Bioswales are channels built to collect and move stormwater runoff while removing dirt, trash, and pollution. They also help groundwater refill, reduce flooding and erosion, and provide a place for pollinators like bees and butterflies to live.

Bioswales are usually covered with plants, mulch, or drought-resistant plants. They have a drainage path with sides that slope gently (less than 6%). The design of a bioswale allows water to stay in the channel for a longer time, which helps remove pollutants, silt, and debris. The shape of the bioswale may be straight or curved, depending on the land’s natural features. Check dams are often added to help water soak into the ground. The way a bioswale is built depends on factors like climate, rainfall, the size of the area, budget, and the types of plants that can grow there.

To work well, bioswales need regular care. Planning for maintenance is important, such as adding filters or large rocks to stop clogging. Each year, checking the soil, looking for problems, and testing the structure helps keep the bioswale healthy and functional.

Bioswales are often placed along streets and around parking lots. These areas collect pollution from cars, which is washed away by the first rain after a dry period, called the first flush. Bioswales or similar systems can be built near parking lots to clean stormwater before it flows into rivers, lakes, or storm drains.

Contaminants addressed

Bioswales help remove pollutants from water using plants and soil. When stormwater flows through a bioswale, pollutants are trapped by the leaves and stems of the plants. These pollutants then move into the soil, where they break down or are broken down by bacteria in healthy soil.

There are several types of water pollutants that bioswales can help manage. These include silt, inorganic contaminants, organic chemicals, and pathogens.

• Silt: The way bioswales and plants are built slows the movement of silt, reducing the cloudiness of water in nearby areas. Filters can be added to catch debris and silt during this process.
• Organic chemicals: Many organic pollutants, such as polycyclic aromatic hydrocarbons, can evaporate or break down over time. Bioswales slow the movement of these materials, reducing their impact on aquatic life. While not all organic material is removed, bioswales greatly lower the amount that reaches waterways.
• Pathogens: These harmful microorganisms lose access to hosts or nutrients, making them easier for other organisms to break down.
• Inorganic compounds: Common inorganic pollutants include macronutrients like phosphates and nitrates. These often come from agricultural runoff caused by overuse of fertilizers. Too much phosphate or nitrate can lead to eutrophication in water bodies. Some bioswale plants take in these excess nutrients.
• Metallic compounds: Heavy metals like mercury, lead, chromium, and cadmium can build up in bioswale structures. Over time, these metals harm the soil. To prevent this, soil must be removed regularly. Some bioswales use hyperaccumulator plants, which take in heavy metals but do not change them. When these plants are cut, their parts may end up in ponds or compost, which can be dangerous because the metals remain toxic.

Best locations

Bioswales can be added in places that need to manage stormwater. They help slow down the speed of water runoff and remove pollution. Bioswales are especially useful in areas with many hard surfaces, like roads, parking lots, and rooftops, because these areas create a lot of runoff during rain. They help remove the first flush of pollutants from this runoff. Bioswales can also be built into road medians, curb cutouts, sidewalks, or other public spaces.

Benefits

Bioswales are simple designs used to slow down the speed of stormwater runoff and remove pollutants from rainwater. They help protect surface water and nearby rivers and streams from too much pollution. When stormwater stays in the bioswale for a longer time, more pollutants are removed. Bioswales also help prevent standing water that might attract mosquitoes. These features can be designed to look nice and can provide homes for animals. Additionally, bioswales help water soak into the ground, which replenishes groundwater supplies.

Maintenance

If bioswales are not properly maintained, it can cost a lot of money to fix them later. Build-up of large rocks, trash, and overgrown plants can harm how well bioswales work. It is helpful to set aside areas during planning to make maintenance easier, such as leaving space for equipment or ensuring safety for workers. Different types of filters, like grass strips or rock inlets, can trap sediments and dirt. However, if these filters are not cleaned, water might bypass the bioswales because they become blocked. Structural inlets are now more commonly used because they are easier to maintain and effective. Avoid using floating mulch and choose plants that need little care to improve bioswale performance. A four-step process can be used to evaluate bioswales: visual inspection, testing how much water they can hold, using artificial water flow, and monitoring their condition.

Regular checks are needed to keep bioswales working well and looking good. How often inspections happen depends on local rules, but they should be done at least once a year. Inspections can include looking at plants, water, and inlets, or using tools to test them. Some groups use checklists to help with visual checks.

There are ways to know if a bioswale needs maintenance. Bioswales are compared to a standard water absorption rate to decide if repairs are needed. A staff gauge measures how quickly water soaks into the ground. Testing soil chemistry helps find if pollutants like phosphorus or high salt levels are present. Checking the amount of pollutants in water that flows into and out of bioswales also shows how well they work.

Maintenance can involve three levels of care. Aesthetic care includes removing weeds that harm other plants, cleaning trash, and keeping the plants neat. Partial repairs are needed if inlets are blocked by dirt or if plants need replacing. Full repairs are required if bioswales no longer clean water properly and their overall function is poor.

Design

Bioswales go through short, very strong periods of rain, flooding, and pollution followed by dry seasons. It is important to choose plants that grow well in bioswales and understand which plants are best for the job.

There are four types of bioswales that can be built based on the needs of the area.

• Short grass bioswales use low-growing grass that looks like lawns. These bioswales are less effective than vegetated bioswales at cleaning stormwater and keeping water in the bioswale long enough to remove pollutants.
• Vegetated bioswales use taller plants, shrubs, trees, and can include rocks to slow down water flow. This helps keep water in the bioswale longer, allowing it to clean pollutants more effectively. Some plants in these bioswales are especially good at removing chemicals from stormwater.
• Low water use bioswales are useful in dry, hot areas. Xeriscape bioswales collect water only during rain or storms and remain dry otherwise.
• Wet bioswales are similar to wetlands because they hold water for a long time. This allows water to seep into the ground instead of flowing directly into storm drains.

Bioswales need soil that has less than 5% clay. The soil should not be polluted before building. Bioswales should be built with a slope along their length to let sediments settle. The maximum slope is 3:1. Enough space must be left to avoid damaging nearby structures. The drain should be at least 6 inches above the ground to keep water in the bioswale for a longer time. Rocks can be used to slow water flow. Filters are needed to prevent inlets from getting blocked by dirt or trash.

Examples

Two early examples of scientifically designed bioswales for large-scale use are found in the western United States. In 1996, for Willamette River Park in Portland, Oregon, a total of 2,330 feet of bioswale was built and installed to capture and stop polluted runoff from entering the Willamette River. Check dams were added to help trap silt, which reduced by 50% the amount of suspended solids entering the river system.

Another large-scale bioswale was created at the Carneros Business Park in Sonoma County, California. Starting in 1997, the project team worked with the California Department of Fish and Game and the County of Sonoma to design a plan to direct surface runoff around the edge of a large parking area. Surface runoff includes water from building roofs, parking lots, and land to the north of the site. A total of two miles of bioswale was built into the project. The goal was to reduce pollutants from entering Sonoma Creek. The bioswale is lined with grass and has a nearly straight shape. The slope downward is about 4%, and the slope across the channel is about 6%.

A more recent project, called the "Street Edge Alternatives" (SEA) project in Seattle, Washington, was completed in 2001. Instead of using traditional pipes, SEA aimed to create a natural landscape similar to the area before development. The street was 11% more pervious than a standard street and included evergreen trees and bioswales. The bioswales were planted on sloped areas with wetland and upland plants. Other landscaping used native and salmon-friendly plants. The project helped reduce stormwater runoff, which protected Seattle’s creek ecosystems.

The New York City Department of Environmental Protection (NYC DEP) has built more than 11,000 curbside bioswales, which are called "rain gardens." Rain gardens are built throughout the city to manage stormwater and improve the quality of water in city waterways. The care of rain gardens is a partnership between the NYC DEP and a group of citizen volunteers called "harbor protectors." Rain gardens are checked and cleaned at least once a week.