University of New Hampshire Stormwater Center
2007 Annual Report
About Vegetated Swales
General Information Performance Data Water Quality Treatment Process Design
Vegetated swales like the one pictured above are widely accepted and implemented. While this system demonstrated better water quality treatment performance than the stone-lined swale previously evaluated at UNHSC, it did not meet EPA treatment requirements and is negatively influenced by cold climate conditions.
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About Vegetated Swales
Vegetated, dry, wet, or stone-lined—stormwater swales are open, channel-like structures that are used to convey stormwater runoff. The vegetated swale evaluated at the UNHSC should not be confused with the more complex “water quality swales,” or “bioswales,” which are often designed with modified soils and subdrains. It is a trapezoidal channel designed for minimal slope and maximum flow velocity. Its ability to remove pollutants is modest at best, and—being vulnerable to large, high-velocity storm flows—its effectiveness will likely decline with age.
Where to Use It
Though their designs vary widely, vegetated stormwater swales are the most commonly employed stormwater management system. They often serve as stormwater drainage infrastructure in lieu of curbs, gutters, and stormwater sewer systems. Frequently, they are located along property boundaries or roads and constructed to suit the natural grade. They can be used wherever the site provides adequate space and elevation.
Design and space constrictions generally restrict the use of swales in urban settings, in areas with highly erosive soils, or where dense vegetative cover is difficult to maintain.
Vegetated swales are widely accepted. Since they are inexpensive and require little site modification, they are the stormwater treatment of choice in many areas. In some places, they are used for pretreatment and combined with another system that provides primary water quality treatment. While designed to function like streams, vegetated swales rarely have a stream’s structural complexity, and as such are prone to sedimentation and erosion. This is compounded by poor—or nonexistent— maintenance, which leads to contaminant export into receiving waters and the degradation of the swale’s channel.
The cost to install a vegetated swale to treat runoff from one acre of impervious surface was $12,000. This does not include maintenance expenditures, which may involve routine inspection and the periodic mowing or removal of collected sediments, as needed. However, our observations reinforce stormwater manual assessments that maintenance requirements for these systems are generally minimal. That being said, vegetated swales do need inspection, since in-system erosion can lead to failure and/or the export of pollutants.
You can access more information about the design for this system by contacting UNHSC.
Table of Contents
About the Center
About the Field Site
How We Evaluate Performance
Stormwater Treatment Performance Comparison
How to Read this Report
Stormwater Treatment System Data
Resources for Land & Water Management
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This publication was produced in partnership with the UNH/NOAA Cooperative Institute for Coastal & Estuarine Environmental Technology.