About Vegetated Swales
General Information Performance Data Water Quality Treatment Process Design




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System Performance

Water Quality Treatment
The vegetated swale was moderately effective in removing the pollutants commonly associated with stormwater performance assessment. Its performance appeared somewhat better than that of the stone-lined swale tested in the same location the previous year. This system’s median annual average for removal of total suspended solids was in the sixty percent range—below EPA’s recommended level for removal. Likewise, it did not meet regional ambient water quality criteria for removal of petroleum hydrocarbons. Its ability to treat metals was generally strong, but dramatically declined in winter when vegetation was dormant and sediment removal was low. Researchers observed no removal for dissolved inorganic nitrogen or total phosphorus. Like all systems monitored at UNHSC, it does not treat chloride.

The chart at top right reflects the system’s removal of total suspended solids, total petroleum hydrocarbons, dissolved inorganic nitrogen, total phosphorus, and zinc. Values represent results recorded over one year, with the data further divided into summer and winter components.

Water Quantity Control
Swales typically exhibit no peak flow reduction or lag time. Studies have shown they are often prone to destabilization by large storm flows, a tendency that may negatively affect downstream geomorphology. In the figure at bottom right, the vegetated swale demonstrates limited peak flow reduction with short lag times during the summer months. What little impact the swale demonstrated with respect to water quantity control diminished with the onset of winter.

Maintenance
The maintenance requirements of vegetated swales include periodic inspection for erosion and channel destabilization, removal of trash, leaves, and other natural debris, and sediment buildup. Some may also require periodic mowing. No maintenance was performed on the vegetated swale during its year of evaluation at the UNHSC field site.

Cold Climate
In general, winter conditions significantly limit the vegetated swale’s ability to treat water quality and manage water quantity. While it exhibited modest removal of sediment, petroleum products, and metals in the summer, the onset of winter eliminated or significantly reduced its effectiveness. Its performance also varied dramatically in winter months, likely as a result of icing within the system.