Environmental Engineering Reference
In-Depth Information
being violated. Field reconnaissance indicates that the
roadway segment has a litter and street refuse deposi-
tion rate of 10 g/m·day, a dry deposition rate of 1 g/m
2
·d,
a traffic emission rate of 0.05 g/axle·m, a traffic density
of 50 axles/d, and a road condition factor of 0.8. The
height of the curb is 20 cm, the average traffic speed is
60 km/h, and the average wind speed is 8 km/h. It is
estimated that the urban washoff coefficient is 0.1 mm
−1
.
Ten days before a recent 2-hour storm-produced 50 mm
of rainfall, the solids on the roadway segment was esti-
mated to be 0.6 kg/m. (a) Estimate the mass of solids
entering the lake from the storm runoff. (b) What is the
equilibrium level of solids on the roadway?
m t
( )
=
am
( )exp(
0
−
k it
)
(6.21)
U
where the availability factor,
a
, is given by Equation
(6.19) as
a
=
0 057 0 04
.
+
.
i
1 1
.
=
0 057 0 04 25
.
+
.
(
)
1 1
.
=
1 44
.
Since
a
> 1, take
a
= 1. Substituting
a
= 1,
m
(0) = 98 g/m,
k
U
= 0.1 mm
−1
,
i
= 25 mm/h, and
t
= 2 hours into Equation (6.21) yields the residual
mass along the curb at the end of the storm as
m
( )
=
1 98
(
)exp[ ( . )(
−
0 1 25 2
)( )]
=
0 67 g/m
.
Since the mass of solids remaining along the
roadway after the storm is 0.67 g/m, and the pre-
storm mass along the roadway is 98 g/m, the wash-
off is equal to 98 g/m − 0.67 g/m = 97.3 g/m. Since
the roadway segment is 100 m long,
Solution
(a) From the data given, LIT = 10 g/m·d, ATMFL = 1 g/
m
2
·d, SW = 8 m, TE = 0.05 g/axle·m, TD = 50 axels/d,
RCC = 0.8,
H
= 20 cm, TS = 60 km/h, WS = 8 km/h,
m
(0) = 0.6 kg/m = 600 g/m,
k
U
= 0.1 mm
−1
, and
i
=
50/2 mm/h = 25 mm/h. The mass of solids per unit
curb length,
m
(
t
), as a function of time,
t
, is given by
Equation (6.9) as
total solids in storm runoff
=
100
m
×
97 3
.
.
g/m
=
9730
g
=
9 73
kg
Therefore, it is estimated that the storm washes
approximately 10 kg of solids from the roadway
pavement into the lake.
(b) The equilibrium mass of solids,
m
eq
, on the roadway
pavement is given by Equation (6.10) as
p
(
)
+
m t
( )
=
1
−
e
−
ξ
t
m e
( )
0
−
ξ
t
(6.20)
ξ
The solids input into curb storage,
p
, is given by
Equations (6.12-6.15) as
p
44
0 16
m
=
=
=
275
g/m
.
eq
ξ
.
p
=
p
+
p
+
p
r
a
t
The accumulated mass on the roadway is expected
to approach 275 g/m
1
2
1
2
in the absence of any
=
LIT
+
(
ATMFL SW
)(
)
+
(
TE TD RCC
)(
)(
)
wash-off.
1
2
1
=
10
+
( )( )
1 8
+
( .
0 05 50 0 8
)(
)( . )
2
=
44
g/m d
⋅
6.2.3 Stormwater Control Measures
The removal coefficient,
ξ
, is given by Equation
(6.8) as
Stormwater control measures, which are also widely
known as
best management practices
(BMPs), are
defined as devices, practices, or methods for removing,
reducing, retarding, or preventing targeted stormwater
runoff constituents, pollutants, and contaminants from
reaching receiving waters. This definition of SCMs dis-
tinguishes between
structural SCMs
, which are either
engineered and/or bioengineered solutions for manag-
ing stormwater primarily on a site-specific basis, and
nonstructural SCMs
, which are primarily tools to guide
the placement of development or tools to modify actions.
There are a variety of nonstructural and structural
SCMs that are used to attenuate urban pollution prior
to discharge into receiving waters. These methods
generally fall into one of the following five categories:
(1) prevention, (2) source control, (3) hydrologic
ξ=
0 0116
0 0116
.
e
e
−
0 08
.
H
TS WS
(
+
)
=
.
−
0 08 20
.
(
)
(
60 8
+
)
=
0 16
.
d
−
1
Substituting
p
= 44 g/m·d,
ξ
= 0.16 d
−1
,
t
= 10 days,
and
m
(0) = 600 g/m into Equation (6.20) gives the
mass of solids along the roadway at the beginning
of the storm as
44
0 16
[
]
+
m t
( )
=
1
−
e
−
( .
0 16 10
)(
)
600
e
−
( .
0 16 10
)(
)
=
98
g/m
.
The mass of solids along the roadway at a time
t
after the start of the storm is given by Equation
(6.18) as
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