Environmental Engineering Reference
In-Depth Information
and summed. These calculations are summarized in the
following table:
accumulation rate is greatest at the time immediately
following the last runoff or street-sweeping event, and
then decreases and eventually approaches a constant
rate due to several factors, such as wind and vehicle
effects or the chemical and biological decay of some
constituents. An expression for estimating the removal
coefficient,
ξ
(d
−1
), is given by (Novotny et al., 1985)
Unit
Loading
Rate (kg/
ha/year)
Percent
of Total
(%)
Area
(ha)
Soil
Group
Total
(kg/yr)
Land Use
ξ=
0 0116
.
e
−
0 08
.
H
TS WS
(
+
)
(6.8)
Residential
24.3
62
B
1.01
24.5
Commercial
8.9
23
B
1.79
16.0
Industrial
4.1
10
B
1.51
6.1
where
H
is the curb height (cm), TS is the traffic speed
(km/h), and WS is the wind speed (km/h). The coeffi-
cient of correlation between
ξ
predicted by Equation
(6.8) and
ξ
derived directly from measured data was
reported to be 0.86. Equation (6.7) can be integrated to
yield the following expression for the pollutant buildup
as a function of time:
Open space
2.0
5
B
0.09
0.2
Total
39.3
100
46.8
Based on these results, it is expected that the catch-
ment runoff will contain a TP load of 46.8 kg/yr.
p
(
)
+
m t
( )
=
1
−
e
−
ξ
t
m e
( )
0
−
ξ
t
(6.9)
ξ
6.2.2.2 Buildup-Wash-Off Models.
Although wet
weather urban runoff originates from both pervious and
impervious surfaces, most urban runoff comes from
impervious surfaces and typically only larger storms
yield appreciable runoff from pervious areas. Pollutant
loads from impervious areas are generally related to the
accumulation of solids on roadways, and a two-step pro-
cedure is commonly used to estimate pollutant loads
from these areas. In the first step, a model is used to
quantitatively predict the buildup of solids as a function
of time, and in the second step a model is used to quan-
titatively predict the wash-off of the accumulated solids.
To obtain the loading of pollutants, the solids loads
are multiplied by the pollution content of the solids.
Although exact calculation of pollutant accumulation
and wash-off on street surfaces is highly uncertain, the
models described below have been incorporated into
urban watershed models, and the users of such models
should be familiar with the basic concepts embodied in
these models.
where
m
(0) is the initial load of solids. In these types of
processes, there is always a tendency to attain an equi-
librium, whereby Equation (6.7) yields
dm
dt
=
0 when
p m
=
ξ
(6.10)
eq
and Equation (6.7) can be written in terms of the equi-
librium solids accumulation,
m
eq
, as
dm
dt
= −
ξ(
m m
−
)
(6.11)
eq
This expression elucidates the fact that the accumula-
tion rate of solids in street curb storage can be either
positive or negative, depending on whether the initial
pollutant load,
m
(0), is greater or smaller than the equi-
librium load
m
eq
. Statistical evaluation and validation of
the buildup equation, Equation (6.9), are described by
Novotny et al. (1985), and these results showed that the
removal coefficient,
ξ
, in medium-density residential
areas was fairly constant, attaining values of about 0.2-
0.4 d
−1
, meaning that approximately 20-40% of the
solids accumulated near the curb on the street surface
is removed daily by wind and traffic.
The input,
p
, of solids into curb storage from the
three major sources: refuse deposition (
p
r
), atmospheric
dry deposition (
p
a
), and traffic (
p
t
) can be combined into
the following loading estimate:
Buildup Models.
Almost all street refuse can be found
within 1 m (3 ft) of the curb, and accumulation of pol-
lutant mass is typically expressed per meter of curb. The
mass balance accumulation function for street solids is
typically expressed in the form
dm
dt
= −ξ
p m
(6.7)
where
m
is the amount of pollutants in curb storage
(ML
−1
),
t
is time (T),
p
is the sum of all pollutant inputs
(ML
−1
T
−1
), and
ξ
is a removal coefficient (T
−1
). Equation
(6.7) quantifies a process in which the pollutant
p
=
p
+
p
+
p
t
(6.12)
r
a
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