High water level/ extreme floods
Moderate water level/regular floods
Flood vulnerability differs from inundation levels in terms of occurrence probability, influenced by factors such as water levels and time scales Inundation occurs when an urban area, like a road, is already submerged, while flood vulnerability indicates areas that may be at risk of flooding due to worsening conditions, such as increased rainfall or higher tides Understanding this distinction aids in analyzing flood situations and projecting potential impacts on urban environments, which is essential for enhancing resilience For example, a road redesigned at a higher elevation may still be considered vulnerable if rising tides exceed this new height However, focusing solely on current inundation levels can lead to short-term solutions that overlook the broader implications for flood resilience.
In relation to flood impacts on transportation, a vulnerable segment can result in
The inaccessibility of travel over longer routes or larger networks can significantly impact commuting, especially when linked to paths or entrances of resilient buildings Despite being elevated above current flood levels, these structures still face disruptions from flooding, affecting residents' activities Therefore, while higher elevation may mitigate localized flooding, adapting to vulnerable levels is crucial for addressing potential disruptions across the entire system This necessitates a thorough assessment of flood vulnerability through simulations of possible extreme floods in the future.
2.2.2 Increasing flood exposure: uncontrolled urbanisation and concentration of people and assets on floodplains
The rapid expansion of urban areas in coastal regions poses significant challenges, particularly regarding flood risk management Without proper planning, more residences become vulnerable to flooding, as evidenced by the severe losses in cities like New Orleans and Manila due to illegal settlements and unregulated development on low-lying land Although new urban designs aim for increased resilience, the volatility of flooding factors has intensified, largely due to climate change Poor spatial planning on floodplains exacerbates the exposure of populations and assets to flooding, driven by the pressure for economic growth that often leads local governments to overlook flood vulnerability assessments in development approvals This uncontrolled urbanization not only heightens exposure to environmental shocks but also strains infrastructure distribution and maintenance.
2.2.3 Susceptibility to flooding: uncertain changes in urban hydro-meteorology
Uncertainty of climate change versus limitations of flood protection systems
Flood magnitude is influenced by urban hydro-meteorological changes, including river levels and local rainfall patterns Climate change contributes to rising sea levels and the intensification of tropical cyclones, leading to increased heavy rainfall and storm surges.
From 1993 to 2009, global sea levels rose at an average rate of 3.3 mm per year, with projections indicating an increase of approximately 74 cm by the end of the 21st century This rise contributes to the growing frequency of tidal flooding, highlighting the urgent need to address climate change impacts.
2014) Such changes will have long term effects on urban areas; the Royal Society (2014)
Extreme weather events have increasingly impacted cities globally since the early 2000s, with predictions indicating this trend will persist Urban areas, particularly those in coastal regions, are particularly vulnerable due to their proximity to river basins and estuaries, which heightens the risk of heavy rains and tidal flooding Notable examples include the flooding in Manila in 2009 and Thailand in 2011, which resulted from a combination of excessive rainfall and elevated river tides.
Coastal cities are enhancing their flood defenses, including dams and drainage systems, to combat rising flood risks However, these systems have limited capacity and can be overwhelmed by unpredictable climatic factors While hurricanes in the tropical North Atlantic can be forecasted, pinpointing their exact impact locations remains challenging due to incomplete predictive capabilities Increased reliance on these resistant systems may heighten cities' vulnerability to flooding, especially when faced with unforeseen events.
Flood resilience
Cities at risk must enhance their resilience to extreme events, such as large-scale floods, through effective planning Strategies for evacuation and emergency management can significantly reduce vulnerability This raises critical concerns for governments regarding practical design, stress management, and emergency preparedness Urban areas must be equipped to handle worst-case scenarios, which includes understanding the stress thresholds of their systems, adapting to unforeseen changes, and reorganizing their structures for survival.
Since Holling's (1973) introduction of ecological resilience, researchers have sought methods to address potential urban environmental disasters, particularly in light of increasing losses noted by Coaffee and Lee (2016) Community resilience, defined as the capacity to endure and recover from disasters with minimal impact (Berke and Campanella, 2006; Cutter et al., 2003), faces challenges in assessing social resilience due to the complexities of individual and group responses to shocks (Rogers et al., 2012) This evaluation is further complicated in mega-cities of developing nations, where comprehensive datasets on social development are often lacking For instance, Ho Chi Minh City officially recorded approximately 7.2 million residents in 2015, yet the actual population is estimated to be around 10 million, largely due to a significant influx of migrant laborers Currently, urban population management relies on outdated paper-based systems, although a new national population statistics program has recently been initiated.
Accessing data on individuals residing in flood-prone areas is challenging due to central government restrictions, which significantly hampers research efforts Consequently, understanding social resilience in these communities falls outside the scope of this study.
In this article, resilience theories are categorized into two main frameworks: engineering-based and ecology-based (Holling, 1996; Liao, 2012) The engineering perspective emphasizes the capacity to maintain a system's stability close to an equilibrium point, while the ecological approach offers a different understanding of resilience dynamics.
“emphasises conditions far from any equilibrium steady state” (Holling, 1996, p 33; Holling
In the study of ecological dynamics, contrasting aspects such as efficiency and existence, as well as constancy and change, have been highlighted by researchers including O'Neill et al (1973) and Holling (1996) This dichotomy reflects the tension between predictability and unpredictability in ecological systems, emphasizing the engineering perspective focused on the system's ability to maintain stability amidst these fluctuations.
The concept of "bounce-back" to normal status emphasizes the importance of survival capacity in ecological systems (Wang and Blackmore, 2009; Walker et al., 2004) This perspective prompts critical discussions on the feasibility of planning resilient urban transport systems that can effectively address natural disasters, particularly extreme flooding.
Liao (2012) defines "urban resilience to floods" as a city's ability to endure flooding and reorganize to reduce fatalities and injuries while preserving its socioeconomic identity This concept emphasizes the importance of "floodability" and urban system reorganization, advocating for a reduction in threshold values to enhance the capacity for socioeconomic fluctuations However, the acceptance of occasional flooding may limit urban development, as properties designed to withstand floods can still face significant disruptions in daily commuting, affecting urban activities such as work, education, commerce, and healthcare Liao's research lacks clarity on how a city can effectively manage these challenges.
42 organises its floodable areas in terms of spatial dimensions in planning, which should demonstrate particular benefits in practice
Several unanswered questions remain regarding the establishment of flooding thresholds for urban areas and transport structures, as well as the integration of resilient models into city planning and management systems The disparity between theoretical models and their practical applications is largely attributed to insufficient testing to validate their feasibility The practical value of these models increases with their application to specific cases, such as cities and towns This presents a challenge for urban researchers, who must find effective methods to translate their conceptual models into actionable plans for individual cities.
Enhanced built-in resilience is essential for urban planning, management, and design to effectively address challenges in cities (White and Howe, 2002; Godschalk, 2003) Broader approaches to urban contexts can help explain the increasing prevalence of resilient models aimed at mitigating hazards such as urban flooding However, many contemporary resilience models require further adaptation to account for spatial disparities (Coaffee and Lee).
Further research on resilience within urban contexts is essential to develop spatial-temporal models that offer customized solutions for urban flooding Enhancing transport resilience is anticipated to play a crucial role in complementing overall urban flood resilience.
A resilient city is a complex adaptive system that includes sub-urban systems, where their individual resilience contributes to the overall resilience of the city for long-term development The resilience of urban infrastructure, particularly transportation, is a key component of this physical system, encompassing four critical factors.
The four essential properties of resilient systems include: i) Robustness, which refers to the physical strength needed to endure disturbances; ii) Redundancy, allowing for the substitution of system components; iii) Resourcefulness, the ability to identify issues and mobilize necessary resources; and iv) Rapidity, which denotes the capacity for timely restoration after a disruption.
Rapid urbanization has driven the need for significant construction of urban infrastructure, such as roads and railways, designed to meet predicted demand To maintain stability in urban activities, new investments in infrastructure are essential, emphasizing the importance of robustness in transitioning from resistance to engineering resilience Additionally, redundancy plays a crucial role by providing alternative options for critical elements, enabling adjustments to the system scale to ensure continued functionality.
The resilience of an ecological system is crucial for its overall survival Berdica (2002) emphasizes that having multiple transportation options and routes can effectively reduce the adverse effects on certain areas of the system during disturbances.
Resourcefulness and rapidity in urban development can evolve over time, influenced by the specific conditions of each city Evaluating a city's resource sufficiency and restoration capacity is challenging without prior shocks, as true resilience is only evident during significant events (Desouza and Flanery, 2013) Alternatively, cities can prepare for potential challenges by simulating worst-case scenarios (Coffee and Lee, 2016).
Flood resilience development for urban transport systems
2.4.1 Definition, and perspectives of development
Urban transport systems encompass both internal components, including transport structures like roads, pedestrian crossings, and terminal stations, as well as vehicles and equipment, and external components such as government and customers These systems feature various modes of transport, including road, rail, sea, and air The transport network is crucial for the economy, significantly influencing spatial relationships between locations and facilitating the connection of people’s activities across different regions globally.
Evidence from the 19th century highlights that economic growth is significantly driven by factors that were previously absent in underdeveloped regions.
The development of transport systems is crucial as it reflects the varying shares of different transport modes, which adapt to practical demands Roads remain the primary mode for individual mobility, while railways effectively cater to high-capacity travel needs Motorisation has become the predominant form of urban commuting in the EU due to its convenience, leading to significant growth in both road and rail transport over recent decades Urban road systems are essential for daily mobility, the transport of goods, and serve as critical lifelines during city disruptions or recovery efforts.
Maritime transport is more cost-effective for longer distances, while air transport has become increasingly competitive due to rapid innovations Despite this, road transport remains the most popular mode, with rail expected to capture a growing share of urban travel This shift reflects broader trends in urban development, responding to significant changes in social functions across cities globally.
Cities are continuously evolving, serving as hubs for both people and businesses while facilitating complex transport networks essential for various activities Despite advancements in information technology, evidence suggests that urban commuting will not decrease In developing countries, the demand for urban transport is increasing due to several factors: a growing population leading to more transport trips, the expansion of urban areas resulting in longer journeys, rising incomes that encourage greater travel, and heightened commercial and industrial activities.
As urban populations grow and cities expand, the complexity of transport systems increases, necessitating the development of extensive routes connecting established centers to new residential areas This spatial expansion demands an enlarged transport network, prompting investments in infrastructure such as main roads, bridges, and tunnels The construction of new roads often catalyzes housing and commercial development, exemplified by extending main roads from older town areas to newly emerging suburbs To facilitate this integration, a resistant approach is frequently employed in new developments, ensuring connectivity between old and new areas, such as raising road surfaces to accommodate flood levels Rodrigue et al (2006) highlight the intricate nature of this transportation evolution.
Contemporary networks often remain unchanged or only slightly modified despite extensive urban development over long periods, sometimes exceeding a century A prime example is France's highway network, which evolved from national roads established in the early 20th century, tracing back to Roman-built roads The evolution of a location illustrates the cumulative interactions among transport infrastructure, economic activities, and the built environment.
Climate change has significantly impacted urban transport systems, leading to unforeseen consequences Extreme weather events, particularly heavy rainfall, have made road and rail transport vulnerable to flooding, resulting in physical damage and operational disruptions across various segments of the transport network.
The impacts of flooding on roads and railways can lead to significant direct repair costs and indirect economic losses from the closure of vital transport links For instance, in London, flooding that causes road closures can result in costs exceeding £100,000 per hour during peak times, as evidenced by the floods in July 2007 Additionally, sea-level rise (SLR) exacerbates flooding challenges for urban infrastructures in coastal areas, making them more vulnerable compared to inland regions.
Flooding, a significant external threat linked to adverse weather and natural disasters, disrupts urban infrastructures, particularly transportation systems (Mattsson and Jenelius, 2015) Such disruptions not only damage physical structures but also lead to widespread operational chaos in urban areas Wang (2015) categorizes these disruptions into disasters, day-to-day variations, and ongoing long-term changes, with normal incidents occurring more frequently (Campanella et al., 2006) For instance, a heavy rain event in August 2008 overwhelmed Mexico City's drainage system, resulting in severe traffic collisions (Lankao, 2010) Although disasters are infrequent, their aftermath can linger for years, affecting community memory and resilience The legacy of Hurricane Katrina exemplifies this, as the costly reconstruction of New Orleans did not entice former residents to return due to fears of future flooding (Campanella et al., 2006).
Urban transportation is increasingly vulnerable to flooding due to climate change, yet Divall (2012) contends that this does not necessitate a reduction in travel As urban transportation systems become more complex and intense, it is crucial for governments, especially in developing countries, to focus on planning resilient urban transport systems This resilience should align with existing infrastructures to ensure the continuous operation of the entire system amidst environmental challenges.
2.4.2 Developing flood resilience in transport planning
The resilience of the transport system is crucial for the overall resilience of urban areas, especially as cities face increasing vulnerability to flooding As a vital component of urban infrastructure, the transport system must be included in resilience enhancement efforts, characterized by four key properties (Tierney and Bruneau, 2007) It is essential to maintain a minimum operational level during sudden disturbances to mitigate adverse impacts and ensure a swift recovery to prevent long-term consequences (Xenidis and Tamvakis, p 3449) This underscores the importance of proactive planning in the transport sector to foster resilience, allowing cities to sustain urban commuting even in challenging conditions.
In the UK, transportation resilience is characterized by the Department for Transport (DFT, 2014, p 8) as the capacity of the transport network to endure extreme weather conditions, maintain operations during such events, and swiftly recover from their impacts This resilience is essential for ensuring the continued functionality and reliability of the transport system.
- Continued transportation of people and goods;
- Quick restoration of services and routes to normality;
- Sufficient and thorough communication to transport users
The three objectives of a resilient transport system are particularly relevant in the context of the UK's long-established infrastructure In contrast, developing countries, which have more recently planned their infrastructure as a catalyst for economic growth, may approach these objectives differently During flood events, a resilient transportation system is crucial for minimizing widespread disruptions and maintaining vital connections across different areas Therefore, prioritizing the continuity of transportation functions is essential, while restoration and communication can be addressed over the long term as cities secure stronger financial resources.
Flood resilience development relying on key properties
A resilient urban transport system relies on robustness and redundancy to enhance its capacity to withstand challenges Engineering solutions, such as constructing elevated roads 4 to 6 meters above ground level, can provide alternative travel routes during flooding, ensuring critical pathways remain accessible This approach aligns with ecological objectives while emphasizing the importance of engineering resilience As noted by Xenidis and Tamvakis (2012), a transport system's ability to regain balance and return to normalcy hinges on its robustness and capacity to adapt when faced with disruptions.
Additionally, redundancy is considered as an objective to develop a resilient transport system
It can be measured by the extent of routes and modes potentially being substituted as elements
Summary
2.5.1 A vision for transport development constrained by flooding, in line with rapid urbanization driven by ultimate economic growth in emerging coastal cities
The expansion of urban areas, driven by economic growth, has led to the development of extensive transport systems that connect new suburbs to city centers Coastal cities, often characterized by low-lying areas and numerous river channels, face significant threats to their transport networks from flooding Moderate floods can disrupt connectivity between urban areas, while extreme floods hinder evacuation efforts, resulting in severe losses for isolated regions To address these challenges, it is crucial to learn from case studies of other cities that have faced similar issues with uncontrolled settlements in flood-prone areas Thus, emerging coastal cities must develop transport systems that not only accommodate increasing commuting demands but also mitigate flood impacts Achieving a balance between horizontal and vertical transport system development is essential for long-term efficiency, particularly in developing countries facing urbanization and flood hazards.
2.5.2 The relationship between flood vulnerability and flood resilience
Understanding the relationship between vulnerability and resilience is crucial for mitigating the impacts of flooding Enhancing flood resilience is particularly vital for the transportation systems of emerging coastal cities, which face increasing susceptibility to flood events.
Climate change is exacerbating flood risks through higher tides and increased rainfall, particularly in emerging coastal cities of developing countries where urbanization on low-elevated land is prevalent Elevating entire transport networks is impractical, making elevation classification crucial for robust and redundant flood-resilient development This necessitates assessing flood vulnerability, especially for critical road networks connecting old centers with new development areas Identifying vulnerable routes involves comparing flood surface simulations with the transport network, which requires integrating hydrological modeling and spatial analysis using GIS.
2.5.3 Developing flood resilience in transport planning
Resilience has become a vital approach in addressing urban challenges, particularly in cities prone to flooding However, existing models for enhancing flood resilience in transport systems require further development within a spatial planning framework Current research reveals a significant gap in comprehensive models that cover the entire process from concept to implementation, specifically for transport systems in vulnerable coastal cities This article proposes the Flood Resilience Transport System (FRTS) model as a crucial focus for investigation, aiming to enhance the resilience of urban transport systems and provide actionable insights for revising transport development plans in flood-prone areas.
An effective urban planning framework can enhance the flood resilience of transport systems in emerging coastal cities in Southeast Asia by focusing on robustness and redundancy This approach aims to coordinate new resilient developments with existing flood-resistant transport infrastructure, addressing the challenges posed by rapid urbanization and economic growth By improving these two properties, we can ensure a stable transport system that aligns with the efficiency objectives of the Flood Resilient Transport System (FRTS) Furthermore, this model supports ecological development while fostering resilience over the long term, establishing essential principles for effective planning.
- Horizontal development, referred to spatial distribution broken down to three levels, with the focus on city level, subsequently divided into different zones;
- Vertical development, referred to elevation classification corresponding to flood thresholds, which can have a link to the value of vulnerable assessment
- For such spatial organisational structure, critical nodes need to be identified and suitably engineered to support the possibility of transport transfers
In Ho Chi Minh City, conducting a vulnerability assessment is essential to pinpoint urban areas and transport sections at risk of flooding, both now and in the future This assessment can be effectively carried out using a combined methodology that takes into account hydro-meteorological changes and existing transport development plans Additionally, evaluating the feasibility of this model requires an analysis of current flood impacts and the conditions of transport development across various cities through on-ground studies The subsequent chapter will detail the research methods necessary for this analysis.