Editing IPCC:AR6/SROCC/Chapter-4 (section)

===== 4.4.2.2.4 Effectiveness of hard and sediment-based protection =====

There is ''high confidence'' that well designed and maintained hard and sediment-based protection is very effective in reducing risk to the impacts of SLR and ESL (Horikawa, 1978 <sup>[[#fn:r1572|1572]]</sup> ; USACE, 2002 <sup>[[#fn:r1673|1673]]</sup> ; CIRIA, 2007 <sup>[[#fn:r1674|1674]]</sup> ). This includes situations in which coastal megacities in river deltas have experienced, and adapted to, relative SLR of several metres caused by land subsidence during the 20th century (Kaneko and Toyota, 2011 <sup>[[#fn:r1675|1675]]</sup> ; Esteban et al., 2019 <sup>[[#fn:r1676|1676]]</sup> ; Box 4.1). In principle, there are no technological limits to protect the coast during the 21st century even under high-end SLR of 2 m (Section 4.3.3.2), but technological challenges can make protection very expensive and hence unaffordable in some areas (Hinkel et al., 2018 <sup>[[#fn:r1677|1677]]</sup> ). Examples include southeast Florida, because protected areas can be flooded by rising groundwater through underlying porous limestone (Bloetscher et al., 2011 <sup>[[#fn:r1678|1678]]</sup> ). Gradually rising water tables behind defences is also an issue, which can be managed by increasing pumping and drainage (Aerts, 2018 <sup>[[#fn:r1679|1679]]</sup> ). Maintaining this effectiveness over time requires regular monitoring and maintenance, accounting for changing conditions such as SLR and widespread erosional trends in front of the defences. There will always be residual risks, which can be reduced, but never eliminated, by engineering protection infrastructure to very high standards, such as so-called ‘unbreakable dikes’ (de Bruijn et al., 2013).

It is difficult to assess at what point in time and for which amount of SLR technical limits for coastal protection will be reached. Parts of Tokyo have been protected against five metres of relative SLR during the 21st century (Kaneko and Toyota, 2011 <sup>[[#fn:r1680|1680]]</sup> ) and it has been argued that it is possible to preserve territorial integrity of the Netherlands even under 5 m SLR, using current engineering technology (Aerts et al., 2008 <sup>[[#fn:r1681|1681]]</sup> ; Olsthoorn et al., 2008 <sup>[[#fn:r1682|1682]]</sup> ). This suggests that under RCP2.6, technical limits to adaptation will be rare even under longer-term SLR. Protecting against high-end SLR will be increasingly technically challenging as we move beyond the 21st century. This is not only due to the absolute amount of SLR, but also due to the very high rates of annual SLR (e.g., 10–20 mm yr –1 ''likely'' range under RCP8.5 in 2100), which challenge the planning and implementation of hard protection because major protection infrastructure requires decades to plan and implement (Gilbert et al., 1984 <sup>[[#fn:r1683|1683]]</sup> ; Burcharth et al., 2014 <sup>[[#fn:r1684|1684]]</sup> ). In summary, the higher and faster SLR, the more challenging coastal protection will be, but quantifying this is difficult. In any case, before technical limits are reached, economic and social limits will be reached because societies are neither economically able nor socially willing to invest in coastal protection (Sections 4.4.2.2 and 4.3.3.2; Hinkel et al., 2018; Esteban et al., 2019).

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