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

==== 6.3.4.4 Coastal Flood Protection ====

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Coastal ecosystems including coral and oyster reefs, coastal forests including mangroves and other tree species, salt marshes and other types of wetland habitat, seagrass, dunes and barrier islands can reduce impacts of coastal flooding and storms ( ''robust evidence'' , ''high agreement'' ) (Zhao, Roberts and Ludy, 2014; [[#Boutwell--2016|Boutwell and Westra, 2016]] ; Narayan et al., 2017; Yang, Kerger and Nepf, 2015; Bridges et al., 2015; [[#World%20Bank--2016|World Bank, 2016]] ) (see also Section CCP2 Cities and Settlements by the Sea). Recent literature highlights the value of nature-based approaches for coastal protection in terms of avoided damages and human well-being (Narayan et al., 2017; Silva et al., 2016a). NBS can protect coasts from flooding through reducing the wave energy by drag friction, reducing wave overtopping by eliminating vertical barriers, and absorbing floodwaters in soil (Arkema, Scyphers and Shepard, 2017; Dasgupta et al., 2019; Zhu et al., 2020). For example, coastal and marine vegetation and reefs can dissipate wave energy, attenuate wave heights and nearshore currents, decrease the extent of wave run-up on beaches, and trap sediments (Ferrario et al., 2014; Bridges et al., 2015). These effects result in lower water levels and reduce shoreline erosion, which in turn has potential to save lives and prevent expensive property damages (Narayan et al., 2017).

Researchers, practitioners and policy-makers are increasingly calling for the use of nature-based approaches to protect urban shorelines from coastal hazards ( [[#Cunniff--2015|Cunniff and Schwartz, 2015]] ; Bilkovic et al., 2017). The expectation is that coastal ecosystems can help stabilise shorelines, protect communities against storm surge and from tidal-influenced flooding, while providing other co-benefits for people and ecosystems. However, vegetation along protected coastlines with higher frequency, lower intensity coastal hazards ( [[#National%20Research%20Council--2014|National Research Council, 2014]] ) may be more effective for stabilising shorelines and reducing risk to coastal communities and properties, and benefits will depend on local hydrology of the coastal region. [[#Narayan--2017|Narayan et al. (2017)]] estimate that coastal wetlands alone reduced direct flood damages by USD 625 million during Hurricane Sandy in the USA in 2012. Similarly, researchers found that villages with wider mangroves between them and the coast experienced significantly fewer deaths than villages with narrow or no mangroves during a 1999 cyclone in India ( [[#World%20Bank--2016|World Bank, 2016]] ). Recently, Arkema et al. (2017) noted that the number of people, poor families, elderly and total value of residential property most exposed to hazards along the entire coast of the USA can be reduced by half if existing coastal habitats remain fully intact.

Coastal habitats also have limitations in their ability to protect coasts from extreme events. Some studies suggest reduced effectiveness of vegetation and reefs for coastal protection from large storm waves and surge (Möller et al., 2014; Guannel et al., 2016) and there is active debate in the literature about the ability of ecosystems to mitigate the impact of tsunamis (Gillis et al., 2017). Further research is needed to understand and quantify coastal protection services provided by these hybrid green-grey solutions, especially in urban areas (Bilkovic et al., 2017). Additionally, in some coastlines, water may be too deep or waves too high for some species such as mangroves to grow, thrive and provided needed NBS.

Maximising the adaptation benefits of NBS for improving coastal flood protection research requires that cities seek to restore and conserve the vegetation and reef types that are appropriate for the exposure setting and in sufficient abundance to be effective. In particular, planners and managers can use vegetation in protected bays as alternatives to hard infrastructure for shoreline stabilisation. However, the influence of ecosystems on flooding and erosion is variable and depends on a suite of social, ecological and infrastructural factors that vary within and among urban areas (Narayan et al., 2017; Ruckelshaus et al., 2016; Bridges et al., 2015). Additionally, long-term planning to restore or ensure resilience of individual species and ecosystems that may themselves be damaged or destroyed during extreme events is needed in order for urban green and blue infrastructure to continue providing NBS over the longer term.

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