Coastal cities and sea level rise
This article deals with the potential impact of climate change on cities that are located on the coast and therefore vulnerable to sea level rise and extreme conditions at sea. The focus is on coastal cities in low-income countries which are exposed to the greatest risks.
- 1 Vulnerability to climate change and sea level rise
- 2 Causes of increasing vulnerability
- 3 Most vulnerable cities
- 4 Adaptation measures
- 5 Adaptation examples
- 6 References
Vulnerability to climate change and sea level rise
Coastal cities have experienced tremendous growth in recent decades, especially in Africa and Asia , see Fig. 1. Whereas in the past coastal zones were sparsely inhabited, vast urban centers have developed in short time. An important part of the economy of coastal states is concentrated in these centers. Two-thirds of the global population is expected to live in cities by 2050 and already an estimated 800 million people live in more than 570 coastal cities vulnerable to a sea-level rise of 0.5 meters by 2050 (WEF, 2019). Many coastal cities have grown organically, without proper planning and due attention to the vulnerability that results from their location by the sea. This vulnerability already manifests itself at extreme weather conditions, which go along with flooding, loss of life  and damage to the buildings along the coast.
The article Sea level rise describes the consequences of climate change for sea-level rise and the associated increase in the frequency of high water under extreme conditions. It is expected that this frequency will increase by a factor of 100 in many places in the coming century and even more in the following period. Flood risks are not limited to urban areas situated below the high water level at sea; higher areas are affected by a reduction in the discharge capacity of rivers and drainage canals that evacuate river and stormwater to the sea. Without timely measures, the consequences for almost all coastal cities in the world will be dramatic.
The increase in the frequency and severity of floods has a strong disruptive influence on the community and the economy in coastal cities. Some estimates were made of these risks, to assess which measures are needed and to determine their urgency, see section #Most vulnerable cities. There are also indirect consequences because of less favorable economic prospects as the risks to which coastal cities are confronted are increasing. Declining investments in high-risk areas and related fall in income from industry and tourism can also lead to economic and social disruption . Coastal cities with weak governance, a poor and growing population and a large influx of migrants are particularly sensitive to these risks.
Causes of increasing vulnerability
The causes of increasing vulnerability are multiple and not only related to climate change. A number of important causes that enhance vulnerability to climate change are briefly discussed below.
The geographical setting
Many coastal towns lie in low-lying coastal plains, often near estuaries or lagoons . They have grown from first settlements on elevated soils (dune areas, rock formations, ancient elevated deposits) along the coast. When higher ground was no longer available, urban expansion took place in surrounding areas: wetlands, marshes or lagoons that were drained or filled up, see Fig. 2. These areas are often below the high-water level of the nearby sea or river and collect water from the surrounding higher grounds in the event of heavy rainfall. They are therefore very susceptible to flooding. The most vulnerable areas usually have the greatest population density and are inhabited by families with the least resources. Such geographical and social conditions are an important factor for the vulnerability of many major coastal cities and for their sensitivity to climate change.
The low-lying coastal areas in which urban expansion has taken place often have a soft soil, which largely consists of clay and organic material. These soils cannot bear great weight and therefore require adequate foundation of buildings and infrastructure, which is often not present. In addition, these soils are sensitive to compaction due to drainage and oxidation. Extraction of groundwater (and in some cases gas or oil) reinforces the subsidence. In several coastal megacities, subsidence of several centimeters per year has been measured in places (Fig. 3) Cite error: Closing
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<ref> tag, mainly due to groundwater extraction. Such a strong subsidence significantly increases the sensitivity of these areas to flooding. Subsurface seawater intrusion is an additional problem caused by extraction of groundwater and soil subsidence, see the article Groundwater management in low-lying coastal zones.
Demographic growth in developing countries is particularly strong in rural areas. Water scarcity, especially in Africa, reduces the availability of suitable agricultural land , while mechanization reduces the need for manpower. These trends make that new generations can no longer earn a living. They flock to the large urban centers along the coast to find employment. Many of them lack education and do not have the means to settle in safe places and often live unregistered in slums in the least safe areas, including the seashore and river banks (illustrated in Fig. 4). The habitants of these areas do not have sanitary facilities, safe drinking water and are the first victims of flooding. They form a highly vulnerable group because, due to their precarious living conditions, they cannot make provisions against the effects of climate change .
In low-income countries, it is difficult to provide public facilities in the coastal megacities to the rapidly growing population. This not only concerns protection against flooding, but also waste removal, sewage collection and treatment and safe drinking water. Treatment plants are often defective; they have insufficient capacity or are even missing completely. The discharge of waste from households and industry is poorly regulated and insufficiently enforced. Drainage canals are often obstructed with garbage, as illustrated in Fig. 5. The population is therefore exposed to harmful substances that cause diseases and premature death. Higher temperatures increase the risk of infectious diseases. The greatest risks of exposure occur during floods in which waste water is spreading everywhere. Degraded water quality also has a strong negative impact on fishing, which is important for food supply and for employment and income of a substantial part of the population.
Degradation of natural protection
Marshes, mangroves and coral reefs offer natural protection against flooding in extreme weather conditions. The growth of megacities often came at the expense of this natural protection. Wetlands have been reclaimed for urbanization and mangroves have been harvested for timber and to make way for fish ponds. Dunes and beaches are often partially excavated to extract sand for landfill; corals and shell banks are used as raw materials for construction . Although these latter practices are often prohibited, enforcement fails to prevent this. The impoverishment of the natural ecosystem that results from these practices causes a further accelerated deterioration of the protection that nature offers.
Failing governance is a major problem in developing countries. Steering the aforementioned issues is an enormous challenge, in particular the issue of rapid population growth and the mass influx of poorly educated migrants. Tax collection systems often fall short for providing the financial resources needed to cope with the legacy of poor infrastructure and inadequate coastal zone planning and to make necessary investments (including health care and education). There is a lack of well-trained staff and therefore lack of competence within government institutions. The governmental organization is generally weak and institutions do not work well together. Legal provisions are not well aligned with existing problems, legal provisions are not being properly enforced, land ownership rights are unclear, decision-making processes are poorly organized, without proper involvement of civil society and administrative procedures are insufficiently effective. There are often important cultural differences between representatives of different population groups, which hinder standing up for a common interest. The overwhelming amount of short-term problems pushes long-term developments into the background. Therefore, anticipating the effects of climate change does not have the priority that is required.
Most vulnerable cities
Various lists of highly vulnerable coastal cities have appeared in the literature. The ranking depends on the criteria used. Table 1 gives an (non-exhaustive) overview of coastal cities that are often cited for their vulnerability to climate change. For each coastal city, it is indicated which aspects influence vulnerability the most. The overview is based on an the inventories of the World Wide Fund For Nature (WWF, 2009), Hanson (2011), Neumann et al. (2015), , Dhinan et al. (2019), Hallegatte et al. (2013).
The table shows that the exposure to flood risks is already very high in many cities today (existing flood protection measures being taken into account). The consequences of flooding are serious in all cases. Low-income countries are less well organized and have fewer resources than rich countries to take measures for reducing vulnerability. Drainage canals clogged with garbage and squatter settlements on river banks and beaches, as often observed in low-income countries, are indicators of weak governance and low adaptive capacity. Table 1 further shows that without additional measures, there is a strong or very strong increase in flood exposure of the population due to climate change in all coastal cities.
An overview of climate adaptation measures is given in the article Climate adaptation policies for the coastal zone. Below we will discuss in more detail measures that are particularly relevant for coastal cities in low-income countries. These measures respond to the vulnerabilities discussed in the previous section #Causes of increasing vulnerability.
Climate adaptation stands or falls with the ability of administrations to take in time appropriate adaptation measures. Authorities need well-trained staff and therefore have to invest in training and capacity building programs. A major obstacle to risk reduction measures are the high costs and the lack of an immediate tangible effect. Broad support for such measures often arises only after a disaster has occurred. Anticipatory measures that require shared sacrifices will not easily receive broad support in communities where social inequality is strong. However, it is possible to incorporate climate adaptation in measures that deliver direct social benefits, such as: reduction of social inequality through equitable taxes and income redistribution, investments in education and health care, good affordable housing and improvement of the infrastructure for water and sanitation . Measures can be designed such that they contribute to reduce the potential impact of climate change, e.g. by increasing the resilience of citizens, by providing faster and better emergency aid, by building flood-proof homes, by securing critical infrastructure and by preventing the dispersal of hazardous substances. Such measures can be implemented step by step, depending on the resources available. Measures that can be realized at short term are the implementation of organizations for early warning, emergency interventions and rescue. See also the articles Integrated Coastal Zone Management (ICZM) and Climate adaptation policies for the coastal zone.
City planning is a crucial instrument for increasing the resilience of coastal cities to flooding. City planning must first of all prevent development in zones that are most sensitive to flooding. A very effective measure is the establishment and enforcement of setback areas along the coast and river banks, see the article Setback area. These areas are often already built on and relocating the local population may be needed. This is not easily done and requires not only good alternative housing. The current situation is that residents of unsafe areas often prefer to accept flood risk than give up their home, postponing the need for relocation as long as possible by ad-hoc measures. People who live precariously in vulnerable areas often feel neglected and distrust authorities. To obtain support and cooperation, authorities have to communicate intensively as an understanding, listening and reliable partner with inhabitants in planning and relocation processes.
Adaptation of the local infrastructure is necessary to better manage storm and flood water. This can be realized by giving water more space in the city, as illustrated in Fig. 6. In addition, an efficient and well-maintained drainage infrastructure is required. A general planning principle for dealing with storm water and flash floods is: (1) space for water retention / absorption upstream of the city, (2) space for water storage in the city and (3) high-capacity canals / drains for fast water discharge downstream of the city .
Water supply from remote sources must ensure that no groundwater needs to be pumped up, in order to reduce subsidence. Building regulations must ensure that houses are resistant to flooding and that residents can secure themselves on higher floors; existing buildings have to be refitted if necessary. These adaptations are costly and require appropriate funding mechanisms. . A broad spectrum of issues is related to urban water management (see Fig. 7); therefore, an integrated approach is required. See also Groundwater management in low-lying coastal zones.
Limiting rural exodus
Limiting the migration from the countryside to the urban centers along the coast requires political priority for rural development. Agricultural policy should tackle existing obstacles to rural development. It comprises a broad spectrum of measures, for example, investing in knowledge development and knowledge sharing of efficient modern farming techniques, creating financial mechanisms for their implementation and creating insurance mechanisms against crop failures, regulating land ownership, improving agricultural product marketing mechanisms, stimulating the development of local food processing industries, improving water supply and irrigation practices and investing in infrastructure for transport, warehousing, cold storage and wholesale markets . By reducing migration to urban centers, policies to improve prosperity and economic growth in agricultural areas are therefore an important complement to policies for reducing the vulnerability of coastal towns.
Protection against flooding from the sea
Sea level rise is a major threat to coastal cities worldwide. Large parts of many coastal cities are situated today below the water level reached at sea during a 1/100 storm; the frequency of exceedance of these storm levels may increase by a factor 10 or 100 during the 21st century. Constructions to protect against flooding and overtopping waves have to be adapted accordingly.
Different types of constructions can be considered for protection against flooding from the sea. Hard structures are used most often. An overview of such constructions with criteria of application, advantages and drawbacks can be found in several articles in the category Hard structures. The costs of coastal defenses depend on the intended level of protection, i.e. the size and strength of the structure required to keep the probability of flooding below a certain value. An overview of cost estimates is given in Jonkman et al. (2013) and Aerts (2018). The costs for ensuring a high level of protection are considerable, but in many cases much lower than the avoided costs of damage in the event of flooding. Costly defense measures can also be justified to protect people and to avoid strong social impacts. Some highly exposed coastal cities are listed in Table 1, but this list is far from exhaustive.
Soft coastal defense options have become more popular in recent decades, see the articles in the category Soft coastal interventions and some examples in the article Climate adaptation policies for the coastal zone. These soft measures are usually more resilient and easier to maintain, but cannot be used everywhere, e.g. because materials are not available or because space is insufficient. Van Coppenolle and Temmerman (2019)  have made an inventory of the potential of coastal cities to implement soft coastal protection measures, showing that this is a feasible option for many coastal cities. An example of an artificial dune as soft coastal protection measure is shown in Fig. 8. Other soft nature-based coastal protection measures, such as mangroves, marshes or reefs, can provide cost-effective solutions to reduce the wave impact on urbanized coasts, see also the articles Natural shore protecting barriers and Shore protection vegetation.
The metropolis of Bangkok with more than 14 million inhabitants is built in the broad coastal plain of the Chao Phraya River. Located just a few meters above sea level it is often subject to flooding. Floods have become worse in recent decades due to the rapid subsidence caused by massive groundwater extraction and the replacement of canals and urban water spaces with roads and buildings. A comprehensive set of measures has been elaborated to increase the flood resistance of the city :
- flood retarding in the upstream river reaches by diverting water towards temporary water retention areas;
- developing new water storage capacity inside the city by creating open spaces and green areas as potential water storage areas;
- improving community-based adaptation and disaster preparedness and communication;
- improving the urban flood defense system by upgrading existing drainage systems;
- enhancing emergency preparedness and response through monitoring and communication;
- capacity building for disaster risk reduction.
The implementation and effectiveness of these measures will depend crucially on generating broad public support and interprovincial cooperation .
Sponge Cities initiative in China
A 'Sponge City' is a city that has the capacity to integrate urban flood risk management into its urban planning policies and designs, based on appropriate planning and legal frameworks and tools. Sponge cities implement, maintain, and adapt their infrastructure systems to collect, store, and purify (excess) rainwater. A Sponge City will not only be able to deal with too much water, but will also re-use rain water to reduce the impacts of drought. The anticipated benefits of a Sponge City are :
- a reduction of the economic losses due to flooding;
- an enhancement of the livability of cities, and
- the establishment of an environment where investment opportunities in infrastructure upgrading and engineering products and new technologies are created and fostered.
In China, 16 pilot cities were selected to become Sponge Cities, including the coastal megacities of Tianjin, Shanghai, and Shenzhen. The Directive on promoting Sponge City Construction of 2015 sets the target that 20% of the urban areas of Chinese cities should absorb, retain, and re-use 70% of the rainwater by 2020. By 2030, this percentage should rise to 80%. The general objectives of the concept are:
- restoring the city’s capacity to absorb, infiltrate, store, purify, drain, and manage rainwater and
- regulating the water cycle as much as possible to mimic the natural hydrological cycle.
Jakarta, the capital of Indonesia, is among the most densely urbanized and most vulnerable coastal megacities in the world. This vulnerability is due on the one hand to the massive land conversion of rivers, canals, and wetlands, which reduces flood retention and discharge capacities, and on the other hand to land subsidence at rates of up to 25 cm per year, mainly driven by groundwater extraction. Measures proposed to protect Jakarta against flooding, triggered by the big flood of 2007, are primarily based on engineering interventions:
- river and canal regulation, the broadening of water ways and the clearance of river banks, which are frequently encroached by informal settlers;
- the restoration and expansion of flood reservoirs and
- a new coastal flood protection wall, the 'giant seawall' also known as 'Great Garuda Project' (see Fig. 9).
This dike with a length of 25 km turns Jakarta Bay into an enclosed reservoir, with a pumping station of 730 m3/s for discharging peak river runoff. The financing of the project (estimated at more than 40 billion US$) is based on the estimated revenues derived from the development of new estates for commercial and residential purposes on reclaimed islands in Jakarta Bay. The project is controversial, however, because of its potential impacts on the environment (pollution, sedimentation, ecology) and on small-scale fishing and aquaculture on which many poor households rely. There are also concerns that the project will increase the gap between the haves and have-nots in the city, that it does not tackle the root of the vulnerability issue and that it is inflexible for responding to future uncertain economic and environmental developments. It is not yet certain that the project will be completed. Great Garuda is the first large-scale adaptation project designed to protect a coastal megacity against the threat of relative sea-level rise. The technical, economic and social issues addressed in this project therefore offer highly relevant lessons for other coastal megacities .
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