Coastal flooding occurs when dry and low-lying land is submerged (flooded) byseawater.[1] The range of a coastalflooding is a result of the elevation of floodwater that penetrates the inland which is controlled by thetopography of the coastal land exposed to flooding.[1][2] The seawater can flood the land via several different paths: direct flooding, overtopping or breaching of a barrier.[3] Coastal flooding is largely a natural event. Due to theeffects of climate change (e.g.sea level rise and an increase inextreme weather events) and an increase in the population living in coastal areas, the damage caused by coastal flood events has intensified and more people are being affected.[4]
Coastal areas are sometimes flooded by unusually high tides, such asspring tides, especially when compounded by high winds andstorm surges. This was the cause of theNorth Sea flood of 1953 which flooded large swathes of theNetherlands and the East coast ofEngland.
When humans modify the coastal environment this can make coastal flooding worse.[1][5][6][7] Extraction of water from groundwater reservoirs in the coastal zone can instigatesubsidence of the land, thus increasing the risk of flooding.[5] Engineered protection structures along the coast, such assea walls, alter the natural processes of the beach. This can lead toerosion on adjacent stretches of the coast which also increases the risk of flooding.[1][7][8]
Reduction and control of coastal flooding is carried out using structural methods to hold back or redirect flood waters. Non-structural methods include coastal management, behavioral and institutional response to adapt to the processes. Natural defenses include physical features likegravel bars andsand dune systems, but alsoecosystems such assalt marshes,seagrass andmangrove forests which have a buffering function.Mangroves,wetlands andseagrass meadows are often considered to provide significant protection against storm waves, tsunamis, and shorelineerosion through their ability to attenuate wave energy.[6][9][10] To protect the coastal zone from flooding, the natural defenses should, therefore, be protected and maintained in for exampleMarine Protected Areas (MPAs).[11]
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Theseawater can flood the land via several different paths:
Coastal flooding can result from a variety of different causes includingstorm surges created by storms likehurricanes andtropical cyclones, rising sea levels due to climate change andtsunamis.
Storms, includinghurricanes andtropical cyclones, can cause flooding throughstorm surges which are waves significantly larger than normal.[1][14] If a storm event coincides with thehigh astronomical tide, extensive flooding can occur.[15] Storm surges involve three processes:
Wind blowing in an onshore direction (from the sea towards the land) can cause the water to 'pile-up' against the coast; this is known as wind setup. Lowatmospheric pressure is associated with storm systems and this tends to increase the surface sea level; this is a barometric setup. Finally increasedwave breaking height results in a higher water level in thesurf zone, which iswave setup. These three processes interact to create waves that can overtop natural and engineered coastal protection structures thus penetrating seawater further inland than normal.[15][16]
The sea level has been rising since the end of the last ice age, which was around 20,000 years ago.[17] Between 1901 and 2018, the averagesea level rose by 15–25 cm (6–10 in), with an increase of 2.3 mm (0.091 in) per year since the 1970s.[18]: 1216 This was faster than the sea level had ever risen over at least the past 3,000 years.[18]: 1216 The rate accelerated to 4.62 mm (0.182 in)/yr for the decade 2013–2022.[19]Climate change due to human activities is the main cause.[20]: 5, 8 Between 1993 and 2018, meltingice sheets andglaciers accounted for 44% ofsea level rise, with another 42% resulting fromthermal expansion ofwater.[21]: 1576
Sea level rise lags behind changes in theEarth's temperature by decades, and sea level rise will therefore continue to accelerate between now and 2050 in response to warming that has already happened.[22] What happens after that depends on future humangreenhouse gas emissions. If there are very deep cuts in emissions, sea level rise would slow between 2050 and 2100. The reported factors of increase in flood hazard potential are often exceedingly large, ranging from 10 to 1000 for even modest sea-level rise scenarios of 0.5 m or less.[23] It could then reach by 2100 between 30 cm (1 ft) and 1.0 m (3+1⁄3 ft) from now and approximately 60 cm (2 ft) to130 cm (4+1⁄2 ft) from the 19th century. With high emissions it would instead accelerate further, and could rise by 50 cm (1.6 ft) or even by 1.9 m (6.2 ft) by 2100.[24][20][18]: 1302 In the long run, sea level rise would amount to 2–3 m (7–10 ft) over the next 2000 years if warming stays to its current 1.5 °C (2.7 °F) over the pre-industrial past. It would be 19–22 metres (62–72 ft) if warming peaks at 5 °C (9.0 °F).[20]: 21
Tidal flooding, also known as sunny day flooding[25] or nuisance flooding,[26] is the temporary inundation of low-lying areas, especially streets, during exceptionallyhigh tide events, such as atfull andnew moons. The highest tides of the year may be known as theking tide, with the month varying by location. These kinds of floods tend not to be a high risk to property or human safety, but further stress coastal infrastructure in low-lying areas.[27]
This kind of flooding is becoming more common in cities and other human-occupied coastal areas assea level rise associated withclimate change and other human-related environmental impacts such ascoastal erosion andland subsidence increase thevulnerability of infrastructure.[28] Geographies faced with these issues can utilizecoastal management practices to mitigate the effects in some areas, but increasingly these kinds of floods may develop into coastal flooding that requiresmanaged retreat or other more extensiveclimate change adaptation practices are needed for vulnerable areas.Coastal areas can be significantly flooded as the result oftsunami waves[29] which propagate through theocean as the result of the displacement of a significant body of water throughearthquakes,landslides,volcanic eruptions, andglacier calvings. There is also evidence to suggest that significant tsunami have been caused in the past bymeteor impact into the ocean.[30] Tsunami waves are so destructive due to thevelocity of the approaching waves, the height of the waves when they reach land, and thedebris the water entrains as it flows over land can cause further damage.[29][9]
Depending on the magnitude of the tsunami waves and floods, it could cause severe injuries which call for precautionary interventions that prevent overwhelming aftermaths. It was reported that more than 200,000 people were killed in the earthquake and subsequent tsunami that hit the Indian Ocean, on December 26, 2004.[31] Not to mention, several diseases are a result of floods ranging from hypertension to chronic obstructive pulmonary diseases.[31]
Thecoastal zone (the area both within 100 kilometres distance of the coast and 100 metres elevation of sea level) is home to a large and growing proportion of the global population.[5][7] Over 50 percent of the global population and 65 percent of cities with populations over five million people are in the coastal zone.[32] In addition to the significant number of people at risk of coastal flooding, these coastal urban centres are producing a considerable amount of the globalGross Domestic Product (GDP).[7]
People's lives, homes, businesses, and city infrastructure like roads, railways, and industrial plants are all at risk of coastal flooding with massive potential social and economic costs.[33][34][35] The recentearthquakes andtsunami inIndonesia in 2004 and inJapan in March 2011 clearly illustrate the devastation coastal flooding can produce. Indirect economic costs can be incurred if economically important sandybeaches are eroded resulting in a loss oftourism in areas dependent on the attractiveness of those beaches.[36]
Coastal flooding can result in a wide variety of environmental impacts on different spatial and temporal scales. Flooding can destroy coastal habitats such as coastalwetlands andestuaries and can erode dune systems.[13][5][36][32] These places are characterized by their highbiological diversity therefore coastal flooding can cause significantbiodiversity loss and potentially speciesextinctions.[29] In addition to this, these coastal features are the coasts natural buffering system against storm waves; consistent coastal flooding and sea-level rise can cause this natural protection to be reduced allowing waves to penetrate greater distances inland exacerbating erosion and furthering coastal flooding.[5] "By 2050, "moderate" (typically damaging) flooding is expected to occur, on average, more than 10 times as often as it does today, and can be intensified by local factors."[37]
Prolongedinundation ofseawater after flooding can also causesalination of agriculturally productive soils thus resulting in a loss of productivity for long periods of time.[1][36] The effects of the soil salinization, which is brought on by sea levels rising and differed precipitation patterns, impacts agricultural production, ultimately leading towards food and water shortages.[citation needed] Foodcrops andforests can be completely killed off by salination of soils or wiped out by the movement of floodwaters.[5] Coastal freshwater bodies includinglakes,lagoons, and coastal freshwateraquifers can also be affected bysaltwater intrusion.[13][5][32] This can destroy these water bodies as habitats for freshwater organisms and sources of drinking water for towns and cities.[5][32]
Flood management or flood control are methods used to reduce or prevent the detrimental effects offlood waters. Flooding can be caused by a mix of both natural processes, such asextreme weather upstream, and human changes to waterbodies and runoff. Flood management methods can be either of thestructural type (i.e. flood control) and of thenon-structural type. Structural methods hold back floodwaters physically, while non-structural methods do not. Buildinghard infrastructure to prevent flooding, such asflood walls, is effective at managing flooding. However, it is best practice withinlandscape engineering to rely more onsoft infrastructure andnatural systems, such asmarshes andflood plains, for handling the increase in water.
Flood management can includeflood risk management, which focuses on measures to reduce risk, vulnerability and exposure to flood disasters and providing risk analysis through, for example,flood risk assessment.[38]Flood mitigation is a related but separate concept describing a broader set of strategies taken to reduce flood risk and potential impact while improving resilience against flood events.
Asclimate change has led to increased flood risk an intensity, flood management is an important part ofclimate change adaptation andclimate resilience.[39][40] For example, to prevent or manage coastal flooding,coastal management practices have to handle natural processes liketides but alsosea level rise due to climate change. The prevention and mitigation of flooding can be studied on three levels: on individual properties, small communities, and whole towns or cities.If human systems are affected by flooding, an adaption to how that system operates on the coast through behavioral and institutional changes is required, these changes are the so-callednon-structural mechanisms of coastal flooding response.[41]
Building regulations, coastalhazard zoning, urban development planning, spreading the risk throughinsurance, and enhancing public awareness are some ways of achieving this.[5][41][36]Adapting to the risk of flood occurrence can be the best option if the cost of building defense structures outweighs any benefits or if the natural processes in that stretch of coastline add to its natural character and attractiveness.[8]
A more extreme and often difficult to accept the response to coastal flooding is abandoning the area (also known asmanaged retreat) prone to flooding.[13] This however raises issues for where the people andinfrastructure affected would go and what sort ofcompensation should/could be paid.
There are a variety of ways in which humans are trying to prevent the flooding of coastal environments, typically through so-called hard engineering structures such asflood barriers,seawalls andlevees.[8][42] That armouring of the coast is typical to protect towns and cities which have developed right up to the beachfront.[8] Enhancing depositional processes along thecoast can also help prevent coastal flooding. Structures such asgroynes,breakwaters, and artificial headlands promote the deposition of sediment on the beach thus helping to buffer against storm waves and surges as the wave energy is spent on moving the sediments in the beach than on moving water inland.[42]
Coastal areas do provide natural protective structures to guard against coastal flooding. These include physical features likegravel bars andsand dune systems, but alsoecosystems such assalt marshes,seagrass andmangrove forests have a buffering function.Mangroves,wetlands andseagrass meadows are often considered to provide significant protection against storm waves, tsunamis, and shorelineerosion through their ability to attenuate wave energy.[6][9][10] To protect the coastal zone from flooding, the natural defenses should, therefore, be protected and maintained in for exampleMarine Protected Areas (MPAs).[11]
Reducing global sea level rise is one way to prevent significant flooding of coastal areas. This could be minimised by further reducinggreenhouse gas emissions. However, even if significant emission decreases are achieved, there is already a substantial amount of sea level rise into the future.[5] Internationalclimate change policies like theParis Agreement are seeking to mitigate the futureeffects of climate change, including sea level rise. In addition, more immediate measures of engineered and natural defenses are put in place to prevent coastal flooding.
Examples of countries with existing coastal flooding problems include:
Hurricane Katrina made landfall as a category 3cyclone on theSaffir–Simpson hurricane wind scale, indicating that it had become an only moderate level storm.[16] However, the catastrophic damage caused by the extensive flooding was the result of the highest recorded storm surges inNorth America.[16] For several days prior to the landfall of Katrina, wave setup was generated by the persistent winds of thecyclonic rotation of the system. This prolonged wave set up coupled with the very low central pressure level meant massive storm surges were generated.[45] Storm surges overtopped and breached thelevees and floodwalls intended to protect the city from inundation.[6][16][45] Unfortunately,New Orleans is inherently prone to coastal flooding for a number of factors. Firstly, much of New Orleans is below sea level and is bordered by theMississippi River therefore protection against flooding from both the sea and the river has become dependent on engineered structures.Land-use change and modification to natural systems in the Mississippi River have rendered the natural defenses for the city less effective.Wetland loss has been calculated to be around 1,900 square miles (4,920 square kilometres) since 1930. This is a significant amount as four miles of wetland are estimated to reduce the height of astorm surge by one foot (30 centimeters).[6]
2004 Indian Ocean earthquake and tsunami: Anearthquake of approximately magnitude 9.0 struck off the coast ofSumatra,Indonesia causing the propagation of a massivetsunami throughout theIndian Ocean.[9] This tsunami caused significant loss of human life, an estimate of 280,000 – 300,000 people has been reported[29] and caused extensive damage to villages, towns, and cities and to the physical environment. The natural structures and habitats destroyed or damaged includecoral reefs, mangroves, beaches, and seagrass beds.[9] The more recent earthquake and tsunami inJapan in March 2011 (2011 Tōhoku earthquake and tsunami) also clearly illustrates the destructive power of tsunamis and the turmoil of coastal flooding.
Fig. 2b
Press Release Number: 21042023.
This corresponds to a mean sea-level rise of about 7.5 cm over the whole altimetry period. More importantly, the GMSL curve shows a net acceleration, estimated to be at 0.08mm/yr2.
Box SYN-1: Sustained warming could lead to severe impacts
