Temporary and Permanent Stormwater Filtration

Temporary and permanent stormwater filtration

Credit: ISTOCK/MICHAELPENHALLOW

“In California, in the Bay area, treatment is everything,” says Kelly Rogers, general superintendent of offsite development for Robson Homes. “We’re a company building single-family homes within the city limits of very large cities in the Bay area.” The firm’s projects are often referred to as infill development. In many instances, Robson Homes projects begin with demolition of older structures for replacement with modern home sites; however, on occasion the company breaks new ground, converting sites from agricultural uses or from an undeveloped condition to a residential neighborhood.

Rogers says stormwater capture and water-quality management are central focal points of any project. “It is mandated by the state, by the cities, and by the Water Quality Board of California. There is no option.” He adds, however, “There are different mathematical equations on how you can do it.”

Protecting the Bay
Considering the expense and effort involved in adding water-quality management to each of his construction projects, Rogers says, “You’d think that, being a builder, I’d be against it, but I am a tree hugger at heart. I’ve been doing this for almost 40 years, and I can tell you that back in the ’70s and ’80s we were hooking up downspouts and pipes, area drains in all the yards—the water running off the roofs and from the curbs and gutters into these area drains and into the city storm system would reach the tributaries, creeks, and rivers so fast you could time it with an egg timer. You’d see a little babbling creek that was just trickling along, and we’d get a half-inch of rain in the area and that creek was in a torrential flood. It had risen four feet, scouring the edges of the creek, creating all this silt runoff flowing into the greater San Francisco Bay. That was a very big problem,” he acknowledges.

“Now we are mandated to hold our storm runoff. What it’s doing is it’s stopping the erosion of the creeks and tributaries that flow into the San Francisco Bay.”

To perform this function of slowing down the water during the early years of the mandate, Rogers says, he experimented with different systems, including one he describes as plastic milk crates assembled into massive arrays of detention cells. “You could stack them and rack them. You could get them four feet tall, five or six feet tall. You could put this maze of trays underground and put a bladder around it and backfill it.”

Rogers notes that when the regulators heard about the system, “they loved it, at first.” However, he says, “It was pricey and labor intensive to install.” When its long-term maintenance needs became apparent, regulators began to reassess the technology. “The negative was that you couldn’t go in and totally clean them out. The cavities were so small you couldn’t get to all of them,” he notes.

“The sweet deal of the StormCapture unit is they’re basically concrete vaults connected together, and you can use a ladder to get down into them and vacuum them every couple of years to clean out all the silt they collect.” Roger notes that he has been using the StormCapture System by Oldcastle exclusively for several years.

“These tanks are pretty big, and at the bottom they have a three-quarter-inch hole, so you can imagine how slowly it lets the water out.”

He says, “The project I am doing right now requires over 30,000 cubic feet of water storage.” The modules come in tops and bottoms with end panels so they can be stacked, making it possible to achieve vast storage capacity. Fabricated from precast concrete, they are durable and resilient. The walls are a standard dimension, but extra reinforcing steel can be added. Although, as Rogers says, “You can’t build houses on top of them,” they can be specified with a rating for use underneath a road and can be used under open spaces such as parks.

He notes that good planning is the key to successfully implementing a StormCapture System. The modules, which are manufactured in two sizes, can be arranged to accommodate almost any desired footprint, corresponding to the subsurface area available onsite. Another key question, he says, is “How deep is it going to be installed?” The StormCapture System he installed on the Hobbs project in Freemont, CA, “has 22 feet of dirt on top of it,” he says. “That was done because it was on a very steep slope. There was probably a month of negotiations between the structural engineer working for Oldcastle and my soils engineers.”

Rogers likes to begin decision-making on the design and implementation of drainage systems very early in a project. He notes that there are many steps to go through before installation of a customized stormwater detention facility. It is not unusual, he says, for the process to take a few months with behind-the-scenes work ranging from initial design through several stages of engineering and regulatory approvals to fabrication and delivery of the components. With ample lead time, however, he says, “When I am mass-grading a project, I can put the vaults in during the grading operation.”

Credit; BIO CLEAN
Kraken filter unit

The Hobbs project, consisting of 64 new homes on lots ranging from 3,000 to 4,000 square feet each, is being built on an 8.5-acre hillside parcel that was most recently used as farmland. Rogers explains that California stormwater regulations require, at minimum, that developers mimic the existing hydrology of a site. He notes that the task is a bit less challenging on urban sites that have large existing impermeable areas or large structures slated for demolition and replacement. “If you take a property that was a commercial site, which had buildings on it and parking lots, you get credit for that. This property was all grassy farmland. When it rained really hard for weeks at a time, not much water left that site; it all soaked into the ground.”

Comparing stormwater management requirements of a recent project he completed—converting a medical office complex to a residential use—Rogers says, “I hardly had to install any of the StormCapture Systems, because it always had water runoff. All I had to do then was treat the water.”

In contrast, he says, “We needed so much storage for the Hobbs project that we designed it so that there are two holding facilities, one positioned to the left and one positioned to the right of the entryway to the complex.” The area above the detention system was envisioned as open space. “As we have evolved, we try to place them off of the right of way, below bioswale areas, so we don’t have to spend the money to have the H-20 rating for having them under the street. We try to get them off into landscape areas.” With this setup, he says, the areas dedicated to stormwater management can also serve as a park-like amenity when the project is complete. As an added benefit, this placement outside the right-of-way avoids having stormwater storage compete for space with other essential infrastructure elements, such as sewer lines, water mains, and stormwater conveyance that share the right-of-way.

Rogers says a pipe connects the two storage areas, “so that when one is filling up, the one across the street is filling at the same rate.”

He says, “We put a manhole outside the lot. The manhole has a steel plate that serves as a weir.” As he describes, water drains through a 3/4-inch hole in the bottom of the chamber. “During heavier rain events, a foot above that is a three-inch hole that drains water into the subsurface, and the weir itself is only four feet tall. If we have a major 100-year event, the water will just flow over the steel plate and go out to the site, so that the system never overflows.”

The Hobbs installation took only two days. With 6 inches of clean crushed rock on the bottom of the excavation, workers hoisted the precast modules into position by crane. Because the units are large and made of rough concrete, he says, a good fit at the joints could leave about 3/4 of an inch of wiggle room where technicians then taped off the seams. A filter fabric is wrapped around the box, and the installation is backfilled with base rock. Rogers says the StormCapture System can be installed with the seams loosely fitted to allow water to leak into the grade below. “That’s the ideal situation; that’s what we did in this case. If you’re in an area where you can’t allow that, then we install a reinforced polyethylene liner underneath so that it is watertight.”

Balancing Volume, Flows, and Filtration
Nashville, TN, is a river city. It’s both scenic and historic, with waterfront parks. Notably, the entire original downtown was settled along the banks of the Cumberland River. That river is still integral to daily life of Nashville. It’s the source of drinking water and a focal point for fishing, boating, and recreation, says Michael Kusch, regional sales manager for Bio Clean. Looking back over the past decade, Kusch says, “The area is heavily built up, with very little pervious area remaining, which has contributed to major flooding downtown.”

According to Kusch, the target pollutant affecting the area is “sediment of a fine particle size.”

“With a small rainfall event, there is no soil downtown to soak that up. It all gets funneled down the streets, down the storm drains, and it creates a lot of velocity. You don’t want that coming out at the last outfall, creating a lot of erosion and turbulence on the Cumberland River,” he says.

Under stormwater regulations in the Nashville area, new projects are required to capture and hold the first inch of rainfall and hold it onsite for treatment, slowing its velocity, allowing sediment to settle out, and reducing the tendency to scour riverbanks at outfalls. Capture and storage must be designed to handle the average rain event, which is locally designated as 2.54 inches of rainfall per hour.

Engineers in the region often design for underground detention to hold that water and “throttle” it out slowly into smaller water-quality filter units and to let these filtration units drain down slowly over 72 hours. That allows the heavier particles to filter out by gravity, and the finer particles, oils, and heavy metals can be picked up through the filter units, which are sized much smaller than the detention basins. The original proposed drainage and filtration system for Burkitt Commons, a large mixed-use commercial infill site, followed a similar philosophy. The site plan was broken down into three different drainage sub-watershed basins, each draining to separate storage and filtration systems.

That initial drainage and filtration system design consisted of two underground detention systems of corrugated metal pipe (CMP). System A was designed as 655 linear feet of 42-inch-diameter CMP with an 8×8 Kraken filter unit. Basin C was also designed for underground detention using 270 linear feet of 60-inch CMP and an 8×8 Kraken filter. Basin B was a small-footprint site with no room for underground storage.

Kusch describes systems A and C as “volume-based design,” drainage, and filtration. However, there were a number of considerations that argued against using a volume-based methodology on this project. “Nashville sits on solid limestone,” explains Kusch, “so it’s expensive to excavate and expensive to dig.”

Credit: BIO CLEAN
Kraken filter unit

In addition to mitigating the expense of excavation, there was another good reason to consider a different approach, according to Sean DeCoster, a principal at Civil Site Design Group. DeCoster explains that the site is close to the bottom of the Mill Creek watershed, which encompasses 108 square miles.

Storm flow calculations showed that if implemented at the Burkitt Commons site, a volume-based detention design might increase rather than reduce the potential for high-volume discharges into the Mill Creek outfalls during a heavy storm. As DeCoster explains, the 29-mile Mill Creek would be reaching peak volume in the vicinity of Burkitt Commons at exactly the same time a traditional onsite volume-based design detention basin would begin releasing stored stormwater. Rather than reducing peak flow at the outfall to the Cumberland River, water detained for timed release would augment flows by discharging into the stream as the flow from upstream neared its high-water mark.

Working with DeCoster, “We took the original detention design and changed it to a peak flow design,” explains Kusch.

According to Kusch, a system designed based on a peak flow rationale would discharge stormwater from the site continuously during a rain event, allowing peak flows from the site to pass to the outfall long before the arrival of the Mill Creek crest in the vicinity of Birkitt Commons.

Although this design would necessitate upsizing the filtration systems of Basin A and Basin C to handle larger volumes of water more quickly, it totally eliminated the need for underground detention, “which saved a ton of money,” says Kusch.

Describing the filtration system, DeCoster says, “It’s a cartridge-based system. We have 188 cartridges in one unit, 130 cartridges in another, and 140 in a third. The water flows in and fills up the area where the cartridges are. The cartridges serve to clean out any solids or pollutants as per the local requirements. An underdrain manifold allows the water to drain down under the cartridges and head on downstream, and off the site.” He continues, “The cartridge-based water-quality system is fairly common in the area if you need to reach the 80% total suspended solid requirement.”

Kusch points out that, in the Nashville metro area, “They are trying to capture a really fine sediment particle size—down to 75 microns. Normal hydrodynamic separators do not meet that requirement, so they use a more sophisticated filter system.”

He says there can be advantages beyond monetary savings to using the peak flow method rather than capture and detention for stormwater management. “Anytime you have underground storage you have to maintain it—you have to put a man inside those underground chambers, which is always a scary thing to do. You have that OSHA mandate about confined space entry that always has to be dealt with.

“The beautiful part of the Bio Clean Kraken unit, and why they were chosen over other technologies, is because the maintenance is so simple. Because they are membrane filters and not media filters, they can simply be pulled out, rinsed off with a garden hose or a low-pressure water source, and then they can be put back in. They last eight to ten years depending on how dirty the site is. They don’t have to be replaced like media does every few years, so from a maintenance standpoint they are the cheapest technology out on the market today.” Describing the Kraken technology as comparable to an automobile oil filter, he says, “It’s just a series of folded layers. If you were to cut open a filter and spread out the membrane, a section of 30 inches by 10 inches would yield about 170 square feet of material. It’s got a tremendous amount of surface area to catch all these particles and heavy metals.”

Kusch says engineers should always consider that there are two ways to design water-quality filter units: “volume-based flow and peak flow.” He explains, “Sometimes you can eliminate your underground detention by simply upping the size of your filter units, and sometimes that’s the cheapest way to do it.”

Now Screening in Los Angeles
Marsa Chan, civil engineering associate with the Department of Public Works and Los Angeles Sanitation—Watershed Protection Program, has been involved in a multi-year effort in the city to reach a goal of eliminating trash discharged into the city’s water bodies such as the Los Angeles River, Ballona Creek, Santa Monica Bay, Dominguez Channel, and Echo and Machado Lakes. Installing trash-blocking catch basin inserts is a major component of the agency’s plan.

Chan says that to comply with the trash total maximum daily load (TMDL), which designates zero trash as its target goal, the project, now in phase 4 in the city, is focused on identifying any gaps where storm drain inlets (e.g., catch basins) have not yet been outfitted with trash-blocking catch basin devices. The current project phase is budgeted for retrofitting 2,600 catch basins over three years. However, Chan says roughly 10% of these catch basins, because of the small size or configuration of their openings, will not accept the city’s regular stainless steel screen covers. Chan is targeting these sites for installation of new screen covers from Hydra TMDL.

With a variety of different catch basin designs in service throughout the city, Chan’s division sought the flexibility in addressing unusual situations with one product. Hydra TMDL provides that flexibility. It can fit storm inlets with curb openings as narrow as 5 inches, “making it a good application in those special cases.” Chan says Hydra TMDL can also be installed in many of the catch basins where, because of space constraints, technicians have found it impractical to install mounting units on the inside of the structure.

“They work like regular curb opening screens,” explains Chan. The design is reminiscent of “piano keys,” she says, with a steel wire providing the tension and keeping them in a closed position but flexible enough to allow storm flow to enter the catch basin and then return to a closed position. That keeps the trash at the curb, outside the catch basin, where the city’s street sweepers come by and collect the debris.

Installation is very straightforward, Chan says. Preparation requires obtaining accurate measurements of the curb inlet height and width. “When the crew has that data, they send that information to Hydra TMDL, where they cut and fabricate the units to specifications.”

According to Chan, hillside locations represent one of the prime opportunities for using the Hydra TMDL. These areas had posed a challenge to using the typical stainless steel curb opening screens because of the locking mechanism employed, limiting inlet capacity. The Hydra TMDL, which relies on tension, allows it to maintain inlet capacity but also keeps trash out of the catch basin. EC_bug_web

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