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

===== 4.4.2.2.3 Cost of hard and sediment-based protection =====

There is ''medium evidence'' and ''medium agreement'' on the costs of hard protection. Data on the costs of hard defences is only available for few countries and unit costs estimated from this data vary substantially depending on building/fill material used, labour cost, urban versus rural settings, hydraulic loads, etc. (Jonkman et al., 2013 <sup>[[#fn:r1587|1587]]</sup> ; Lenk et al., 2017 <sup>[[#fn:r1588|1588]]</sup> ; Aerts, 2018 <sup>[[#fn:r1589|1589]]</sup> ; Nicholls et al., 2019 <sup>[[#fn:r1590|1590]]</sup> ). In general, there has been limited systematic data collection across sites, although useful national guidance does exist in some cases (Environment Agency, 2015 <sup>[[#fn:r1591|1591]]</sup> ). Defences depend on good maintenance to remain effective. For some types of infrastructure such as surge barriers, maintenance costs are poorly described and hence more uncertain (Nicholls et al., 2007 <sup>[[#fn:r1592|1592]]</sup> ). Protection-based adaptation to saltwater intrusion is more complex than adaptation to flooding and erosion, and there is less experience to draw upon.

Based on these unit cost estimates, and different assumptions on future protection, global annual protection costs have been estimated to be 12–71 billion USD considering coastal dikes only (Hinkel et al., 2014 <sup>[[#fn:r1593|1593]]</sup> ) and about 40–170 billion USD yr -1 considering coastal dikes, river dikes and storm surge barriers, under RCP2.6, and about 25–200 billion USD yr -1 considering coastal dikes only (Tamura et al. 2019 <sup>[[#fn:r1594|1594]]</sup> ) under RCP8.5. If protection is widely practised through the 21st century, the bulk of the costs will be maintenance rather than capital costs (Nicholls et al., 2019 <sup>[[#fn:r1595|1595]]</sup> ).

<span id="table-4.7"></span>
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'''Table 4.7'''

'''Table 4.7:''' Capital and maintenance costs of hard protection measures.

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{| class="wikitable"
|-
| Measure
| Capital cost (in million USD unless stated otherwise)
| Annual Maintenance Cost (% of capital cost)
|-
| Sea Wall
| 0.4–27.5 km -1 length and metre height (Linham et al., 2010)
| 1–2% per annum (Jonkman et al., 2013)
|-
| Sea Dike
| 0.9–69.9 km -1 length and metre height (Jonkman et al., 2013; Nicholls et al., 2019; Tamura et al., 2019)
| 1–2% per annum (Jonkman et al., 2013)
|-
| Breakwater
| 2.5–10.0 km -1 length (Narayan et al., 2016)
| 1% per annum (Jonkman et al., 2013)
|-
| Storm Surge Barrier
| 0.9–2.7 (Jonkman et al., 2013) or 2.2 (Mooyaart and Jonkman, 2017) million EUR per metre width
| 1% per annum (Mooyaart and Jonkman, 2017) or 5–10% per annum (Nicholls et al., 2007)
|-
| Saltwater
Intrusion Barriers

| Limited knowledge
|}
<!-- END TABLE -->

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Sediment-based measures are generally costed as the unit cost of sand (or gravel) delivery multiplied by the volumetric demand. Unit costs range from 3–21 USD m <sup>–</sup> ³ sand , with some high outlier costs in, for example, the UK, South Africa and New Zealand (Linham et al., 2010 <sup>[[#fn:r1596|1596]]</sup> ; Aerts, 2018 <sup>[[#fn:r1597|1597]]</sup> ). Costs are small where sources of sand are plentiful and close to the sites of demand. Costs are further reduced by shoreface nourishment approaches. The Netherlands maintains its entire open coast with large-scale shore nourishment (Mulder et al., 2011 <sup>[[#fn:r1598|1598]]</sup> ) and the innovative sand engine has been implemented as a full-scale decadal experiment (Stive et al., 2013 <sup>[[#fn:r1599|1599]]</sup> ). The capital costs for dunes are similar to beach nourishment, although placement and planting vegetation may raise costs. Maintenance costs vary from almost nothing to several million USD km <sup>–1</sup> , although costs are usually at the lower end of this range (Environment Agency, 2015 <sup>[[#fn:r1600|1600]]</sup> ).

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