Also known as infiltration swales, biofilters, grassed swales, or in-line biorentention, bioswales are vegetated open channels specifically designed to attenuate and treat stormwater runoff for a defined water volume. Like open ditches, they convey larger stormwater volumes from a source to a discharge point, but unlike ditches, they intentionally promote slowing, cleansing and infiltration along the way. A sloped base to facilitate this water movement distinguishes bioswales from rain gardens.

There are some design variations of the bioswale, including grassed channels, dry swales and wet swales. These designs may also include an underlying rock reservoir, with or without a perforated underdrain. The specific design features and treatment methods differ in each variation, but all are considered improvements on traditional drainage ditches.

Each type of swale incorporates modified geometry and other design features to allow it to treat and convey stormwater runoff. A typical swale bottom is flat in cross section, 600 to 2400 mm wide, with a 1-2% longitudinal slope, or dished between weirs on steeper slopes. Bioswale side slopes are usually no more than 3:1, horizontal to vertical.

Bioswale vegetation is typically lawn grasses, but more and more of the low volume swales being built in North America are finished with a combination of grasses, perennials, shrubs, groundcover and trees in order to meet other community goals in addition to stormwater management.

Types of Bioswales

Grassed Channels

These are similar to a conventional drainage ditch, with the major differences being flatter side slopes and longitudinal slopes, and a slower design velocity for water quality treatment of small storm events. Grass channels are the least expensive option, but also provide the least reliable pollutant removal. The best application of a grassed channel is as pretreatment to other structural stormwater treatment practices.

A major difference between the grassed channel and other stormwater treatment practices is the method used to size the practice. Most stormwater treatment practices are sized by volume of runoff. That is, the process captures and treats a defined water quality volume, or the volume of water. The grassed channel, on the other hand, is based on flow rate (i.e., a peak flow from the water quality storm; this varies from region to region but a typical value is the one inch storm), grass channels should be designed to ensure that runoff takes an average of ten minutes to flow from the top to the bottom of the channel.

Wet Swales

These swales intersect the groundwater, and behave almost like a linear wetland cell. The design variation incorporates a shallow permanent pool and wetland vegetation to provide stormwater treatment. Wet swales are rarely used in residential settings because the shallow standing water is often unpopular with homeowners.

Dry Swales

Dry swales incorporate a deep fabricated soil bed into the bottom of the channel. Existing soils are replaced with a sand/soil mix that meets minimum permeability requirements. An underdrain system is also placed under the soil bed. Typically, the underdrain consists of a layer of gravel encasing a perforated pipe. Stormwater treated by the soil bed flows into the underdrain, which conveys treated stormwater back to the storm drain system.

Advantages of Bioswales

Runoff Volumes

Even where soils have very poor hydraulic conductivity (around 1 mm/h), a 4 m swale/trench can reduce the volume of runoff from a typical local road to about 25% of total rainfall. In general, infiltration facilities along roads are more effective than on-lot infiltration facilities because there is typically less concentration of runoff (i.e. the ratio of impervious area to infiltration area tends to be lower).

Pollutant Removal

As stormwater runoff flows through bioswales, pollutants are removed through filtering by vegetation and soils. Above ground plant parts (stems, leaves, and stolons) slow flow and thereby encourage particulates and their associated pollutants to settle. The pollutants are then incorporated into the soil where they may be immobilized and/or decomposed.  In particular, bacteria within healthy soils can help break down carbon-based pollutants like motor oil.

Study data suggest relatively high removal rates for some pollutants, but negative removals for some bacteria, and modest removal capability for phosphorous. Phosphorus removal in biorentention soils increases with the depth of the facility. Sorption onto certain components of the soils is the likely mechanism. Low pH or oxygen conditions can cause phosphorus to de-sorb however, so the design should allow for dewatering, and pH should be monitored annually if Phosphorus is a concern. Nitrate removal is highly variable. Where it is a concern an elevated under-drain design that creates a fluctuating aerobic/anaerobic zone below the drain pipe can be used to enhance the de-nitrification process.

Some of the oldest biorentention facilities in the US seem to have developed soils structures and functions that actually enhance pollutant removal ability. 

Groundwater Recharge

Grassed channels and dry swales provide some groundwater recharge if a high degree of infiltration is achieved by the practice. Wet swales typically do not contribute to groundwater recharge, as infiltration is lessened by the accumulation of organic debris on the bottom of the swale.

Siting

Grassed swales can be applied in most development situations, including residential areas, office complexes, rooftop runoff, parking and roadway runoff, parks and green spaces. Swales are well-suited to treat highway or residential road runoff because of their linear nature and because they are designed to receive stormwater runoff via distributed sheet flow, which travels through a grassy filter area at the swale verges. Bioswale design can easily incorporate driveway crossings.

Provision of underground overflow allows use of the technique in most soils, including clay with infiltration rates as low as 0.6mm/hr.

Retrofitting

One common retrofit opportunity is to use grassed swales to replace existing drainage ditches. Ditches are traditionally designed only to convey stormwater away from roads. In some cases, it may be possible to incorporate features to enhance their pollutant removal or infiltration using check dams (i.e., small dams along the ditch that trap sediment, slow runoff, and reduce the longitudinal slope).

One well-publicized example of a retrofit roadway is the Seattle Street Edge Alternative (SEA) project.  The drainage goals for this project included conveyance, flood control, and minimizing the flow of stormwater off-site. The designers sculpted the project area to move water away from the roadway and homes and into planted swales along both sides of an existing road. They replaced impervious road surface area with bioswales, and helped address traffic goals in the community, at the same time improving local aesthetics and increasing the amount of urban forest to intercept rainfall.

Size

Individual grassed channels are generally designed for drainage areas of less than two hectares. If grass channels are used to treat larger areas, the flow velocity within the bioswale becomes too great to treat runoff or prevent erosion in the channel. Literature suggests swale areas of about 10-20% of upstream impervious area.

Limitations of Bioswales

Pollutant Removal

If designed improperly, bioswales will have very little pollutant removal.  They also do not seem to be effective at reducing bacteria levels in stormwater runoff. 

Pollutant Load

Highly contaminated runoff can be generated by some land uses where pollutant concentrations exceed those typically found in stormwater. These hot spots include commercial nurseries, recycling facilities, fuelling stations, industrial storage, marinas, some outdoor loading facilities, public works yards, hazardous materials generators (if containers are exposed to rainfall), vehicle service and maintenance areas, and vehicle and equipment washing and steam cleaning facilities. With the exception of the dry swale design (see Design Variations), hotspot runoff should not be directed toward grassed channels. Swales infiltrate stormwater and can intersect the water table, thereby increasing the risk that hotspot runoff will become a threat to groundwater quality.

All contaminated runoff should be prevented from entering municipal storm drain systems by using best management practices for the specific industry or activity.

Groundwater Recharge

Designers should identify potential high pollutant sources, particularly industrial/commercial hotspots that would dictate pre-treatment or source control upstream of a bioswale. Restrictions on the depth to groundwater depend on the type of channel used. Some sources suggest a minimum depth from base of drain rock reservoir to water table of 600 mm to prevent a moist swale bottom, or groundwater contamination.

Siting

While some sources recommend that bioswales should be used on sites with relatively flat slopes (i.e., less than 4%), others note that the use of properly spaced weirs can allow siting on slopes up to 10%.  When slopes become too steep, runoff velocities become fast enough to cause erosion, and prevent adequate infiltration or filtering in the channel.

Maintenance

Maintenance requirements are similar to those for ditches: inspecting for bank slumping & erosion, replanting any bare patches where vegetation has been unsuccessful or removed, maintaining ideal vegetation heights by mowing, and removing garbage. Additionally, sediment build-up within the bottom of the swale should be removed once it has accumulated to 25% of the original design volume.

Design

Sources suggest a thick vegetative cover is needed for proper bioswale function. Water level fluctuation, long-term inundation, erosive flow, excessive shade, poor soils, and improper installation were found to be the most common causes of low vegetation survival in a King County study. The Seattle SEA street project has had good success with vegetation survival. Neighbours have agreed to care for their boulevards, and careful plant selection was based on non-invasive, low maintenance plants suited to the moisture regime of their location within the bioswales.

Because of the linear nature of bioswales, stormwater should ideally enter via sheet flow.  Pre-treatment (such as grassed verges) and erosion control must be part of the design in order to avoid sedimentation of the channel.