Environmental Engineering Reference
In-Depth Information
where
V is the average velocity (m/s)
R is the hydraulic radius (m), defined as A/P, where A is the cross-sectional area of the
flow and P is the length of the wetted portion of flow boundary (the wetted perimeter)
S is the slope of the water surface (m/m)
n is the roughness coefficient (available from published tables, dimensionless)
(2) the extra volumes of runoff generated can cause local flooding, especially in first-
order streams. Many early studies on stormwater problems reported these events
(Novotny 2003).
From a qualitative perspective, stormwater is polluted water, and the type of land use
influences the quality of stormwater (Table 12.3).
Industrial land uses exhibit the highest levels of total suspended solids. Solids are
responsible for transporting heavy metals in stormwater, and the higher levels of metals
present at industrial sites confirm this relationship. Fecal coliform levels are highest at
residential sites, and this is likely the result of animal wastes.
After stormwater traverses a given land use, there is a direct relationship between
stormwater quantity and the quality of the receiving water body. Figure 12.8 illustrates
this relationship.
When impervious area increases, runoff volume increases and this leads to a deteriora-
tion of stormwater quality. A decline in the quality of the receiving water body ensues. As
indicated by the small boxes at the bottom of columns 1 and 2, there is also a relationship
between stormwater and groundwater. As shown in column 1, when receiving water qual-
ity decreases, there is a decline in groundwater quality. The box at the bottom of column 2
identifies a relationship between increases in impervious surface and a decline in ground-
water quality. Problems to ecosystems (denoted by “C” on the diagram) result from all of
the quantitative and qualitative characteristics of stormwater. Reducing stormwater flow
TABLE 12.3
Land Use and Stormwater Quality
Parameter
a
Residential
Commercial
Industrial
% Impervious
37.0
83.0
75.0
Temperature (°C)
16.4
16.0
17.9
Total Suspended Solids (mg/L)
49.0
42.0
78.0
Fecal Coliform (mpn/100 ml)
8345.0
4300.0
2500.0
Total Nitrogen Kjeldahl (mg/L)
b
1.4
1.6
1.4
Total Phosphorus (mg/L)
0.3
0.22
0.26
Cadmium (μg/L)
0.5
0.89
2.0
Chromium (μg/L)
4.6
6.0
14.0
Lead (μg/L)
12.0
18.0
25.0
Zinc (μg/L)
31.5
59.0
112.0
Source:
Pitt, R. et al.,
The National Stormwater Quality Database
(NSQD, version
1.1), Department of Civil and Environmental Engineering, University
of Alabama, Tuscaloosa, AL, 2005.
a
Sample size varied slightly for some of the parameters. Overall, the average
sample size for residential was 1050 sites, commercial had 500 sites, and
industrial 525.
b
Kjeldahl nitrogen is the sum of ammonia-nitrogen and organic nitrogen.
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