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Army Corps says large coastal barrier not justified for West Ashley or James Island; study timeline accelerated to 2028
Summary
The U.S. Army Corps of Engineers told Charleston's flood committee it has accelerated the inland flood risk study and will seek 35% design maturity for some features by a 2028 chief's report. The Corps said large-scale coastal barriers similar to the Battery extension lack economic justification in West Ashley and James Island and will instead pursue smaller- and nature-based measures.
The U.S. Army Corps of Engineers (Charleston District) told the Basin Flood Action Committee on Thursday that its Charleston inland flood risk management study has been accelerated and will aim to reach a chief's report with some features at 35% design maturity in 2028, about a year earlier than previously scheduled.
"We are now looking at the summer, or 2028 to get that final report with the chief of engineer's signature on it," Carissa, a Corps project manager, told the committee. She said the accelerated schedule is intended to move some projects more quickly toward preconstruction and design.
Carissa said the study evaluated alignments for large coastal barrier structures in West Ashley and James Island similar to the 12-foot Battery extension proposed for the peninsula but found they were not economically justified. "There is not an economic justification for a large scale coastal barrier in West Ashley or James Island," she said, and added that to protect 50—25% of storm-surge damages in those areas a barrier would likely need to be 13 to 15 feet in elevation, which the team found did not align with costs and benefits.
Instead, the Corps will continue to evaluate smaller-scale surge measures, marsh and nature-based improvements (in partnership with contractor Biohabitats), targeted stormwater conveyance upgrades around Lockwood and the medical district, potential road-raising, flood walls around critical facilities such as the Plum Island wastewater management site, and optimization of tidal gates and pumps associated with the Battery extension so they can also accommodate inland rainfall events.
The team also presented compound-flooding analysis that combines storm surge and heavy rainfall. Carissa said transects show interior West Ashley locations can experience a larger combined effect, though in some transects the difference in water-surface elevation when surge and rainfall occur together was under a foot compared with single-hazard scenarios.
Committee members asked about features north of Wagner Terrace and how the Corps' authorized coastal storm risk management (CSRM) projects and separate city initiatives overlap. Carissa and Corps staff said some areas north of Wagner Terrace are addressed under authorized CSRM features and that the study will avoid duplicating active city efforts. Members asked the Corps to provide benefit-cost figures for public context; the peninsula project retained a cited benefit-cost ratio of about 10:1, while staff said West Ashley and James Island produce much lower ratios, which factored into the decision not to pursue a continuous large barrier there.
Carissa said the Corps is coordinating closely with the Battery extension team to ensure tidal gates and pumps are optimized for combined tidal and rainfall events. The study will continue to develop and evaluate alternatives against engineering, cost, environmental, cultural and social criteria.
Next steps: the Corps will continue modeling and public engagement over the summer, work with the city and Biohabitats on nature-based alternatives, and advance selected project features to 35% design maturity to speed delivery of construction-ready projects.
