Enhancing the Shores of the Potomac

This Spring, boaters traveling just south of the Harry W. Nice Bridge in Charles County, MD, will have a more pristine view of the steep, picturesque shores of the Potomac River. In Spring 2002, construction crews relocated and anchored an outfall—carrying treated effluent from the Cliffton on the Potomac Wastewater Treatment Plant—more than 1,000 feet into the river.
KCI engineers began working with the Charles County Department of Public Works in June 2000 to design plans for relocating the effluent outfall, located near Newburg, MD. “The outfall was visible on the beach at low tide,” noted KCI project team member Cynthia C. Brush. “Charles County was concerned about a variety of environmental issues and initiated the project in partnership with the Maryland Department of the Environment MDE to meet NPDES permitting requirements.” The challenging task involved relocating the existing outfall approximately 1,100 linear feet offshore, to a minimum submerged depth of 36 feet below mean low tide, minimizing potential impacts on adjacent oyster bars.

Project Planning—KCI gathered as-built drawings and conducted site inspections to assess the condition of existing facilities. Field teams videotaped the gravity line adjacent to the outfall and performed wetland delineation, bathymetric, and hydrographic surveys. KCI used the data to develop a preliminary concept and alignment plan. A portion of the effluent outfall line from the wastewater treatment plant is force main, so KCI conducted a hydraulic analysis of the pumps to assess the potential impact of the outfall extension on the overall system and to determine the size and number of required diffuser ports. “The video investigation of the outfall’s gravity line revealed a series of 45° bends traversing the 85-foot slope toward the shoreline and existing outfall point,” explained Project Manager John R. Kovacs, PE. “We considered the possibility of replacing the existing gravity main in the new design and evaluated non-corrosive materials for use underwater.”

Final Design—The final plans incorporate 1,200 linear feet of 12-inch, high-density polyethylene (HDPE) pipeline, 900-feet of which is subaqueous, and an additional 200-feet of submerged pipeline. “We recommended a directional drilling technique for installing the new gravity line over the steep slope that would be minimally invasive,” said Brush. The final 200-feet of pipeline was fitted with a series of six elastomer, “duckbill” diffuser valves and circular reinforced concrete anchors.

Construction Phase—KCI provided construction inspection services for the outfall project, involving both surface and subsurface investigations. HDPE pipe was fused and floated out into the Potomac from a local marina, one mile southeast of the site, which served as the launch point for the underwater operation. KCI’s professional engineer-divers partnered with the contractor to oversee the underwater construction efforts. Throughout the project, KCI worked with MDE, the U. S. Army Corps of Engineers, and the U. S. Coast Guard, as well as other regulatory agencies, to ensure that the project would comply with the spirit and intent of the final design and environmental permitting requirements.

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