WATERSHEDS - Water Quality Engineering in Natural Systems - page 204

imperviousness), storm sewers may not be needed, and
storm drainage can be accomplished by roadside swales,
grassed waterways, small creeks, and canals. This type of
natural drainage is typical of suburban communities.
Watersheds with imperviousness less than 10-20%
are typically considered to have minimal impact on the
hydrology and water quality of receiving streams, while
imperviousness greater than 30-50% inevitably causes
biological degradation of receiving streams (NRC,
2009). however, even at low levels of imperviousness,
the distribution of urban development within a water-
shed can be a significant factor affecting hydrologic
impacts on stream segments within the watershed. In
watershed applications, the term “imperviousness” typi-
cally refers to the total impervious area (TIA), which is
the sum of the directly connected impervious area
(DCIA) and the nondirectly connected impervious area
(NDCIA). however, characterizing a watershed by TIA
overlooks the fact that the DCIA is primarily respon-
sible for hydrologic and water-quality impacts on receiv-
ing streams. This limitation notwithstanding, the impact
of imperviousness (i.e., TIA) on stream water quality is
vividly demonstrated in Figure 6.2, which is referred to
in practice as the impervious cover model (Scheuler et
al., 2009). Figure 6.2 makes it clear that although water-
shed impervious cover is a strong factor influencing
stream water quality, other watershed metrics such as
forest cover, road density, and riparian continuity will
also affect water quality. As a consequence, impervious
cover should not be the sole metric used to predict
stream quality, particularly when watershed impervious
cover is very low. Based on the conditions under which
Figure 6.2 was derived from field studies, it is important
to keep in mind that: (1) use of Figure 6.2 should gener-
ally be restricted to application in first- to third-order
streams; (2) Figure 6.2 might not work well in water-
sheds with major point sources of pollutant discharge,
or extensive impoundments or dams located within
the stream network; and (3) Figure 6.2 is best applied
to watersheds located within the same physiographic
region.

In areas with storm sewers, the two primary modes
of drainage are without pretreatment and with pre-
treatment. In drainage systems without pretreatment,
sometimes called positive drainage systems , stormwa-
ter runoff is collected and conveyed directly to the
nearest water body. In drainage systems with pretreat-
ment, stormwater is collected and detained or partially
retained onsite before it enters stormwater conduits.
This type of sedimentation pretreatment or infiltration
removes a significant amount of pollution prior to the
collected runoff being discharged into offsite receiving
waters.
Streams in urban environments are usually stressed
physically, chemically, and biologically. The increased
magnitude and frequency of runoff increases the fre-
quency of a stream reaching its critical erosive velocity,
at which point the stream begins to erode. This causes
deepening, widening, straightening, and sedimentation
problems. Most urban stream corridors have been
straightened, enclosed, or channelized. Such practices
increase channel slopes, which tends to transport the
problems downstream (DeBarry, 2004), and removes
habitat for essential aquatic species, thus degrading the
biodiversity.

6.2.1 Sources of Pollution

The major sources of water pollution in urban areas are
as follows.

Atmospheric Deposition. Atmospheric deposition of
pollutants is divided into wet and dry surface
loading. Wet loading originates from contaminants
absorbed from the air by rain and snow, whereas
dry loading results from atmospheric fallout.
Atmospheric loading originates from both local
and distant sources, and industrial, urban, trans-
portation, and agricultural activities are the most
frequent contributors to the pollution content
of atmospheric deposits. In most larger cities,
the deposition rate of atmospheric particulates in
wet and dry fallout is in the range of 7-30 g/
m 2 ·month. higher deposition rates occur in con-
gested downtown and industrial zones, and lower
rates are typical of residential and other low-
density suburban zones. Location-specific studies
of atmospheric deposition rates usually cannot be
extended to other areas.
Street Refuse. Particles with sizes larger than dust
(>60 µ m) are considered as street refuse or street
dirt. These deposits can be further divided into
median-sized deposits (60 µ m-2 mm) and litter
(>2 mm). Sources of street dirt are numerous
and very often difficult to control; however, it is

Excellent

Good

Fair

Range of water quality

Poor

10 30 40 70 80 90 100

0 20 50 60

Watershed impervious cover (%)

Figure 6.2. Impact of imperviousness on stream water quality.