There are several elements that need to be examined during any harbor deepening project, one of which includes where to put the material that is dredged from depths of the sea.
During the Feasibility and Pre-construction Engineering and Design phases of the Charleston Harbor Post 45 Deepening Project, the Charleston District developed several options, with input from stakeholder and partner agencies, for the placement of the dredged material. One aspect that was important to consider was the most cost-effective and environmentally-acceptable location for the dredged sediment. The final solution was a combination of placement areas for the dredged material, including using inshore and offshore placement sites, and, most creatively, building artificial reefs.
“Building reefs using the limestone rock that is being dredged from the entrance channel of the Harbor has several benefits,” said Holly Carpenter, project manager for Post 45. “We are building eight reefs that will each provide at least 33 acres of habitat in the Charleston Harbor for fish and invertebrates.”
Two of the reefs are intended to mitigate for existing hard bottom habitat that will be lost during construction in the entrance channel. The other six artificial reefs are being constructed as a “beneficial use” of the dredged material.
When the placement of dredged material provides an additional benefit, such as creating habitat, it is considered a beneficial use. With navigation projects, the Corps can explore possible beneficial uses of dredged material and implement them when they are the least-cost and an environmentally-acceptable placement option. The creation of six additional beneficial use reefs is a win-win for the Post 45 project as it is the most economical way to place the dredged material while also benefiting other natural resources.
The mitigation reefs are currently being constructed using the Dredge New York, a mechanical excavator dredge, which digs into the limestone rock and loads approximately two dump trucks of material onto a scow that transports and drops the material into the area of reef construction.
“We started building the reefs in June and expect to complete the mitigation reefs by the end of September,” said Carpenter. “The beneficial-use reefs should be completed by 2020.”
The locations of the artificial reefs were strategically planned based on survey data and input from stakeholders, such as the South Carolina Artificial Reef Program, commercial fishing outfits, and others, as to avoid particular areas and not conflict with the transportation uses of the Harbor, while promoting the potential for ecological success on the reefs.
Once the reefs are constructed, they will evolve to become fully functioning reef habitat over time. First, the stationary organisms, such as algae, sponges and soft corals, will attach themselves to the surface of the rocks. There are limited hard structures on the bottom of the ocean, so this is very valuable for habitat creation. Next, marine invertebrates such as shrimp, crabs and starfish, will be attracted to the reefs for food and protection, followed by juvenile fish who also need food and protection from bigger fish. Eventually the larger fish, such as red drum and sea bass, will come looking for food and to reproduce. This is economically important to South Carolina because many of these species are commercially and recreationally fished.
It should take about three to four years for the stationary organisms to colonize and mature, and, if everything goes according to plan, the invertebrates and fish will follow shortly afterwards.
“We are building the reefs in a three-dimensional structure using native material from the Charleston Harbor, and we think this will be very attractive to marine life and there will be even more productive bottom habitat available after the harbor is deepened,” said Bethney Ward, biologist.