Environmental Engineering Reference
In-Depth Information
modification, (4) reduction of pollutants and flows in the
conveyance systems, and (5) end-of-pipe pollution con-
trols. Prevention practices prevent the deposition of pol-
lutants in the urban landscape; source control practices
prevent pollutants from coming into contact with pre-
cipitation and stormwater runoff, hydrologic modifica-
tion minimizes the runoff formation from precipitation,
reduction of pollutants and flows in the conveyance
systems involve special channel features for pollutant
attenuation, such as swales and particle removal struc-
tures, and end-of-pipe controls include such treatment
features as wetlands that are immediately prior to dis-
charge into a receiving water body. SCMs are often
mandated by local drainage regulations.
sweeping (more than 1 week apart) tends to have
a poor effect irrespective of the pollutant removal
efficiency of the sweeper. In many cases, street
sweeping is practiced for its aesthetic rather than
water-quality benefits.
Erosion Control. Erosion from pervious areas is
often a source of pollution. Temporary or perma-
nent seeding of grass, sodding, and mulching are
used to reduce erosion. Such measures are impor-
tant and mandated in some areas for control of
pollution caused by construction activities. Cover-
ing an exposed area with any of a number of avail-
able mulches generally increases surface roughness
and storage, protects the surface against rainfall
impact, and subsequently reduces erosion.
Control of Surface Application of Chemicals. Mea-
sures to control surface application of chemicals
include control of herbicide use on pervious
grassed areas (lawns and golf courses). Control of
pollution caused by chemical use by individual
homeowners on their lands is difficult due to a lack
of legal instruments.
Control of Urban and Highway Pollution During
Winter. Deicing chemicals, such as sodium chlo-
ride (NaCl), are the predominant cause of pol-
lution from winter runoff. To diminish the
environmental threats of deicing chemicals and
abrasives, the selection of deicing compounds and
their application rates must be judicial and tar-
geted. The practice of prewetting road surfaces
with liquid (not granular) slurry of a chemical has
been found to be an effective way to reduce appli-
cation rates.
6.2.3.1  Source  Control  Measures.  Source control
measures are the most effective in controlling pollution
from urban runoff. These measures commonly include
reduction of pollutant accumulation on the impervious
surfaces, reduction of erosion of pervious lands, and on-
site runoff infiltration. Some of the most advanced sub-
urban developments have zero-discharge stormwater
management, where urban runoff is dissipated by a suc-
cession of SCMs that promote storage, infiltration, and
evapotranspiration. The most effective source control
measures are described below.
Removal of Solids from Street Surfaces. Removal of
solids from street surfaces is a commonly prac-
ticed source control measure. This includes litter
control programs and street cleaning. Litter
includes paper, vegetation residues, animal feces,
bottles, broken glass, and plastics. In the fall season,
leaves are typically the most dominant component
of street litter, and it has been shown that litter
control programs can reduce the amount of depo-
sition of pollutants by as much as 50%. The average
tree drops 14.5-26 kg (30-60 lb) of leaves per
year, and the leachate from leaves and lawn clip-
pings is a source of phosphorus in urban runoff.
Pet waste can be a source of fecal bacteria, nutri-
ents, and oxygen-demanding compounds in urban
runoff when allowed to be deposited on sidewalks
and urban streets. Many communities have ordi-
nances that regulate pet waste and require proper
disposal of the waste by pet owners. Street flushing
is another practice that washes streets by water
jets delivered from tanker trucks. Flushing cleans
the entire street and not just a narrow strip near
the curb. Sweeping is more common in the United
States, while flushing is more practiced in Europe.
The effect of street sweeping on reducing pol-
lutant loads is related more to frequency of sweep-
ing and less to sweeper efficiency. Infrequent
6.2.3.2  Hydrologic Modiications.  hydrologic modi-
fications of urban watersheds include measures and
practices that reduce the volume and intensity of urban
runoff entering the storm sewer or combined sewer
system. hydrologic modifications can be divided into
the following three practices:
Practices that enhance infiltration,
Practices that increase on-site storage
Practices that reduce the size of impervious areas
directly connected to the sewer system
In applying practices that enhance infiltration, care
must generally be taken since the longevity of these
practices can be severely limited by lack of pretreat-
ment, poor construction practices, application in un-
suitable sites, lack of regular maintenance, and faulty
design. The most common hydrologic modifications are
described in more detail below.
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