Page 472 - Fair, Geyer, and Okun's Water and wastewater engineering : water supply and wastewater removal

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430 Chapter 12 Urban Runoff and Combined Sewer Overflow Management

Street tree, typ., offset
Rock or concrete check to street side, 2.5 ft
dams@ 12´ intervals from back of curb

12˝ min. area
Min. 12˝ flat area w/max 4:1 slope
next to sidewalk
Standard curb
w/curb cut spillways
Top of sidewalk
elev. street
gutter ele Street surface

6˝ min. from
curb cut to
12˝ overlap bottom of swale
Line street edge w/
3:1 max. side slopes
impermeable
membrane or clay
12˝ sandy loam topsoil
Rock trench width 3 ft. minimum
Swale width 7 ft. minimum in public ROW

Use woven monofilament If needed per design procedure:
1
3
filter fabric, Geotex WM- 1 2 ˝ 4 ˝ washed drain rock, except
111F or fabric with equivalent Not to scale in tree wells, minimum void ratio
strength and permeability, (V%) 30%, trench width (3 ft minimum)
to separate topsoil from drain and depth to be determined per
rock, no fabric in tree wells. surface infiltration facility design procedure
Figure 12.16 Cross-Section of Vegetated Swale. Conversion factors: 1 1 in. 25.4 mm;
1 ft 0.3048 m.
(Source: City of Portland BES)
Limitations on Use. Vegetated swales might be difficult to retrofit in already developed
areas. They can replace curb and gutter drainage systems, but work best in low-slope areas
with soil that is not susceptible to erosion.
12.5.5.2 Filter Strips Filter strips, shown in Figure 12.17, are similar to vegetated swales.
Runoff entering these systems, however, generally is sheet flow, is evenly distributed across
the filter strip, and flows perpendicular to the filter strip. Because these systems can accept
only overland sheet flow, level spreading devices are used so that water is not ponded.

Pollutant Removal. Pollutant removal in filter strips depends on the filter strip’s length,
size, slope, and soil permeability; the size of the watershed; and the runoff velocity. Filter
strips are most effective at removing pollutants such as sediment, organic material, and
some trace metals, and less effective at removing dissolved pollutants such as nutrients.

Design Considerations. The major design aspects of filter strips that can be effectively
changed are the length, width, slope, and vegetative cover of the strip. Greater pollutant
removal is achieved with filter strips that are long and flat. A level spreading device must be
incorporated in the design of a filter strip to ensure that concentrated flow does not enter and
create a channel. If concentrated flows enter a filter strip, they can cause erosion of the vegetation