Going Underground

Sometimes, when it comes to stormwater detention and treatment, the best place to go is underground.

“Like everything else, cost is the main concern,” says Gil Colman of Colman Engineering PLC in Harrisonburg, VA. Typically, land-based best management practices (BMPs) such as ponds, vegetated swales, and filter strips tend to be less expensive than manufactured ones. However, when space is limited and the value of the land is high, it’s often more cost-effective to build detention underground. And even where the economic value of the land is lower–for example, at schools and parks–school districts and local governments often place a premium on grassy areas and landscaping.

Another concern is maintenance. Runoff should be treated upstream of detention structures when water-quality treatment is required. The structures should include traps or filters to remove large debris. They should be accessible to large maintenance vehicles and to maintenance personnel. Many of these structures qualify as confined spaces and need OSHA-required confined space equipment and procedures. There should be a detailed maintenance and inspection plan as well as a budget to cover the cost.

Underground structures, whether pipe systems of metal or plastic, arched chambers of plastic, or vaults of reinforced concrete, store stormwater runoff and release it at a controlled rate to mitigate downstream flooding and erosion. They provide little stormwater treatment by themselves; however, they can be combined with water treatment systems. On the other hand, some systems that are used mainly for water treatment also may be used for detention.

Underground structures also can be used with aboveground detention ponds as well as green infrastructure–for example, permeable pavers, vegetated swales, and rain gardens.

Meadow Glen Middle School
When the officials at Lexington School District One in South Carolina were deciding on a stormwater detention system for the new middle school, one of their priorities was to provide a playing field outside the cafeteria to encourage the students to get outside and be active during lunch breaks, says John Fleming, P.E., of Hussey Gay Bell & DeYoung (HGBD) in Columbia, SC.

“The original design called for an open pond,” says Fleming, who led the design team for HGBD. “They decided they wanted more space outside to create a place for the students to play.”

Meadow Glen Middle School is a completely new 1,000-student school in Lexington, a suburb of Columbia. Because it was a new development and the downstream waterway is impaired, county stormwater regulations required the school district to install a system that removed 90% of total suspended solids (TSS) in the stormwater, he says. The area gets a large amount of rain and the stormwater has the potential to have a lot of velocity, but it contains no pollutants other than sediment.

HGBD used StormTech’s new, larger MC-4500 injection molded polypropylene chambers, with two Isolator Rows. The Isolator Rows catch the first flush of sediment before stormwater overflows into the adjacent chambers.

“As a firm, we’ve used StormTech for many years for a lot of projects,” says Fleming. “We’re very familiar with it. We’ve never had any problems.”

The project also incorporates three two-stage open ponds.

The school sits on approximately 77 acres of hilly, formerly wooded land with a mixture of soils, predominantly loam. The school development will disturb approximately 38 acres.

“A lot of retaining walls had to be built on the site,” says Fleming. “Now it’s terraced on two levels. One is 10 to 12 feet lower than the other, to reduce the amount of earthwork on the site. From the school looking over the top, no one will necessarily see the field.”

The StormTech portion of the project began mid-March 2011 and was finished in three weeks. It called for 320 MC-4500 chambers and 38 MC-4500 end caps, for a total storage volume of 55,970 cubic feet. The StormTech chambers, or arches, take up an area the size of the approximately two basketball courts. Stormwater discharges to a wetland, an unnamed tributary to Twelve Mile Creek, which is on the current 303(d) list of impaired waters for macroinvertebrates, says Fleming.

The locations of the utilities was a challenge. “There are a lot of proposed utilities in the area. We did an outline design, and StormTech did the drawing to scale to make sure everything would fit. They were on top of getting us what we needed.”

The project originally specified StormTech’s smaller MC-3500 chambers, but design changes unrelated to the project required that the frontage road adjacent to the school be raised from its original elevation. Fleming changed the layout to the new, larger MC-4500 chambers both to accommodate the increase in fill that resulted from the rise in elevation and to maintain the layout of the original storm drain.

“The height was a bonus,” he says. “The ability to build StormTech in the configuration we needed was pretty important. Their new version fit exactly what we needed. And if StormTech hadn’t had these bigger ones, we probably would have had to look at other solutions, which could have been a 50-plus percent cost increase.”

Coogler Construction of Irmo, SC, excavated a pad the size of the chamber system, leveled it, and placed a nonwoven filter fabric on the subgrade and up the excavated walls. Crews then placed a 12-inch-deep layer of gravel from a local site on top of the fabric. Because the installation was so large, it was important to get the gravel correctly placed, leveled, and compacted so the bottom of the chambers, especially the two Isolator rows, would be on the proper grade, Fleming says.

Crews then installed the Isolator Rows, the first rows water enter in the chamber beds, at each inlet point, wrapping a nonwoven geotextile around them to separate them from the gravel. The main chambers were installed with 6 inches of space between adjacent rows, and additional gravel was placed between the rows. Crews then backfilled gravel over the entire chamber area, wrapped the fabric around the sides and over the top, and backfilled over the filter fabric to complete the chamber envelope.

One reason the installation went so quickly was that the end cap of each chamber was pre-molded at the factory. This prevented the contractor from having to custom-drill them in the field. A manifold pipe connects the chambers together through the holes to allow water to move more evenly through the system.

Maintenance and sediment removal are crucial in underground systems. StormTech has maintenance plans for the school district to follow, and the two Isolator Rows have access ports for maintenance. Sediment can be removed with a Jet-Vac.

“The project went smoothly,” says Fleming. “We ordered the chambers, they showed up fast, and Coogler put them in ground fast.”

The project’s largest low-impact development (LID) elements are the three open two-stage detention ponds in another area of the school grounds. Stormwater flows from pipes in the parking lots and other areas where water collects into the forebays of the ponds, where much of the sediment drops out of suspension.

“The forebays are fairly deep,” he says. “There are survey markers so the district knows when to clean them out. It will probably be about every five years.”

Stormwater overflows into the second stage of the ponds. It discharges through an outfall box into level spreaders, pipes laid on top of the ground, then flows through the pipes and hits the contour of the earth, which reduces the its velocity to the wetland.

The Canadian National Intermodal Yard
At the Canadian National (CN) Intermodal yard in Markham, IL, containers and trailers are transferred between trucks and rail cars and shipped across the US and Canada. Truck traffic had been growing, and by the end of 2010 the truck processing was automated to increase processing capacity and reduce waiting time for trucks entering and exiting the facility.

Remprex Engineering Services LLC, which provides automated gate system design, build, and maintenance services to the intermodal industry, as well as structural and electrical design, project management, and onsite engineering, was chosen for the job. “This has been an existing railroad yard for years,” says Chad Hewitt, P.E., the project design and management consulting engineer for the project. Because it was paving areas that weren’t previously paved, CN was required to install stormwater detention to collect and detain runoff and divert it into the county drainage ditch.

CN chose a SingleTrap system from StormTrap in Morris, IL, a modular precast system of reinforced high-strength concrete, manufactured by StormTrap’s sister company, Utility Concrete Products LLC. “In this case, it was definitely because of the ability to put the system underground and not sacrifice driving or parking space,” says Hewitt. “There wasn’t room for an open pond that would be large enough. It was also because of the ability to expand the system in the future, if we decided to expand the area where the trucks come off the highway and drive up to the kiosk.” In addition, CN had a short deadline, and the StormTrap system could be installed in just one day.

The project won third place in the Underground category of the National Precast Concrete Association’s 2010 CUP (Creative Use of Precast) Award. The award is based on innovation, cost benefits, schedule savings, and ease of construction.

The yard takes up some 350 acres and is located in a well-developed residential and industrial area on the south side of Chicago. The land is flat, and there are few pollutants in the stormwater. There is, however, a large amount of imported sand from when the yard was built up years ago, Hewitt says.

StormTrap designed the detention system and provided support through the preconstruction period. Before stormwater enters the system, sediment falls out in several upstream catch basins. Stormwater flows into the structure and exits to an open detention pond immediately beside it. The pond is approximately 400 feet by 80 feet and may be converted into additional underground detention in the future as site expansions dictate. From there, the stormwater empties into a county-controlled drainage ditch through a 4-inch restrictor outlet.

The entire project began the end of August 2010 and was finished in early November.

Remprex began the installation by excavating a hole approximately 100 by 130 feet and about 5 feet deep. Crews hit an obstacle almost immediately.

“This was an old railroad yard,” says Hewitt. “We encountered a lot of old railroad ties that were unexpected.” Crews had to over-excavate to remove all the ties, some of which were 12 to 15 feet long.

Once they removed the ties and leveled the working area, they poured a 7-inch-thick concrete slab 90 feet by 105 feet in the bottom of the hole. They used a crane to lower the 78 precast structures into the hole and assembled them on the slab.

StormTrap units are available in custom sizes. Each of these is 4 feet 8 inches tall. The inside pieces look like rectangular concrete tables. Some were as large as 5 feet by 15 feet 4 inches. The outside perimeter pieces, 6 feet, 7 inches by 15 feet, 4 inches, have one side that becomes part of the wall enclosing the system. The system stores 35,847 cubic feet of stormwater in a small footprint, thus optimizing cost and efficiency.

Remprex used two types of sealants to make the system waterproof: an 8-inch-wide elastomeric polymer to seal the vertical and horizontal joints between the pieces, and a ¾-inch preformed butyl rubber mastic sealant to seal the joint between the exterior wall of the pieces and the concrete pad.

Crews placed 9 to 12 inches of imported aggregate on top and paved over the aggregate, and the surface was ready for use. It took just one day to install the system and another couple of days to do the backfill and paving. “It falls together pretty good as long as everything is square and level,” says Hewitt.

StormTrap also was chosen because of its ease of maintenance, Hewitt says, although he doesn’t expect much sediment to enter the structure. Nevertheless, it should be inspected on a yearly basis. A manhole allows easy access for inspection and maintenance, which consists of hosing out and vacuuming sediment and debris. “It was a pleasure working with StormTrap,” he says. “The installation went very well and was very quick.”

Fairfield Inn
The new Fairfield Inn in Harrisonburg, VA, boasts of spacious suites, a fitness center, and an indoor pool. And although hotel guests can’t appreciate it, it also has a sophisticated stormwater treatment system under the front drive and parking areas.

“The development of this urban site required stormwater detention as well as water-quality measures,” says Colman, P.E., CFM, who designed the project for Blackwell Engineering in Harrisonburg. In 2010, he opened his own civil and water resources engineering firm, Colman Engineering PLC in Harrisonburg, which offers a wide range of design and consulting services for public and private sector clients.

Colman chose the vault type MWS-Linear, a hybrid stormwater filtration system made by Modular Wetland Systems in Oceanside, CA, and supplied by ACF Environmental. It combines bioretention and bioremediation with structural water treatment BMPs.

This system offered the best efficiency and fit for the site, he says. While the initial cost is comparable to similar products, it comes out ahead because of its efficiency in removing pollutants as the volume of water needing filtration increases. In addition, he was able to route all the runoff through just one unit. Another product would have required additional units to provide the equivalent removal efficiency.

“The pollutant regulated by the State of Virginia in this area of the state and for this type of development is total phosphorus,” says Colman. “The total removal efficiency required was 76%, and the expected efficiency provided by the unit was just over that. The unit also controls several other pollutants, including hydrocarbons and suspended solids, which carry heavy metals such as copper, lead, and zinc.”

Colman designed the system so that stormwater flows from several curb inlets around the site to underground detention pipes from ADS, through the Modular Wetland System, and out to the city’s storm sewer system.

The design process started in mid-2008 and the site was finished in 2010.

The hotel is on a 1.5-acre infill lot inside the city limits of Harrisburg. Most of the lot and the land above it are highly impervious. Although rainfall isn’t especially heavy, substantial amounts of runoff from both on- and offsite ran through the lot and ended up in the ecologically fragile Chesapeake Bay.

“During the design process it was determined that all offsite runoff was coming from existing detention facilities,” says Colman, “so we were able to route it directly into the city’s stormwater system, bypassing our site entirely. What was left was mainly stormwater from our own site.”

He used an underground system because of the amount of building and parking space that was required on the site. In an urban area, this almost always leads to a conflict with existing utilities, he says, and this project was no different. He avoided some utilities by carefully routing and resizing the conveyance piping system. He had to reroute some others, though, such as an existing 12-inch water line under the road, so the hotel’s new discharge storm pipe could connect to the city’s storm system across the street.

The ADS detention unit consists of 140 linear feet of 3-foot storage pipe with manifolds at each end. It holds about 6,000 cubic feet of water and releases it to the modular wetland system at a controlled flow rate. The pipe also has additional outlets, which release excess water at a controlled rate for a 2-, 10-, and 100-year storm. Much of the sediment carried by the stormwater drops out in this pipe before it reaches the wetland system.

The installation, which was done by the subcontractor, Quality Excavating Inc. of Harrisonburg, was straightforward.

“The excavation is minimal when it comes to installing the wetland units,” says Colman. The subcontractor excavated as little as 1 foot beyond the size of the 22- by 5-foot unit on all four sides to a depth of approximately 6 feet. The unit itself is 5.5 feet high.

The subcontractor placed 4 to 6 inches of gravel for the base, then compacted, leveled, and graded it so the pipes of the wetland system would align with the incoming pipes from the detention unit and the outgoing pipes to an onsite manhole that connects to the city’s storm system and from there to the Chesapeake Bay.

“We had to make sure the unit would be perfectly in place,” he says. “Perhaps the only challenge was positioning the delivery truck for the crane to be just right for dropping it into place.”

The wetland unit is a four-stage treatment train and comes in two sections: the main catch basin chamber and the wetland, or bioremediation, chamber. The catch basin chamber directs stormwater first through a screen to remove solids such as litter, foliage, and sediment, then to a separation chamber where suspended solids settle out of the water, and finally through the BioMediaGREEN filter, which is made of lightweight blocks that offer higher flow rates and better performance than traditional granular filter media. This chamber provides about 69.6% phosphorus removal efficiency and also captures fine TSS, metals, nutrients, and bacteria.

The wetland, or bioremediation, chamber is installed separately. Here, runoff passes through a granular, porous medium that contains both microbial organisms and the root systems of the plants growing on the surface. It removes another 6.8% of phosphorus as well as any remaining suspended solids in the water. Biological nutrient uptake takes place through the plant roots. This chamber also has internal bypass pipes that direct high flows around the perimeter and wetland filter directly into the discharge chamber.

The wetland system can be used to store runoff after a large storm event as well as treat it. In this volume-based design, the high-flow bypass is external and located before the predetention system.

Once Quality Excavating had seated the unit, crews sealed the connecting pipes and backfilled the sides with crushed stone. They covered part of the unit with pavement and part with sod.

“The project went exceptionally well,” says Colman. “The location and installation of the Modular Wetland System took no more than an hour.”

The system does require maintenance, he says, as does any other filtration system. The screen filter should be inspected at least twice a year, and the separation chamber should be cleaned once a year. The BioMediaGREEN filter can last as long as two years, while the medium in the wetland chamber can last as long as 20 years.

“I really like this unit,” he says. “I haven’t seen anything else like it. It does a really good job.”