Crab Bank Sedimentation Study and Habitat Restoration
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Project Objective

As part of the Charleston Harbor Deeping Project, the United States Army Corps of Engineers (USACE) elected to use 660,000 cubic yards of dredge spoil to help rehabilitate the Crab Bank Seabird Sanctuary, once a flourishing nesting site. The proposed location of Crab Bank was just offshore of Shem Creek, a vital asset of Mount Pleasant, South Carolina. Because the Town of Mount Pleasant (Town) had concerns about the location chosen for the Crab Bank project, they hired ATM, a Geosyntec Company, to evaluate the potential impacts to the creek.

Geosyntec’s Scope of Services

ATM studied the migration of the original Crab Bank and reviewed coastal conditions that might affect sediment transport patterns in the area. ATM produced numerical models of alternate placement volumes and locations to find configurations that could mitigate potential negative impacts while achieving the goals of the restoration project. To accomplish this, an Environmental Fluid Dynamics Code (EFDC) model was developed that provided increased resolution in the area of the proposed land reclamation project, calibrated the updated model to local data, and assessed the impacts of proposed design alternatives.
ATM could not determine whether USACE had studied the fate of the material placement or considered impacts to the Town’s resources. The modeling showed significant sedimentation near the mouth of Shem Creek and in an adjacent historical small boat channel. Coupled with historical northerly sediment migration toward Shem Creek, the modeling affirmed the Town’s concerns.

Notable Accomplishments

As a result of our comprehensive study, the Town provided substantive comments to USACE and recommended a major shift in the disposal location to reduce the sedimentation rates in Shem Creek. On April 15, 2021, USACE Charleston District issued a joint press release with the South Carolina Department of Natural Resources (the local project sponsor) and the Town of Mount Pleasant to announce that they were relocating placement of the dredged materials 1,400 feet farther away from the mouth of Shem Creek. The revised placement footprint significantly reduced sedimentation in areas of concern, was far more favorable for the Town, and restored 30 acres of nesting habitat for seabirds.