Designing and Maintaining a Bioswales

Designing a Bioswale

For best results, enhance and utilize existing natural drainage swales whenever possible. Maintain riparian areas before and during construction to act as “natural” bioswales.

Existing swales can be enhanced with native plants. The thicker and heavier the grasses, the better the swale can filter out contaminants. Additionally, subgrade drains and amended soils may be needed to facilitate infiltration. Other considerations when designing or maintaining bioswales:

• Costs vary greatly depending on size, plant material, and site considerations. Bioswales are generally less expensive when used in place of underground piping.
• Deep-rooted native plants are preferred to promote water infiltration and reduce maintenance. They are also better at controlling erosion.
• Evaluate site soil conditions. Soil infiltration rates should be greater than one-half inch per hour.
• Avoid soil compaction during installation.
• A parabolic or trapezoidal shape is recommended with side slopes no steeper than 3:1.
• Collaborative projects with neighbors and adjacent property owners are usually more effective at reducing runoff.
• Swales should be sized to convey at least a 10-year storm (or about 2.4 inches in 24 hours).

Maintaining a Bioswale

Once established, bioswales require less maintenance than turf grass because they need less water and no fertilizer. Native grasses and forbs are adapted to Montana’s climate and soil conditions. Native plants also resist local pests, disease, and weed infestations.

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Last Modified: 07/05/2007