Tackling a Construction Site's Rough Terrain
Saturday, October 01, 2011
National Pollutant Discharge Elimination System (NPDES) Phase II regulations have made stormwater treatment a chief priority for construction projects. Getting terrain to cooperate in order to comply with environmental protection requirements, however, is not always easy. For a recent stormwater management installation at a Tractor Supply retail center in Putnam, CT, the designer of the system, CPH Engineers of Vernon, CT, found the solution to be a 40-foot-long water-quality unit made from corrugated HDPE pipe.
The New England-area site presented some challenges such as steep grades, a high water table, and a contiguous stream and wetland area at the rear of the lot. These elements constricted land use and restricted excavation work to a very limited area. Once CPH completed the storm sewer design, the only space available to place the water-quality unit was along the highest contour adjacent to a steep backfilled slope. The stormwater quality system was designed for the retail center with 126 parking spaces in a lot covering more than 7 acres with some 32,000 square feet of retail space that is anchored by a 19,000-square-foot Tractor Supply store.
“In this northeast area of Connecticut, we have to design around hills, slopes, and rocky terrain,” explains Alan Carpenter, P.E., of CPH Engineers, the company’s lead engineer for the New England area. “More and more of the water-quality issues relating to discharge are becoming increasingly sensitive as the Connecticut Department of Environmental Protection [DEP] and local wetland commissions try to clean up water resources.
“Controlling the amount and the quality of runoff coming off the site, and maintaining that discharge as cleanly as possible, is very important. We want to do our part environmentally to make sure we’re not making the downstream conditions worse both from water-quantity and -quality measures.
“In Connecticut, EPA has recommendations on how to design water-quality systems to attain an 80% total suspended solid removal, and that’s the goal,” he continues. “Normally, the sites we design are well in excess of that, probably closer to 95 to 100% removal before any discharge.”
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| The 40-footlong, 36-inchdiameter Counter Flow Technology stormwater control unit is put in place. |
“With EPA’s Phase II requirements in effect, engineers such as Al Carpenter are eager for creative, reliable, and cost-effective solutions to provide stormwater treatment in compliance with the new regulations,” says Tony Radoszewski, executive director of the Plastics Pipe Institute (PPI), a nonprofit advocacy group for the industry. “Able to handle a large volume of water and debris, a water-quality unit made from HDPE provides an economical solution that is also light in weight for ease of handling, which is especially important in difficult terrain, and it can last for upwards of 75 to 100 years.”
The system, a Counter Flow Technology (CFT) Water Quality Unit from Lane Enterprises in Camp Hill, PA, a PPI member company, it is 36 inches in diameter and is fed by two manhole risers. Capacity is 3,000 gallons. Custom built by Lane following the requirements from CPH Engineers, the 40-foot-long unit is made from corrugated HDPE pipe with sections inside to take stormwater runoff in and, in a controlled manner, release it. It also has chambers to contain solids, sediment, and floatable debris, which are cleaned out.
The CFT unit is designed to handle the water flow expected to be treated for the first flush of water. “All we had to do was provide the water flow information to Lane, and they designed the system necessary to handle that flow and fit the terrain,” says Carpenter.
“From what I understand of the concrete products, they’re not designed to handle a specific volume of flow. You just order an off-the-shelf tank size, connect the pipe, and whatever you get you get,” continues Carpenter. “But the HDPE products are engineered for a calculated flow and to handle the expected solids and floatables that will be contained in the system. That is a major design benefit.
“The HDPE unit provided the level of treatment to help us get to the removal level recommended by the DEP,” he explains. “You get a certain level in the catch basin. Then you get another level by using the water-quality unit. And you get another level in the detention area. It all adds up to the level of removal that is your goal. And when you’re presenting these aspects to a wetlands commission, the more you can exceed the goal, the better off you’ll be.
“Most designers understand that this is their goal—to make sure they’re using the design elements that either DEP recommends or engineers come up with to obtain that goal. Again, the better cost effectiveness of the materials and products, the more measures you can put in, and the better the water quality will be that is exiting the site.”
Carpenter also says clients realize that more elements of a “treatment train” are required because of the increasing need for improved water quality. “There is a cost/benefit analysis that you go through with clients,” he says. “They understand they have to pay to incorporate these elements, and we try to control those costs as much as possible. Most environmentally responsible corporate retailers, such as Tractor Supply Company, realize the short- and long-term benefits of a water control system plus other initiatives, and they are willing to incur reasonable cost in their pollution prevention measures to protect the environment.”
He further explains the inner workings of the unit. “It has baffles that retain the floatables inside the unit, and a chamber that allows solids to be trapped that are carried in the stormwater. The unit handles the first flush of water of the site, which generally carries most of the pollutants.
“Just think about a parking lot that has been sitting in a dry condition for a certain amount of time,” he says. “Here in New England, snow removal and maintenance of a parking lot occurs quite a lot in the winter and spring months, so there’s a buildup of solids and hydrocarbons that gets carried into the catch basins on the site. If it is not collected, it would be carried into the water body as opposed to being treated primarily at the site. The unit traps materials that travel in the stormwater runoff at the site. A certain amount goes into that first flush, which contains most of the solids, hydrocarbons, floatables, trash, and all. Because the CFT unit maintains a certain water level, the floatables and hydrocarbons stay on the surface and are kept inside by a baffle for later cleanout.”
One manhole is located in the solids section and the other in the floatable section so each area can be easily cleaned out. CPH is recommending two clean-outs a year, one in late spring, the other before the start of winter.
Product Versatility
Using corrugated HDPE pipe to make the CFT permitted the unit to be located on top and parallel to an unaccommodating slope, while providing additional benefits: a shallower trench capable of providing minimum cover; the ability to have equipment and personnel working on level surfaces throughout the installation; and an installation location directly beneath a level parking lot surface versus a steep slope, maintaining the accessibility needed for future inspection and maintenance.
Water-quality units such as the CFT can be incorporated in a wide variety of storm sewer alignments. It is capable of being installed on either side of the alignment, whether parallel, perpendicular, or at 90-degree storm sewer bends. Because storm sewer alignments are determined before considerations for a water-quality unit, this feature is essential to prevent unnecessary site plan modifications or additional pipe.
“Versatility in designing a system for stormwater quality control is important today because it provides for cost savings while meeting environmental requirements,” says PPI’s Radoszewski.
“National environmental legislation often requires that municipalities establish regulations limiting the quantity of stormwater being discharged from newly developed or redeveloped sites. Where land costs are high or discharge limits have been established, underground detention systems and water-quality management units are the most cost-effective solution because the ground surface over these systems can be used for parking or as a ‘green-zone’ as is the case here in Connecticut.
“Stormwater quality units provide a means for controlling the quality of discharge from a newly developed site,” he continues. “The purpose of these structures is to make certain the discharge from a site complies with national regulations concerning the concentrations of pollutants in stormwater. These structures are typically constructed in conjunction with stormwater management systems. The ease with which products made from HDPE pipe can be fabricated, makes it ideal for both types of systems.”
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