Bio-Swales

You’ve likely heard this term at least once but may not have known what it was. A simple definition of a bioswale would be “any vegetated swale, ditch, or depression that conveys storm water.” Simple enough! But there is a lot more that goes into the planning and design of these nature-mimicking solutions.

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Why do we need them?

 

Bioswales have become more important as building is increasing leaving less land in its natural condition. Cities and counties are passing new legislation to require and regulate bioswales in common interest developments.

Bioswales mimic natural habitats as they are designed to filter out pollutants and debris from storm water before it enters the watershed or storm sewer. Therein, they reduce storm water runoff and improve storm water quality. In common interest developments bioswales are positioned near roadways and parking lots as there is substantial automotive pollution that settles on the paving.

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How do they work?

Bioswales have a slope sufficient enough to ensure water will flow in the desired direction but not so great that water rushes down. The purpose of the bioswale design is to maximize the time water spends in the swale allowing the plant material and soil to soak up the pollutants and catch the debris therein slowing and filtering the stormwater.

In planning community developments, bioswales are designed to handle the amount of water that is anticipated from the planned areas. A well thought out bioswale is made of soil that is absorbent, usually containing higher amounts of nitrogen and phosphorus to increase water retention.

Plant material in bioswales is chosen based on its ability to withstand heavy watering and drought conditions, use of nutrients efficiently, prevention of erosion, and absorption of pollutants. In selecting vegetation, the following factors should be considered: climate; expected pollutants; expected depth of water; and season with the most water runoff. There are plants to meet the different needs of the bioswale; plants to absorb the heavy metals; plants for erosion control; plants to absorb the nitrogen and so on. Plant material that is dormant October through May would reduce the speed of the storm water runoff but won’t be as beneficial to the bioswale since its absorption will be low when there is likely to be the most rain water needing to be filtered. Some local landscape ordinances provide lists of appropriate plants and grasses for bioswales.

You often see filters or large rocks placed in bio-swales. These are used to prevent clogging of the system, catching larger debris.

What does Maintenance look like?

Bioswales need to be maintained. The slope of a bioswale can increase over time causing displacement of the soil and plant material. This erosion can be abated or slowed by replacing plant material and ensuring plant growth on the slope. The plants’ roots help to hold the slope in shape as they grow into the ground as anchors.

Maintenance of a bioswale includes removing debris caught in the plant material, filters, and rocks; installing new plant material as older plant material declines; and adding new soil.

JPA offers bio-swale maintenance services as part of our landscape maintenance. Our crews monitor the bioswales to ensure they meet city standards. We also complete inspection reports required by locals.

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