Environmental Engineering Reference
In-Depth Information
10
-3
A B C D
E
F
15
10
-4
25
50
10
-5
75
100
10
-6
150
200
10
-7
0.05
0.1
1
10
100
x
max
(km)
Figure 9.3
(
uc
max
/
Q
p
) versus
x
max
for six stability classes and a range of plume centerline heights (
H
=
15
to 200 m).
For quick estimation of maximum ground-level pollutant concentrations (with ground reflec-
tion) it is convenient to use Figure 9.3. Here
c
max
is the maximum concentration in
gm
−
3
,
u
µ
is wind speed in m s
−
1
, and
Q
p
must be reckoned in
gs
−
1
. The lettered parameters on top of
the curves are the stability categories; those on the side are the plume centerline heights
H
after
the plume has risen to its equilibrium value. It is seen that for high stacks (
H
µ
50 m), the unstable
categories cause maximum concentrations on the ground relatively near to the stack, and stable
categories cause weak maximum concentration far from the stack.
It must be emphasized that the Gaussian plume model is only an approximation. It works best
on level ground. Because the wind speed
u
appears in the denominator, the GPE cannot be used for
calms, when the wind speed is less than approximately 2 m/s. Also, the modeling distance should
not be extended further than 20-30 km, because wind direction and speed, as well as the dispersion
characteristics (atmospheric stability category), may change over longer distances. Because the
GPE is a steady-state model, the emission rate
Q
p
and plume rise
≥
h
must also remain constant.
Experience shows that within a limited distance and on level terrain, the GPE gives concentration
contours on the ground that are within a factor of two of measurements. In valleys, hills, and urban
areas, aerodynamic obstacle effects need to be considered. Numerous equations exist that work
reasonably well when corrections for terrain complexities are incorporated into the Gaussian plume
model.
9.2.4.3 Plume Rise
There are several empirical equations that allow the estimation of plume rise
h
from known
stack exit conditions. Here we give the Briggs plume rise equation, which is mostly used in
