3.4.3. Mangrove Restoration

Coastal mangroves are salt-adapted trees and shrubs that grow in tropical or subtropical areas [71]. Mangrove restoration usually entails reforestation of species, and restoration success depends on local circumstances such as hydrology, length of the planting period ("time for survival"), and seedling quality. Mangrove survival also depends on the degree of salinity, which depends much on the amount of available sediment and freshwater inflow to compensate for salinity [72].

Most mangrove-restoration projects analyzed in the scientific literature have an area of 10–120,000 ha. Construction costs (material, labor) and water depth are important factors that determine mangrove restoration costs. A meta-analysis by Narayan et al. [21] shows that the average restoration costs vary from \$1081 to \$3244/ha. Bayraktarov et al. [19] report lower and upper limits of \$1506–49,324/ha, respectively. The lower value for developing countries is confirmed by an older study by Lewis [73] and more recently by Hakim [66] for a case study in Indonesia (\$858/ha). Land-purchase costs are not included in these estimates. Furthermore, in a flood-management case study in Vietnam, a comparison was made between mangrove restoration and technical solutions such as breakwaters [74]. For this study, the cost for planting mangroves at water depths of 1–1.8 m varies between \$25–200/m of coastline, which is cheaper than the cost for developing breakwaters (\$125–475/m coastline [21]). Maintenance costs can be high for some species, as small seedlings are vulnerable to wave impacts, which require protection measures (e.g., breakwaters) and thinning and pruning activities. For Indonesia, maintenance costs have been estimated at 10% of initial investment, or around \$85/ha [66]. For Vietnam, this has been estimated at \$7.1/ha [67]. Such maintenance costs need to be considered for at least four years after planting seedlings.

### 3.4.4. Coral, Oyster and Artificial Reef Restoration

Artificial coral reefs reduce wave energy and coastal erosion and protect shorelines against flooding (Beck et al. [60]). Their effectiveness for reducing flooding is determined by reef width (relative to the average wave length) and reef depth (relative to the average wave height) [19]. Existing natural coral reefs can be restored by planting coral on degraded areas [75]. As for transplanting new coral on degraded areas, only 65% transplanted species survive. Bayraktarov et al. [19] use an average of 54,200 coral transplants to populate one hectare. Restoration cost varies from \$2,311,296/ha in developed to \$112,953/ha in developing countries [19]. For the restoration of oyster reefs, used oyster and clam shells from farmers and restaurants are placed in the water, with a cost of \$72,273/ha [19]. Mangroves and other coastal vegetation is often planted in conjunction with oyster restoration to provide surface area to inhabit.

An artificial reef is a man-made structure that mimics some of the characteristics of a natural reef, such as promoting marine life and biodiversity [76]. Artificial reefs can be constructed from rocks, wood, old tires, or submerged shipwrecks sunk to the sea floor. For the 2598 artificial reef projects in Florida, concrete secondary-use materials are used most (43%) followed by concrete modules (24%), steel materials (such as steel towers and military equipment) (17%), steel vessels and barges (11%), and natural rock (primarily limestone boulders) (3%) [77]. One option is to deploy fabricated modules of concrete or natural materials such as limestone boulders. For such solutions, a study in the United Kingdom estimates costs at \$30,263–90,789/100 m of new reef. The cost to prepare, tow, and deploy a steel vessel is around \$10,000–80,000 [68].

### *3.5. Channel Management and Nature-Based Solutions for Riverine Systems*

Maintaining the conveyance of discharge is an important aspect of managing flood levels, as channels that have filled up via natural sedimentation processes lower discharge capacity and hence increase flood risk. Channel managemen<sup>t</sup> refers to activities that aim at retaining flow capacity and water levels in river systems for different users (e.g., shipping, ecosystems, and agriculture). Measures to maintain discharge capacity include periodic dredging, river widening, and creating new side channels (Table 7).
