Improvement of the coral growth and cost-effectiveness of hybrid infrastructure by an innovative breakwater design in Naha Port, Okinawa, Japan
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Hybrid infrastructure combining gray and green infrastructure should be more cost-effective than gray infrastructure; however, its cost-effectiveness and cost-effective methods for its construction are not clear. Here we used pro-environment breakwaters, having the original breakwater function of wave attenuation along with the additional function of providing marine-life habitats, as a model for estimating the cost-effectiveness of hybrid infrastructure. We defined effectiveness as the area of coral cover on the breakwaters. We compared coral areas between a normal (control) breakwater, and pro-environment breakwaters (PBs) with artificial tide pools (ATPs) at shallow depth (PB<sub>shallow</sub>) or ATPs at deeper depth (PB<sub>deep</sub>). The coral area increased by ~10% on PB<sub>shallow</sub> and ~20% on PB<sub>deep</sub> compared to the control. PB<sub>deep</sub> had the largest increase in coral area, resulting from the installation of ATPs, which accounted for ~40% of the increase in coral area. PB<sub>deep</sub>, with greater breakwater surface area and ATPs at depths more appropriate for corals than PB<sub>shallow</sub>, increased the cost-effectiveness by ~10% compared to the control. Our finding that the cost-effectiveness of ATP installation is comparable to that of coral transplantation to natural reefs suggests that ATPs are cost-effective for coral habitat restoration.
