Improving Stormwater Products
IN FAR TOO MANY AMERICAN CITIES, it’s a common sight: the quickly formed, temporary lakes and rivers that pop up after a good-sized downpour. These “waterways” run alongside urban streets, sometimes engulfing entire lanes, and surprising motorists who go splashing through them. They’re formed by overwhelmed sewer systems, their grates and inlets clogged with leaves and trash.
As annoying as this may be, that’s not the biggest problem with these spontaneously-occurring streams. It’s that the water collected in them has washed over the asphalt streets and is full of petrochemical and heavy metal deposits.
This water has cascaded over the tops of buildings and the concrete sidewalks below, and carries with it bird and dog droppings. It’s trickled off of people’s lawns, and is rife with residues of fertilizers, herbicides and pesticides.
No, the problem with stormwater isn’t the temporary inconvenience it causes motorists. It’s that, eventually, all of the gunk that’s in it finds its way into our wetlands, streams, rivers, lakes and oceans, causing untold harm to fish, wildlife, and ultimately, us.
Stormwater used to leach into the soil slowly and naturally. But as urban development replaced the Earth’s crust with hard, impervious surfaces, runoff was born.
Stormwater became a nuisance, much like a Victorian-era child, to be “seen and not heard.” So we dug channels and installed conduits to pipe it out of our way, as fast as possible, with little thought as to where all that water was going and what it was carrying with it.
From Puget Sound to Chesapeake Bay, it’s been obvious for some time that we need to handle stormwater runoff in a different way. That “different way” is called Low Impact Development (LID). The U.S. Environmental Protection Agency’s website defines LID as “an approach to land development (or re-development) that works with nature to manage stormwater as close to its source as possible.”
Employing the principles of LID doesn’t mean that we have to tear up all of our roads and buildings and start living in teepees. The idea is simple; instead of sending stormwater away, we keep it in the neighborhood, filter its impurities and let it replenish the watershed. It’s how the natural landscape used to function before we paved over it. It means regarding stormwater as a resource—a source of irrigation— rather than as a waste product to be shipped off and forgotten. LID principles can be applied to both new and existing development, from low density to ultra-high-density urban settings.
So how do we put the LID on stormwater? “We need to first start by asking questions,” says Vaikko Allen, director of stormwater regulatory management for Contech Engineered Solutions in West Chester, Ohio. “We need to ask, ‘Can we filter the water to the point where it’s hazardless to our groundwater and soil, and keep the precious resource around?’” Fortunately, the answer to that question is a resounding “Yes!” It can be accomplished by following current stormwater best management practices (BMPs). You’ve probably employed some of them already.
They include bioretention cells; curb and gutter elimination; grassed swales; green parking design; infiltration trenches; inlet protection devices; permeable pavement; rain barrels and cisterns; riparian buffers; sand and organic filters; soil amendments; stormwater planters; tree box filters; vegetated filter strips and vegetated roofs. We don’t have room to discuss all of these techniques in detail, but we’ll give four of them a closer examination.
One way to handle the problem of impervious surfaces is to replace them with permeable ones. Permeable pavements allow water to leach through to the ground beneath rather than running off into the gutter. There are several different types of permeable pavement products available: pervious concrete and asphalt, permeable interlocking concrete pavers, and plastic-reinforced grass pavement. All of them have gravel storage layers underneath, which also function as structural supports. The deeper the gravel layer, the more water that can be stored.
Peter “Paver Pete” Baloglou, director of education and information at Techo-Bloc, a maker of permeable masonry products headquartered in Pen Argyl, Pennsylvania, explains how these products work. “All water that enters the pavement flows through the joints in the paver blocks to the bedding layer, then to the base and sub-base layer, where it’s eventually piped out into the existing stormwater system. The water that’s piped out is now filtered water.”
When water passes through permeable pavement, many pollutants are trapped inside it, or removed as the water passes out of it into the surrounding soil. Studies have shown that when permeable pavements are used, runoff volume is reduced by as much as 60 percent. And the water that does run off shows much lower concentrations of heavy metals such as zinc and copper.
“Permeable pavements are able to absorb and take in that first initial flush of water from a rain event,” says Baloglou. “That’s what carries the most petroleum dissolute, nitrates, phosphates and heavy metals. The pavers stop them from washing into our local streams and waterways.”
Permeable pavements’ main drawback is their higher initial cost of installation versus traditional asphalt and concrete surfaces. However, the overall system cost may actually be less, as peak runoff rate, and the need for other BMPs such as pipes, is reduced.
Rain gardens are another natural means of stormwater capture.
“The simplest explanation is that a rain garden is a bowl-shaped crater filled with amended soils and plants that like getting their feet wet,” says Aaron Clark, PhD, project manager for 12,000 Rain Gardens in Puget Sound, a program of the Seattle, Washington-based environmental organization Stewardship Partners.
A rain garden is fairly easy to construct, according to Clark. In general, you dig a hole two feet down, and then fill it with 18” of amended soils. The best type is a blend of sand and compost. You can use the onsite soil if it’s sandy enough, and just amend it with compost. What you’re aiming for is a very absorbent, forest-like spongy soil that can slow down the rain, giving it a chance to soak in and slowly recharge the groundwater, filtering it at the same time.
The hole needn’t be as big as the Grand Canyon. “One of my favorite rain gardens was built by a contractor here in Seattle,” said Clark.
“It’s the smallest one I’ve ever seen, but also one of the most effective ones. It’s only two feet in diameter, connected to a series of cisterns. It captures about 2,000 gallons of roof runoff. Once they’re full, they overflow into drip tubing out to the irrigation system.”
In general, the dimensions of a rain garden are about one-tenth of the contributing area’s size. For instance, if you’re using the rain garden to handle roof runoff, you’ll need 100 square feet of rain garden to handle 1,000 square feet of roof. The bigger the site, the more rain garden(s) you’ll need.
But doesn’t all that toxic material, the heavy metals and chemicals, eventually seep into the water table? Clark says no. “The heavy metals, particularly hydrocarbons, as well as phosphates, are reduced 88 to 90 percent by just 18 inches of soil.” If it’s necessary to collect water in less than 18 inches of soil, an underdrain is used. This is also used if the soils underneath the rain garden are really compacted, or a type of soil that just doesn’t absorb water well.
“As you get down into the soil, at the two- or three-foot level, you get into an anaerobic, oxygen-deprived environment,” continues Clark. “There, you get a whole new suite of bacteria that takes care of those heavy-metal components. In a typical rain garden with no underdrain in it, before you hit the water table, everything’s been broken down.”
The pollutants themselves act as plant food. “The nitrates and phosphates are nutritive to the plants,” adds Clark, “and the metals are critical components to some biological plant functions. The plants convert them into enzymes and things that are no longer harmful; they become natural components of the plants.”
What about toxins eventually building up in the soil? “There are actually quite a few studies out that looked at that,” says Clark. “In most cases, it would take 70 to 100 years to reach levels of toxins in the soil that the USDA would say you can’t grow food in. In essence, there’s really no accumulation.”
Vegetated (green) roofs
Green or vegetated rooftops are nothing new; they’ve been used in Europe since the 1970s, but are just catching on in the U.S. A green roof consists of waterproofing and drainage mats, special growth media, and plants that are able to withstand extreme climates. Green roofs have been shown to reduce not just runoff, but overall temperatures of roofs and buildings, thus lowering energy usage and cost. They’ve also been shown to diminish the “urban heat island” effect caused by densely packed urban development. They even make roofs last years longer.
There are two kinds of green roofs, extensive and intensive. Extensive green roofs are like vegetated carpets. They’re covered in engineered soil from three to five inches thick, composed of natural soil and expanded (superheated) clay, shale or slate, with low-lying vegetation growing across their surfaces. Intensive green roofs are less common, because they’re more expensive to construct than extensive ones. They’re garden-like, with a much deeper soil layer for growing trees and shrubs. They also require much more mainte nance, such as irrigation and fertilization. Both types are capable of capturing 65 percent of yearly rainfall.
Charles Sinkler, owner of Apex Green Roofs in Somerville, Massachusetts, believes that installing green roofs can have a strong impact in reducing stormwater pollution. “You’re catching water at one of the major runoff sources, a building. You’re able to catch, hold and filter the majority of your rainfall onsite. The water is filtered through the plants and soil layers, with a slow discharge, so we’re not overloading our stormwater systems.”
Here’s how they work. As rain falls, the soil media captures some of the water and later releases it back to the atmosphere through evapotranspiration. Plastic cups under the media catch additional rain for later uptake by the vegetation. Once this media is saturated, rainwater flows down to the roof’s surface via a drainage network that takes it through gutters to the ground. In addition to reducing the total amount of runoff, the media and plants filter and capture pollution particulates deposited from the air as well as the water.
“The roof structure must be structurally sound to be able to hold at least 15 pounds per square foot of additional weight,” says Sinkler. “You must make sure an architect signs off that the roof can hold that weight. You also want to make sure the roof is flood-tested and electronically leak-tested.”
We’ve all captured rainwater, intentionally or not. After a storm, any container without a drain that’s been left outdoors right-sideup is full of water. That’s all rainwater harvesting is, plus a system for getting the water back out for use.
Rain barrels and cisterns are relatively low-cost, low-tech devices that can reduce both the peak volume and rate of runoff. They are a good source of chemically untreated “soft water” for landscape, free of most sediment and dissolved salts. Rain barrels are usually placed on roofs or outside of buildings at roof downspouts. Cisterns are essentially bigger rain barrels that can store water in much larger volumes, in manufactured tanks, often buried underground. This non-potable water can be used for toilet flushing, irrigation, or both.
A multifaceted solution
The challenge facing Puget Sound illustrates the overall problem with stormwater. “The biggest single threat to Puget Sound is stormwater,” says Clark. “It accounts for approximately 90 percent of the pollution in the water.
Researchers at Washington State University have been looking at the runoff into the Sound, trying to figure out what’s causing the biggest problem.”
“Is it the brake-pad dust? The motor oil? The antifreeze? The dog waste?” asks Clark. “When they pull those things out individually, they find that, yes, each of those things has an effect. But that doesn’t explain their combined impact. There’s a synergistic effect to the ‘stormwater cocktail’ as well.”
The challenge of improving stormwater management is multifaceted. And a multifaceted problem requires multiple solutions.
As Clark puts it, there’s no single “silver bullet” solution that will fix it once and for all. That’s why all of the BMPs—inlet protection, bioswales, permeable pavers, tree box filters, reducing curbs and gutters or any of the others, alone or in combination, have to be considered, depending upon the situation.
“We need to undo the highimpact stormwater systems we have and replace them with lowimpact ones,” says Clark. “And, instead of a gray infrastructure, move towards a green infrastructure.”
To make that happen, we who build infrastructures need to be aware of all the options open to us. Our stormwater systems will only improve if we change the way we think about them.