Evaluating the feasibility and advantage of a multi‑purpose submerged breakwater for harbor protection and benthic habitat enhancement at Kahului Commercial Harbor, Hawai‘i: case study

摘要

Construction of breakwaters provides an engineering solution for coastal protection. However, little effort has been madetoward understanding the ecological impact on local coral reef ecosystems and developing engineering structures thatwould enhance the coral reef environment. A submerged breakwater proposed for Kahului Commercial Harbor, Hawai‘i,provided an opportunity to design a multi-purpose ‘reef structure’ to mitigate wave impacts while providing new coralreef habitat. This design involved ecological and environmental considerations alongside engineering principles, servingas a model for environmentally sound harbor development. This field study evaluated environmental conditions and reefcommunity composition at the proposed site in a gradient extending outward from the harbor, using in situ data withmultivariate analyses. Benthic and topographic features in the area were assessed using a towed drop camera system torelate to biological factors. Results that support breakwater topography should follow the natural spur and groove anddepth of the adjacent reef and orient with wave direction. A deep area characterized by unconsolidated substrata andlow coral cover would be replaced with the shallow, sloping hard bottom of the breakwater, and provide an exemplaryarea for corals to flourish while protecting the harbor from large ocean swells. Surfaces on shallow sloping hard bottomsreceive higher levels of irradiance that benefits coral growth. Optimal levels of water motion facilitate sediment removaland promote coral recruitment and growth. The design of the Kahului Harbor submerged multi-purpose structure servesas a model for design of shoreline modification that enhances, rather than degrades, the local coral reef environment.