Improvement of the coral growth and cost-effectiveness of hybrid infrastructure by an innovative breakwater design in Naha Port, Okinawa, Japan

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 (PBshallow) or ATPs at deeper depth (PBdeep). The coral area increased by ~10% on PBshallow and ~20% on PBdeep compared to the control. PBdeep had the largest increase in coral area, resulting from the installation of ATPs, which accounted for ~40% of the increase in coral area. PBdeep, with greater breakwater surface area and ATPs at depths more appropriate for corals than PBshallow, 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.