Team Slows Stormwater Flow off Giant Parking Lot

Designers of the new 150-acre Jack Kent Cooke Stadium parking lot knew that the first half inch of rain, known as the “first flush,” can sweep all manner of pollutants from paved surfaces into creeks and streams.

Designers of the new 150-acre Jack Kent Cooke Stadium parking lot knew that the first half inch of rain, known as the “first flush,” can sweep all manner of pollutants from paved surfaces into creeks and streams.

They used a variety of techniques to protect the site’s two watersheds, which included a rare 60-million-year-old fossil bed on the banks of a creek that was in the middle of the construction site.

The Maryland-National Capital Park and Planning Commission (M-NCP&PC) bought the property in part to protect the fossils, known as the Brightseat Formation. They sold it to the late Jack Kent Cooke on the condition that he do the same.

“The first flush on the stadium parking lot creates 2.5 million gallons of water. Our design had to hold the water temporarily on-site to prevent downstream flooding, provide water quality systems, and protect the fossils,” said Al Arnold, P.E., of Greenhorne & O’Mara, Inc. (G&O), site designers for the Jack Kent Cooke Stadium. “M-NCP&PC wanted the same amount of stormwater to get to Fossil Creek as before - no more, no less. That created an engineering challenge. It would have been easier to just divert water away from the creek.”

The site lays across a watershed divide. Approximately half is in the Anacostia River watershed, which drains to the Potomac River and then to the Chesapeake Bay at Point Lookout, at the southwestern tip of Maryland. The other half is in the Patuxent River watershed, which cuts through the heart of Maryland and joins the Chesapeake Bay at Solomon’s Island. Protecting the bay is a priority of the Chesapeake Bay Foundation, local environmental agencies, and the water-loving, crab-eating public.

Both of these watersheds have chronic downstream flooding problems due to older developments built before flood prevention and stormwater management codes were in place. The watershed divide limited the design team’s options for where to direct water around the fossils, which are located in the Beaver Dam Creek watershed.

“Because of the incredibly fast-track design-build schedule for the stadium and the project’s high visibility, it was a non-stop brainstorming session,” Arnold said.

The Pond Design

The extent of the design covered the entire site including the parking lot, the widening of two county roads and improvements on two major state roads.

The Pond Design

Three ponds were built to handle stormwater volume up to the 100-year storm. During storm events, 20 percent of the first flush is diverted to “Pond 2,” a large, year round visual amenity on the adjacent Prince George’s Sports Complex site currently under construction. Greenhorne & O’Mara is the site designer for the Complex as well.

The Pond Design

“Pond 2 needs a continuous supply of water so it won’t dry up,” said Arnold. “Fossil Creek must also have water to keep the wetland plants alive, but not too much or the fossil layer would erode.”

The Pond Design

After Pond 2 and Fossil Creek are sufficiently watered, the remaining stormwater is delivered to Ponds 1 and 3, which may be dry part of the year. All three ponds hold the stormwater, letting silt and debris settle and giving vegetation time to break down pollutants before the water leaves the site.

The Pond Design

Several steps are taken to clean the water before it even gets to the ponds. Waste quality grass filters in the center of the parking lot receive the first flush. Under the grass are sand and gravel filters encased in filter cloth. Water is cleaned as it passes through these filters to the piping system underneath.

The Pond Design

G&O also designed water quality strips that clean runoff and serve as parking spaces, rather than traditional grassy medians. For bigger storms, standard storm drains were installed to manage the additional water volume.

GOtrench Goes to Work

Under the parking lot, a complex series of pipes direct the flow. When the stormwater reaches Fossil Creek, it meets with an exclusive G&O design. Called “GOtrench,” it includes a perforated 48-inch pipe surrounded by gravel and encased in filter cloth.

GOtrench Goes to Work

No matter what size the storm, only a small, steady amount of water is allowed to trickle out to feed Fossil Creek. Five GOtrenches keep Fossil Creek moist, “recharging” the groundwater, while protecting the fossils. The remaining stormwater flows past to Pond 1.

GOtrench Goes to Work

An additional GOtrench serves Pond 3, the only pond in the Patuxent watershed. For further cleansing action, Pond 3 has two forebays and Pond 1 has a 2- to 3-foot deep pool as its buffer. Forebays are natural wetland contours that stormwater travels through on its way to the ponds. Vegetation in the forebays helps to clean and filter the runoff. The Pond 1 pool is vegetated to serve a similar function.

GOtrench Goes to Work

“A watershed divide, a rare fossil layer, and a 2.5-million-gallon first flush. Civil engineers get a design challenge like this maybe once in their careers,” Arnold said.

GOtrench Goes to Work

Despite the sheer volume of water flowing off the parking lot when it rains, the amount of water flowing off the site is the same as that from the pre-existing farm through the 2-year storm. For larger storms, the amount of water allowed off-site is actually less than when the site was farmland.

GOtrench Goes to Work

“The Jack Kent Cooke Stadium’s stormwater management design will actually help reduce existing downstream flooding problems when major storms occur,” Arnold said.

GOtrench Goes to Work

The Stadium opened on time with maximized usable parking area – 950 additional spaces, construction cost savings of $250,000, reduced off-site water quantity for large storms (15 percent less), cleaner runoff than the uncontrolled farm, and preserved fossils.

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