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12.5 Urban Runoff Control Practices 429

Figure 12.15 Roof Garden of Rockefeller Center, NY
(Source: Newyork-http://upload.wikimedia.org/wikipedia/en/7/75/
Rockefeller_ 2007.jpg)

pollutant removal and are therefore used as pretreatment for the removal of suspended
solids from runoff prior to more intensive treatment by other practices. The two major
types of vegetative practices commonly used in urban areas are vegetated swales and filter
strips (both sometimes referred to as biofilters). Native vegetation is recommended since it
requires less site preparation and maintenance.
12.5.5.1 Vegetated Swales Vegetated swales are channels covered with vegetation to re-
duce erosion of soil during storms (Fig. 12.16). They are used to replace conventional
catch basin and pipe network systems for transporting runoff to surface waters. Stormwater
runoff flows through the swale reducing runoff velocity and promoting the removal of sus-
pended solids.

Pollutant Removal. Infiltration of the runoff and associated pollutants is the most impor-
tant pollutant removal process accomplished by vegetated swales. Vegetated swales also
remove pollutants through filtering by the vegetation and settling of solids in low-flow
areas. Because of these pollutant removal mechanisms, swales are most effective at remov-
ing suspended solids and associated pollutants, such as metals. The infiltration mechanism
also removes bacteria.

Design Considerations. Pollutant removal in vegetated swales can be increased by re-
ducing runoff velocity: reducing the slope, increasing the vegetation density, and installing
check dams to promote ponding. Also, the underlying soils should have a high permeabil-
ity to help promote infiltration.

Maintenance Requirements. Maintenance is aimed at preserving dense vegetation and
preventing erosion of underlying soils. This maintenance includes regular mowing, weed
removal, and watering during drought periods and after initial seeding. In conjunction with
mowing, the cut material should be removed.