Bioswales

What are bioswales?Bioswales are storm water runoff conveyance sys-tems that provide an alternative to storm sewers.They can absorb low flows or carry runoff fromheavy rains to storm sewer inlets or directly to sur-face waters. Bioswales improve water quality byinfiltrating the first flush of storm water runoff andfiltering the large storm flows they convey.

The majority of annual precipitation comes fromfrequent, small rain events. Much of the value ofbioswales comes from infiltrating and filteringnearly all of this water.

Designing a bioswaleFor best results, enhance and utilize existing natu-ral drainage swales whenever possible. Existingswales can be enhanced with native plants. Thethicker and heavier the grasses, the better theswale can filter out contaminants. Additionally,subgrade drains and amended soils may be neededto facilitate infiltration.

Bioswales2 0 0 5. . . absorb and transport large runoff events

A bioswale featuring native vegetation shows its fall colors.

Other considerations when designing ormaintaining bioswales:

• Costs vary greatly depending on size,plant material, and site considera-tions. Bioswales are generally lessexpensive when used in place ofunderground piping.

• Deep-rooted native plants are pre-ferred for infiltration and reducedmaintenance.

• Soil infiltration rates should begreater than one-half inch per hour.

• A parabolic or trapezoidal shape isrecommended with side slopes nosteeper than 3:1.

• Avoid soil compaction during instal-lation.

• Swales should be sized to convey atleast a 10-year storm (or about 4.3inches in 24 hours).

Bioswales

Bioswales

A road ditch can serve as a bioswale. The rock trench and wetland vegetation arenotable features, along with the natural drainageway in the background that serves asa bioswale for residential runoff.

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to the environment?• protects sensitive areas• increases habitat for wildlife by preserving

trees and vegetation• protects local and regional water quality by

reducing sediment and nutrient loads• reduces streambank and channel erosion by

reducing the frequent surges/bounces of high-er flows from storm sewer discharges

• reduces frequent high and low flows associat-ed with surface runoff, stabilizing stream flowvolumes by restoring ground water dischargesinto receiving waters

• may reduce potential for flooding

to residents?• increases community character• improves quality of life• more access to trails and open space• pedestrian-friendly

to developers?• reduces land clearing and grading costs• reduces infrastructure costs

(streets, curbs, gutters, sidewalks)• increases community marketability

to communities?• balances growth needs with environmental

protection• reduces infrastructure and utility

maintenance costs

Why is LID important:

Low Impact DevelopmentTraditionally, storm water management hasinvolved the rapid conveyance of water viastorm sewers to surface waters. Low ImpactDevelopment (LID) is a different approachthat retains and infiltrates rainfall on-site.The LID approach emphasizes site designand planning techniques that mimic the nat-ural infiltration-based, groundwater-drivenhydrology of our historic landscape.Bioswales are one component of LID.

For More InformationFind more information about low impact development andbioswales by visiting the following websites:

www.iowasudas.orgwww.lid-stormwater.netwww.cwp.orgwww.iowastormwater.org

Maintaining a bioswaleOnce established, bioswales require less maintenance than turfgrass because they need less water and no fertilizer. Nativegrasses and forbs are adapted to Iowa rainfall patterns. Nativesalso resist local pests and disease.