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Tsunami
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'''Coastal cities and sea level rise'''
  
  
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{{Definition|title=Tsunami
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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.
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|definition= Series of long waves caused by a strong local disturbance of the water mass. The wavelength is typically much larger than the wavelength of wind-generated waves and much smaller than the wavelength of tidal waves. }}
 
  
  
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== Vulnerability to climate change and sea level rise==
  
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[[Image:PopulationCoastalCities.jpg|thumb|400px|left|Figure 1: Population of coastal cities around 1950 and in 2020. Adapted from Barragan et al. (2015) <ref name=BA>Barragan, J.M. and de Andres, M. 2015. Analysis and trends of the world's coastal cities and agglomerations. Ocean & Coastal Management 114; 11-20 </ref>. ]]
  
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Coastal cities have experienced tremendous growth in recent decades, especially in Africa and Asia <ref name=BA></ref>, 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)<ref>WEF 2019. The Global Risks Report 2019, 14th Edition. World Economic Forum</ref>. 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 <ref>Jonkman, S.N. and Vrijling, J.K. 2008. Loss of life due to floods. J Flood Risk Management 1: 43–56</ref> and damage to the buildings along the coast.
  
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==Tsunami causes and occurrence==
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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<ref name=I>IPCC, 2019. Special Report on the Ocean and Cryosphere in a Changing Climate [Eds.: H.-O. Pörtner, D.C. Roberts, V. Masson-Delmotte, P. Zhai, M. Tignor, E. Poloczanska, K. Mintenbeck, M. Nicolai, A. Okem, J. Petzold, B. Rama and N. Weyer]</ref>. 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.
  
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[[Image:RingofFireUSGS.jpg|thumb|300px|left|Figure 1: Ring of fire around the Pacific. Image USGS.]]
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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 <ref name=VB>Vivekananda, J. and Bhatiya, N. 2016. Coastal Megacities vs. the Sea: Climate and Security in Urban Spaces. BRIEFER 30: 1-12</ref>. Coastal cities with weak governance, a poor and growing population and a large influx of migrants are particularly sensitive to these risks.
  
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The occurrence of a tsunami can have several causes<ref name=RV>Röbke, R.B. and Vött, A. 2017. The tsunami phenomenon. Progress in Oceanography 159: 296–322</ref>. The main cause, responsible for the strongest tsunamis, are seaquakes (submarine earthquakes): the sudden uplift of the seabed due to shifting earth plates. This mainly occurs in subduction zones, where an earth plate slides under an adjacent earth plate causing vertical motion of the submarine crust along the subduction line. The most important subduction zones lie along the edges of the Pacific Ocean, the so-called ring of fire (see Fig. 1). The strength of the tsunami depends not only on the vertical and horizontal size of the seafloor uplift, but also on the speed at which the seafloor rises. The strongest response occurs when this speed is comparable to the local wave propagation speed. The tsunami wave radiates in both directions perpendicular to the subduction line.
 
  
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A second important cause is submarine gravity mass wasting. Such  slides can be triggered by seismic activity at a sloping seabed (e.g., the shelf break) that is covered with a thick layer of unconsolidated material. The tsunami wave propagates in the direction of the slide and is strongest if the speed of the debris flow is comparable to local wave propagation speed. Gas formation (methane) due to the anaerobic degradation of organic material can contribute to the loss of stability of the sediment layer on the sloping seabed <ref>Tinivella, U., Accaino, F. and della Vedova, B. 2008. Gas hydrates and active mud volcanism on the south Shetland continental margin, Antarctic Peninsula. Geo-Mar. Lett. 28: 97–106</ref>.
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==Causes of increasing vulnerability==
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Volcanic activity and subaerial landslides can also cause tsunamis, but these are usually not as strong. A tsunami can in rare cases result from the impact of a large meteorite.
 
  
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Another type of tsunamis,  so-called meteotsunamis, can arise from atmospheric disturbances. Sudden local pressure fluctuations can generate a small setup or set-down of the water level that is amplified if the atmospheric front propagates at a speed comparable to the wave propagation speed or if the spatial/temporal scales coincide <ref>Monserrat, S., Vilibić, I. and Rabinovich, A.B. 2006. Meteotsunamis: atmospherically induced destructive ocean waves in the tsunami frequency band. Nat. Hazards Earth Syst. Sci. 6: 1035–1051</ref>. The amplitude of meteotsunamis is usually small, but it can increase strongly due to resonance effects in semi-closed basins (e.g., harbor seiches, which are damaging for moored ships).
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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.
  
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===The geographical setting===
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Many coastal towns lie in low-lying coastal plains, often near estuaries or lagoons <ref>McGranahan, G., Balk, D. and Anderson, B. 2006. The rising tide: assessing the risks of climate change and human settlements in low elevation coastal zones. Environ Urban 12: 17–38</ref><ref name=N> Neumann, B., Vafeidis, A. T., Zimmermann J. and Nicholls, R. J. 2015. Future coastal population growth and exposure to sea-level rise and coastal flooding – a global assessment. PLOS ONE 10: 1-34</ref>. 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. Such geographical conditions are an important factor for the vulnerability of many major coastal cities and for their sensitivity to climate change.
  
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==Tsunami characteristics==
 
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A tsunami usually consists of a number of waves (up to about ten) of different amplitude <ref>Grilli, S.T., Harris, J.C., Tajalli Bakhsh, T.S., Masterlark, T.L., Kyriakopoulos, C., Kirby, J.T. and Shi, F. 2013. Numerical simulation of the 2011 Tohoku tsunami based on a new transient FEM Co-seismic source: comparison to far- and near-field observations. Pure Appl. Geophys. 170: 1333–1359</ref>. The third, fourth or fifth wave are often the highest <ref> Murty, T.S. 1977. Seismic sea waves. Tsunamis. In: Stevenson, J.C. (Ed.), Bulletin of the Fisheries Research Board of Canada, vol. 198</ref>. The characteristic wave period is different for each tsunami, but usually much greater than the period of wind-driven waves (i.e., more than a few minutes) and much smaller than the tidal period (i.e., less than two hours). The wave height can be a few meters at the place of origin, but when the tsunami starts propagating over the ocean the wave height quickly becomes smaller and is then usually less than one meter<ref> Bolt, B.A., Horn, W.L., Macdonald, G.A. and Scott, R.F. 1975. Geological hazards. Earthquakes—Tsunamis—Volcanoes—Avalanches—Landslides—Floods. Springer, New York</ref>. The wavelength is much greater than the water depth and the energy loss during ocean propagation is small (approximately inversely proportional to the wavelength). The propagation speed <math>c</math> can therefore be approximated with the formula <math>c = \sqrt {gD}</math>, where <math>D</math> is the average depth and <math> g </math> the gravitational acceleration. Tsunamis travel at great speed across the ocean (<math>c</math> in the order of a few hundred m/s) covering distances of many thousands of kilometers without great energy loss.
 
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The energy flux <math> F </math> can be estimated with the formula <math>F = cE \approx (1/8) \rho g H ^ 2 \sqrt {gD}</math>, where <math>E</math> is the tsunami wave energy, <math> H </math> the tsunami wave height and <math>\rho</math> the water density. When the tsunami reaches the coastal zone, the water depth decreases sharply as does the propagation speed <math> c </math>. However, frictional energy dissipation being relatively modest, the energy flux <math>F</math> remains almost the same. This implies <math>H_{shore}/H_{ocean} \approx (D_{ocean}/D_{shore})^{1/4}</math>: the wave height <math>H</math> of the tsunami is strongly increased, up to about 5 times the wave height on the ocean. The wave height can be amplified even more in marine inlets, such as bays, fjords or harbours, where the tsunami energy flux is concentrated and accelerated due to strong funnel effects <ref name=RV></ref>. The tsunami waves are also distorted when entering shallow water; when the rear of the wave train is still traveling in deeper water it gets closer to the front of the wave train which is already travelling in shallow water. Differences in propagation speed of wave crest and wave trough contribute to steepening of the wave front on wide continental shelfs - wide compared to the tsunami wavelength.
 
  
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When the tsunami wave arrives at the coast it runs up the shoreface (schematically depicted in Fig. 6). If the slope is steep (the width of the shoreface being much smaller than the wavelength of the tsunami, i.e. less than 10 km), the tsunami 'feels' the shoreface as a straight wall. The tsunami is then almost completely reflected and the wave height is doubled. The tsunami floods the coast as a surging wave with a relatively low speed that can be outrun. The wave run-up is not much higher than the wave height on the foreshore.
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[[Image:ExpansionCoastalCities.jpg|thumb|800px|center|Figure 2: Schematic representation of the characteristic geographical setting of coastal towns that were built in delta plains, close to estuaries or lagoons. Left panel: The original settlement was built on high ground. Right panel: For urban expansion the surrounding lowlands (marshland, lagoons) were reclaimed by drainage and landfills. These areas are vulnerable to flooding.]]
  
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[[Image:Tsunami Phuket 2004.png|thumb|400px|left|Figure 2: The Indian Ocean tsunami of December 26, 2004, invading the coast at Phuket Thailand as a breaking bore. Picture from a camera lost on the beach. Image Flickr creative commons.]]
 
  
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[[Image:SubsidenceCoastalCities.jpg|thumb|400px|left|Figure 3: Subsidence in a few coastal megacities  <ref name=D>Deltares, Sinking cities  https://www.deltares.nl/app/uploads/2015/09/Sinking-cities.pdf</ref>.]]
  
  
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If the shoreface slope is gentle (the length of the foreshore being of the same order or greater than the wavelength), the tsunami transforms into an undular bore, with short waves riding on the main tsunami wave. When reaching the shore, the tsunami transforms further into a breaking bore (a wall of water, see Fig. 2) that propagates at great speed with a very fast rising water level. The wave run-up is much higher than the wave height at the toe of the shoreface (a factor of 2 and possibly more).
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===Soil subsidence===
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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, see Fig. 3 <ref name=D><ref> Syvitski, J.P.M., Kettner, A.J., Overeem, I., Hutton, E.W.H., Hannon, M,T., Brakenridge, G.R., Day, J., Vörösmarty, C., Saito, Y., Giosan, L. and Nicholls, R.J. 2009. Sinking deltas.</ref>. Such a strong subsidence significantly increases the sensitivity of these areas to flooding.      
  
  
  
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===Population growth===
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Demographic growth in developing countries is particularly strong in rural areas. Water scarcity, especially in Africa, reduces the availability of suitable agricultural land <ref> Koutroulis, A.G., Papadimitriou, L.V., Grillakis, M.G., Tsanis, I.K., Warren, R. and Betts, R.A. 2019. Global water availability under high-end climate change: A vulnerability based assessment. Global and Planetary Change 175: 52–63</ref>, while mechanization reduces the need for manpower. These trends make that new generations can no longer earn a living<ref> Seto, K.C., 2011. Exploring the dynamics of migration to mega-delta cities in Asia and Africa: contemporary drivers and future scenarios. Global Environ. Change 21: S94-S107</ref>. 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 the most vulnerable group because, due to their precarious living conditions, they cannot make provisions against the effects of climate change <ref name=VB> </ref>.
  
  
  
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The above qualitative description suggests that the character of a tsunami is related to the surf similarity parameter <math>\xi</math> <ref name=MG> McGovern, D.J., Robinson, T., Chandler, I.D., Allsop, W. and Rossetto, T. 2018. Pneumatic long-wave generation of tsunami-length waveforms and their runup Coastal Engineering 138: 80–97</ref><ref name=LF>Larsen, B.E. and Fuhrman, D.R. 2019. Full-scale CFD simulation of tsunamis. Part 1: Model validation and run-up Coastal Engineering 151: 22–41</ref><ref name=MS>Madsen, P.A. and Schäffer, H.A. 2010. Analytical solutions for tsunami runup on a plane beach. J. Fluid Mech. 645: 27–57</ref>,
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[[Image:SlumsMonrovia.jpg|thumb|800px|center|Figure 4: Slums on a sandspit at the coast of Monrovia (Liberia).]]
  
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<math>\xi = \Large\frac{S}{\sqrt{2 A_0/L_{\infty}}}\normalsize = \Large\frac{S}{\omega} \sqrt{\frac{\pi g}{A_0}}\normalsize , </math>
 
  
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where <math>A_0</math> is the tsunami wave height above mean sea level at the toe of the shoreface slope, <math>L_{\infty} = g T^2/(2 \pi)</math> is the tsunami wave length at infinite depth, <math>S</math> is the shoreface slope and <math>\omega</math> is the tsunami wave period. Surging waves correspond to high values of <math>\xi</math> and breaking waves to low values.
 
  
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[[Image:BandAcehTsunamiLeadingDepression.jpg|thumb|300px|left|Figure 3: Retreating shoreline before arrival of the major wave of the Indian Ocean tsunami at Banda Aceh on December 26, 2004. Image C.E. Synolakis <ref name=SB>Synolakis, C.E. and Bernard, E.N. 2006. Tsunami science before and beyond Boxing Day 2004. Phil. Trans. R. Soc. A. 364: 2231–2265</ref>.]]
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===Water quality===
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[[Image:AccraCloggedDrainageCanals.jpg|thumb|300px|right|Figure 5: Clogged drainage canal in Accra (Ghana). Photo credit TU Delft.]]
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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<ref> Penning de Vries, F.W.T., Acquay, H., Molden, D., Scherr, S.J., Valentin, C. and Cofie, O. 2002. Integrated Land and Water Management for Food and Environmental Security. Global Environmental Facility. Comprehensive Assessment Research Paper Colombo, Sri Lanka</ref>. 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.
  
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===Degradation of natural protection===
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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<ref>Agardy, T. and Alder, J. (lead authors) 2005. Millennium Ecosystem Assessment Chapter 19 Coastal Systems. Washington, D.C., Island Press</ref>. Dunes and beaches are often partially excavated to extract sand for landfill; corals and shell banks are used as raw materials for construction <ref>Wilkinson, C. 2001. Status of coral reefs of the world 2000. Queensland (Australia): Australia Institute of Marine Science</ref>. 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.   
  
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===Weak governance===
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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<ref> Diop, S. and Scheren, P.A. 2016. Sustainable oceans and coasts: Lessons learnt from Eastern and Western Africa. Estuarine, Coastal and Shelf Science 183: 327-339</ref><ref> Tabet, L. and Fanning, L. 2012. Integrated coastal zone management under authoritarian rule: An evaluation framework of coastal governance in Egypt. Ocean & Coastal Management 61: 1-9</ref><ref> Le, T.D.N. 2019. Climate change adaptation in coastal cities of developing countries: characterizing types of vulnerability and adaptation options. Mitigation and Adaptation Strategies for Global Change, https://doi.org/10.1007/s11027-019-09888-z</ref><ref> Varis, O. 2006. Megacities, development and water. Int. J.Water Resour. Dev. 22: 199-225</ref>. 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. 
  
  
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==Most vulnerable cities==
  
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A distinction is often made between two types of tsunamis: tsunamis where the largest wave crest precedes the largest wave trough (sometimes called leading-elevation N-wave or LEN-wave) and tsunamis where the largest wave crest is preceded by the largest wave trough (sometimes called leading-depression N-wave or LDN-wave). Both types of tsunami occur in practice. In case of a LDN-wave, the sea retreats over a large distance and part of the shoreface falls dry before arrival of the wave crest, see Fig. 3. This is an important signal for bathers that a high tsunami wave is approaching. A LDN-wave approaches the shore at a lower speed than a similar LEN-wave, but the runup speed on land is in both cases of the same order.  
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[[Image:ClimateChangeVulnerabilityCoastalCities.jpg|thumb|500px|left|Table 1: Exposure and vulnerability of coastal cities to flood risks exacerbated by sea level rise. Dark red: very high vulnerability (very high exposure / very high socio-economic sensitivity / weak adaptive capacity / very strong increase of population at risk); Yellow: high vulnerability / weak-medium adaptive capacity; Green: medium-low vulnerability /  medium-strong adaptive capacity.]]
  
  
  
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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)<ref> WWF 2009. Mega-Stress for Mega-Cities, A Climate Vulnerability Ranking of Major Coastal Cities in Asia</ref>, Hanson (2011)<ref name=Ha>Hanson, S., Nicholls, R., Ranger, N., Hallegatte, S., Corfee-Merlot, J., Herweyer, C. and Chateau, J. 2011. A global ranking of port cities with high exposure to climate extremes. Climatic Change 104: 89–111</ref>, Neumann et al. (2015)<ref name=N></ref>, <ref> Wang, G., Liu, Y., Wang, H. and Wang, X. 2014. A comprehensive risk analysis of coastal zones in China. Estuarine, Coastal and Shelf Science 140: 22-31</ref>, Dhinan et al. (2019)<ref> Dhiman, R., VishnuRadhan, R., Eldho,  T. I. and Inamdar, A. 2019. Flood risk and adaptation in Indian coastal cities: recent scenarios. Applied Water Science 9:5</ref>, Hallegatte et al. (2013)<ref name=Ha> Hallegatte, S., Green, C., Nicholls, R.J. and Corfee-Morlot, J. 2013. Future flood losses in major coastal cities. Nature Climate Change 3: 802-806</ref>.
  
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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.
  
  
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==Tsunami impact==
 
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Tsunamis can cause enormous damage to coastal infrastructure, crushing houses, deracinating trees and making many casualties, see Fig. 4. From historical records it has been estimated that more than a million people have been killed worldwide by tsunamis <ref>National Geophysical Data Center/World Data Service, 2015. Global historical tsunami database. Tsunami event data. <http://www.ngdc.noaa.gov/hazard/tsu_db.shtml></ref>. In low-lying coastal areas (less than 10 m elevation above mean sea level) tsunamis can penetrate several kilometers inland. There are even records of tsunamis that have penetrated several tens of kilometers inland and have reached levels of several tens of meters. The flood water volume is approximately equal to the water volume above beach level of the incident tsunami wave <ref>Bryant, E.A., 2008. Tsunami: The Underrated Hazard, second ed. Springer, Berlin</ref>. Runup velocities of strong tsunamis are very high; velocities of the order of 5 – 8 m/s are common and speeds up to 20 m/s have even be observed<ref>Nanayama, F. and Shigeno, K. 2006. Inflow and outflow facies from the 1993 tsunami in southwest Hokkaido. Sed. Geol. 187: 139–158</ref>. The destructive power of tsunamis is largely due to the many debris that are dragged by the flow. Upon travelling inland the flow gradually slows down. The largest debris are deposited first whereas the finest sediments are transported further inland. Sedimentary records showing upward fining deposits over large distances are a distinctive mark of tsunami events. The return flow to the sea (backwash) can also be very strong and dangerous; it follows the strongest topographic slope and is often concentrated in channels, with velocities that are often higher than during the uprush <ref>Le Roux, J.P. and Vargas, G. 2005. Hydraulic behavior of tsunami backflows: insights from their modern and ancient deposits. Environ. Geol. 49: 65–75</ref>.
 
  
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[[File:Tsunami Phuket 2004 S.Kennedy.jpg|thumb|left|300px|Fig. 4a. Impact of the Indian Ocean tsunami of December 26, 2004 at Phuket, Thailand. Image S. Kennedy, Flickr creative commons.]]
 
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[[File:Tsunami Japan 2011 Flickr J.Teramoto.jpg|thumb|left|300px|Fig. 4b. Destruction at the Honshu coast, Japan, by the tsunami of March 11, 2011. Image J. Teramoto, Flickr creative commons.]]
 
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[[File:Tsunami Japan 2011 S.Yoshida.jpg|thumb|left|300px|Fig. 4c. Destruction at the Honshu coast, Japan, by the tsunami of March 11, 2011. Image Y. Yoshida, Flickr creative commons.]]
 
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==Why are tsunamis so devastating==
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==Adaptation measures==
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The period of tsunami waves is typically in the range 300 – 6000 s (angular frequency in the order of 0.001 – 0.02 <math>s^{-1}</math>). Therefore, these waves are rather insensitive to the effect of earth's rotation, which corresponds to a frequency smaller than 0.00015 <math>s^{-1}</math>. Tsunami waves in the ocean have wavelengths typically in the order of 100 to 500 km and on the continental shelf in the order of 1 – 10 km. This implies that they are refracted and diffracted by seabed structures with spatial scales of these orders of magnitude. More specifically, this means that tsunami waves refract to the continental shelf and approach the shore more or less perpendicularly. The energy flux of tsunami waves is thus directed toward the coast. This contrasts with tidal waves that are too long for being refracted to the coast. Due to their very low frequency, tidal waves are bent along the coast by the effect of earth's rotation (see [[Coriolis and tidal motion in shelf seas]]). The energy flux of tidal waves, which is comparable in strength to the energy flux of tsunami waves, is therefore not directed towards the coast, but along the coast. Hence, the impact of tidal waves on the coast is much less than the impact of tsunami waves.
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Just like tsunami waves, wind-generated waves are also refracted to the coast. Their energy density on the ocean is generally higher than the energy density of tsunami waves. However, the energy propagation speed of wind waves is much lower and so is the corresponding energy flux. Hence, wind waves are much less amplified when travelling into shallow water than tsunami waves. Moreover, wind waves lose a great deal of their energy due to breaking in the surf zone before they reach the shore. This explains why tsunamis have a far more devastating effect on the coast compared to tidal and wind waves.
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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 section [[#Causes of increasing vulnerability]].
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===Governance===
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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 <ref name = VB></ref>. 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]].  
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===City planning===
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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. Setback areas must be defined and enforced 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. To obtain support and cooperation, authorities should communicate intensively as an understanding, listening and reliable partner with inhabitants in planning and relocation processes.  
  
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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 <ref>Hillen, M.M. and Dolman, N. 2015. Towards water adaptive cities. World Engineers Summit on Climate Change (WES) July 2015, Singapore</ref>.
  
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==Tsunami observation and warning==
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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. <ref>Satterthwaite, D., Huq, S., Pelling, M., Reid, H. and Lankao, P. R. 2007. Adapting to Climate Change in Urban Areas. IIED, Rockefeller Foundation. www.iied.org/pubs/display.php?o=10549IIED </ref>. A broad spectrum of issues is related to urban water management (see Fig. 7); therefore, an integrated approach is required. See also [[Testpage4|Groundwater management in low-elevation coastal zones]].
−
Experimental measurements of real-life tsunamis running up the coast do not exist. Installing observation equipment at specific locations in anticipation of a tsunami makes no sense. Tsunami's are very rare events for each specific coastal location, even in areas that are very sensitive to submarine earthquakes. Experimental information about tsunamis that actually occurred is based on the effects observed in the coastal area after being hit by a tsunami. In some cases, eyewitness reports or amateur films are available. Although very important, this is insufficient for gaining in-depth understanding of the hydrodynamic processes that occur when a tsunami hits the coast. For this reason, understanding tsunamis is primarily based on modelling, as described in the next section.  
 
  
−
Tectonic activity that generates tsunamis is recorded by many seismic stations around the world. However, submarine earthquakes do not necessarily cause a tsunami. Tsunamis traveling across the ocean are very long waves of small amplitude that cannot be easily detected visually or by ships. For tsunami warning, an extensive network of monitoring stations has been installed in ocean areas where tsunamis can occur. The largest network is the DART system <ref> Deep-ocean Assessment and Reporting of Tsunamis, https://www.ndbc.noaa.gov/dart/dart.shtml </ref> consisting of an array of stations in the Pacific Ocean, see fig. 5a. Each station consists of a seabed pressure recorder that detects the passage of a tsunami. The data is sent by sonar signal to a moored buoy. This buoy sends a radio signal via satellite to the Pacific Tsunami Warning Center, see Fig. 5b. In this way, a tsunami traveling across the ocean can be detected and an early warning can be issued to countries where the tsunami will land, allowing timely evacuation of risk populations. The information collected by the monitoring network also enables to estimate the nature and intensity of the tsunami and the possible impact on the coast.
 
  
 
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[[File:TsunamiMonitoringNetwork.jpg|thumb|left|400px|Fig. 5a. The global tsunami monitoring network.]]
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[[File:RoomForWaterCoastalCities.jpg|thumb|left|400px|Figure 6: Examples of room for water in Dutch cities.]]
 
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[[File:TsunamiWarningDARTsystem.png|thumb|left|400px|Fig. 5b. The DART monitoring system. Image NOAA.]]
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[[File:WaterAssessmentAmsterdam.jpg|thumb|left|500px|Figure 7: Assessment of policy objectives related to urban water management for the city of Amsterdam<ref>Koop, S. H. A., and Van Leeuwen, C. J. 2015. Assessment of the Sustainability of Water Resources Management: A Critical Review of the City Blueprint Approach Water Resources Management 29: 5649-5670</ref>.]]
 
|}
 
|}
  
 +
===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, etc. <ref> FAO 2017. The State of Food and Agriculture - leveraging food systems for inclusive rural transformation. http://www.fao.org/3/a-i7658e.pdf </ref>. 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.
  
−
==Tsunami models==
+
===Protection against flooding from the sea===  
−
Due to the lack of field observations, one has to rely on models for gaining insight into the hydrodynamic processes involved in tsunami events. The propagation of tsunami waves over the continental shelf and the shoreface can be simulated in hydraulic scale models based on Froude scaling, as long as boundary-layer processes are of minor importance. Scale effects occur when energy dissipation (wave breaking, bed friction) comes into play. This imposes a limitation on the simulation of the inland runup process in the laboratory. The same limitation applies to simple mathematical models that do not include wave breaking and friction. Detailed process-based numerical models are required for simulating these effects. A thorough model study of this type has been carried out by Larsen and Furman (2019)<ref name=LF></ref>.
+
Sea level rise is a major threat to coastal cities worldwide<ref>Wong, P.P., I.J. Losada, J.-P. Gattuso, J. Hinkel, A. Khattabi, K.L. McInnes, Y. Saito, and A. Sallenger 2014. Coastal systems and low-lying areas. In: Climate Change 2014: Impacts, Adaptation, and Vulnerability. Part A: Global and Sectoral Aspects. Contribution of Working Group II to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Field, C.B., V.R. Barros, D.J. Dokken, K.J. Mach, M.D. Mastrandrea, T.E. Bilir, M. Chatterjee, K.L. Ebi, Y.O. Estrada, R.C. Genova, B. Girma, E.S. Kissel, A.N. Levy,S. MacCracken, P.R. Mastrandrea, and L.L. White (eds.)]. Cambridge University Press, pp. 361-40</ref>. Large parts of many coastal cities are situated today below the water level reached at sea during a 1/100 storm<ref>Muis, S., Verlaan, M., Winsemius, H.C., Aerts, J.C.J.H. and Ward, P.J. 2018. A global reanalysis of storm surges and extreme sea levels. Nat. Commun. 7:11969</ref><ref name=Ha></ref>; the frequency of exceedance of these storm levels may increase by a factor 10 or 100 during the 21st century<ref name=V>Vousdoukas, M.I., Mentaschi, L., Voukouvalas, E., Verlaan, M., Jevrejeva, S., Jackson, L.P. and Feyen, L. 2018. Global probabilistic projections of extreme sea levels show intensification of coastal flood hazard. Nature communications, 9 (1), 2360</ref><ref name=I></ref>. Constructions to protect against flooding and overtopping waves have to be adapted accordingly.  
−
 
 
−
A major problem for predicting the impact of tsunamis lies in the fact that the initial tsunami-generating disturbance cannot be predicted and is generally not well known. When a tsunami wave is detected on the ocean, the time is too short for an accurate calculation of the impact before the tsunami lands on the coast. Only theoretical or semi-empirical relationships can be used.  
 
  
−
[[Image:TsunamiScheme.jpg|thumb|400px|left|Figure 6: Schematization of the shoreface and beach used in the tsunami runup model of Madsen and Schäfer (2010)<ref name=MS></ref>.]]
+
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 [[:Category: Hard structures|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)<ref>Jonkman, S.N., Hillen, M.M., Nicholls, R.J., Kanning, W. and van Ledden, M. 2013. Costs of adapting coastal defences to sea-level rise—New estimates and their implications. J. Coast. Res. 29: 1212–1226</ref> and  Aerts (2018)<ref> Aerts, J.C.J.H. 2018. A Review of Cost Estimates for Flood Adaptation. Water 10, 1646 </ref>. 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<ref name=Ha></ref>. Costly defense measures can also be justified to protect people and to avoid strong social impacts<ref name=N></ref>. Some highly exposed coastal cities are listed in Table 1, but this list is far from exhaustive. 
  
−
Relationships for the tsunami runup height and runup velocity, validated with laboratory experiments and advanced numerical models, are based on the theoretical work of Madsen and Schäfer (2010)<ref name=MS></ref>. They solved analytically the non-linear shallow-water equations (mass and momentum balance) over a sloping bed,
 
  
−
<math>\Large\frac{\partial \zeta}{\partial t}\normalsize + \Large\frac{\partial}{\partial x}\normalsize [(D+\zeta -Sx)u] = 0 , \quad \Large\frac{\partial u}{\partial t}\normalsize + u \Large\frac{\partial u}{\partial x}\normalsize + g \Large\frac{\partial \zeta}{\partial x}\normalsize =0 , </math>
+
[[Image:KatwijkCoastalProtection.jpg|thumb|300px|left|Figure 8: The coastal village Katwijk (Netherlands) has been protected by an artificial dune built in front of the seashore boulevard with sand extracted far offshore. A parking space has been created for beach tourists on the inside of this multifunctional structure.]]
  
−
where <math>t</math> is time, <math>x</math> is the spatial coordinate in the propagation direction, <math>D</math> is the average shelf depth, <math>S</math> is the average shoreface and beach slope, <math>\zeta</math> is the surface elevation and <math>u</math> is the depth-averaged current velocity. The corresponding schematization is shown in Fig. 6. Madsen and Schäfer considered an incident tsunami wave of the form
+
Soft coastal defense options have become more popular in recent decades, see the articles in the category [[:Category: Soft coastal interventions|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) <ref> Van Coppenolle, R, and Temmerman, S. 2019. A global exploration of tidal wetland creation for nature-based flood risk mitigation in coastal cities. Estuarine, Coastal and Shelf Science 226, 106262</ref> 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<ref>Narayan, S., Beck, M., Reguero, B., Losada, I., Van Wesenbeeck, B., Pontee, N., Sanchirico, J., Ingram, J., Lange, G. and Burks-Copes, K. 2016. The Effectiveness, Costs and Coastal Protection Benefits of Natural and Nature-Based Defences. PLoS ONE 11, e0154735</ref>.
  
−
<math>\zeta(0,t) = A_0 \cos(\omega t)  .</math>
 
  
−
For a non-breaking tsunami wave they arrived at the following expressions for the maximum runup height <math>R</math> and the maximum runup velocity <math>V</math>:
 
  
−
<math>R = 2 \pi^{3/4} A_0 (D / A_0)^{1/4} \xi^{-1/2} , \quad V = 2 \pi^{5/4} (A_0 / D) \sqrt{g A_0} \xi^{-3/2} .  </math>
 
−
 
−
These simple expressions represent fairly well detailed numerical model results that include wave breaking, bed friction and turbulent energy dissipation<ref name=LF></ref>. The numerical model gives somewhat higher values of <math>R</math> and <math>V</math>, but in practice the values might be lower due to energy dissipation on obstacles that are generally present the runup area.
 
  
−
The numerical model <ref name=LF></ref> also provides results for <math>R</math> and <math>V</math> in the case of a breaking tsunami. These results are best approximated by
 
  
−
<math>R = A_0 \xi , \quad V = 2 \sqrt{g A_0} .</math>
 
  
−
The first expression corresponds to the formula established by Hunt (1959) <ref>Hunt, I.A. 1959. Design of seawalls and breakwaters. J. Waterw. Harbors Division ASCE 85: 123–152</ref> for swash uprush (see the article [[Swash zone dynamics]]) and the second expression corresponds to the initial flow velocity after dam break (see the article [[Dam break flow]]).
 
  
−
Non-breaking tsunamis occur for large <math>\xi</math>-values <math>\xi > \xi_b</math>, corresponding to a steep shoreface/beach slope and long-period tsunamis. Breaking tsunamis occur for small <math>\xi</math>-values <math>\xi < \xi_b</math>, corresponding to gentle shoreface/beach slope and short-period tsunamis. According to the numerical model <ref name=LF></ref>, <math>\xi_b \approx 3 (D / A_0)^{1/6}</math>.
+
==Adaptation examples==
  
−
The above relationships are the result of recent studies (up to 2019) of tsunami irruption on a coast. However, further underpinning of the above results is needed, given that previous investigations produce results that differ in some respects <ref name=MG></ref>. Synolakis <ref> Synolakis, C. E. 1987. The runup of solitary waves. J. Fluid Mech. 185: 523–545 </ref> modeled the tsunami as a solitary wave of height <math>H</math>. This model, which was validated with other laboratory measurements, yields for the runup height of a tsunami the relationship
+
===Bangkok===
 +
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 <ref>Resilient Bangkok. 100 Resilient Cities. https://www.100resilientcities.org/wp-content/uploads/2017/07/Bangkok_-_Resilience_Strategy.pdf</ref>:
 +
# 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 <ref>Thanvisitthpon, N.,  Shrestha, S. and  Pal, I. 2018. Urban Flooding and Climate Change: A Case Study of Bangkok, Thailand. Environment and Urbanization. Asia 9: 1–15</ref>.
  
−
<math>R = 2.8 H S (H/D)^{5/4} .</math>
+
===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 <ref>Zevenbergen, C., Fu, D. and Pathirana, A.  2018. Transitioning to Sponge Cities: Challenges and Opportunities to Address Urban Water Problems in China. Water 10, 1230; doi:10.3390/w10091230</ref>:
 +
* 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.
  
−
A purely empirical rule for the maximum runup of a tsunami was proposed by Plafker (1964, unpublised). This rule states that the maximum runup will not exceed twice the height of the seafloor deformation resulting from a submarine earthquake <ref name=SB></ref>.  
+
===Jakarta===
 +
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 [https://en.wikipedia.org/wiki/Giant_Sea_Wall_Jakarta '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<ref>Wade, M. Hyper-planning Jakarta: The Great Garuda and planning the global spectacle. Singapore Journal of Tropical Geography 40: 158–172</ref>. 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 <ref>Garschagen, M.,  Surtiari, G.A.K. and Harb, M. 2018. Is Jakarta’s New Flood Risk Reduction Strategy Transformational? Sustainability 2018, 10, 2934; doi:10.3390/su10082934</ref>.
  
 +
[[Image:GreatGaruda.jpg|thumb|700px|center|Figure 9: Lay-out of the Great Garuda Project, Jakarta.]]
  
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==Related articles==
 
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: [[Swash zone dynamics]]
 
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: [[Dam break flow]]
 
−
: [[Tidal bore dynamics]]
 
  
  
 
==References==
 
==References==
 
<references/>
 
<references/>

Revision as of 16:22, 22 March 2020

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.


Vulnerability to climate change and sea level rise

Figure 1: Population of coastal cities around 1950 and in 2020. Adapted from Barragan et al. (2015) [1].

Coastal cities have experienced tremendous growth in recent decades, especially in Africa and Asia [1], 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)[2]. 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 [3] 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[4]. 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 [5]. 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 [6][7]. 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. Such geographical conditions are an important factor for the vulnerability of many major coastal cities and for their sensitivity to climate change.


Figure 2: Schematic representation of the characteristic geographical setting of coastal towns that were built in delta plains, close to estuaries or lagoons. Left panel: The original settlement was built on high ground. Right panel: For urban expansion the surrounding lowlands (marshland, lagoons) were reclaimed by drainage and landfills. These areas are vulnerable to flooding.


Figure 3: Subsidence in a few coastal megacities [8].


Soil subsidence

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, see Fig. 3 Cite error: Closing </ref> missing for <ref> tag. Such a strong subsidence significantly increases the sensitivity of these areas to flooding.


Population growth

Demographic growth in developing countries is particularly strong in rural areas. Water scarcity, especially in Africa, reduces the availability of suitable agricultural land [9], while mechanization reduces the need for manpower. These trends make that new generations can no longer earn a living[10]. 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 the most vulnerable group because, due to their precarious living conditions, they cannot make provisions against the effects of climate change [5].


Figure 4: Slums on a sandspit at the coast of Monrovia (Liberia).


Water quality

Figure 5: Clogged drainage canal in Accra (Ghana). Photo credit TU Delft.

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[11]. 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[12]. Dunes and beaches are often partially excavated to extract sand for landfill; corals and shell banks are used as raw materials for construction [13]. 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.

Weak governance

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[14][15][16][17]. 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

Table 1: Exposure and vulnerability of coastal cities to flood risks exacerbated by sea level rise. Dark red: very high vulnerability (very high exposure / very high socio-economic sensitivity / weak adaptive capacity / very strong increase of population at risk); Yellow: high vulnerability / weak-medium adaptive capacity; Green: medium-low vulnerability / medium-strong adaptive capacity.


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)[18], Hanson (2011)[19], Neumann et al. (2015)[7], [20], Dhinan et al. (2019)[21], Hallegatte et al. (2013)[19].

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.



Adaptation measures

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 section #Causes of increasing vulnerability.

Governance

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 [5]. 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

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. Setback areas must be defined and enforced 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. To obtain support and cooperation, authorities should 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 [22].

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. [23]. 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-elevation coastal zones.


Figure 6: Examples of room for water in Dutch cities.
Figure 7: Assessment of policy objectives related to urban water management for the city of Amsterdam[24].

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, etc. [25]. 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[26]. Large parts of many coastal cities are situated today below the water level reached at sea during a 1/100 storm[27][19]; the frequency of exceedance of these storm levels may increase by a factor 10 or 100 during the 21st century[28][4]. 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)[29] and Aerts (2018)[30]. 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[19]. Costly defense measures can also be justified to protect people and to avoid strong social impacts[7]. Some highly exposed coastal cities are listed in Table 1, but this list is far from exhaustive.


Figure 8: The coastal village Katwijk (Netherlands) has been protected by an artificial dune built in front of the seashore boulevard with sand extracted far offshore. A parking space has been created for beach tourists on the inside of this multifunctional structure.

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) [31] 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[32].




Adaptation examples

Bangkok

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 [33]:

  1. flood retarding in the upstream river reaches by diverting water towards temporary water retention areas;
  2. developing new water storage capacity inside the city by creating open spaces and green areas as potential water storage areas;
  3. improving community-based adaptation and disaster preparedness and communication;
  4. improving the urban flood defense system by upgrading existing drainage systems;
  5. enhancing emergency preparedness and response through monitoring and communication;
  6. capacity building for disaster risk reduction.

The implementation and effectiveness of these measures will depend crucially on generating broad public support and interprovincial cooperation [34].

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 [35]:

  • 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:

  1. restoring the city’s capacity to absorb, infiltrate, store, purify, drain, and manage rainwater and
  2. regulating the water cycle as much as possible to mimic the natural hydrological cycle.

Jakarta

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:

  1. river and canal regulation, the broadening of water ways and the clearance of river banks, which are frequently encroached by informal settlers;
  2. the restoration and expansion of flood reservoirs and
  3. 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[36]. 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 [37].

File:GreatGaruda.jpg
Figure 9: Lay-out of the Great Garuda Project, Jakarta.


References

  1. ↑ 1.0 1.1 Barragan, J.M. and de Andres, M. 2015. Analysis and trends of the world's coastal cities and agglomerations. Ocean & Coastal Management 114; 11-20
  2. ↑ WEF 2019. The Global Risks Report 2019, 14th Edition. World Economic Forum
  3. ↑ Jonkman, S.N. and Vrijling, J.K. 2008. Loss of life due to floods. J Flood Risk Management 1: 43–56
  4. ↑ 4.0 4.1 IPCC, 2019. Special Report on the Ocean and Cryosphere in a Changing Climate [Eds.: H.-O. Pörtner, D.C. Roberts, V. Masson-Delmotte, P. Zhai, M. Tignor, E. Poloczanska, K. Mintenbeck, M. Nicolai, A. Okem, J. Petzold, B. Rama and N. Weyer]
  5. ↑ 5.0 5.1 5.2 Vivekananda, J. and Bhatiya, N. 2016. Coastal Megacities vs. the Sea: Climate and Security in Urban Spaces. BRIEFER 30: 1-12 Cite error: Invalid <ref> tag; name "VB" defined multiple times with different content
  6. ↑ McGranahan, G., Balk, D. and Anderson, B. 2006. The rising tide: assessing the risks of climate change and human settlements in low elevation coastal zones. Environ Urban 12: 17–38
  7. ↑ 7.0 7.1 7.2 Neumann, B., Vafeidis, A. T., Zimmermann J. and Nicholls, R. J. 2015. Future coastal population growth and exposure to sea-level rise and coastal flooding – a global assessment. PLOS ONE 10: 1-34
  8. ↑ Deltares, Sinking cities https://www.deltares.nl/app/uploads/2015/09/Sinking-cities.pdf
  9. ↑ Koutroulis, A.G., Papadimitriou, L.V., Grillakis, M.G., Tsanis, I.K., Warren, R. and Betts, R.A. 2019. Global water availability under high-end climate change: A vulnerability based assessment. Global and Planetary Change 175: 52–63
  10. ↑ Seto, K.C., 2011. Exploring the dynamics of migration to mega-delta cities in Asia and Africa: contemporary drivers and future scenarios. Global Environ. Change 21: S94-S107
  11. ↑ Penning de Vries, F.W.T., Acquay, H., Molden, D., Scherr, S.J., Valentin, C. and Cofie, O. 2002. Integrated Land and Water Management for Food and Environmental Security. Global Environmental Facility. Comprehensive Assessment Research Paper Colombo, Sri Lanka
  12. ↑ Agardy, T. and Alder, J. (lead authors) 2005. Millennium Ecosystem Assessment Chapter 19 Coastal Systems. Washington, D.C., Island Press
  13. ↑ Wilkinson, C. 2001. Status of coral reefs of the world 2000. Queensland (Australia): Australia Institute of Marine Science
  14. ↑ Diop, S. and Scheren, P.A. 2016. Sustainable oceans and coasts: Lessons learnt from Eastern and Western Africa. Estuarine, Coastal and Shelf Science 183: 327-339
  15. ↑ Tabet, L. and Fanning, L. 2012. Integrated coastal zone management under authoritarian rule: An evaluation framework of coastal governance in Egypt. Ocean & Coastal Management 61: 1-9
  16. ↑ Le, T.D.N. 2019. Climate change adaptation in coastal cities of developing countries: characterizing types of vulnerability and adaptation options. Mitigation and Adaptation Strategies for Global Change, https://doi.org/10.1007/s11027-019-09888-z
  17. ↑ Varis, O. 2006. Megacities, development and water. Int. J.Water Resour. Dev. 22: 199-225
  18. ↑ WWF 2009. Mega-Stress for Mega-Cities, A Climate Vulnerability Ranking of Major Coastal Cities in Asia
  19. ↑ 19.0 19.1 19.2 19.3 Hanson, S., Nicholls, R., Ranger, N., Hallegatte, S., Corfee-Merlot, J., Herweyer, C. and Chateau, J. 2011. A global ranking of port cities with high exposure to climate extremes. Climatic Change 104: 89–111 Cite error: Invalid <ref> tag; name "Ha" defined multiple times with different content
  20. ↑ Wang, G., Liu, Y., Wang, H. and Wang, X. 2014. A comprehensive risk analysis of coastal zones in China. Estuarine, Coastal and Shelf Science 140: 22-31
  21. ↑ Dhiman, R., VishnuRadhan, R., Eldho, T. I. and Inamdar, A. 2019. Flood risk and adaptation in Indian coastal cities: recent scenarios. Applied Water Science 9:5
  22. ↑ Hillen, M.M. and Dolman, N. 2015. Towards water adaptive cities. World Engineers Summit on Climate Change (WES) July 2015, Singapore
  23. ↑ Satterthwaite, D., Huq, S., Pelling, M., Reid, H. and Lankao, P. R. 2007. Adapting to Climate Change in Urban Areas. IIED, Rockefeller Foundation. www.iied.org/pubs/display.php?o=10549IIED
  24. ↑ Koop, S. H. A., and Van Leeuwen, C. J. 2015. Assessment of the Sustainability of Water Resources Management: A Critical Review of the City Blueprint Approach Water Resources Management 29: 5649-5670
  25. ↑ FAO 2017. The State of Food and Agriculture - leveraging food systems for inclusive rural transformation. http://www.fao.org/3/a-i7658e.pdf
  26. ↑ Wong, P.P., I.J. Losada, J.-P. Gattuso, J. Hinkel, A. Khattabi, K.L. McInnes, Y. Saito, and A. Sallenger 2014. Coastal systems and low-lying areas. In: Climate Change 2014: Impacts, Adaptation, and Vulnerability. Part A: Global and Sectoral Aspects. Contribution of Working Group II to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Field, C.B., V.R. Barros, D.J. Dokken, K.J. Mach, M.D. Mastrandrea, T.E. Bilir, M. Chatterjee, K.L. Ebi, Y.O. Estrada, R.C. Genova, B. Girma, E.S. Kissel, A.N. Levy,S. MacCracken, P.R. Mastrandrea, and L.L. White (eds.)]. Cambridge University Press, pp. 361-40
  27. ↑ Muis, S., Verlaan, M., Winsemius, H.C., Aerts, J.C.J.H. and Ward, P.J. 2018. A global reanalysis of storm surges and extreme sea levels. Nat. Commun. 7:11969
  28. ↑ Vousdoukas, M.I., Mentaschi, L., Voukouvalas, E., Verlaan, M., Jevrejeva, S., Jackson, L.P. and Feyen, L. 2018. Global probabilistic projections of extreme sea levels show intensification of coastal flood hazard. Nature communications, 9 (1), 2360
  29. ↑ Jonkman, S.N., Hillen, M.M., Nicholls, R.J., Kanning, W. and van Ledden, M. 2013. Costs of adapting coastal defences to sea-level rise—New estimates and their implications. J. Coast. Res. 29: 1212–1226
  30. ↑ Aerts, J.C.J.H. 2018. A Review of Cost Estimates for Flood Adaptation. Water 10, 1646
  31. ↑ Van Coppenolle, R, and Temmerman, S. 2019. A global exploration of tidal wetland creation for nature-based flood risk mitigation in coastal cities. Estuarine, Coastal and Shelf Science 226, 106262
  32. ↑ Narayan, S., Beck, M., Reguero, B., Losada, I., Van Wesenbeeck, B., Pontee, N., Sanchirico, J., Ingram, J., Lange, G. and Burks-Copes, K. 2016. The Effectiveness, Costs and Coastal Protection Benefits of Natural and Nature-Based Defences. PLoS ONE 11, e0154735
  33. ↑ Resilient Bangkok. 100 Resilient Cities. https://www.100resilientcities.org/wp-content/uploads/2017/07/Bangkok_-_Resilience_Strategy.pdf
  34. ↑ Thanvisitthpon, N., Shrestha, S. and Pal, I. 2018. Urban Flooding and Climate Change: A Case Study of Bangkok, Thailand. Environment and Urbanization. Asia 9: 1–15
  35. ↑ Zevenbergen, C., Fu, D. and Pathirana, A. 2018. Transitioning to Sponge Cities: Challenges and Opportunities to Address Urban Water Problems in China. Water 10, 1230; doi:10.3390/w10091230
  36. ↑ Wade, M. Hyper-planning Jakarta: The Great Garuda and planning the global spectacle. Singapore Journal of Tropical Geography 40: 158–172
  37. ↑ Garschagen, M., Surtiari, G.A.K. and Harb, M. 2018. Is Jakarta’s New Flood Risk Reduction Strategy Transformational? Sustainability 2018, 10, 2934; doi:10.3390/su10082934
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