Stormwater Detention and Treatment

Aug. 20, 2014

Protecting a local drinking water supply. Retaining and using stormwater onsite. Meeting regulatory requirements. Treating stormwater in as small a footprint as possible. These reasons–and a combination of others–underscore a number of post-construction stormwater BMP projects in the United States.

Saving Space
Regulatory permits and real estate footprint are the two major considerations underscoring the choice of Triton S-29 chambers for a stormwater project for a new parking lot to be constructed at the headquarters of a company in Maplewood, MN.

Darren C. Schwankl, P.E., LEED-AP, is with TKDA, an engineering architecture firm in St. Paul, MN. 3M Facilities Civil Engineering is a client of TKDA, and Schwankl says Triton had proven itself in a previous installation on the 3M campus.

“The number-one consideration was the local regulatory permit requirements mandated for rate control and volume reduction,” says Schwankl. “One of the reasons we chose Triton is that we had success using it in the past. This is our second installation here at 3M and our third installation overall for a company; we’ve done some work for another client here locally and had a double-stacked Triton system installed.”

The Triton S-29 chamber features 29-cubic-foot storage volume. Some 2,034 of the S-29 chambers are being used to cover a 32,450-square-foot area beneath the new parking lot, with the void space and the rock around it comprising 85,350 cubic feet.

Credit: Triton
Installation of subsurface Triton chambers

Manufactured from a soy-based resin and from recycled materials, the Triton chambers are designed to be 46% lighter per cubic foot and up to 46% larger per linear foot, in contrast to other options.

“This underground system addresses both our rate control and our volume reduction pieces to the permit,” says Schwankl.

The biggest selling point for the underground system from 3M’s standpoint is the ability to maintain use of the real estate for something other than a traditional excavated stormwater pond, says Schwankl.

“In this case, we were able to meet permitted requirements, but also still maintain usable space for the parking lot and building, which is the driver of this project,” he adds.

Converting about 6.5 acres of what was once woods and meadows to a parking lot signaled the need to mitigate the flow rate, says Schwankl. Local requirements also called for groundwater recharge as part of the permit mandate.

“We look at volume reduction as an integral piece of the permit,” says Schwankl, adding that the Triton system works well in that regard and is able to filtrate a certain volume of runoff from the impervious surface.

In the first water retention project at 3M, the company was in need of a comprehensive stormwater solution to facilitate upgrades to a parking lot and building entrance to support a community walking path on its campus. Existing structures and landmarks could not be disturbed, and the path’s design required flexibility to fit with existing infrastructure. Space limitations called for an underground system that offered maximum storage capacity. Project managers had to protect municipal storm sewers. The project also had to be carried out with minimal disruption to the facility’s ongoing operations.

Triton SWS chambers were selected for the project because they were environmentally friendly and easy to install and maintain while offering the desired footprint and storage capacity.

Although that project was on a smaller scale than the current one, it entailed similar requirements in working with local governmental units, says Schwankl.

“In that case, rate control was not a factor because we actually reduced the amount of impervious area with the project, but we still needed the Triton system to manage the volume reduction component of the permit,” he says.

“This new building addition, on the flip side of the campus, is a larger-scale project with a laboratory and office building complex being constructed on one of the last remaining pieces of green space on the campus. The system is much larger in scale than the first one.”

The chambers are onsite awaiting installation, which was to have taken place during spring 2014, but the abnormally harsh winter has pushed out the date.

“This is a long-range project,” says Schwankl, adding that it’s slated to take two years to complete and the site needs to be stabilized before construction begins.

The system will filter the first flush of runoff to the sand bed, Schwankl says. “We don’t have any empirical data on discharge, water clarity, suspended solids, or anything tangible to point to, but we’ve got an 18-inch-thick clean sand filtration bed that we run all of that initial first flush through and that’s our layer that gives us that water-quality piece as well as our infiltration piece.”

Pollutants of concern are another consideration. “That obviously includes heavy metals, oil, and the grit and sediment from wintertime when we use a lot of sand and salt up here. The salinity of the runoff as it is discharged to a wetland here immediately adjacent to the project site was considered,” says Schwankl.

“We do have an extensive vegetation system,” he says. “Not that it’s all considered pretreatment, but it’s certainly breaking up the concentration of the parking lot runoff, which is our primary concern.”

3M will be the responsible for maintenance as part of its overall campus maintenance procedures. Maintenance includes accessing the header rows for visual inspection and measuring the depth of sediment. Any accumulated sediment is jetted back to the front manholes, which are cleaned out with a vacuum truck.

If the first Triton installation is any indication, Schwankl expects a good outcome with the second one. “We’ve had good success with the one we’ve had in the ground,” he says. “It allows us to comply with the permit requirements yet still allows us to be able to use that real estate for something other than stormwater management.”

A Facelift for the Pier
The Navy Pier in Chicago has undergone many changes since its construction began in 1914; it has served as a commercial pier and entertainment venue, a Navy training center, a college classroom for the University of Illinois for returning war veterans in 1946, and finally as a public gathering place.

Today, the Navy Pier–the city’s top tourist attraction–features public art sculptures, an interactive fountain, an embarkation point for tour and excursion boats, and a public space from which to watch lakefront events. It also contains parks, gardens, shops, and restaurants. In conjunction with its centennial, a portion of the Navy Pier is getting a facelift for some of its exterior public elements.

A competition in early 2012 led to the selection of a design concept by a team led by James Corner of James Corner Field Operations that includes a reworked streetscape, wider pedestrian space, and moving tour-boat moorings to improve the view from a new central stairway centered on the Ferris wheel.

“We wanted to make it more publicly accessible, more like an urban park and a little bit less like a carnival or an amusement park. Sustainability is an important consideration in the project and, given its location, stormwater management is a very important element of sustainability,” says John P. Fehlberg, P.E., a civil engineer with Primera Engineers in Chicago. Fehlberg is the project manager for Primera Engineers as part of the architectural firm’s James Corner Field Operations team on what is called the Pierscape project.

The stormwater element of the project is critical, because the pier is located at the junction of the Chicago River and Lake Michigan, Fehlberg points out. The tourist destination attracts up to eight million visitors annually.

Most of the stormwater management being done focuses on rainwater harvesting for irrigation. “There is a lot of new planting going in–a lot of new trees and landscaping–and rather than use treated water to pump around, we wanted to use captured stormwater runoff,” says Fehlberg. To that end, he and his team have chosen to use StormTrap stormwater chambers.

Fehlberg says the team is “very selective” in the water being harvested. The site, Gateway Park, has a heavily trafficked road running through it. “We separated the roadway drainage into a separate system,” he says. “For the landscape irrigation water, we tried to capture runoff from areas where it was deemed that the runoff would be a little cleaner and wouldn’t have road salts in it.”

The team is using two separate StormTrap precast concrete underground chambers. One is used to meet the regulatory requirements from Chicago’s Department of Water Management, the local stormwater management authority.

“We’re essentially providing stormwater detention in that chamber,” says Fehlberg. “Then we’re using another underground chamber of precast concrete vaults for rainwater harvesting.”

StormTrap block units are approximately 6 feet tall, 7 to 8 feet wide, and 14 to 16 feet long, and the chambers are assembled from those units, he points out. A poured-in-place concrete foundation slab is put in place first, with the units set on top.

“The two chambers for stormwater management and rainwater harvesting, chamber one and chamber two, are from roughly 15 units to 25 units,” adds Fehlberg.

Rainwater is collected through traditional drainage structures and piped into one of the chambers, depending on whether it is for stormwater detention or rainwater harvesting. The irrigation system draws water from the rainwater harvesting chamber, where the water has been treated and filtered. The water is pumped to the landscaped areas slated for irrigation.

Fecal coliform is a primary pollutant of concern in the water bodies as a result of an occasional combined sewer overflow (CSO) event.

“Chicago is unique as it’s the only watershed of its kind that I know of in that the Chicago River, which originally flowed from land into Lake Michigan, was reversed,” says Fehlberg. “Now it goes from Lake Michigan inland and eventually back to the Mississippi River. Part of the problem with the combined sewer overflows is that when we get really heavy rains, the river wants to go its natural course to flow back to Lake Michigan.”

All of the manmade systems constructed over the past century aren’t enough to hold that action back, “so what happens in the really big storms is that so much that has been collected and contained spills over into Lake Michigan and pollutes the lake,” he adds.

Although such an event doesn’t happen very often, when it does, it necessitates closing the beach for a few weeks, Fehlberg says.

A few challenges have arisen on the project. Design took place in 2013 and construction started in the fall, but was delayed because of the frequent deep freezes Chicago experienced throughout the challenging winter of 20132014. And because it’s a heavily used tourist area, the site has to remain open throughout the project, with only small areas closed at any given time.

“We want to minimize the construction footprint during the high tourist season,” says Fehlberg. “It’s been important to get as much done as we can during the winter.”

Another challenge is inherent in the pier structure itself. “There is no ground beneath the pier. It’s a structure supported over the water,” he points out. “It’s difficult to run utilities. The potable water lines were a challenge. It was easier to run irrigation lines in a utility trench that runs the length of the project along the pier and cuts over through the deck slab where we had to reach the landscaped areas to be irrigated.”

Because the Navy Pier project site has been developed and redeveloped for many varying purposes over its history, the team encountered previously unknown underground obstructions during construction, with several underground utilities either unmapped or not in their mapped locations.

“This required several revisions to the layout of the underground stormwater management and rainwater harvesting chambers on the fly during construction,” says Fehlberg. “The modular nature of the StormTrap system helped us adapt to and overcome these surprises efficiently, and made StormTrap a good fit for this challenging application.”

Properly maintained, the system is expected to serve its purpose going forward, Fehlberg says. “Each of the underground chambers we’re providing is equipped with service access where those maintaining it can enter it and wash it out, freeing it of sediment,” he says.

Quabbin Reservoir
The Quabbin Reservoir is one of the largest manmade public water supplies in the United States and is the largest inland body of water in Massachusetts. The reservoir, built between 1930 and 1939, is the primary drinking water supply to the metropolitan Boston area, servicing nearly 2.2 million people. When full, the Quabbin holds approximately 412 billion gallons of water.

The Massachusetts Department of Conservation and Recreation (DCR) Division of Water Supply Protection has stewardship of about 81,000 acres of protected lands surrounding the reservoir, managing the undeveloped tracts of land to preserve the Quabbin Reservoir’s high water quality.

The reservoir also serves as a site for passive recreational pursuits for visitors, protected habitat for wildlife, and the preservation of the historical significance of the communities lost during the original creation of the reservoir.

The Quabbin Administration Building and Visitor’s Center is staffed and operated approximately 360 days annually, typically serving 60,000 visitors, including students who take in programs on topics connected to drinking water, Quabbin history, watershed management, and wildlife. The administration building also serves as a DCR headquarters facility, housing administrative staff, two 10-stall garages behind the main building for vehicles, maintenance machinery, and work areas for equipment upkeep. A hangar that was originally intended to house amphibious aircraft to patrol the reservoir is now used to store boats and vehicles.

In 2012, a need arose to reconstruct deteriorated paved roads and parking areas around the hangar service area. The project also presented an opportunity to make stormwater improvements.

A Stormceptor treatment unit, STC 900, was selected by DCR engineers for its high pollutant removal efficiency and small footprint. Its ability to provide oil/water separation and hydrocarbon spill containment was a plus because of the close proximity of the existing outfall to the reservoir.

The Stormceptor system is a water-quality-treatment device used to remove total suspended solids and free oil from stormwater runoff. It takes the place of a conventional junction or inlet structure within a storm drain system. It is manufactured with precast concrete components and a fiberglass disc insert in accordance with AASHTO and ASTM standards. The Stormceptor, licensed by Imbrium Systems to Rinker Materials, was manufactured by Rinker Materials’ Concrete Pipe Division in Westfield, MA, and installed by Warner Brothers of Sunderland, MA, under contract with the Commonwealth of Massachusetts.

The DCR also implemented an emergency shutoff in the downstream piping for extra protection to protect the drinking water supply in the case of a significant accidental spill.

About the Author

Carol Brzozowski

Carol Brzozowski specializes in topics related to resource management and technology.

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