3.3.2. Dune Restoration

Dunes are usually located right at the front of the beach and are created by sand deposition due to winds, often on wider beaches of >35 m [55]. Dunes protect mainland against flooding and can provide habitat for plants, birds, and other terrestrial and beach organisms [56]. Dune restoration mostly involves re-planting native dune vegetation and the installation of sand fencing. Fences can be used on the seaward side to trap sand and help stabilize any bare sand surfaces [57]. Native vegetation may be planted to stabilize natural or artificial dunes and to promote the accumulation of sand from wind-blown sources [58]. Invasive non-native vegetation is often removed.

Costs vary from \$7636 to \$13,888/ha for studies in Australia and the United States (Appendix C Table A4). If dunes have been subject to erosion, dune reconstruction involves the placement of sand against the remaining dunes using bulldozers. Construction costs are for labor, new vegetation, and the sand needed to reconstruct the dune area. Some studies involve cleaning activities—for example, after a dune area has been hit by an oil spill. Such projects are more expensive through additional cleaning measures, with costs estimated at \$52,000–76,000/ha (Appendix C). Maintenance costs of restored dunes are estimated at \$333–2526/ha per year [54].

A *groin* is a structure which is oriented perpendicular to a shore and which reduces the flow of sediment along that shore. Retention structures (e.g., groins) can help to capture sand and sustain the lifetime of beach nourishment. Sand collects on the up-drift side of a groin until it is filled and the amount of sand on the beach stays the same [17]. Aerts et al. [16] estimate that the re-conditioning or new development of existing groins for New York City beach-nourishment projects at approximately \$1.6 million per groin, including 15% contingencies. The USACE [29] provides cost estimates for nourishment including groins at \$0.55 million/km.

### *3.4. Nature-Based Solutions for Coastal Ecosystems*

Reducing flood risk by restoring or creating new coastal ecosystems is increasingly seen as an alternative to hard-engineered protection measures. (For an extensive overview of co-benefits of nature-based solutions see Morris et al. [59]) Coastal ecosystems already have a value in flood protection, and research shows that, without mangroves, 18 million people would be flooded every year [60]. However, global mangrove forests decreased by 19% over the period 1981–2005 [21], while over 60% of the world's coral reefs are declining through overfishing, coastal development, and climate change [61]. In a comparative study for the U.S. Gulf coast, nature-based adaptation options could avert up to \$50 billion of the expected flood losses in 2030, with an average benefit–cost ratio >3.5 [62].

Nature-based solutions (Table 6)—including the restoration of degraded coral reefs, or coastal wetlands (e.g., seagrasses, saltmarshes and mangroves)—can reduce flood-water flow and wave height [63]. Wetlands also function like sponges, temporarily storing tidal or flood waters and slowly releasing them, thus reducing flood heights [64]. Furthermore, restoring coastal vegetation and reefs can stabilize shorelines, promote sediment deposition and biodiversity, and reduce erosion [65].
