RESIDENTIAL DEVELOPMENT WITH DIFFICULT SOIL CONDITIONS MEETS STORM WATER REQUIREMENTS

“Charleston Harbor is one of the finest natural harbors on the Atlantic Coast. It is one of the most important economic assets of the state of South Carolina serving both commercial and military navigation,” stated a 1966 Charleston Harbor Water Quality Study.

Fifty three years later that still rings true. Preserving the harbor’s water quality is just as important today, as the city and its economy continue to grown dramatically. Charleston is no stranger to growth or history, as it was advantageously established on the peninsula between the Ashley and Cooper Rivers, and its harbor has been an economic center since early colonial days.

The water quality study is a reminder of the challenges still ahead for South Carolina’s shoreline. Land development and large commercial or industrial sites, in particular, can each have exponential effects on nature’s own management of rainfall and runoff. New impervious development creates immediate point sources for pollution which could contribute to nasty algae blooms and environmental deterioration that spreads from the shoreline, damaging wetlands, waterways and eventually the local tourism industry.

Situation

Brigade Street in Charleston is within reach of the Cooper River, an area with a rich history of tourism and wildlife at the mouth of Charleston Harbor. The new owner/developers and the engineering firm, Thomas & Hutton, were presented with a number of additional storm water design challenges as major plans were in place for a large-scale multi-family home residential development. First, 107 Brigade Street or Pollack Shores development’s known groundwater elevations are shallow, with pre-existing contaminated soils and a drainage pattern toward the adjacent protected wetland. The location and conditions would need feasible water quality solutions that could address high water tables, minimal space, contaminated soils, and strict regulations on nitrogen pollutants.

Challenge

There are heightened storm water concerns and regulations around development sites within Charleston Harbor and this site is particularly challenging. High soil contamination makes nitrogen a major concern for governing agencies as well as total suspended solids (TSS), phosphorus, and heavy metal removals. Beyond the regulations, storm water system(s) would have to address shallow groundwater elevations and adjust to accommodate high flow rates at a shallow depth. Per city storm water regulations, outfall could not go below a certain elevation. A storm water system would have to accommodate shallow flows across the development.

The collection of challenges is difficult to address, but finding a system that could successfully work with the shallow height was essential. Beyond that, engineers also had to design within a small footprint, keep costs down, and source systems that have proven treatment and records of requiring minimal maintenance.

“Many storm water products cannot effectively operate at this flow rate in such a shallow restriction, and would cost too much to install and maintain,” said Scott Sertich, director of engineering for Bio Clean. “They are not economically viable for new developments with site restrictions, space limitations and local regulations.”

Moreover, the storm water system would need verified and proven data showing high removal rates of TSS, phosphorus and heavy metals.

Solution

Lead Designer Brittany Bowers, and Engineer of Record Tony Woody of Thomas & Hutton in Mt. Pleasant, S.C., worked closely with a Bio Clean engineering team to fit each design with simple solutions. Hazardous pre-existing soil conditions are a major reason to use precast vaults as opposed to corrugated metal which could corrode.

First, the Kraken membrane filter was a clear candidate for its ability to sit within relatively small footprints and treat specific levels of nitrogen and large pollution concentrations of TSS (89%), trash, phosphorus and metals.

The product’s individual membrane filters are washable and reusable; easy and inexpensive to maintain. The filter provides higher treatment flow rates in a smaller footprint. Each 7.5-in. diameter filter cartridge has 170 sq ft. of media surface area. With this much surface, the filter can operate at a loading rate of only 0.05 gpm per sq ft of media surface area.

The second system to address the site’s needs is the SciClone hydrodynamic separator. The separator is NJCAT verified, it will treat up to 80% TSS, and it is known for its simple and low-cost installation. It also is maintenance-friendly. Per NJCAT verification, “maintenance frequency for all SciClone models is 96 months.”

The Bio Clean engineering team worked quickly with Thomas & Hutton to solve challenging configuration requests and variations from the standard designs that the systems and Bio Clean’s team could solve without incurring any extra costs. The Kraken and SciClone accommodated requests for pipes in configurations different than 180 degrees from each other, such as an inlet pipe through the sidewall of a filter, or an outlet pipe at 90 degrees in a hydrodynamic separator.

Bio Clean engineers also were able to:

• Lower the rim on the hydrodynamic separator to match finished grade;
• Add an anti-flotation collar and thicken the bases of the filter and hydrodynamic separator in order to overcome buoyancy issues due to high local groundwater;
• Steepen the angle of the inlet pipe into the hydrodynamic separator so that the inlet and outlet pipes would be at the same elevation;
• Shift the membrane filters within the filter to one side of the vault, accommodating two inlet pipes, and saving the developer an added cost of a junction box; and
• Work carefully with the Thomas & Hutton to establish finished grade, top of concrete and pipe elevations. This allowed the filter to fit into the site, function properly and provide access for maintenance.