Sept. 17, 2015 10:20

Choosing Stormwater BMPs

“Space…the final frontier.” Those words, of course, are the familiar opening line of every classic “Star Trek” episode. There, Captain Kirk is referring to the infinite vastness of the galaxy, which was his to roam and explore. In the stormwater game, however, you can’t always count on having big, roomy areas in which to work, especially in highly developed urban settings. Often, you’ll have to install or retrofit devices in very tight spaces.

“Space is the predominant issue in an urban area,” said Doug Hecox, a spokesman for the Federal Highway Administration. “In a rural area, where highways generally are, you’ve got plenty of space.

You won’t have that in an urban setting.”

When Margie Sobczynski, CPESC, owner and founder of Olathe, Kansas-based Erosion Control, Inc., takes on an urban project, space is one of her main considerations. “We look at how much room we’ll have to work with. We also ask, ‘What’s the force of water coming into or out of that small area? What will we need to do to keep it under control or contain it?’ From there, we make a determination as to the best BMPs to put in.”

She often runs into situations where there are tight spaces along curb lines, or her crews have to work along the backs of slopes, as is often the case with urban residential developments. In that scenario, it can be a “nightmare” for big machinery, such as skid steers, to deliver rocks for check dams. In those cases, coir logs have been Sobczynski’s go-to for temporary stormwater control. They’re inexpensive and can be hand-carried into areas where there just isn’t any room for machinery.

“Where you predominantly have city streets, you have a lot of traffic, but not a lot of rights-of-way to the sides of roads,” Hecox added. “There’s no place for the stormwater to go, except into a storm drain, or perhaps to another area where ponding may occur.”

He added that in certain places, such as rural South Carolina, there are retention ponds on the sides of highways. “When there’s runoff, it’ll accumulate in those designated areas where it can be used in some productive way, and not cause mud and erosion,” said Hecox. But you won’t find that sort of thing in the cities.

The Federal Highway Administration defines an ‘ultra-urban environment’ as one where the space available to install a device is less than 0.5 hectares per acre; where the imperviousness of the drainage-area is greater than 50 percent; where the land value is more than $20 per square foot; where the only available space in which to install a BMP is in a rightof-way; and where there’s dense build-out, i.e., lot-line-to-lot-line development.

The urban environment presents other difficulties, too. A rat’s nest of buried utilities, including electrical cables, gas, water and sewer lines, all may limit your BMP choices. Groundwater flow and the hydraulic gradient, with respect to nearby buildings and basements, must also be considered, as well as any underground structures such as steam tunnels.

The EPA’s list of stormwater best management practices (BMPs) is quite extensive. It includes bioretention cells, curb and gutter elimination, stormwater planters, grassed swales, vegetated roofs, green parking design, infiltration trenches, soil amendments, inlet protection devices, permeable pavement, rain barrels and cisterns, riparian buffers, sand and organic filters, tree-box filters and vegetated filter strips.

How many of those BMPs on the EPA’s list are feasible in urban or “ultra-urban” settings? Fortunately, the answer is ‘most of them,’ although making them work may call for some out-of-the-tight-box thinking.

For example, commercial sites frequently have flat roofs; why not take advantage of that?

A vegetated roof is an excellent, and very green, urban stormwater BMP with many other side benefits, such as lower cooling costs for the building. Rerouting gutters is another good, green option; roof gutters are often connected directly to storm sewers. But they can be redirected to drain into rain-storage barrels or underground cisterns, or to irrigate a vegetated roof or even the building’s surrounding landscape.

Large commercial buildings may also have internal drainage systems. These can also be rerouted to cisterns, planters, or vertical storage facilities. Any overflow can be directed to a rain garden or other retention area instead of straight into a sewer system.

Can’t go up? Try going down. Subsurface recharge/storage beds can be placed under sidewalks or parking lots. Permeable pavement can help with this. There may be some room in an underground parking garage for cisterns.

When a new parking lot is being built, that’s an ideal opportunity to install porous pavement that will drain to infiltration beds, rain gardens and under- or above-ground storage. It’s important to pay special attention to the condition of the underlying soil, however. In the urban environment, it’s probably been compacted for many years. There may also be contaminants in the soil.

Retrofitting existing parking lots, adding permeable pavement and/or infiltration beds to them, is of course more complicated, specifically when it comes to connecting to existing inlets. But you don’t have to dig up the entire lot. Small strips of porous pavement, draining to infiltration beds or other catchment areas, can be installed in the downhill area of a lot. Median strips between rows can be converted to small rain gardens.

Tree-box filters and tree trenches

A tree-box filter is a mini bio-retention area, a kind of mini rain garden. When several of these are distributed throughout a site, they can be very effective at controlling runoff, and they don’t take up much space.

A tree-box filter consists of the ‘box,’ an above-ground container, filled with a soil mixture and covered by a layer of mulch. An underdrain should be installed beneath.

Any typical landscape plant can be used, including ornamental shrubs, grasses and flowers.

Treated water flows out of the planter through the underdrain, to a storage tank or retention area, to an overflow catch-basin, into the surrounding soil, or to a stormdrain inlet.

The tree trench is a variation on this theme. Tree trenches are usually planted between roads or parking lots and sidewalks. Trees are planted in series, with their roots connected by an underground, stone or concrete-lined infiltration trench that runs the length of a city block or parking lot.

Runoff from impervious surfaces drains directly into the tree trench, where it’s cleaned by natural filtration. Whether the water is stored, detained, or eventually flows into the sewers, it comes out cleaner than it started.

There are multiple side benefits to tree-box filters and tree trenches. Besides the obvious aesthetic benefits, the stormwater or runoff irrigates the trees or other plant material, and the greenery contributes oxygen and reduces the urban heat-island effect.

“For the most part, the stormwater storage containment is underground and you don’t really see it; you wouldn’t even know it’s there,” said Lisa Goddard, P.E., senior associate and civil engineering consultant specializing in water resources at the SRF Consulting Group in Minneapolis, Minnesota.

She’s seen tree-box filters and tree trenches becoming more prevalent, at least in Minnesota. “Think of how lots of city streets are lined with trees,” she explains. “The tree trench expands on the tree-box concept. The entire soil strip next to the sidewalk or roadway becomes one big stormwater management device, with trees planted every 30 or 40 feet, depending on their size.”

Tree trenches have the added advantage of being hidden underground. “In a lot of areas, they use permeable pavement for the surface. The stormwater management is under the sidewalk. People can cross back and forth over it and not even know they’re walking right over a water-storage facility. It could even have a grass surface.”

Multichamber treatment trains (MCTTs)

The multi-chamber treatment train (MCTT) is a newer BMP on the scene. It consists of a series of units similar to those found in wastewater treatment plants. The first chamber aerates the stormwater as it enters the treatment train and permits preliminary settling of larger-diameter sediment.

Stormwater is then conveyed to an inclined tray settler, where the majority of the particulates are captured. Dissolved air-flotation is then provided to help lift floatables and oil up to absorbent media. The final step entails passing the stormwater through a sand or peat filter.

Lynn Ewoldt, a sales professional at St. Louis, Missouri-based ASP Enterprises, Inc., put together a simplified version of a treatment train for a portion of a sewer-separation job being done for the city of Omaha, Nebraska. Like many cities, Omaha was originally built with combined sewers, stormwater and sewage going into the same pipe. Ewoldt’s contribution was part of a 20- year project to separate them.

At a certain intersection, the contractor ran into a snag trying to lay down sanitary sewer pipes. “He called us, saying, ‘You’ve got to come to the site, the TSS (total suspended solids) levels are very high,’” Ewoldt said. “And it turned out that there were some heavy metals in the water, too.”

There was very little room on the site to do anything. Work had stalled after the crew had dug a pit about 17 feet deep. Overnight, the  pit would fill with groundwater,  and the crew would have to spend  all day pumping it out, instead of laying pipe, which requires dry conditions. To complicate matters, the crew had accidentally cracked the sewer line, so contaminants had entered the mix.

“We’d dabbled in flocculants a little bit, but this was the first time we’d done a treatment train like this,” said Ewoldt, who conceived the idea. After doing some testing, he called the flocculant manufacturer, who flew a representative out to assist.

The treatment train consisted of one large steel tank and three smaller, 275-gallon ones, lined up in succession, in a trench. Flocculant was added to all four tanks through holes in the tops. All of the water in the pit went into the first and largest tank.

The water was then pumped into the three smaller tanks in succession. The agitation from the pump stirred the flocculant up so it could absorb the toxins.

The sediments settled out, and the flocculant removed more and more of the suspended solids each time. Each tank cleansed the water it received from the previous one, with cleaner water going into each successive tank.

“By the very end of the process, we had nearly clear water, but there were still some granules of flocculant in it,” recalled Ewoldt. “So we did a final filtration through an excelsior wattle, which took out the flocculant and some of the heavier particles that hadn’t settled out. We were able to treat the water, lower the TSS count and remove the metals, and they were able to keep their sewer-separation job going.”

Before the cleaned water went back into the storm sewer, he filled an empty plastic water-bottle with it. “This water looked just like it came out of the tap; clear as a bell. One of the kids from the construction company even took a drink of it, though we told him not to.”

Though the treatment train was his idea, Ewoldt doesn’t take all of the credit, saying it was a result of the contractor, the supplier and the manufacturer working together to implement a solution. It not only worked well in the confined space, but this approach was, by far, the least expensive option for both the contractor and the city.

Stormwater chamber systems

Titan Construction, Inc., in Ann Arbor, Michigan, does a lot of stormwater work in highly developed, densely populated areas.

“Like when they want to put a fastfood restaurant in a downtown area, and there’s a bunch of stuff around them,” explained company president Joel Hargrave.

“It’s a big challenge, trying to meet stormwater management requirements, especially when you’re landlocked. And some of the products that are able to ‘go deep’ are rather expensive.”

He’s talking about large concrete stormwater vaults, which are extremely expensive and laborintensive to install. Large cranes are required to move the heavy vaults, and there’s usually no way to get the machines into these tight sites.

So, Titan turns instead to an ecofriendly modular stormwater chamber system. The manufacturer says that this system achieves high pollutant-removal rates through soil filtration and accelerated microbial actions. The chambers, made of soy resin, are very strong and can be stacked to save space. Accessories, such as sediment floors and filter systems, can be added on.

“The chamber system is able to handle the first-flush volumes and any other rain events, and keep them from flooding nearby properties,” said Hargrave. “They’re also able to manage pollutants, sediment and debris. The water is held there and percolates back through the soil column.”

“The product is easy to handle, goes in quickly, and doesn’t require heavy equipment,” he adds. “They can be installed with a labor force of two to four men, which, in confined spaces, is absolutely imperative.”

The company has, in the past, converted parking-lot median strips into grassed swales as a stormwater management technique. However, these require “a lot of maintenance,” which Hargrave says no one cares about after a project is done.

“They’ll all say, ‘I want to make sure the system is maintained,’ but at the end of the day, that takes effort, and there are lots of associated costs. If they’re not maintained, such as by regular mowing and weed removal, they lose their effectiveness.”

Another drawback of grassed swales is that they usually have to be very big to handle the volume of stormwater coming off of most commercial sites. “That’s why everyone’s choosing to go underground nowadays,” said Hargrave.

“These systems are strong enough to put a parking lot over them.”

Urban development isn’t going to stop. Fortunately, neither will the production of newer, more costeffective and space-saving BMPs,to manage the water that inevitably will fall.

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