Green Streets/Bioswales/Rain Gardens

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After at least a century of hard engineering solutions for urban rain/storm water run-off, communities are turning more and more to using a plant-based technology that mimics nature’s wetlands and ponds.

Portland, Oregon, has played a leading role in supporting and developing this concept, with successful demonstration projects now helping control negative storm water events that included flooding and the overflow of sewage into local rivers.

Just holding back the flow of a major rain event is enough justification for continuing the development of green streets, or rain gardens. When the cost of “hard” infrastructure is considered, these plant-based technologies may be the perfect “green” technology to receive early installation.

All these variations…green streets and rain gardens…are built on the concept of bioswales. So, much of the information is based on bioswale research and the success of earlier projects.

A Decade of Development
One of the first scientifically designed, large-scale bioswales was built in 1996 for Willamette River Park in Portland, Oregon. This bioswale, at a total of 2330 lineal feet, was designed to capture pollutant runoff and prevent it from entering the Willamette River. Silt capture was improved by adding intermittent check dams. The dams reduced suspended solids entering the river system by 50 per cent.

Another example of a large, designed bioswale is at the Carneros Business Park, in Sonoma County, California. In 1997 the California Department of Fish and Game and County of Sonoma, working with an environmental design team, created a detailed design that took the surface runoff from the park’s large parking area. The runoff came from the building’s roof and parking lots. There was also an overland flow from properties located north of the project site. A two-mile bioswale was built to reduce runoff contaminants from entering Sonoma Creek. The grass-lined bioswale channel has an almost linear construction, with a down slope gradient of four percent and six percent cross-slope gradient.

Another early project, completed in 2001, is Seattle’s pilot Street Edge Alternatives Project (SEA Streets). Its drainage designs closely mimics the natural landscape compared to traditional piped systems. Impervious surfaces were reduced, now with 11 percent less than a traditional street while providing improved surface detention in swales. SEA also added over 100 evergreen trees and 1100 shrubs. Two years of monitoring show that SEA Street reduced the total volume of storm water leaving the street by 99 percent.

Meanwhile, back in Portland, the Bureau of Environmental Services has created a “green streets” program. In one project, the city retrofitted SW 12th Avenue, near Portland State University, to collect runoff from 8,000 sq ft and running it into a series of four planters. Up to 6 inches of water can be collected in each planter, then the water overflows down the street to the next planter. In 2006, the project won a General Design Award of Honor from the American Society of Landscape Architects.

There are even companies now that focus on the design, construction and promotion of rain gardens and related products, such as rain barrels. This is another great example of how the low-tech use of plants can solve serious environmental problems, instead of billion dollar “hard” solutions such as Portland’s two massive pipe system projects now under construction.

But, despite many similarities, there are some differences between the bioswale variations.

A swale is a low tract of land, that usually exists in a moist or marshy situation and can be a natural landscape feature or one specifically built for environmental reasons. The later is often an open drain system is that manages water runoff.

Bioswales are landscape elements designed and built to remove silt and pollution from surface runoff water. These “swaled” drainage courses are, in a sense, gently sloped ditches that contain plants, compost and/or riprap. The sloped sides are usually less than six percent slope.

As water flows though the typically wide and shallow ditch, so the water spends enough time in the swale, to help trap of silt and pollutants, a bioswale can have a meandering or almost straight channel alignment, based on the lie of the land where it is built.

Bioswales are often built around parking lots due to auto pollution. Potential harmful compound end up being collected on the paving and then flushed by rain. The bioswale, acts as a biofilter, and surrounds the parking lot. As the runoff enters the bioswale, it is cleaned before entering a watershed or storm sewer.
The bioswales can also contain biological factors also contribute to the breakdown of certain pollutants.
Bio-retention ponds, commonly called “rain gardens,” are landscape features that help control rainwater runoff. The runoff comes from roofs, driveways, walkways, compacted lawn areas and other impervious urban surfaces, and cause problems, especially during the large storm events. Structures, low-lying depressions and other landscape constructions that slow and deter running water allow heavy rains to be absorbed into the soil. This prevents the urban situation where the rains flow into storm drains and cause secondary environmental problems. Or it becomes surface water that causes erosion, water pollution, flooding, and diminished groundwater. Some studies claim this can reduce the pollution reaching creeks and streams by up to 30 percent.

Rain garden plants also return water vapor into the atmosphere through the transpiration.

Thus, rain gardens are essentially all landscape features that capture, channel, and divert natural rain and snow that falls on a property. This diverted water may also find other uses, such as stored water returned as irrigation. If designed correctly, an entire landscape or garden can become a rain garden. Individual elements act as components or small-scale rain gardens.

Meanwhile, green streets use small-scale, vegetated bioswales, built along streets that again help control storm water events. These constructed elements create on-site infiltration, while providing attractive streetscapes. They also improve a neighborhood’s livability by adding park-like elements that serve as urban greenways. As mentioned earlier, the City of Portland has officially incorporated green street facilities into all its development, redevelopment or enhancement projects. Besides treating and infiltrating storm water, these projects can also increase tree shade canopy and support native habitat all in the parkways and medians.

It is important to note that these man-made water-control landscapes’ success depends on an adequate “infiltration rate.” This measures, in inches or millimeters per hour, the rate a particular soil absorbs rainfall or irrigation. As the soil becomes saturated, the infiltration rate decreases. When the precipitation rate exceeds the infiltration rate leads to “runoff.” So, the correct soils are also crucial to the overall functioning of all these bioswale variations.

In addition, most of these bioswale-based, water control landscapes are using at least some, if not all, native or related plant choices. This leads to both environmental and cost advantages.

Why Bioswales…Benefits of Water Control Landscapes
These wetland variations, working like their natural versions, have several key benefits that will increase their use as bioswale technology becomes adopted as an effective construction element in urban settings.

Reduced expense for storm water management facilities
In many locations, natural landscaping, like bioswales or rain gardens, can handle and control storm and flood waters. This, in turn, can reduce the need for expensive, “highly engineered” pipes systems and detention facilities. More and more real world projects are showing that drainage swales can cost much less to install than storm sewers.

“Sustainable innovations can actually reduce costs,” explained landscape architect Paul Morris, speaking at an annual meeting of Oregon Landscape Contractor Association. Morris works on planning and sustainable issues for Cherokee Investment Services, Inc., an international development firm that has long recognized the many benefits of incorporating sustainable technologies into their projects.

He said these include storm water run-off (largest environmental problem in US) control using bioswales, rain gardens, green roofs, and capturing the water on site can be less expensive to construct than traditional solutions.

When curbs and gutters are eliminated and curbs are slotted, there can be substantial construction savings. When natural drainage measures increase infiltration of storm water into the local soil, runoff volume is reduced while the need for downstream conveyance and detention structures is reduced.

Other projects found that detention basins, designed with natural landscaping to resemble wetlands or natural lake systems, also reduce costs over conventional basins. These “natural” landscapes eliminate the need for expensive riprap stabilization and low flow channels paved with concrete. Natural vegetation in detention basin bottoms and on side slopes is less costly to maintain than conventional turf landscaping (see next section), and is a more reliable soil stabilizer.

Removing Contaminants
As was indicated in the definition of a bioswale, one reason to slow water down is so it can react with the nearby plants, roots and soil. This has documented several benefits.

First, these plant-based technologies can help control several classes of water pollutants, including silt, inorganic contaminants, organic chemicals and pathogens. This falls under the definition (according to Wikipedia) of “biofiltration.” It is defined as “a pollution control technique using living material to capture and biologically degrade process pollutants.” These process include cleaning waste water, capture chemicals that are potentially harmful, micro biotic oxidation of contaminants in air, or collecting silt from surface water runoff.

This is similar to “bioremediation,” which is defined as “any process that uses microorganisms, fungi, green plants or their enzymes to return the natural environment altered by contaminants to its original condition.” Bioremediation may be employed to attack specific soil contaminants, such as degradation of chlorinated hydrocarbons by bacteria. An example of a more general approach is the cleanup of oil spills by the addition of nitrate and/or sulfate fertilizers to facilitate the decomposition of crude oil by indigenous or exogenous bacteria. (Wikipedia)

With silt, the bioswale or rain garden’s effect is to slow the moving water, reducing turbidity, and allowing the small soil particles to drop out of the water. Thus, the soil is returned to a place where it is beneficial instead of traveling downstream to become a problem.

Meanwhile, inorganic compounds, such as metallic compounds like lead, chromium, cadmium and other heavy metals, are common pollutants, especially in areas of heavy auto use. Lead, from automotive residue (e.g. surface spillage of leaded gasoline) is the most common example.

Other common inorganic polluting compounds include phosphates and nitrates, whose main source is excess fertilization. This often causes “eutrophication,” defined as “an increase in chemical nutrients — compounds containing nitrogen or phosphorus — in an ecosystem from the release of sewage effluent, urban storm water run-off, and run-off carrying excess fertilizers into natural waters. It may occur on land or in water.

However, the term is often used to describe the resultant increase, and thus excessive, plant growth and decay in aquatic environments. This results in a lack of oxygen and results in severe reductions in water quality, fish, and other animal populations, disrupting normal functioning of the ecosystem, In aquatic environments, this enhanced growth creates choking aquatic vegetation or phytoplankton, often known as “algal blooms.”

Meanwhile, common pesticides, frequently over-used in agricultural and urban landscaping, are also seriously detrimental organic chemicals. They can actually poison some organisms and often seriously disturb aquatic ecosystems.

Finally, there are human pathogens that usually come from animal waste in surface runoff water. In just the past few years, it has lead to a several serious diseases in humans, with outbreaks coming from spinach and peanut butter.

Less recognized, but still serious, are comparable diseases that have affected aquatic organisms.

Reduced costs of landscape installation and maintenance
Studies have shown that these “bioremediation” technologies are less expensive than the some other landscaping options. For instance, conventional rolled-sod, turf lawns can have installation costs exceeding $12,000 per acre, while planting grass seeds may cost $4,000 to $8,000 per acre. But, seeding native prairie grasses and forbs costs only $2,000 to $4,000 per acre.
Several publications noted that planting native plants plugs increases installation costs significantly but does give plants a “head start” if desired.

Another plus is that sponsors and volunteers can help control native plant installation costs. Sponsors can even sometimes be a public or private entity with plant propagating capabilities. Volunteers can be recruited to install and maintain native landscapes.

And, natural landscaping just cost less to maintain. Over the first ten years, the combined costs of installation and maintenance for natural landscape can be as little as one fifth of the costs for conventional landscape maintenance. Many projects use a range of native plants already adapted to the region’s soil conditions and climate, including summer heat and drought. Natural landscaping lowers many normal costs including labor, water, fertilizer, herbicides, insecticides, and fungicides, replanting annual flowers, and mowing. In drier climates, natural landscaping lowers the high irrigation costs.

The reduced use of lawn maintenance equipment lowers gas use, an additional benefit. Natural landscapes require simple maintenance, usually just annual mowing or burning, and some weed removal (mostly in the few years after installation)

This, like green roofs and green walls, is becoming a new market for both growers and landscape contractors. With Portland leading the way in this area, it will be one more topic this blog will continue to follow. If you are interested in more details, go to, click on “Stormwater Solutions” under Library.

Watch for the upcoming post that discusses and provides references on the plant material being used in this newest version of sustainable horticulture.

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