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

===== 4.4.2.2.5 Co-benefits and drawbacks of hard and sediment-based protection =====

When space is limited (e.g., in an urban setting), co-benefits can be generated through multi-functional hard flood defences, which combine flood protection with other urban functions, such as car parks, buildings, roads or recreational spaces into one multifunctional structure (Stalenberg, 2013 <sup>[[#fn:r1601|1601]]</sup> ; van Loon-Steensma and Vellinga, 2014 <sup>[[#fn:r162|162]]</sup> ). An important co-benefit of sediment-based protection, such as beach nourishment and dune management, is that it preserves beach and associated environments, as well as tourism (Everard et al., 2010 <sup>[[#fn:r1603|1603]]</sup> ; Hinkel et al., 2013a <sup>[[#fn:r1604|1604]]</sup> ; Stive et al., 2013 <sup>[[#fn:r1605|1605]]</sup> ).

Drawbacks of hard protection include the alteration of hydrodynamic and morphodynamic patterns, which in turn may export flooding and erosion problems downdrift (Masselink and Gehrels, 2015 <sup>[[#fn:r1606|1606]]</sup> ; Nicholls et al., 2015 <sup>[[#fn:r1607|1607]]</sup> ). For example, protection of existing shoreline in estuaries and tidal creeks may increase tidal amplification in the upper parts (Lee et al., 2017 <sup>[[#fn:r1608|1608]]</sup> ). Hard protection also hinders or prohibits the onshore migration of geomorphic features and ecosystems (called coastal squeeze; Pontee, 2013 <sup>[[#fn:r1609|1609]]</sup> ; Gittman et al., 2016 <sup>[[#fn:r1610|1610]]</sup> ), leading to both a loss of habitat as well as of the protection function of ecosystems (see Sections 4.3.2.4 and 4.4.2.2). Another drawback of raising hard structures, also emphasised in AR5, is the risk of lock-in to a development pathway in which development intensifies behind higher and higher defences, with escalating severe consequences in the event of protection failure (Wong et al., 2014 <sup>[[#fn:r1611|1611]]</sup> ; Welch et al., 2017 <sup>[[#fn:r1612|1612]]</sup> ), as experienced in Hurricane Katrina impacted New Orleans (Burby, 2006 <sup>[[#fn:r1613|1613]]</sup> ; Freudenburg et al., 2009 <sup>[[#fn:r1614|1614]]</sup> ). This lock-in results from protection attracting further economic development in the flood zone within defenses, which then leads to further raising defences with SLR, and the growing value of exposed assets.

Seabed dredging of sand and gravel can have negative impacts on marine ecosystems such as seagrass meadows and corals (Erftemeijer and Lewis III, 2006 <sup>[[#fn:r1615|1615]]</sup> ; Erftemeijer et al., 2012 <sup>[[#fn:r1616|1616]]</sup> ). Nourishment practices on sandy beaches have also been shown to have drawbacks for local ecosystems if local habitat factors are not taken into consideration when planning and implementing nourishment and maintenance (Speybroeck et al., 2006 <sup>[[#fn:r1617|1617]]</sup> ). A further emerging issue is beach material scarcity mainly driven by demand of sand and gravel for construction, but also for beach and shore nourishment (Peduzzi, 2014 <sup>[[#fn:r1618|1618]]</sup> ; Torres et al., 2017 <sup>[[#fn:r1619|1619]]</sup> ), which makes sourcing the increasing volumes of beach materials required to sustain beaches in the face of SLR more expensive and challenging (Roelvink, 2015 <sup>[[#fn:r1620|1620]]</sup> ).

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