Editing IPCC:AR6/WGII/Chapter-6 (section)

==== 6.2.2.2 Urban Flooding ====

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Flood risks in settlements arise from hydrometeorological events interacting with the urban system which exposes settlements to river (fluvial) floods, flash floods, pluvial (precipitation-driven) floods, sewer floods, coastal floods and glacial lake outburst floods ( [[#IPCC--2012|IPCC, 2012]] ). Sea level increase and increases in tropical cyclone storm surge and rainfall intensity will increase the probability of coastal city flooding ( ''high confidence'' ) (WGI Box TS14). Globally, the increase in frequencies and intensities of extreme precipitation from global warming will ''likely'' [[#footnote-000|4]] expand the global land area affected by flood hazards ( ''medium confidence'' ) ((Alfieri et al., 2018; Alfieri et al., 2017; Hoegh-Guldberg et al., 2018); [[IPCC:Wg2:Chapter:Chapter-4#4.2.4|Section 4.2.4.2]] ). [[#Mishra--2015|Mishra et al. (2015)]] noted that out of 241 urban areas, only 17% of cities experienced statistically significant increases in frequencies of extreme precipitation events from 1973 to 2012. In the future, there is some evidence that changes in high-intensity short duration (sub-daily) rainfall in urban areas will increase ( ''limited evidence'' , ''medium agreement'' ) (Kendon et al., 2014; Ban, Schmidli and Schär, 2015; Abiodun et al., 2017).

Flooding associated with sea level rise is addressed in more detail in Cross-Chapter Paper 2, with detailed regional examples from Africa discussed in [[IPCC:Wg2:Chapter:Chapter-9#9.3|Section 9.3]] . Coastal flooding associated with sea level rise is exacerbated due to the significant number of people living in subsiding areas. As a result of this, the average coastal resident is experiencing (over the last two decades) rates of relative sea level rise three to four times higher than typical estimates due to climate-induced changes (Nicholls et al., 2021). This process can also result in release of coastal waste into urban areas (Beaven et al., 2020).

Urban flooding risks are also increased by urban expansion and land use and land cover change which enlarges impermeable surface areas through soil sealing, impacting drainage of floodwaters with consequent sewer overflows ( ''high confidence'' ) (Arnbjerg-Nielsen et al., 2013; Ziervogel et al., 2016; [[#Aroua--2016|Aroua, 2016]] ; Kundzewicz et al., 2014). These risks are also driven by increasing societal complexity, urban developmental policy on flood control and long-term economic growth (Berndtsson et al., 2019), including in mega-cities (Januriyadi et al., 2018). The increase in flood risk from urban development can be considerable; based on modelling of two RCP (4.5 and 8.5) scenarios, Kaspersen et al. (2017) noted flooding in four European cities could increase by up to 10% for every 1% increase in impervious surface area. Risks are also compounded by the location of settlements, with greater risks within cities located in low elevation coastal zones subject to sea level rise, potential land subsidence and exposure to tropical cyclones (( [[#Koop--2017|Koop and van Leeuwen, 2017]] ; Hoegh-Guldberg et al., 2018; see also Section [https://www.ipcc.ch/chapter/6#CCP2.2 CCP2.2] ) and within informal settlements, where generally little investment in drainage solutions exists and flooding regularly disrupts livelihoods and disproportionately undermines local food safety and security for the urban poor (Dodman, Colenbrander and Archer, 2017; Dodman et al., 2017; Kundzewicz et al., 2014; Sections 5.4 and 5.8).

Future risks of urban flooding is increasing in conjunction with continued increases in global surface temperature ( ''high confidence'' ) ( [[#IPCC--2019b|IPCC, 2019b]] ; Winsemius et al., 2015; [[#Kulp--2019|Kulp and Strauss, 2019]] ; Hoegh-Guldberg et al., 2018). In particular, Asian cities are highly exposed to future flood risks arising from urbanisation processes. Between 2000 and 2030, rapid urbanisation in Indonesia will elevate flood risks by 76–120% for river and coastal floods, while sea level rise will further increase the exposure by 19–37% (Muis et al., 2015). In Can Tho, Vietnam, current urban development patterns put new assets and infrastructure at risk due to sea level rise and river flooding in the Mekong Delta (Chinh et al., 2017; Chinh et al., 2016). Flooding in urban areas is exacerbated both by the encroachment of urban areas into areas that retain water and by the lack of infrastructure such as embankments and flood walls, as is the case for large areas of Dhaka East (Haque, Bithell and Richards, 2020). [[#Zhou--2019|Zhou et al. (2019)]] have also shown that for the city of Hohhot, China, the increase in impervious surfaces contributes between 2–4 times more to modelled annual flood risk compared with risk induced by climate change.

Global trends in surface water flooding are increasing, which poses risks to vulnerable urban systems depending on current adaptation measures to manage flooding impacts, for example, stormwater management, green infrastructure and sustainable urban drainage systems (Molenaar et al., 2015). The economic risks associated with future surface water flooding in towns and cities are considerable. For example in the UK, expected annual damages from surface water flooding may increase by £60–200 million for projected 2–4°C warming scenarios; enhanced adaptation actions could manage flooding up to a 2°C scenario but will be insufficient beyond that (Sayers et al., 2015). Analyses conducted in South Korea suggests that future flood levels could exceed current flood protection design standards by as much as 70% by 2100, considerably increasing urban flood risk (Kang et al., 2016). Modelling of urban flood damage in the Kelani River Basin in Sri Lanka showed increased frequency of flooding by 2030 could increase potential urban property damage by up to 10.2% (Komolafe Akinola, Herath and Avtar, 2018). Urban flood impacts may also exacerbate health burdens (including disease outbreaks of malaria, typhoid and cholera), which are compounded by damage to medical facilities (e.g., damage to hospitals and disruption of medicinal supply chains), as observed in urban areas of Ghana (Gough et al., 2019). In addition, emerging research shows the cascading consequences of hazard events, in this case urban flooding, on other risks to well-being in ways that are particularly severe for the urban poor, including mental ill-health, incidents of domestic violence impacting children and women, chronic diseases and salinity of drinking water ((Matsuyama, Khan and Khalequzzaman, 2020); [[IPCC:Wg2:Chapter:Chapter-4#4.2.4|Section 4.2.4.5]] ; [[#6.2.4.2|Section 6.2.4.2]] ; Box 7.2; [[IPCC:Wg2:Chapter:Chapter-8#8.4.5.2|Section 8.4.5.2]] ).

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