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

== 13.2 Water ==

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=== 13.2.1 Observed Impacts and Projected Risks ===

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<span id="risk-of-coastal-flooding-and-erosion"></span>
==== 13.2.1.1 Risk of Coastal Flooding and Erosion ====

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Almost 50 million Europeans live within 10 m above mean sea level ( [[#Vousdoukas--2020|Vousdoukas et al., 2020]] ; McEvoy et al., 2021). Without further adaptation ( [[#13.2.2|Section 13.2.2]] ), flood risks along Europe’s low-lying coasts and estuaries will increase due to SLR compounded by storm surges, rainfall and river runoff ( ''high confidence'' ) ( [[#Mokrech--2015|Mokrech et al., 2015]] ; [[#Arns--2017|Arns et al., 2017]] ; [[#Sayol--2018|Sayol and Marcos, 2018]] ; [[#Vousdoukas--2018a|Vousdoukas et al., 2018a]] ; [[#Bevacqua--2019|Bevacqua et al., 2019]] ; [[#Couasnon--2020|Couasnon et al., 2020]] ). The population at risk of a 100-year flood event starts to rapidly increase beyond 2040 ( [[#Vousdoukas--2018a|Vousdoukas et al., 2018a]] ) reaching 10 million people under RCP8.5 by 2100, but it stays just below 10 million people under RCP2.6 by 2150 (Figure 13.5; [[#Haasnoot--2021b|Haasnoot et al., 2021b]] ) assuming present population and protection. The number of people at risk is projected to increase and risk to materialise earlier especially in response to increasing population under SSP5 ( [[#Vousdoukas--2018a|Vousdoukas et al., 2018a]] ; [[#Haasnoot--2021b|Haasnoot et al., 2021b]] ). Under high rates of SLR resulting from rapid ice sheet loss from Antarctica, risks may increase by a third by 2150 ( [[#Haasnoot--2021b|Haasnoot et al., 2021b]] ). Expected annual (direct) damages due to coastal flooding are projected to rise from 1.3 billion EUR today to 13–39 billion EUR by 2050 between 2°C and 2.5°C GWL and 93–960 billion EUR by 2100 between 2.5° and 4.4°C GWL, largely depending on socioeconomic developments (Cross-Chapter Box SLR in Chapter 3; [[#Vousdoukas--2018a|Vousdoukas et al., 2018a]] ) ( ''high confidence'' in the sign; ''low confidence'' in the numbers). UNESCO World Heritage sites in the coastal zone are at risk due to SLR, coastal erosion and flooding ( [[#13.8.1.3|Section 13.8.1.3]] ; Cross-Chapter Paper 4; [[#Marzeion--2014|Marzeion and Levermann, 2014]] ; [[#Reimann--2018b|Reimann et al., 2018b]] ) as are coastal landfills and other key infrastructures in Europe (AR6/SROCC; [[#Brand--2018|Brand et al., 2018]] ; [[#Beaven--2020|Beaven et al., 2020]] ).

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[[File:45f5737249250a5357d80d774e41fd9c IPCC_AR6_WGII_Figure_13_005.png]]

'''Figure 13.5 |''' '''Sea level rise (SLR) vulnerability and national planning in Europe:'''

'''(a)''' map of countries in Europe summarising the amount of SLR each country is planning for, at different time horizons (blue bars), and the present population (2020) at risk of a 100-year coastal flood event (orange bars) ( [[#Haasnoot--2021b|Haasnoot et al., 2021b]] ). The amounts of SLR and time horizons reflect national guidance or planning (local or project-based levels may differ) (McEvoy et al., 2021);

'''(b)''' projected population at risk to experience a 1-in-10-year coastal flood event under RCP2.6-SSP1 and RCP8.5-SSP5 assuming present protection and population levels, as well as population change according to, respectively, SSP1 and SSP5, based on Merkens (2016);

'''(c)''' projected population at risk to experience a 1-in-100-year coastal flood event under RCP2.6-SSP1 and RCP8.5-SSP5, assuming the present protection and population levels, as well as population change according to, respectively, SSP1 and SSP5, based on Merkens (2016) (based on [[#Haasnoot--2021b|Haasnoot et al., 2021b]] ).

Observations indicate that soft cliffs and beaches are most affected by erosion in Europe with, for example, 27–40% of Europe’s sandy coast eroding today, without climate change being identified as the main driver so far ( [[#Pranzini--2015|Pranzini et al., 2015]] ; [[#Luijendijk--2018|Luijendijk et al., 2018]] ; [[#Mentaschi--2018|Mentaschi et al., 2018]] ; [[#Oppenheimer--2019|Oppenheimer et al., 2019]] ). SLR will increase coastal erosion of sandy shorelines ( ''high confidence'' ) ( [[#Ranasinghe--2021|Ranasinghe et al., 2021]] ), but there is ''low confidence'' in quantitative values assessment of erosion rates and amounts ( [[#Athanasiou--2019|Athanasiou et al., 2019]] ; [[#Le%20Cozannet--2019|Le Cozannet et al., 2019]] ; [[#Thieblemont--2019|Thieblemont et al., 2019]] ). Without nourishment or other natural or artificial barriers to erosion, sandy shorelines could retreat by about 100 m in Europe at 4°C GWL; limiting warming to 3°C GWL could reduce this value by one-third ( [[#Vousdoukas--2020|Vousdoukas et al., 2020]] ).

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<span id="risks-related-to-inland-water"></span>
==== 13.2.1.2 Risks Related to Inland Water ====

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===== 13.2.1.2.1 Riverine and pluvial flooding =====

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Precipitation has raised river flood hazards in WCE and the UK by 11% per decade from 1960 to 2010 and decreased in EEU and SEU by 23% per decade ( [[#Douville--2021|Douville et al., 2021]] ; [[#Ranasinghe--2021|Ranasinghe et al., 2021]] ). The most recent three decades had the highest number of floods in the past 500 years with increases in summer ( [[#Blöschl--2020|Blöschl et al., 2020]] ). Economic flood damages increased strongly, reflecting increasing exposure of people and assets ( [[#Visser--2014|Visser et al., 2014]] ; [[#Hoegh-Guldberg--2018|Hoegh-Guldberg et al., 2018]] ; [[#Merz--2021|Merz et al., 2021]] ).

Projections indicate a continuation of the observed trends of river flood hazards in WCE ( ''high confidence'' ) of 10% at 2°C GWL and 18% at 4.4°C GWL, and a decrease in NEU and SEU ( ''medium confidence'' ) with, respectively, 5 and 11% in NEU and SEU for a 100-year peak flow, making Europe one of the regions with the largest projected increase in flood risk ( [[#Di%20Sante--2021|Di Sante et al., 2021]] ; [[#Ranasinghe--2021|Ranasinghe et al., 2021]] ). While there is disagreement on the magnitude of economic losses and people affected, there is ''high agreement'' on direction of change, particularly in WCE ( [[#Alfieri--2018|Alfieri et al., 2018]] ). New research increases confidence in AR5 statements that without adaptation measures, increases in extreme rainfall will substantially increase direct flood damages (e.g., [[#Madsen--2014|Madsen et al., 2014]] ; [[#Alfieri--2015a|Alfieri et al., 2015a]] ; [[#Alfieri--2015b|Alfieri et al., 2015b]] ; [[#Blöschl--2017|Blöschl et al., 2017]] ; [[#Dottori--2020|Dottori et al., 2020]] ; [[#Mentaschi--2020|Mentaschi et al., 2020]] ). With low adaptation, damages from river flooding are projected to be three times higher at 1.5°C GWL, four times at 2°C GWL and six times at 3°C GWL ( [[#Alfieri--2018|Alfieri et al., 2018]] ; [[#Dottori--2020|Dottori et al., 2020]] ). At 2°C GWL, the incidence of summer floods is expected to decrease across the whole alpine region, whereas winter and spring floods will increase due to extreme precipitation ( [[#Gobiet--2014|Gobiet et al., 2014]] ) and snowmelt-driven runoff ( [[#Coppola--2018|Coppola et al., 2018]] ).

Pluvial flooding and flash floods due to intense rainfall constitute most flood events in SEU and a substantial risk in other European regions (Cross-Chapter Paper 4; [[#Llasat--2016|Llasat et al., 2016]] ; [[#Rudd--2020|Rudd et al., 2020]] ). The majority (56%) of flood events between 1860 and 2016 were flash floods ( [[#Paprotny--2018a|Paprotny et al., 2018a]] ). These floods had considerable impacts including danger to human lives, for example, causing total economic damage of 1 billion USD in Copenhagen (Denmark) in 2011 ( [[#Wójcik--2013|Wójcik et al., 2013]] ), damage to private households of more than 70 million EUR in Münster (Germany) in 2014 ( [[#Spekkers--2017|Spekkers et al., 2017]] ) and during the 2021 floods in Belgium, Germany and the Netherlands over 200 deaths, damage to thousands of homes and disrupted water and electricity supply ( [[#Kreienkamp--2021|Kreienkamp et al., 2021]] ). The intensity and frequency of heavy rainfall events is projected to increase ( ''high confidence'' ) (Figure 13.3; [[#Ranasinghe--2021|Ranasinghe et al., 2021]] ). Combined with increasing urbanisation, the risk of pluvial flooding is projected to increase ( [[#Westra--2014|Westra et al., 2014]] ; [[#Rosenzweig--2018|Rosenzweig et al., 2018]] ; [[#Papalexiou--2019|Papalexiou and Montanari, 2019]] ). Small catchments, steep river channels and cities are particularly vulnerable due to large areas of impermeable surfaces where water cannot penetrate ( [[#13.6|Section 13.6]] ).

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===== 13.2.1.2.2 Low Flows and Water Scarcity =====

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The frequency and severity of low flows are projected to increase, making streamflow drought and water scarcity more severe and persistent in SEU and WCE ( ''medium confidence'' ) (Figure 13.3; [[#Ranasinghe--2021|Ranasinghe et al., 2021]] ), but decreases are projected in most of NEU except the southern UK ( [[#Forzieri--2014|Forzieri et al., 2014]] ; [[#Prudhomme--2014|Prudhomme et al., 2014]] ; [[#Schewe--2014|Schewe et al., 2014]] ; [[#Roudier--2016|Roudier et al., 2016]] ; [[#Ranasinghe--2021|Ranasinghe et al., 2021]] ). In EEU, uncertainty about changes in water scarcity pose distinct challenges for adaptation ( [[#Greve--2018|Greve et al., 2018]] ). At 1.5°C GWL, the number of days with water scarcity (water availability as opposed to water demand) and drought will increase slightly in SEU ( [[#Schleussner--2016|Schleussner et al., 2016]] ; [[#Naumann--2018|Naumann et al., 2018]] ), resulting in 18% of the population exposed to at least moderate water scarcity, increasing to 54% at 2°C GWL ( [[#Byers--2018|Byers et al., 2018]] ). Moderate water scarcity is emerging in some parts of WCE ( [[#Bisselink--2018|Bisselink et al., 2018]] ) increasing to16% of the population under 2°C GWL and SSP2 ( [[#Byers--2018|Byers et al., 2018]] ). Under 4°C GWL, areas in WCE experience water scarcity, especially in summer and autumn. Future intensive water use can aggravate the situation, in particular in SEU (Sections 13.5.1, 13.10.3).

Groundwater abstraction rates reach up to 100 million m³ yr –1 across WCE and SEU, and exceed 100 million m³ yr –1 in parts of SEU ( [[#Wada--2016|Wada, 2016]] ). Low recharge rates lead to a depletion of groundwater resources in parts of SEU and WCE ( [[#Doll--2014|Doll et al., 2014]] ; [[#Wada--2016|Wada, 2016]] ; [[#de%20Graaf--2017|de Graaf et al., 2017]] ), increasing the impacts on water scarcity in SEU. Groundwater pumping and declines in groundwater discharge already threaten environmental flow limits in many European catchments, especially in SEU, extending to almost all basins and sub-basins within the next 30–50 years ( [[#de%20Graaf--2019|de Graaf et al., 2019]] ).

The combined effect of increasing water demand and successive dry climatic conditions further exacerbates groundwater depletion and lowers groundwater levels in SEU but also WCE ( [[#Goderniaux--2015|Goderniaux et al., 2015]] ). Declines in groundwater recharge of up to 30% further increase groundwater depletion ( [[#Aeschbach-Hertig--2012|Aeschbach-Hertig and Gleeson, 2012]] ) especially in SEU and semiarid to arid regions ( [[#Moutahir--2017|Moutahir et al., 2017]] ). Even in WCE and NEU, projected increases in groundwater abstraction will impact groundwater discharge, threatening sustaining environmental flows under dry conditions ( [[#de%20Graaf--2019|de Graaf et al., 2019]] ).

The risks for soil moisture drought are projected to increase in WCE and SEU for all climate scenarios ( [[#Grillakis--2019|Grillakis, 2019]] ; [[#Tramblay--2020|Tramblay et al., 2020]] ; [[#Ranasinghe--2021|Ranasinghe et al., 2021]] ). At 3°C GWL compared with 1.5°C GWL, the drought area will increase by 40% and the population under drought by up to 42%, especially affecting SEU, and to a lesser extent in WCE ( [[#Samaniego--2018|Samaniego et al., 2018]] ).

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<span id="water-temperature-and-quality"></span>
===== 13.2.1.2.3 Water Temperature and Quality =====

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Water temperatures in rivers and lakes have increased over the past century by ~1–3°C in major European rivers (CBS, 2014; [[#EEA--2017a|EEA, 2017a]] ; [[#Woolway--2017|Woolway et al., 2017]] ). Warming is accelerating for all European river basins ( [[#Wanders--2019|Wanders et al., 2019]] ) increasing by 0.8°C in response to 1.5°C GWL and 1.2°C for 3°C GWL relative to 1971–2000 ( [[#van%20Vliet--2016a|van Vliet et al., 2016a]] ) aggravated by declines in summer river flow.

(Ground)water extractions or drainage have caused saltwater intrusions ( [[#Rasmussen--2013|Rasmussen et al., 2013]] ; [[#Ketabchi--2016|Ketabchi et al., 2016]] ). During summer, seawater will also penetrate estuaries further upstream in response to reduced river flow and SLR, resulting in more frequent closure of water inlets in the downstream part of the rivers in a period when water is most needed ( ''high agreement, low evidence'' ) (e.g., [[#Haasnoot--2020b|Haasnoot et al., 2020b]] ).

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'''Box 13.1 | Venice and Its Lagoon'''
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Venice and its lagoon are a UNESCO World Heritage Site. This socio-ecological system is the result of millennia of interactions between people and the natural environment. It is exposed to climatic and non-climatic hazards: more frequent floods, warming, pollution, invasive species, reduction of salt marshes, hydrodynamic and bathymetric changes, and waves generated by cruise ships and boat traffic.

The elevation of the average city pedestrian level and of its inner historic area are, respectively, 105 and 55 cm above the present relative mean sea level (RMSL). Consequently, even small surges and compound events cause floods when they coincide with high tide ( [[#Lionello--2021a|Lionello et al., 2021a]] ). During the 20th century, RMSL rose at about 2.5 mm yr –1 due to SLR and land subsidence ( [[#Zanchettin--2021|Zanchettin et al., 2021]] ). The frequency of floods affecting the city has increased from once per decade in the first half of the 20th century to 40 times per decade in the period 2010–2019 (Figure Box 13.1.1a).

In 1973, the Italian government established a legal framework for safeguarding Venice and its lagoon. Construction of the flood protection system started in 2003 and was used for the first time in October 2020 ( [[#Lionello--2021b|Lionello et al., 2021b]] ). This system of mobile barriers (MoSE) closes the lagoon inlets to avoid floods when needed, while under normal conditions they lay on the seabed, thus allowing ship traffic and the exchange between the lagoon and the sea ( [[#Molinaroli--2019|Molinaroli et al., 2019]] ). To prevent flooding of the central monument area, additional measures have been proposed including inlets, expansion of salt marshes and pumping seawater into deep brackish aquifers to raise the city’s level ( [[#Umgiesser--1999|Umgiesser, 1999]] ; [[#Umgiesser--2004|Umgiesser, 2004]] ; [[#Teatini--2011|Teatini et al., 2011]] ).

Without adaptation, potential economic damages between 7 and 17 billion EUR have been estimated for the next 50 years ( [[#Caporin--2016|Caporin and Fontini, 2016]] ). Additionally, the ecosystem is vulnerable to warming ( [[#Solidoro--2010|Solidoro et al., 2010]] ) and SLR (Day Jr et al., 1999; [[#Marani--2007|Marani et al., 2007]] ). The duration of the closure of the lagoon inlets is expected to increase from 2 to 3 weeks yr –1 for RMSL rises of 30 cm, to 2 months yr –1 for 50 cm and 6 months yr –1 for 75 cm (Figure Box 13.1.1b; [[#Umgiesser--2020|Umgiesser, 2020]] ; [[#Lionello--2021b|Lionello et al., 2021b]] ), resulting in disconnection from the sea for most of the time for RMSL rise exceeding 75 cm. Frequent closures of the inlets would prevent ship traffic and in/outflow of water. For Venice, adaptation pathways considering the full range of plausible RMSL (Figure Box 13.1.1c) levels are not available, indicating a long-term adaptation gap. As planning and implementation of adaptation of this extent can take several decades ( [[#Haasnoot--2020b|Haasnoot et al., 2020b]] ; Cross-Chapter Box SLR in Chapter 3), this increases the risk that the city will not be prepared in case of rapid SLR.

[[File:bbec01e830594efa3edaef2d3055e0c6 IPCC_AR6_WGII_Figure_13_Box_13_1_1.png]]

'''Figure Box 13.1.1 |''' '''Venice sea level rise (SLR) and coastal flooding: (a)''' ''evolution of relative and mean sea level in Venice and decadal frequency of floods above the safeguard level in the city centre (Frederikse et al.'' '','' 2020; [[#Lionello--2021a|Lionello et al., 2021a]] ; [[#Lionello--2021b|Lionello et al., 2021b]] ; [[#Zanchettin--2021|Zanchettin et al., 2021]] ); '''(b)''' projected relative SLR at the Venetian coast ( [[#Fox-Kemper--2021|Fox-Kemper et al., 2021]] ); “very likely ” corresponds to 5–95th percentile range, “likely ” to 17–83rd percentile range; '''(c)''' timing when critical relative sea level thresholds will be reached depending on scenarios and confidence level ( [[#Lionello--2012|Lionello, 2012]] ; [[#Umgiesser--2020|Umgiesser, 2020]] ; [[#Lionello--2021a|Lionello et al., 2021a]] ), the upper limit of the medium confidence range under SSP5–8.5 represents a low-likelihood, high-impact storyline, low confidence processes include ice sheet instability; '''(d)''' Landsat view of Venice and its lagoon with the three inlets connecting it to the Adriatic Sea.

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=== 13.2.2 Solution Space and Adaptation Options ===

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In recent decades water management in Europe has increasingly shifted towards integrated and adaptive strategies, with the most noticeable shifts in WCE ( ''high confidence'' ) (e.g., [[#Kreibich--2015|Kreibich et al., 2015]] ; [[#Bubeck--2017|Bubeck et al., 2017]] ). While adaptive strategies are increasingly considered as an approach to strengthen flexibility and implement climate-change adaptation actions, given deep uncertainty about the future ( [[#Ranger--2013|Ranger et al., 2013]] ; [[#Klijn--2015|Klijn et al., 2015]] ; [[#Bloemen--2019|Bloemen et al., 2019]] ; [[#Hall--2019|Hall et al., 2019]] ; [[#Pot--2019|Pot et al., 2019]] ), more traditional water management approaches still dominate across Europe ( [[#OECD--2013|OECD, 2013]] ; [[#OECD--2015|OECD, 2015]] ; [[#Wiering--2017|Wiering et al., 2017]] ). Current measures focus on structural flood protection and water resources supply and play an important role to preserve present land use and development patterns. The long-term effectiveness of such measures is increasingly challenged by their reinforcing path dependency (e.g., flood defence and water supply attract developments which require further protection and supply). This path dependency limits the solution space and may hamper implementation of transformative measures, such as land-use change, to accommodate the water system ( ''medium confidence'' ) (Cross-Chapter Paper 2; [[#Di%20Baldassarre--2015|Di Baldassarre et al., 2015]] ; [[#Kreibich--2015|Kreibich et al., 2015]] ; [[#Alfieri--2016|Alfieri et al., 2016]] ; [[#Gralepois--2016|Gralepois et al., 2016]] ; [[#Welch--2017|Welch et al., 2017]] ; [[#Di%20Baldassarre--2018|Di Baldassarre et al., 2018]] ; [[#Haer--2020|Haer et al., 2020]] ).

Water laws, policies and guidance documents increasingly mainstream climate impacts and adaptation options ( [[#Runhaar--2018|Runhaar et al., 2018]] ; [[#Mehryar--2021|Mehryar and Surminski, 2021]] ), though not everywhere. Differences are apparent, for example, in coastal adaptation where most, but not all, countries are planning for SLR (Figure 13.5; McEvoy et al., 2021). Although the planning horizon of 2100 and 1-m SLR are most common (adjusted for local conditions), there are significant differences between countries (e.g., the high-end SLR value in 2100 ranges from 0.3 to 3 m), which may lead to unequal impacts over time (McEvoy et al., 2021).

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==== 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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<span id="riverine-and-pluvial-flood-risk-management"></span>
===== 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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==== 13.2.2.2 Water Resources Management ====

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Planning adaptation to water scarcity has centred on increasing the availability and supply of freshwater through water storage, diversification of sources and water diversion and transfer ( ''high confidence'' ). Reservoirs are costly, have negative environmental impacts and will not be sufficient under higher warming levels in every place ( [[#Papadaskalopoulou--2015a|Papadaskalopoulou et al., 2015a]] ; [[#Di%20Baldassarre--2018|Di Baldassarre et al., 2018]] ; [[#Garnier--2019|Garnier and Holman, 2019]] ). Wastewater reuse is considered a low-cost and effective measure where wastewater is available ( [[#Lavrnic--2017|Lavrnic et al., 2017]] ; [[#De%20Roo--2020|De Roo et al., 2020]] ), but public acceptance for domestic reuse is presently limited ( ''high confidence'' ) ( [[#Papadaskalopoulou--2015b|Papadaskalopoulou et al., 2015b]] ; [[#Morote--2019|Morote et al., 2019]] ). Increasing desalination capacity is used particularly in SEU but has high energy demands and produces brine waste ( [[#Garnier--2019|Garnier and Holman, 2019]] ; [[#Jones--2019|Jones et al., 2019]] ; [[#Morote--2019|Morote et al., 2019]] ).

Adaptation measures on the demand side include monitoring (e.g., water meters, early warning systems of drought) and regulating demand, for example, water restrictions, water pricing, water saving and efficiency measures, and land management and cover change ( [[#Papadaskalopoulou--2015b|Papadaskalopoulou et al., 2015b]] ; [[#Varela-Ortega--2016|Varela-Ortega et al., 2016]] ; [[#Manouseli--2018|Manouseli et al., 2018]] ; [[#Garnier--2019|Garnier and Holman, 2019]] ). Prolonged water restrictions and prioritising sectoral supply could result in economic losses (e.g., for irrigated agriculture) ( [[#13.5.2|Section 13.5.2]] ; [[#Wimmer--2014|Wimmer et al., 2014]] ; [[#Salmoral--2019|Salmoral et al., 2019]] ). Economic instruments, such as water pricing, can be effective when combined with incentives for water saving and efficiency ( [[#Kayaga--2014|Kayaga and Smout, 2014]] ; [[#Esteve--2018|Esteve et al., 2018]] ; [[#Crespo--2019|Crespo et al., 2019]] ). Water saving and efficiency measures, such as leakage repair, education and improved irrigation, could limit conflicts across sectors but necessitate technological advances and changes in practice together with a willingness to cooperate ( [[#Garnier--2019|Garnier and Holman, 2019]] ; [[#Papadimitriou--2019|Papadimitriou et al., 2019]] ; [[#Teotónio--2020|Teotónio et al., 2020]] ). Increased irrigation efficiency has reduced water scarcity, particularly in SEU ( [[#13.5|Section 13.5]] ; [[#De%20Roo--2020|De Roo et al., 2020]] ), and occur at farm level in WCE and NEU ( [[#Papadaskalopoulou--2015b|Papadaskalopoulou et al., 2015b]] ; [[#van%20Duinen--2015|van Duinen et al., 2015]] ; [[#Rey--2017|Rey et al., 2017]] ) but come with increasing path dependency on supply and trade-offs which may not be sustainable in the long term ( ''high confidence'' ) ( [[#Di%20Baldassarre--2018|Di Baldassarre et al., 2018]] ).

The assessment of the effectiveness and feasibility of adaptation options shows that a portfolio of supply-and-demand measures is needed to reduce water scarcity (Key Risk 3, [[#13.10.3|Section 13.10.3]] ), although locally demand-side measures could be sufficient ( [[#Kingsborough--2016|Kingsborough et al., 2016]] ). Under high warming levels, adaptation to drought and low flows by water saving and efficiency measures may not be sufficient to counteract reduced availability ( ''medium agreement, low evidence'' ) ( [[#Collet--2015|Collet et al., 2015]] ; [[#De%20Roo--2020|De Roo et al., 2020]] ). Successful adaptation in the water sector depends on integrating water considerations into sectoral policies ( [[#Collet--2015|Collet et al., 2015]] ; [[#Papadaskalopoulou--2016|Papadaskalopoulou et al., 2016]] ). Inclusive and participatory approaches where (local) stakeholders are actively involved in the initiation and execution of water management can enhance problem ownership, the quality and democratic legitimacy of processes and decisions, enhance support and accelerate decisions ( [[#Edelenbos--2017|Edelenbos et al., 2017]] ; [[#Begg--2018|Begg, 2018]] ).

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=== 13.2.3 Knowledge Gaps ===

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An assessment of the full solution space of adaptation options and pathways under low to high GWL, including the long term, is lacking. A quantification of the effectiveness of measures in reducing risk is limited in the scientific literature. The available assessments consider adaptation by incremental measures. Transformative options, such as land-use changes, planned relocation from exposed areas or restricting future development, are rarely considered. While high-end scenarios describing ''low confidence'' processes and scenarios beyond 2100 are considered to be useful for risk-averse decision making, in particular coastal adaptation ( [[#Hinkel--2019|Hinkel et al., 2019]] ; [[#Haasnoot--2020b|Haasnoot et al., 2020b]] ), they are rarely considered in practice.

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