Editing IPCC:AR6/WGII/Chapter-13 (section)

==== 13.2.2.1 Flood Risk Management ====

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Across Europe a range of measures have been implemented to address flood risk (Figure 13.6), with protection as the most used strategy ( ''high confidence'' ). Early warning and flood protection have been successful in reducing vulnerability to coastal and riverine flooding ( [[#Jongman--2015|Jongman et al., 2015]] ; [[#Kreibich--2015|Kreibich et al., 2015]] ; [[#Bouwer--2018|Bouwer and Jonkman, 2018]] ). Consequently, fatalities due to river flooding have decreased in Europe, despite similar numbers of people exposed (1990–2010 compared with 1980–1989) ( [[#Jongman--2015|Jongman et al., 2015]] ; [[#Paprotny--2018a|Paprotny et al., 2018a]] ).

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[[File:1f234ee802071bde5e9f871665185836 IPCC_AR6_WGII_Figure_13_006.png]]

'''Figure 13.6 |''' '''Effectiveness and feasibility of water-related adaptation options to achieve objectives under increasing climate hazards''' (Section SM13.9; Table SM13.1 )

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===== 13.2.2.1.1 Coastal flood risk management =====

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Further protection against coastal flooding is considered economically beneficial for densely populated areas ( [[#Lincke--2018|Lincke and Hinkel, 2018]] ; [[#Tiggeloven--2020|Tiggeloven et al., 2020]] ). At least 83% of flood damages due to coastal flooding could be avoided by elevating dykes along ~23–32% of Europe’s coastline by 2100 (RCP4.5-SSP1, RCP8.5-SSP5) ( [[#Vousdoukas--2020|Vousdoukas et al., 2020]] ). Limitations of building flood defences include cost–benefit considerations in rural areas, available land and social acceptability in densely populated areas ( [[#Haasnoot--2018|Haasnoot et al., 2018]] ; [[#Hinkel--2018|Hinkel et al., 2018]] ; [[#Meyerhoff--2021|Meyerhoff et al., 2021]] ).

Nature-based Solutions (NbS) (e.g., wetlands) and sediment-based solutions (e.g., sand nourishment) are increasingly considered for environmental, economic and/or societal reasons (Cross-Chapter Box NATURAL in Chapter 2; [[#Stive--2013|Stive et al., 2013]] ; [[#Pranzini--2015|Pranzini et al., 2015]] ; [[#Pinto--2020|Pinto et al., 2020]] ; [[#de%20Schipper--2021|de Schipper et al., 2021]] ). Coastal wetlands can be effective to reduce wave height and form habitats, but their feasibility and effectiveness is limited for densely populated areas with competing land use, runoff of pollution, sediment-starved deltas like the Rhine Delta ( [[#Edmonds--2020|Edmonds et al., 2020]] ) and rapid SLR ( [[#Kirwan--2016|Kirwan et al., 2016]] ; [[#Oppenheimer--2019|Oppenheimer et al., 2019]] ; [[#Haasnoot--2020b|Haasnoot et al., 2020b]] ). While losses of wetlands could be minor if warming stays below 1.7°C GWL, at high warming or SLR above 0.5 m large-scale losses of these habitats will impact their ecological importance, ecosystem function ( [[#13.4|Section 13.4]] ; KR 1, [[#13.10.2|Section 13.10.2]] ) and their ability to protect coastlines ( [[#Roebeling--2013|Roebeling et al., 2013]] ; [[#van%20der%20Spek--2018|van der Spek, 2018]] ; [[#Wang--2018|Wang et al., 2018]] ; [[#Xi--2021|Xi et al., 2021]] ). A combination with structural defences could reduce risk in urbanised coastal regions ( ''high confidence'' ). Accommodation through elevated or floating houses have been implemented and proposed locally within cities as part of a hybrid strategy together with protection and as a way of innovative urban development ( [[#13.6.2|Section 13.6.2]] ; Cross-Chapter Paper 2; [[#Penning-Rowsell--2020|Penning-Rowsell, 2020]] ; [[#Storbjörk--2021|Storbjörk and Hjerpe, 2021]] ).

Avoidance through restricting new developments in flood prone areas is applied along the coast of WCE and SEU ( [[#Harman--2015|Harman et al., 2015]] ; [[#Lincke--2020|Lincke et al., 2020]] ) and is considered a low-cost alternative to coastal defence at lower SLR. In SEU, an integrated coastal zone management (ICZM) protocol has been developed which requires a setback zone of 100 m from the coast in unprotected areas. Setback zones are projected to reduce impacts considerably in urbanised regions ( [[#Lincke--2020|Lincke et al., 2020]] ). Planned relocation is increasingly considered as a realistic adaptation option in cases of extreme SLR ( [[#Haasnoot--2021a|Haasnoot et al., 2021a]] ; [[#Lincke--2021|Lincke and Hinkel, 2021]] ; [[#Mach--2021|Mach and Siders, 2021]] ), for example, UK Shoreline Management Plans ( [[#Nicholls--2013|Nicholls et al., 2013]] ; [[#Buser--2020|Buser, 2020]] ). Retreat is rarely applied in Europe ( ''medium confidence'' ), though it can have greater benefit-to-cost outcomes than protection, particularly in less populated parts of Europe ( [[#Lincke--2021|Lincke and Hinkel, 2021]] ). Along parts of the coast in the UK (e.g., The Wash), Germany (e.g., Langeoog Island) and the Netherlands (e.g., Westerschelde) retreat has been applied to restore salt marshes and to aid coastal defence ( [[#Haasnoot--2019|Haasnoot et al., 2019]] ; [[#Kiesel--2020|Kiesel et al., 2020]] ; [[#Lincke--2021|Lincke and Hinkel, 2021]] ).

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===== 13.2.2.1.2 Riverine and pluvial flood risk management =====

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Structural flood protection (e.g., levees) is considered economically beneficial in densely populated areas ( [[#Alfieri--2016|Alfieri et al., 2016]] ; [[#Dottori--2020|Dottori et al., 2020]] ) and could reduce flood damage by ~45% as estimated under 1.5°C GWL and ~70% under 3°C GWL ( [[#Dottori--2020|Dottori et al., 2020]] ).

Providing more room for water through NbS is increasingly considered ( [[#Kreibich--2015|Kreibich et al., 2015]] ) as they can reduce risk effectively at lower costs, except in places with limited space or in areas with large protection. Such measures include (forest) restoration for upstream retention, restoration of river channels and widening riverbeds for natural flood retention ( [[#Kreibich--2015|Kreibich et al., 2015]] ; [[#Barth--2016|Barth and Döll, 2016]] ; [[#Wyżga--2018|Wyżga et al., 2018]] ). Natural retention areas are estimated to be the most effective option to reduce riverine flood risk across Europe in the 21st century, followed by protection ( ''low evidence'' ) ( [[#Dottori--2020|Dottori et al., 2020]] ).

Wet and dry proofing of buildings can be applied at household level. While measures taken at household level can reduce the risk of flooding, there is often insufficient investment ( ''medium confidence'' ) ( [[#Bamberg--2017|Bamberg et al., 2017]] ; [[#Aerts--2018|Aerts et al., 2018]] ). Reasons include low awareness or under-estimation of the risk ( [[#Kellens--2013|Kellens et al., 2013]] ), low perceived efficacy of adaptation measures ( [[#van%20Valkengoed--2019|van Valkengoed and Steg, 2019]] ) and lack of financial support ( [[#Kreibich--2011|Kreibich, 2011]] ). In the long term, risk reduction measures by governments are projected to outweigh floodproofing at household level, in particular in WCE, while for near-term household adaptation or regionally in SEU this could reduce risk more effectively ( [[#Haer--2019|Haer et al., 2019]] ). Relocation of households has occurred in response to river flood events (e.g., the 2013 flood events along the Danube River in Austria), with financial compensation playing a crucial role ( [[#Mayr--2020|Mayr et al., 2020]] ; [[#Thaler--2020|Thaler and Fuchs, 2020]] ; [[#Thaler--2021|Thaler, 2021]] ).

Urban drainage infrastructure is designed based on historical rainfall intensities, and thus may not have sufficient capacity for increased future intensities ( [[#Dale--2018|Dale et al., 2018]] ). Adaptation options to pluvial flooding include large retention ponds, local green spaces and green roofs within cities ( [[#Zölch--2017|Zölch et al., 2017]] ; [[#Maragno--2018|Maragno et al., 2018]] ; [[#Babovic--2019|Babovic and Mijic, 2019]] ; [[#Ribas--2020|Ribas et al., 2020]] ).

Early warning systems, insurance and behaviour change can complement protect and accommodate measures to limit residual risk ( ''high confidence'' ). Early warning systems have high monetary benefits ( [[#Pappenberger--2015|Pappenberger et al., 2015]] ). Behavioural adaptation to flooding relies on recognition of the threat and capacity to respond, both of which are often lacking ( [[#13.11.2.2|Section 13.11.2.2]] ; [[#Bamberg--2017|Bamberg et al., 2017]] ; [[#Haer--2019|Haer et al., 2019]] ). Flood risk insurance and compensation systems vary across European countries, ranging from post-disaster payments by governments and compulsory flood insurance, to public–private partnerships where the state acts as reinsurer ( [[#Keskitalo--2014|Keskitalo et al., 2014]] ; [[#Surminski--2015|Surminski et al., 2015]] ; [[#Hanger--2018|Hanger et al., 2018]] ). Risk-based insurance premiums can induce risk-averting behaviour but may become unaffordable to poor households and some households in high-risk zones ( [[#Hudson--2018|Hudson, 2018]] ; [[#Surminski--2018|Surminski, 2018]] ). Increasing future flood risks due to both climatic and socioeconomic change could overburden government budgets ( ''medium confidence'' ) ( [[#13.11.2|Section 13.11.2]] ; [[#Paudel--2015|Paudel et al., 2015]] ; [[#Mysiak--2016|Mysiak and Perez-Blanco, 2016]] ; [[#Schinko--2017|Schinko et al., 2017]] ; [[#Mochizuki--2018|Mochizuki et al., 2018]] ), resulting in unavailable or unaffordable insurance for private customers ( [[#13.8.3|Section 13.8.3]] ; [[#Hudson--2016|Hudson et al., 2016]] ; [[#Surminski--2018|Surminski, 2018]] ), and underfunding and insufficient solvency of insurance companies ( [[#13.6.2.5|Section 13.6.2.5]] ; [[#Lamond--2014|Lamond and Penning-Rowsell, 2014]] ). Local knowledge about disastrous flood events in the past can be lost across generations, leading to (re)-settlement in flood-prone areas ( [[#Fanta--2019|Fanta et al., 2019]] ).

Limits to adaptation to extremely high SLR scenarios have been identified for coastal defences, such as the Venice MoSE barrier (see Box 13.1), Thames Barrier in the UK ( [[#Ranger--2013|Ranger et al., 2013]] ) and the Maeslant Barrier in the Netherlands ( [[#Kwadijk--2010|Kwadijk et al., 2010]] ; [[#Haasnoot--2020b|Haasnoot et al., 2020b]] ). However, the scale and pace of adaptation required to face high-end SLR scenarios along all coasts of Europe has been poorly studied. Given the lead and long lifetime of large critical infrastructures, there is a growing need to look beyond 2100 to support the design of new infrastructures (Cross-Chapter Box SLR in Chapter 3).

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