Beaufort County, SC, was founded in 1769 and covers 923 square miles, 38% of which (347 square miles) is covered by water. As described by the county’s official website, “Nestled between Charleston, SC, and Savannah, GA, Beaufort County is rich with history, culture, and outdoor beauty. Beaufort County is composed of hundreds of barrier and sea islands, and its warm climate, pristine beaches, vibrant Gullah traditions and true southern hospitality welcome visitors from all over the world throughout the year.”
Beaufort County has been one of the South’s high-growth counties over the past few decades because of its substantial US military presence, climate, and location—which make it an exceptionally desirable resort and golf area. Its population of about 86,000 in 1990 nearly doubled to 170,000 by 2015, and is expected to increase by another 35% to 230,000 by 2035. In addition, the local chamber of commerce estimates that nearly 2.7 million people visit Hilton Head Island each year.
The county is served by three airports: Savannah-Hilton Head International Airport, located 40 miles south of Hilton Head Island in Savannah, GA; Charleston International Airport, located 115 miles north of Hilton Head Island in Charleston, SC; and Hilton Head Island Airport, located on the northeast part of the island. The airports in Savannah and Charleston feature runways in excess of 9,000 feet, while the runway at Hilton Head Island Airport is 4,300 feet. Of the three airports, only Hilton Head is physically situated in Beaufort County.
As one would expect of an island, water is very important to Hilton Head. The airport is located in a watershed area that includes the Calibogue Sound and the May River, Cooper River, Broad Creek, and other tributaries and lakes. Comprising nearly 81,000 acres, waters in this watershed area are classified as Outstanding Resource Waters (Class ORW), Shellfish Harvesting Waters (Class SFH), Tidal Saltwaters (Classes SA and SB), and Groundwaters (Class GB).
The airport project’s impact on long-term water quality is addressed in detail in the master plan, specifically the pollution washoff. The plan expressed concern about contaminants in the washoff, including biochemical oxygen demand, chemical oxygen demand, volatile suspended oils, grease, pesticides, polychlorinated biphenyls, total and suspended solids, algal nutrients, heavy metals, salts, asbestos, and coliform bacterial indicators. Rain dislodges particles on the impervious surfaces, which are then carried through runoff to drainage units. The plan also had to comply with the Clean Water Act, and the project required coordination with federal and state agencies relating to water quality.
“This project had some particular challenges,” says Paul Moore, project manager for Ward Edwards Engineering, a civil engineering firm in Bluffton, SC. “There were some dry detention basins and a very large canal that held water. The function of the airport made it necessary to fill in the canal and eliminate standing water onsite. The option of underground detention was very valuable to the design of the site.
“We considered products from three plastic storm chamber manufacturers, as well as concrete arch chambers,” continues Moore. “We ultimately chose Cultec chambers because theirs were the lowest-cost solution from other projects we’ve designed in the area that utilized large-scale underground detention systems. The grant funding for the project required us to allow for ‘equivalent but equal’ designs, so we were open to contractors supplying equivalent designs using other manufactured chamber systems, but the cost ended up being more than the Cultec system. From a design point of view, we liked the Cultec feed connector that allowed rows to be connected at intervals along the linear length. Given that the project called for very long rows, the feed connectors will allow flow to go from one chamber row to the next without adding in additional storm drain junction boxes and headers.”
While the project was fairly routine, construction crews could work only between 10 p.m. and 5 a.m.; so the contractor had to cover the ground each night, making it appear to pilots and passengers arriving the next day as if nothing occurred. The installation involved two different Cultec chamber models: the 330XLHD and 902HD. The systems are designed to infiltrate as much water as possible, but have an overflow to an existing swale.
“When it rains here, it rains a lot,” says Terry Jakovac of Quality Enterprises USA Inc., the contractor who installed the stormwater system. “That means that there can be a lot of standing water during and after big rainstorms. The idea here was to have a place to store all that water, and the systems we installed should do exactly that.”
“Another unusual aspect of this installation is that we needed the chamber system to function both as a conveyance and as a detention system,” adds Moore. “The system was designed as a long, liner system that would store water during low-flow conditions to help with water quality treatment, but then convey water downstream during high-flow periods. The permeable nature of the system also allowed for continued interaction with the groundwater and temporary surface storage in the swale above the system.
“There are other BMPs in use at the site such as wet detention, permeable paving, and dry detention. These other BMPs are located outside of the Runway Safety Area [RSA]. The chamber system was needed for use within the RSA where BMPs with standing water are not permitted,” says Moore. FAA regulation FAA AC 150/5300-13, Change 17, requires, among other things, that RSAs be drained by grading or storm sewers to prevent water accumulation.
“We wanted our systems to feature the largest storage volume chamber we could fit with sufficient cover for each system,” he adds. “While the runway is flat, the elevations drop as you move south to north. This meant we could use the taller chambers on the south end where the surface grades were higher, and the shorter chambers on the north end.”
The southern system (system 1) used 646 of the 48-inch-high Cultec Recharger 902HD chambers arranged in a bed area of 18,681 square feet with an effective storage depth of 5.75 feet (before additional cover). The chambers were placed on a 9-inch base of stone, covered by an additional 12 inches of stone. System 1 is surrounded by a 12-inch stone border around the system perimeter. Total storage capacity exceeded 68,000 cubic feet, with more than three-quarters of the water storage being provided by the stormwater chambers and Cultec’s unique internal manifold system; the balance of storage is provided within the stone voids.
The northern system (system 2) featured the 30.5-inch-high Cultec Recharger 330XLHD chambers; 1,000 pieces of the 330XLHD chambers were configured into a narrow five-row-wide system 1,403 feet long. The chambers occupy an area of just over 36,000 square feet. The chambers lay above and beneath 6-inch layers of stone, and the outside perimeter of the system is surrounded by a 12-inch-wide stone border. With an effective storage bed depth of 3.54 feet (without additional cover), this system has a total storage capacity of 84,420 cubic feet, with 63% of the storage accomplished by the chambers and manifold, and the balance stored within the stone voids.
The project received financial support from the FAA’s Airport Improvement Program, which provides grants for the planning and development of public-use airports with commercial services. Eligible projects include those improvements related to enhancing airport safety, capacity, security, and environmental concerns.
