These eco-sensitive strategies help keep pollutants out of streams and lakes, replenish groundwater and green the built environment.
NATURAL LANDSCAPES naturally slow the movement of stormwater, and capture and filter some of it as it percolates back into the groundwater supply. But the built environment is dominated by impervious surfaces. Paved surfaces, roofs and building façades change the movement of water over the landscape and increase the volume, speed and temperature of the runoff. Rushing stormwater picks up pollutants, fertilizers and pesticides and can also cause flooding and erosion.
The EPA offers a “Stormwater Calculator,” available to urban planners, architects, designers and homeowners, which helps the user identify strategies from reducing stormwater impacts from the site. Whether designing a new home or a remodel, Low Impact Development (LID) strategies can mitigate many of these negative stormwater impacts by slowing and/or capturing stormwater flows onsite.
Rain Gardens and Swales
Rain gardens are shallow, landscaped depressions, which retain and filter stormwater. These gardens ideally include a variety of native trees, shrubs and herbaceous plants. During storms, runoff ponds above the mulch and soil, then filters through the soil matrix, where plant roots take up water and filter nutrients and other pollutants.
Plants at the bottom of the depression must be able to tolerate both wet and dry conditions; upland species will thrive at the edges. Sometimes rain gardens are designed with an underdrain system—a perforated pipe encased in a gravel bed—that collects filtered runoff at the bottom of the bed and directs it to the storm drain system.
A bioswale, sometimes called a grassed channel or biofilter, is a vegetated, open channel designed to treat and weaken stormwater runoff. Vegetation slows down stormwater as it flows along such a channel, encouraging sedimentation, filtering through the subsoil, and/or infiltration into underlying soils.
Swales vary in size, but should be at least three feet deep in the center. Though some are triangular or rectangular, swales often have a trapezoidal or parabolic cross section with gently sloping sides. Designing the channel with flat side slopes increases the wetted perimeter. This slows runoff velocity and provides more contact with vegetation, encouraging filtering and infiltration. Like rain gardens, bioswales can support a variety of native plants. A grassed swale should be planted with hardy ground covers, native grasses or sedges that can withstand both wet and dry conditions.
Placing large boulders at the edges or in the middle of the “stream” helps intercept stormwater. Photo credit:imgkid
Runoff on sloping sites or sites with poor drainage can cause erosion. A dry creek bed can capture some of this runoff and redirect the flow to a permeable area. This consists of a shallow, meandering trench lined with landscape fabric and filled with rock. In general, dry creeks should be twice as wide as they are deep. Though any rock can be used, round river rocks are most attractive. Use large rocks on the sides of the trench, which will direct water toward the middle.
The green roof system consists of a waterproofing layer, a soil or substrate layer and a plant layer. It can be constructed piecemeal, or purchased as a system.
Photo credit: Capitol Green Roofs
Green roofs absorb, store and evapotranspire precipitation, managing stormwater and reducing overall peak flow discharges. They can also potentially reduce discharge of pollutants such as nitrogen and phosphorous, help mitigate the urban heat island effect and soften urban landscapes. Finally, green roofs insulate buildings from outside temperatures and noise.
Green roofs can be installed on new construction or retrofitted on roofs with up to a 20 percent slope. A building must be able to support the loading of green roof materials under fully saturated conditions. Plants should suit local climatic conditions, and can range from sedums, grasses and wildflowers on extensive roofs to shrubs and small trees on intensive roofs.
Capturing and Redirecting Rainwater
Downspouts often direct rainwater from roofs onto paved surfaces, such as driveways. They may also connect directly to the sanitary sewer or storm drain system via a pipe in the ground. Disconnecting the downspout and rerouting the flow into a rain barrel or cistern, or to a lawn, garden or other permeable area can help reduce the amount of polluted runoff flowing to local creeks, rivers and other water bodies. In cities with combined sewer and stormwater systems, this practice can reduce the occurrence of sewer overflows and the volumes of water requiring wastewater treatment. In fact, many cities are starting to offer incentives for disconnecting downspouts, or are making the practice mandatory.
A rain barrel captures water from a roof and stores it for later use on a lawn, garden or indoor plants. Collecting roof runoff in rain barrels reduces the amount of water that flows into the stormwater system. Holding it until after a storm keeps that volume from contributing to the “storm surge”—the large flow of runoff that can cause flooding, erosion and carry pollutants to waterways.
Most rain barrels are simply 55-gallon drums, but any rust-proof, chemical-free opaque container is suitable. Rainwater enters the barrel through a filtered entry gate cut into the top; a spigot installed near the bottom of the barrel allows the user to control flow out of it. The barrel can also be elevated on concrete cinderblocks to increase pressure and flow.
An overflow pipe on the side of the barrel can route to another barrel or to a permeable area in the yard. Because the rainwater isn’t filtered or chlorinated, it can be better for landscaping plants than municipal water.
Several innovations are turning impervious areas into stormwater treatment zones. Permeable pavers consist of impervious units—typically brick or concrete—designed with small openings between permeable joints. The openings typically comprise five to 15 percent of the paver surface area, and allow stormwater to enter a crushed stone aggregate bedding layer and base that supports the pavers while providing storage and runoff treatment. Permeable pavers are attractive, durable, easily repaired, low-maintenance and can withstand heavy vehicle loads. They work well in walkways, patios, sidewalks, driveways, parking lots and low-volume roadways.
Pervious concrete, also known as porous, gap-graded or enhanced-porosity concrete is concrete with reduced sand or fines. This leaves stable air pockets in the concrete, allowing stormwater to flow through the concrete and enter a crushed stone aggregate bedding layer and base, which supports the concrete while providing storage and runoff treatment. Admixtures can be added to the product to enhance strength and increase setting time.
Porous asphalt, also known as pervious, permeable, “popcorn” or open-graded asphalt, is standard hot-mix asphalt—but like pervious concrete, it contains reduced sand or fines, which create stable air pockets in the product. These interconnected voids allow stormwater to flow through the asphalt and enter a crushed stone aggregate bedding layer and base, which support the asphalt while storing and treating runoff.
An infiltration trench or infiltration galley is a rock-filled trench with no outlet that receives stormwater runoff. There, runoff is stored in the void space between the stones until it infiltrates through the bottom and into the soil matrix. Pollutants are filtered as the runoff moves through the soil.
A network of pillars, piles or structural cells supports the pavement above while protecting the soil below from compaction, so it can accommodate tree roots and filter and manage stormwater runoff.
Because they are thin and linear, infiltration trenches are often used alongside driveways or under roof driplines and in under-utilized areas. The trenches are usually be at least 12 inches deep, lined with non-woven geotextile fabric and backfilled with gravel. Sometimes they include a sand filter near the bottom of the trench. They work best with sandy or loam soils. Ideally, stormwater should pass through some type of pretreatment structure, such as a swale or detention basin, before entering an infiltration trench.
Treating Stormwater with Trees
As in natural landscapes, trees can play an important role in mitigating stormwater runoff in cities, by reducing the volume that enters stormwater and combined sewer systems. Each tree is a mini-reservoir, capturing and storing rainfall in its canopy, and releasing water to the atmosphere via evapotranspiration. In addition, tree roots and leaf litter create soil conditions that promote infiltration of rainwater into the soil. Trees also take up water, pollutants and nutrients through their roots, and transform pollutants into less harmful substances.
Big trees with large, dense canopies manage the most stormwater, but bigger trees also require a greater volume of soil to support them. In general, a large tree—one with a 16-inch diameter at breast height—requires at least 1,000 cubic feet of uncompacted soil.
Once established, trees are remarkably self-sufficient, but urban environments pose challenges; in particular, soils are often poor or compacted. Suspended pavement or structural cell systems can be used to facilitate healthy tree growth in plazas, street medians and other hard surfaces.