Editing IPCC:AR6/WGII/Cross-Chapter-Paper-2 (section)

==== CCP2.3.2.1 Hard Engineering Measures ====

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Hard engineering protective measures are commonly used to reduce coastal flooding and to drain or store excess water from intense precipitation. Many coastal cities, in particular densely populated and high-resource archetypes, have planned and are planning to continue a protection-based strategy, comprising, for example, breakwaters, sea walls and/or dikes, which could be raised or complemented with large barriers, or with ‘super-levees’ enabling construction on top of them ( ''high confidence'' ; Table SMCCP2.1; [[#Takagi--2016|Takagi et al., 2016]] ; [[#Haasnoot--2019|Haasnoot et al., 2019]] ; [[#Hall--2019|Hall et al., 2019]] ; [[#Esteban--2020b|Esteban et al., 2020b]] ).

Protection is effective in the short- to medium-term for many coastal cities, and can be cost effective in the 21st century (Section CCP2.4.2), but residual risk remains because protection can fail. Even under RCP8.5, technical limits to hard protection may only be reached after 2100 in many regions, but socioeconomic and institutional barriers may be reached before then ( [[#Hinkel--2018|Hinkel et al., 2018]] ). With progressive SLR, protection eventually becomes unaffordable and impractical ( [[#Strauss--2021|Strauss et al., 2021]] ). Combining hard engineering measures with nature-based solutions, spatial planning and early warning systems can help to contain residual risk ( [[#Du--2020|Du et al., 2020]] ). Protective works do not prevent salinisation and higher groundwater levels ( [[#Alves--2020|Alves et al., 2020]] ), and can lead to loss of coastal habitat (Cross-Chapter Box SLR in Chapter 3; [[#Achete--2017|Achete et al., 2017]] ; [[#Cooper--2020|Cooper et al., 2020]] ). Hard protective measures also create long-term path dependency as they last for decades and attract new development, locking in impact and exposure as C&amp;S grow, with the expectation of ongoing protection (Chapter 3; [[#Di%20Baldassarre--2015|Di Baldassarre et al., 2015]] ; [[#Gibbs--2016|Gibbs, 2016]] ; [[#Griggs--2019|Griggs and Patsch, 2019]] ; [[#Siders--2019a|Siders, 2019a]] ).

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