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

==== 6.3.5.8 Coastal Management ====

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Physical coastal management infrastructure has significant benefits in reducing flood and erosion losses and damage from storms. Physical infrastructure includes seawalls, dikes, breakwaters, revetments, groynes and tidal barriers. Adapted infrastructure can alter risks in morphologically connected areas, and lead to increased residual risk by encouraging more construction in the coastal zone (Miller, Gabe and Sklarz, 2019; [[#Ludy--2012|Ludy and Kondolf, 2012]] ). The infrastructure is highly cost effective for large settlements, but not always for small settlements (Tiggeloven et al., 2020) and can be inaccessible to poorer communities (Fletcher et al., 2016; [[#Pelling--2019|Pelling and Garschagen, 2019]] ).

Anticipated costs for this vary widely. For example, [[#Hinkel--2014|Hinkel et al. (2014)]] calculate that adaptation costs to maintain current global levels of coastal flood protection would be 1.2–9.3% of gross world product but protect assets in human settlements of USD 21–210 billion; [[#Tiggeloven--2020|Tiggeloven et al. (2020)]] calculate the cost of adaptation to be USD 176 billion (although this would provide a benefit–cost ratio of 106 under RCP8.5); while [[#Nicholls--2019|Nicholls et al. (2019)]] estimate that global coastal protection would cost substantially more, up to USD 18.3 trillion between 2015 and 2100 for RCP8.5 (this includes ranges of unit costs and maintenance costs which have often been ignored).

Coastal protection infrastructure such as dikes and sluice gates can inhibit salinity intrusion through careful management of water levels, this can provide co-benefits for flood risk reduction and agricultural productivity, but can also have negative impacts on ecosystems (Renaud et al., 2015). Managed aquifer recharge can be effective if the objective is to secure freshwater drinking supply (Hossain, Ludwig and Leemans, 2018).

Physical infrastructure can provide substantial benefits, be constructed quickly and has enabled coastal cities and settlements around the world to flourish and grow. Multifunctional physical infrastructure can also provide economic and social co-benefits. These include integration of transport, recreation, agriculture (e.g., cattle pasture), founding for wind turbines, housing, office or industry into the coastal management infrastructure (Anvarifar et al., 2017; [[#Kothuis--2017|Kothuis and Kok, 2017]] ). However, physical infrastructures can also disrupt natural processes, often leading to undesirable impacts such as pollution, degradation of ecosystems and displacement of erosion and flood risk to other locations (Wang et al., 2018b; [[#Dawson--2015|Dawson, 2015]] ; Nicholls, Dawson and Day, 2015). Coastal management strategies that take a hybrid approach, integrating physical and natural infrastructure, provide the best opportunities for managing risk and achieving wider socioeconomic and environmental benefits ( [[#Depietri--2017|Depietri and McPhearson, 2017]] ; Morris et al., 2018; Schoonees et al., 2019; Powell et al., 2019).

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