Water Conservation by Stormwater Management
We've let our stormwater get away from us. These water conservation practices can help clean it up and encourage it to stick around.
STORMWATER RUNOFF is rain or snowmelt that flows over the land without percolating into the soil. Stormwater occurs naturally, especially during large rain events, but nature’s sponge—the water-absorbing cover of trees, shrubs and other vegetation hugging our planet—usually takes care of the rest. Unfortunately, we’ve turned our world into a hard place. Paved sidewalks, asphalt parking lots, concrete curbs, streets, driveways, roofs and building facades—all of these impervious surfaces change the natural movement of water over the landscape, and increase the volume, speed and temperature of the runoff.
What does this mean? Depending on the location, stormwater drains directly into local creeks, rivers, lakes and bays, or, in cities and towns with combined sewer and stormwater systems, into pipes that carry it to the wastewater treatment plant. Fast water picks up unwanted hitchhikers—oil from driveways and streets, fertilizers and lawn pesticides from yards, sediment from construction sites, random chemicals and bits of trash. All of these can pollute local waterways. In addition, high volumes of swift-moving water can cause flooding and erode banks, introducing sediment into a water body. Sediment clouds the water, making it hard for fish and other aquatic creatures to find food; it can also clog gills and smother the eggs of frogs and fish.
In the case of combined sewer and stormwater systems, large volumes of stormwater can overwhelm treatment plants, causing sanitary sewer overflows that release untreated sewage into streams, lakes and bays.
Finally, when stormwater rushes over landscapes, less of it percolates into the ground. This means it’s not available to recharge groundwater, and can’t takes advantage of the filtering and cleaning services provided by plants and soil.
The good news is, cities and towns are starting to address this problem on the local and regional scales. There’s a lot homeowners, builders and designers can do, too. Incorporating Low Impact Development (LID) strategies into new projects or retrofitting them into developed sites can go a long way to mitigating our stormwater impacts. By encouraging rainwater to slow down and stick around for a while, these strategies—adapted from EPA Best Management Practices—help keep pollutants out of streams and lakes, replenish groundwater, and, in the case of combined systems, ease the burden to treatment plants. They also green and beautify our built environments, and make them just a little softer.
Also known as “bioretention areas,” rain gardens are shallow, landscaped depressions which retain and filter stormwater, much in the same way as forested ecosystems.
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.
While they consume a fairly large amount of space (approximately five percent of the area that drains to them), rain gardens are versatile. They work well on small residential lots, parking lot medians and in highly urbanized settings, and can be adapted to different climates and soils. They also provide wildlife habitat and food for birds, bees and other pollinators.
Rain gardens should be installed on shallow slopes, usually about 5 percent, to ensure stormwater will move through them. 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.
The choice of plantings adds both to the functioning and aesthetic value of rain gardens. It’s important to choose plants that can withstand the rain garden’s “hydrologic regime.” Plant species at the bottom of the depression that can tolerate both wet and dry conditions; plant upland species at the edges.
In contrast to traditional asphalt or metal roofing, green roofs absorb, store, and later evapotranspire precipitation, managing stormwater and reducing overall peak flow discharges. If implemented on a wide scale, they can significantly reduce the volume of stormwater entering local waterways, and in urban areas with combined sewer systems, reduce the incidence of combined sewer overflows. As a general rule, extensive green roofs will absorb 50 percent of rainfall.
Green roofs have the potential to reduce discharge of pollutants such as nitrogen and phosphorous via soil microbial processes and plant uptake. They help mitigate the urban heat island effect and give “natural relief” to hard urban landscapes. Durable and long lasting, green roofs also increase thermal insulation of buildings and provide acoustic insulation.
Green roofs are classified as extensive, semi-intensive or intensive. Extensive green roofs include six inches or less of growing medium, and are generally low-maintenance. Intensive green roofs require more than six inches of substrate, need more maintenance and are intended for public use. Semi-intensive green roofs are a hybrid between the two.
Green roofs can be installed on new construction or retrofitted to existing residential, commercial or public buildings, on roofs with up to a 20 percent slope. The roof system consists of a waterproofing layer, a soil or substrate layer and a plant layer. They can be constructed layer by layer, or purchased as a system. 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.
Downspouts often direct rainwater from roofs onto paved surfaces, like driveways. They may also connect directly to the sanitary sewer or storm drain system via a pipe in the ground.
Downspout disconnection is the process of disconnecting the downspout from the pipe or paved area and rerouting the flow into a rain barrel or cistern, or to a lawn, garden or other permeable area.
Disconnecting downspouts 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. It’s also a great way to conserve water.
But even if you don’t use the stored rainwater on landscaping, 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. Rain barrels can be installed on a leveled surface right below gutter downspouts.
Rainwater enters the barrel via 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.
Even a small rain event draining a relatively modest roof area can quickly fill a typical rain barrel; however, several barrels can be connected in series to increase capacity. You will need to plan for overflow by installing an overflow pipe through a hole on the side of the barrel nearer the top. This pipe 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.
A bioswale, also known as a grassed channel, dry swale or biofilter, is a vegetated, open channel designed to treat and attenuate 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. A small forebay should be used at the front of the swale to trap incoming sediments; you can also install gravel check dams across the swale to slow stormwater as it travels down the channel.
Swales should be installed in areas with slight slopes of two to four percent. Their linear design makes them ideal along roads and driveways, but they can be incorporated anywhere with enough space, including alongside parking lots.
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.
A simple grassed swale can potentially recharge the groundwater and is inexpensive, but doesn’t provide as much pollutant removal as a dry swale. A dry swale consists of a permeable, fabricated soil bed with an underdrain system—a gravel layer encasing a perforated pipe, which accepts stormwater treated in the soil bed.
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 are filled with highly permeable, small-sized aggregates. These joints allow stormwater to enter a crushed stone aggregate bedding layer and base that supports the pavers while providing storage and runoff treatment. The subgrade soil also plays a role. Studies have found beneficial bacteria—which feed on nutrients like nitrogen and phosphorus—growing on established aggregate bases.
Permeable pavers are attractive, durable, easily repaired, low-maintenance and can withstand heavy vehicle loads. They’re also versatile, and work well in walkways, patios, sidewalks, driveways, parking lots and low-volume roadways.
Permeable pavers are typically installed over a two-inch layer of small-sized, open-graded aggregate, with an open-graded base reservoir of crushed stones beneath. This layer stores water and provides a transition between the bedding and subbase, which consists of larger stone sizes.
The subbase also stores water in the spaces among the stones, and adds extra support. An underdrain—a perforated pipe that ties into an outlet structure—can be installed over clay soils with low infiltration rates to facilitate water removal from the base and subbase. You can also install a geotextile layer between subbase and subgrade to prevent the migration of soil.
Permeable Concrete and Porous Asphalt
Permeable pavement transforms areas that were a source of stormwater into a treatment system, and can effectively reduce or eliminate runoff that would have been generated from an impervious paved area.
Pervious concrete, also known as porous, gap-graded or enhanced porosity concrete, is concrete with reduced sand or fines. The reduced fines leave stable air pockets in the concrete—a total void space of between 15 and 35 percent—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.
When properly constructed, pervious concrete is durable and low-maintenance, and has a low life cycle cost. Admixtures can be added to the product to enhance strength, increase setting time or add other properties. The thickness of pervious concrete ranges from four to eight inches, depending on the expected traffic loads. To compensate for the lower structural support capacity of clay soils, additional subbase depth is often required. The increased depth also provides additional storage volume to compensate for the lower infiltration rate of the clay subgrade. Underdrains are often used when permeable pavements are installed over clay.Infiltration Trench
An infiltration trench (a.k.a. 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.
Because they are thin and linear, infiltration trenches are often used alongside driveways or under roof driplines, and in under-utilized areas. They should be between 12 and 18 inches wide, and at least eight inches deep. Infiltration trenches are typically filled with a 1.5- to 2.5-inch diameter crushed rock over a six-inch sand filter, then topped with non-woven geotextile fabric and a two-inch layer of pea gravel. To extend the life of the trench, you can line the sides with fabric, too.
Ideally, stormwater should pass through some type of pretreatment structure, such as a swale or detention basin, before entering an infiltration trench. Although these trenches can remove pollutants and recharge the groundwater, they don’t visually enhance a site. They also have a relatively high failure rate.
In natural landscapes, trees play critical roles in controlling stormwater runoff and protecting surface waters from sediment and nutrient loading. But trees can also 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. In these systems, a network of pillars, piles, or structural cells supports the pavement or intended ground surface. The suspension system supports the weight and forces from above while protecting the soil below from compaction, so it can accommodate tree roots and filter and manage stormwater runoff. Suspended pavement can also accommodate the large volumes of soil needed for big tree growth.
Replacing Lawns with Natives
Homeowners tend an estimated 40 million acres of turf. If classified as such, lawns would rank as the fifth largest crop in the country, based on area. Lawns need frequent fertilizing to stay green, and produce significant amounts of nutrient-rich stormwater runoff, which can potentially cause oxygen-starved conditions in streams, lakes and estuaries. Lawns also require large amounts of pesticides—an estimated 70 million pounds are applied annually—which can be taken up by stormwater and contaminate streams, lakes and other water bodies.
Replacing lawn with native gardens—also called rain gardens, xeriscaping or naturescaping—is a strategy with multiple benefits. Native plants enjoy natural resistance to local pests and are adapted to local climates; consequently, they require no fertilizing and little water once established. In fact, native plants can reduce the impact of fertilizers through direct uptake of nutrients—nitrogen and phosphorous—from runoff that otherwise would contaminate the water supply.
Natives also manage stormwater better than turf grasses. The deeper root systems of many native plants can increase the soil’s capacity to store water, thereby reducing the volume of runoff and, consequently, flooding. The deep, thick root systems also ward against soil erosion. Finally, compared to a lawn monoculture, a diverse arrangement of natives provides superior habitat and forage for local wildlife. The installation cost of a native garden is competitive with establishing a new lawn, but over the long term, they’re much less expensive to maintain.
Everyone contributes to stormwater runoff and pollution, but because the stormwater system is largely out of sight, it’s out of the minds of most people.
“It’s the forgotten infrastructure,” says Jillian McCarthy, Stormwater Coordinator for New Hampshire’s Department of Environmental Services. “There’s a widespread assumption that stormwater gets treated before it enters water bodies.” Many communities have tried to correct this misperception by recruiting citizens to “beautify” storm drains with stencils, slogans and colorful paintings.
Education and outreach are important components of the stormwater pollution prevention plans. Larger towns and cities have been required to develop these plans since 1990, but as Phase 2 of the EPA’s Stormwater Program comes online, smaller cities must create them as well. Getting public support—in part by raising awareness—will make the job of implementing these plans a lot easier.
“One of the most important things is finding strategies that fit the homeowner’s site and lifestyle,” says McCarthy. “There’s no silver bullet. A practice that works at my house may not work at my neighbor’s.” She hopes that before too long, rain gardens and rain barrels will be as commonplace as recycling bins.
Put Up a (Green) Parking Lot
Parking lots are a major sources of stormwater runoff. During a storm, rain sheets off large, uninterrupted swaths of impervious asphalt and concrete, picking up speed and pollutants, especially oil, as it flows. Combining several stormwater management strategies can transform parking areas into stormwater treatment reservoirs. Using a combination of permeable pavement, pavers, gravel or even grass for the parking spaces, and incorporating swales and rain gardens to break up impervious areas can cool down temperatures, help slow the flow and filter out pollutants.