Retention and Detention Systems

Oct. 1, 2010

The need to detain and retain stormwater in limited spaces has been stimulating manufacturers and designers to come up with a remarkable variety of products and increasingly creative ways to use them. These include underground boxes and chambers, usually of concrete or some form of synthetic, in all kinds of sizes and configurations.

Whether the project is a large site, such as a school or apartment complex, or a small one, such as two homes unfortunate enough to be in the path of sediment-laden stormwater, the goal is the same: to slow the stormwater down and allow it to either infiltrate into the ground or flow at a controlled rate into a storm sewer system. Because sediment settles in this slow-moving water, these systems have the added benefit of removing trash and some pollutants before the water is released.

Radnor Middle School
When the school district in Radnor, PA, built a new middle school, it solved a decades-old flooding problem at the same time. The project included a new stormwater infiltration system, stormwater treatment units, porous pavement/infiltration parking lots, vegetated roofs, and a rain garden, all within the school grounds.

“Radnor is a very old suburb of Philadelphia,” says Brian Kane of Horst Excavating in Lancaster, PA, the project manager for the stormwater project. “It isn’t particularly hilly or rainy, but it’s so built up upstream that the rain all hits the pavement. They’ve had some major water issues there.”

The school bore the brunt of much of it. “It flooded every time it rained,” he says. “Alongside the brick culvert that takes stormwater away from the school there were whitecaps in the field. It was a major concern.”

The new school building, a geothermal unit, and the stormwater systems, which include StormTank Stormwater Storage Modules by Brentwood Industries in Reading, PA, and Contech’s CDS (continuous deflective separation) stormwater treatment units, were designed in conjunction with each other. The entire project put the school on track for Leadership in Energy and Environmental Design (LEED) Gold status.

“The intent is to keep stormwater out of the culvert and infiltrate as much as possible into the ground,” says Kane. “Now it goes into storm drains to this system. Most leaches down the bottom. What doesn’t goes through pipes to the culvert that’s always taken it away in the past.”

A 125-acre residential area and a 64-acre urbanized downtown district drain into the site. From there, stormwater flows to Ithan Creek and eventually into the Delaware River, says Daniel Wible, water resources project manager for CH2M Hill Inc. in Philadelphia. Wible designed the stormwater system for Cahill Associates, which is now part of CH2M Hill. The area receives approximately 45 to 50 inches of rain a year, usually only about an inch per weather event, and the soils drain relatively well, but the school has a long history of
flooding.

“Some years ago, the natural stream systems were piped and buried in a not-always-sensible fashion,” says Wible. “A brick arch culvert 7 to 8 feet wide and 4 to 6 feet deep conveys stormwater through the site. Over time, as development occurred, neither the culvert nor the pipes were big enough. Any large storm would end up flooding the school. They were constantly pumping water out of the basement, which was built deeper than the culvert bottom.”

The water table is only 8 to 10 feet below the surface, so Wible designed a shallow, below-grade system to at least retain, if not infiltrate, the stormwater. StormTank’s 97% void space provided the maximum amount of storage and was cost effective as well, he says. “It ended up being the most appealing option of the several considered.”

The entire project took a certain amount of cooperation and coordination, Kane says, in terms of both timing and authority. “Everybody had to do their part. It was kind of like putting a puzzle together.”

In 2005, Horst began excavation for the new school, in the approximate southwestern area of the site. The existing school, which was on the eastern side of the property, remained in use until 2007, when the new school and the geothermal system to the north were finished, under contract with the school district.

Horst installed the StormTank system on top of the geothermal unit, which consists of 100 wells, 500 feet deep, under contract with the township. Once the installation got up to grade, the school contract came back into effect for the vegetation. In 2007, the old school was demolished. The most sustainable approach was to leave the basement walls in place and fill the basement with the demolition debris, Wible says. Crews removed the hazardous materials, used a jaw crusher to crush the remaining building rubble to a uniform gradation of 2 to 3 inches, installed pipe connections to the below-grade system, and filled the basement with the rubble. They stockpiled as much of the excess rubble as they could to use as backfill later.

“You’re seeing that more and more,” says Kane, “especially with green building. We try to save as much material as possible. The more you can use onsite, the more cost effective it is.”

The stormwater portion of the project consists of two storage/infiltration beds. Bed #1 contains the StormTank system above the geothermal unit and was designed for infiltration. Bed #2 begins at the southern border of bed #1 and was designed primarily for detention (with some infiltration). The sizing of the beds was based on a 2-year storm (3.55 inches). Construction of the beds began in early May 2008 and ended in the spring of 2009.

“We were able to do the pipe work and the StormTanks through the winter,” Kane says. “We knew the area was a water nightmare, and we had to be smart when the storms came. There were a couple of rains that got ugly. We just made sure we prepared for them by covering the inlets and utilizing silt fence, diversion berms, and rock filters.”

Bed #1 is about 57,000 square feet and includes 1.5 feet of StormTank and 1 foot of aggregate, with a total storage volume of around 2.4 acre-feet, says Wible. Horst put down about 6 inches of sand to protect the wellheads, a layer of the geotextile, and then the StormTank units. The tops, bottoms, and sides of the units are made of rugged, lightweight polypropylene and the columns are of PVC. They’re rated for surfaces such as parking lots and athletic fields.

Horst wrapped the geotextile around the sides and the top of the units. Crews placed aggregate around the perimeter of the geotextile and 12 inches above it, then backfilled to grade with fill from the stockpile of rubble.

The presence of the geothermal unit didn’t have any effect on the installation of the StormTank system because it was far enough underneath, Kane says. “We knew we were close and we had to be careful, but we all worked together.”

Stormwater runoff is treated before it enters the StormTank system, so there is no concern about pollutants in the stored water finding their way down the wells and contaminating the groundwater. Each of four main stormwater lines leads to one of four CDS stormwater treatment units, where the runoff is screened and separated, and sediment, debris, oil, and grease are trapped. Piping takes the filtered runoff to the StormTank system.

The system can be maintained through manholes, Wible says, but it shouldn’t be necessary because of the pretreatment system.

The construction of bed #2, which is immediately to the south of bed #1, was much more straightforward. “There was already a sediment basin from the construction of the school,” says Kane. “We cleaned out the sediment, lined it with geotextile and stone, and backfilled with stockpiled soil.”

This bed is 41,710 square feet and about 6 feet deep, Wible says. It’s filled with 2-inch aggregate, which has 40% void spaces. HDPE piping around the perimeter of the basin distributes the stormwater. The approximate total storage is 2.7 acre-feet.

Crews placed a minimum of 1 foot of topsoil on both beds. Before they were sodded, a sand-slit system was put in into field, Kane says. “The ground tends to get compacted. They put grooves about 1/2 to 1 inch wide in the field every 10 feet, both ways. Then they put a sandy mixture in the grooves. Over the years, it doesn’t get compacted.”

The area now consists of two new athletic fields, one for soccer and one multipurpose, and a new porous asphalt parking lot, Wible says. The only impervious surface above the beds is a basketball court.“This project went very well,” says Kane. “We’re pleased with the outcome. It seems to be doing a very good job.”
Sutton Hill Apartments
The developers of Sutton Hill Apartments in Middletown, NY, also had to build stormwater detention. They added 10 apartment buildings as well as a new parking lot and recreation areas to the complex, says John Queenan, P.E., project manager with Lanc & Tully Engineering and Suveying P.C. in Goshen, NY, and the lead designer for the project.The complex is in a relatively dense, mostly residential area about an hour from New York City. It’s near a commercial area and the famous Catskills and Poconos mountains. The site has wetlands and silty clay soil. It averages a moderate 48 inches of rainfall per year, but because of the hills and the large impervious area, stormwater flows at a relatively high velocity. The stormwater on this site contains typical pollutants, he says, including trash, oil, nitrogen, phosphorus, and salts. To satisfy state regulations, the system has to treat and detain stormwater from a 100-year storm. Queenan did a complete site plan and chose StormTrap from StormTrap LLC in Morris, IL.

“The developer didn’t want a large detention pond,” he says. “It would have been right in the middle of the property. This is a very dense site. Safety becomes a consideration, and detention ponds take up a lot of land. StormTrap gave us the largest volume for the type of structure we needed. We needed a concrete structure because it was going to be under the new parking lot and that provided for the easiest design configurations.”

Queenan designed the stormwater system in 2006. It includes dry swales and a small bioretention area. Both are covered with a reinforced turf matting that prevents erosion before grass grows through the mats.

The StormTrap system was installed in early 2009 and collects most of the stormwater from the 12-acre site. Runoff will flow across the parking lot, which isn’t built yet, to a central catch basin at the lowest point of the site, and from there to the StormTrap system. It will be treated and will infiltrate through sand filters under the system. The overflow will discharge to the existing municipal system and from there to the Monhagen Brook.

As the contractor, Decker and Decker Construction, excavated the approximately 130-foot by 150-foot area, groundwater began to fill the pit. Crews dug a trench around the excavation to prevent more water from seeping in and pumped out the pit. The final depth varied between 6 and 10 feet because the parking lot will be sloped so stormwater drains to the StormTrap system.

The groundwater level posed a second challenge, this time for Queenan.

“The water table is high,” he says. “We weren’t able to let water infiltrate into the ground, and we didn’t want it sitting there. It was challenging at first to figure out how we were going to do this.” Other designers have used shallow underground sand filters, but they usually end up freezing in the winter, he says. He designed this system with sand filtration inside.

This system consists of 176 separate units varying between 4 and 5 feet in height. It holds 62,685 cubic feet or 468,916 gallons of stormwater. One set of StormTrap units is built on poured reinforced concrete pads and the other on poured concrete strip footings with 12 inches of clean sand in the space between them. All stormwater that comes into the system flows through inlet pipes into isolation rows built on the concrete pads. As it slows down and settles, trash accumulates.

From there, a small wall with 6-inch-diameter orifices allows the stormwater to flow into the sand filtration units, which filter the smaller pollutants. After the excavation was done in this area, underdrains that discharge into the city system were installed and covered with 6 inches of 3/4-inch stone, which was covered with Mirafi 500X filter fabric, by TenCate.

Stormwater that doesn’t infiltrate in this unit flows through orifices and a weir wall into the stormwater quantity section, which also has a poured concrete pad, and from there through an outlet structure and outlet pipe and into the city system.

A manhole in the isolation room leads to steps to the concrete floor, where trash can be swept with a broom and removed with a vacuum. Depending on the amount of sediment they receive, these units should be cleaned out every three to five years. A manhole gives access to each sand filtration row, and there’s a cleanout for every underdrain, which should be flushed out about once a year.

“The project went very well,” says Queenan. “StormTrap is very easy to install. We give the company the footprint and the size of the unit, and they break it down into blocks. The blocks come numbered. I believe the setting down was done in about a two to three day period.”

Livingston County, MI
For years, every time it rained, stormwater from the properties on a private road uphill from Joe Miskovich’s property drained into a ditch that led to a culvert and then down a steep ravine, where the flow gained in velocity. When it reached the driveways of Miskovich and his neighbor, it hit like a fire hose. Ken Recker, chief deputy drain commissioner for Livingston County, MI, where the homes are located, was involved in the negotiations between the landowners for the stormwater easements and also reviewed the design engineer’s submittal for the project. “This is 90 vertical feet of fall in a 500-foot stretch,” says Recker. “When the grade is 20%, water is going to flow. It’s just a question of how much soil will go with it.”

The problem took Miskovich completely by surprise. “I bought the house in December 2002,” he says. “In the spring and summer of 2003, it rained for two days. It looked like someone was pouring chocolate milk down my driveway.” The problem had begun when the wrong diameter culvert was installed with the wrong pitch, he says. About 35 to 55 feet of the hillside had eroded, and he was concerned about pollutants, like fertilizer and pesticides, being attached to the sediment.

He began researching stormwater solutions and found a chamber product he thought would work, but the county had concerns about maintenance issues, he says. He used his experience designing products for the auto industry to come up with a chamber system that has a main header row that captures most of the sediment carried by the water. Along with river rock and concrete pipes, it was installed as part of the stormwater solution.

“That was the reason I started Triton,” says Miskovich, who is president of the company.

The two properties are in a residential area where further development uphill is unlikely. Annual rainfall is about 30 inches a year and can be heavy at times. The area consists of very sandy and loamy soils, Miskovich says. “It’s perfect for being able to capture water and hold it for a period to let it infiltrate back into the ground.”

The project was done in November 2009. Water from the culvert now flows underground, through a bed of river rock and into concrete catch basins that lead to concrete pipes. Most of the sediment is trapped by sediment sumps in the concrete structures, which have access ports for cleanout. The pipes lead to the Triton chamber system, which in this case is essentially one long row of header rows. The chambers cover an area of approximately 15 by 45 feet and extend across both properties. Each of the 10 chambers holds 43 cubic feet of stormwater. Excess runoff discharges across the street and into the lake as it did previously.

“This is a hybrid system,” says Miskovich. “Header rows capture the sediment. We drilled holes in the floor and in the geotextile so we could get the benefits of a main header row and the benefits of distribution chambers without having to branch off.”

He notes that the Triton system has a lot of design flexibility, which allows contractors to make decisions in the field that improve the system.

Another advantage is that it’s a green product, says Recker. The chambers are made of soy oil, which also means they aren’t subject to the fluctuations in the price of oil.

The installation went very quickly, Miskovich says. The contractor, Bob Myers Excavating of Brighton, excavated 5 feet deep and backfilled with angular crushed aggregate between 3/4 inch and 2 inches. Once crews drilled the holes in the sediment floors to allow stormwater to infiltrate, they lay them on the aggregate.

They finished backfilling around and above the chambers to approximately 18 inches over the top of the system. Then they made another change. In the design, the overflow area was going to be covered in river rock, but it was in an area where snowplows would be plowing in the winter. Instead, they covered the 6-foot- by 16-foot-long area with blacktop and the remaining area with river rock.

The system has to be monitored to make sure it’s working the way the engineer designed it, Recker adds. The header farthest from the concrete structure upstream contains a sediment sump for sediment collection and a large access port, which allows for easy inspection and maintenance. It’s the only row that needs to be cleaned out. Any sediment or debris that does collect is easily cleaned out with a standard vacuum truck. It should be inspected about once a year, Miskovich says.

“It was a pretty big endeavor,” he adds. “I was hoping Triton would solve the whole problem, but because of the topography, it wasn’t possible. I greatly appreciated the efforts of my neighbor and the municipality to work together to get this problem resolved.”
Bahamas Airport ExpansionWhen Lynden Pindling International Airport in the Bahamas embarked on a $400 million airport terminal expansion and airside and landside improvements, Stantec was responsible for designing the airport’s new stormwater management systems.

The site presented a variety of challenges, including a high level of groundwater and an extremely flat terrain. There was little opportunity for infiltration or for conveying stormwater into piped systems. The engineers found their solution in Cultec’s underground stormwater systems. The Recharger 180 HD chamber provided the largest storage volume and was the best fit in the given depth restriction scenario.

“Cultec allowed us to select from a range of chamber sizes to accommodate the site topography,” says Patrick Clark, P.E., of Stantec. “The systems also provided enough storage capacity to handle at least six inches of runoff over the entire drainage area, as required by the Bahamas stormwater regulations.”

For phase one of construction, two Cultec systems provide 42,068 cubic feet of storage. Located in the landside areas, the systems retain stormwater until dewatering is completed via a dozen new deep drainage wells.

The systems function independently to distribute and balance the stormwater runoff. But they are also interconnected in the event that any one system reaches capacity or overflows through the outlet control structure. Additionally, the systems are connected to several open swales and retention ponds distributed throughout the site to allow for excess storage capacity. The open detention ponds are drained with drainage deep wells for rapid dewatering and provide for infiltration and evaporation.

The underground stormwater systems contributed to solving yet another project challenge. The hard durable rock needed for the installation had to be imported from Jamaica. According to Cultec vice president Fred Dotson, the company’s systems use less stone than other comparable systems on the market, lowering the overall installation costs.

Both systems were installed using Cultec ‘s unique internal manifold, which allows manifolding to take place at any point within the stormwater system, thereby condensing the system while decreasing the required footprint. It allows for maximum design flexibility and eliminates the need for costly fabricated pipe manifolds.

Specific to the project, Cultec’s systems work together with other best management practices to capture and store large amounts of runoff onsite. According to Clark, the underground systems also prevented the loss of valuable square footage that would have otherwise been needed for open storage areas.

About the Author

Janet Aird

Janet Aird is a writer specializing in agricultural and landscaping topics.

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