Edited by Len Phillips

Nature uses vegetated depressions, wetlands, marshes, etc. to clean rainwater runoff by removing sediments, turbidity, heavy metals, and other pollutants. When rainwater runoff leaves a site, unless there is effective erosion protection such as vegetation, soil also leaves the site with the largest particles of sediment and suspended solids to the smallest particles of dissolved solids. When the velocity of the runoff stream becomes slow, sediment and suspended solids will settle out. Pollutants, nutrients, including nitrogen and phosphorus, and the smallest soil particles are removed by vegetation uptake, by natural flocculation from decomposing vegetation, by biota consumption, and by ionic attraction around the root structure.

In an urban setting, runoff is controlled with pipes and drainage systems that force rainwater to run directly into large bodies of water without any sediment or pollutant removal. One solution to this problem is the construction of a bioswale system. This is a system designed to protect local and regional water quality by reducing sediments, pollutants, and nutrient loads.

Bioswale is the term generally given to any vegetated swale, ditch, or depression that conveys water runoff. The majority of annual precipitation comes from frequent, small rain events. Bioswales improve water quality by infiltrating these first flush of rain events. Bioswales also increase habitat for wildlife by preserving trees and vegetation while reducing the potential for flooding. Bioswales reduce stream bank and channel erosion by reducing the frequent surges and bounces of fast flows from storm sewer discharges.

Where to use bioswales:

• in parking lots for the absorption of automotive pollutants
• at downspouts to slow and direct rooftop rainwater

• along the edge of any hard impervious surface to slow rainwater runoff

The costs of designing and maintaining bioswales vary greatly depending on size, plant material, and site considerations. Bioswales are generally less expensive to build when used instead of underground piping. This also reduces infrastructure and utility maintenance costs. Bioswales provide good treatment of rainwater runoff without the extensive maintenance required for some other rainwater treatments. The effectiveness of bioswales is dependent upon the retention time of the rainwater in the bioswale. The longer the retention times, generally, the higher the removal efficiency. The type of vegetation and the life cycle that the vegetation is in will also affect the pollutant removal rate.

Types
The fully vegetated bioswale and the open channel bioswale (roadside) are the two basic types of bioswales based upon the amount of vegetation. Bioswales are also typed according to their general cross-sectional shape, "U", "V", and "trapezoid". Generally, the "U" and "V" shaped swales are roadside ditches that have become naturally vegetated and they are usually open channeled. Open channels do not add much more than infiltration and collecting sediments and result in very little bioremediation of the rainwater. The trapezoidal vegetated bioswale is the most effective bioswale at removing pollutants in the rainwater runoff.

Design
When bioswales are designed, they should address a certain storm event such as the two-year or ten-year 24-hour storm event. The design capacity of the swale should handle around 90% of the storm events. Bioswales should be designed and built with the following considerations in order to be valuable for removing pollutants:

• The first consideration is to enhance and utilize existing natural drainage swales whenever possible.
• Next consider what the design capacity of the bioswale should be. Both the peak flow and duration storm events should be investigated.
• The two-year 24-hour storm event should be the minimum that the bioswale is designed to treat.
• Systems that can accommodate these peak flows are more than adequate to convey small storms, which occur frequently.
• Small storms, because of their frequency and cumulative impacts, make the largest contribution to total annual runoff and have the greatest impact on water quality.

• When designing a swale to handle a certain storm event occasionally a storm event of greater volume than the design capacity will occur and result in the runoff overflowing the swale. Some thought should be given to measures that can be taken to reduce the problems such overflows would create.

Soils
Serious consideration should be given to the type of soils and the amount of compacted soils on the site. Constructed bioswales can be successfully built and operated in most soil types provided the topsoil in the area is optimized for the maximum porosity. Soils will filter out most pollutants in any water that flows through the soil by chemical attractions. The subsoil can be sand, gravel, rock, and should contain organic materials such as compost or composted biosolids. The degree of the soils ability to continually remove pollutants depends to some extent on whether or not the pollutant is biologically degradable. Soil infiltration rates should be greater than one-half inch per hour.

Capacity
The wide, flat bottomed swale maximizes the available treatment area for pollutant removal while also providing ease of maintenance. In order to be able to mow the vegetation in a bioswale, the bottom should be at least two feet wide. The maximum free width of the bioswale bottom should be less than eight feet wide to avoid ridges and gullies and to ensure sheet flow. The bioswale should be at least six inches deeper than the maximum design flow depth. This additional depth is known as "freeboard" and provides a safety factor to prevent the bioswale from overflowing onto adjacent areas if the channel becomes obstructed or if runoff volumes exceed the design capacity. The side slopes should be no steeper than 3:1.

Vegetation
Sedimentation comes about through reduced flow, ion exchange, and natural flocculation. Slower velocities within a bioswale allow higher water retention times which allows for the settling of larger particles and pollutants. Many of the particles suspended in the rainwater are negatively charged. There is a positive charge at the base of the vegetation in the bioswale that attracts these negatively charged particles. Vegetation will consume pollutants and transfer them into their plant cells. As vegetation ages and dies, a natural flocculant is released, which also attracts negatively charged pollutant particles and causes them to settle out.

The selection of vegetation should be in accordance with the pollutants to be removed, the desired capacity, and residence time of the rainwater and pollutants in the bioswale. Trees should only be planted along the top edge of the bioswale to provide shade that will minimize the temperature increase of the water. A lower canopy of shrubs and grasses should be planted underneath the trees and on the side slopes.

Select species that can tolerate standing water and fluctuating water levels. Vegetation selected for bioswales should be native to the region or an improved cultivar of these native plants. Deep-rooted plants are preferred for infiltration and reduced maintenance.

Some types of vegetation to consider are:

• rushes for heavy metal and nutrient uptake,
• reed-grass for collecting total suspended solids (TSS) and erosion control,
• common reed for TSS, nutrient uptake, and chemicals,
• water-starwort for absorption of toxics,
• water-purslane for filtering and uptake of toxics,
• bur-reeds for pollutant uptake,
• clover for erosion control and nitrogen capture,
• Mexican mosquito-fern for uptake of toxics,

• cattail for phosphorus removal is on the order of 80%. Cattails are very good for removing pollutants from rainwater but have several drawbacks such as being highly invasive, having a dormant cycle that can extend for a year or more, and being overly restrictive of flow in the bioswale requiring abnormally high amounts of maintenance.

Other plants to consider include the following:
Bioswale Bottom
Scientific Name             Common Name
Agrostis tenuis             Colonial Bentgrass
Carex sp.                     Sedge
Deschampsia cespitosa Tufted Hairgrass
Eleocharis palustris       Creeping Spikerush
Epilobium densiflorum   Dense Spike-Primrose
Hypericum sp.              St. John's-Wort
Juncus sp.                   Rush
Mimulus guttatus          Common Monkeyflower
Potentilla sp.                Cinquefoil
Ranunculus sp.             Buttercup
Sagittaria latifolia         Wapato
Saxifraga sp.                Saxifrage
Scirpus sp.                   Bulrush

Many of the grasses mentioned above may be used on the side slopes of the bioswale but some should not be used in the bottom due to their low tolerance for submergence.

Bioswale Side Slopes
Scientific Name            Common Name
Abies grandis               Grand Fir
Alnus rubra                  Red Alder
Bromus carinatus          California Brome
Cornus stolonifera         Red osier Dogwood
Crataegus douglasii       Black Hawthorne
Deschampsia cespitosa  Tufted Hairgrass
Elymus glaucus             Blue Wild rye
Festuca sp.                  Fescue Grass
Fraxinus latifolia           Oregon Ash
Lonicera involcrata        Black Twinberry
Oemlaria cerasiformis    Indian Plum
Physocarpus capitatus   Ninebark
Rosa sp.                      Rose
Salix sp.                      Willow
Sambucus racemosa      Red Elderberry
Spiraea douglasii           Douglas' Spirea
Symphoricarpos albus    Snowberry
Thuja plicata                Western Red Cedar

If needed, assistance can be obtained in the selection of the vegetation from a local wetlands biologist and/or from the local NRCS Field Office .

Maintenance
Vegetation in the bioswale should be trimmed every year or two to prevent woody species from taking over. Clippings from plants should be disposed of properly as they may have absorbed pollutants and hazardous toxins. Fertilizers and herbicides are a source of pollutants that are being removed by the vegetation, so the use of fertilizers and herbicides for maintenance of the bioswale should be avoided.

Sources

• "Bioswales", News/Bioswales, USDA, 2005.

• Jurries, Dennis PE, "Biofilters for Storm Water Discharge Pollution Removal", Oregon Department of Environmental Quality, January, 2003.

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