The new Martin’s Food Store complex is one of the first centers in Washington County, Md., to use a subsurface structure to control storm water runoff. The system was built using corrugated high-density polyethylene (HDPE) pipe to solve the multi-pronged problem of a small footprint, an increase in water volume and the area’s challenging karst topography.
Previously, the property contained a Gold’s Gym and utilized a surface detention pond for runoff. The newly invigorated center has six retail stores in addition to the Martin’s anchor. Construction of the subsurface storm water detention system started on site in February 2008 and was completed three weeks later.
Project Considerations
As construction of the center was being planned, the Maryland State Highway Administration (SHA) was also making plans to widen nearby Pennsylvania Avenue. The SHA required runoff from this road expansion to travel to the Martin’s system, adding an additional burden.
The food store complex is one the first centers in the county to use a subsurface structure to control storm water runoff.
“We had an existing footprint of about eight acres for the site that included the surface detention basin,” said David J. Habowski of J. Michael Brill & Associates, responsible for the site’s development. “The question was how to fit more parking and more retail space in the same area while taking care of an increase in storm water runoff and dealing with the sinkhole situation. …We didn’t have a lot of ground area to construct a surface pond, so we definitely had to utilize a subsurface system.”
The site’s geological proneness to sinkholes ruled out the option to infiltrate, prompting project leaders to utilize large-diameter pipe rather than an open-bottom or perforated system, according to Habowski.
The components specified were primarily HDPE pipe because Washington County does not allow the use of metal piping underground. Metal pipe can corrode, whereas HDPE pipe is impervious to corrosion and will provide a watertight system, according to the Plastics Pipe Institute (PPI).
Before the Martin’s Food project and the road expansion, water was running into storm drains under adjacent North Pointe Drive and down the opposite side of the street, where there is another shopping center with a pond.
“Maryland requires that the first flush be handled to protect the land from excessive erosion caused by the increased flow of water,” Habowski said. “This is a standard called the CPv—channel protection volume—that requires runoff volume from a one-year-frequency, 24-hour storm be captured and discharged in a controlled manner during no less than a 24-hour period. So, you have to set an orifice down at the bottom of the outlet structure. For the system’s subsurface outlet structure, we have a 6-by-9-ft box which has a baffle and orifice at the bottom to handle the CPv. The weir plate is cut out in the baffle to handle certain storm events and acts as a trash rack as well.”
The components were primarily HDPE because Washington County does not allow metal piping underground.
System Design
The HDPE pipe detention system weir plates are baffles with a ‘V’ at the top so that when water comes over the top, it spills into the next area. “This acts as a trap,” said Mike Batie, P.E., CFM, technical and engineering manager for PPI. “It separates oils and greases that may get into the system and will be taken out later, as well as organics and trash. This saves the owner a lot of cleanout and maintenance for the system and helps to meet EPA [U.S. Environmental Protection Agency], state and city regulations.”
Half of the water from the site goes directly into the underground system of 48-in. HDPE pipe sections connected by a manifold system. The other half goes through a sand filter that cleans sediment from the water and releases it into the subsurface. According to the plans, the facility will be inspected during a dry period twice a year (March and September); a vacuum truck will clean up sediment and debris.
The system is rated to handle 72,428 cu ft of water and has a footprint of more than 39,000 sq ft. In the structure that enables the system’s capacity to handle a 100-year storm event, it uses 6,200 ft of 48-in. corrugated HDPE pipe from PPI member company Advanced Drainage Systems Inc. The system is set up on a grid pattern with the pipe in a crosshatch design. Runoff comes in from five manholes to the manifold and into the branches, also called laterals.
“You can see from the volume requirement for water storage that the footprint for the system would have required a vast area of valuable land if the open pond was kept,” said Tony Radoszewski, executive director of PPI. “This probably would have meant that one or several of the rent-paying stores might not have been built if a typical retention pond was used.”
“Because each HDPE pipe section is around 20 ft long, there are fewer joints,” Radoszewski added. “This reduces labor and easily provides a secure, watertight system.”
