Geoscience Reference
In-Depth Information
19.2.1 d ilution C apaCity oF r unning W aters
Within limits, dilution is an effective means of dealing with a discharged wastestream.
Immediately beyond the point of discharge, the process of mixing and dilution begins. However,
complete mixing does not take place at the outfall. Instead, a waste plume is formed that gradu-
ally widens. The length and width (dispersion) of the plume depend upon running water geom-
etry, flow velocity, and flow depth (Gupta, 1997). Beyond the mixing zone, the dilution capacity of
running waters can be calculated using the principles of mass balance relation, using worst-case
scenario conditions, and a 7-day, 10-year low low for stream low condition. A simplistic dilution
equation can be written as
QC QC
QQ
+
+
ss
ww
C
=
(19.1)
d
s
w
where
C d = Completely mixed constituent concentration downstream of the effluent (mg/L).
Q s = Stream flow upstream of the effluent, cubic feet per second (cfs).
C s = Constituent concentration of upstream flow (mg/L).
Q w = Flow of the effluent (cfs).
C w = Constituent concentration of the effluent (mg/L).
EXAMPLE 19.1
Problem: A power plant pumps 25 cfs from a stream with a flow of 180 cfs. The discharge of the
plant's ash pond is 22 cfs. The boron concentrations for upstream water and effluent are 0.053 and
8.7 mg/L, respectively. Compute the boron concentration in the stream after complete mixing.
Solution:
QC QC
QQ
+
+
(
180
25
)(.
0 053
)( )( .)
+
22
87
ss
ww
C
=
=
= 1.13 mg/L
d
(
180
−+
25
)
22
s
w
19.3 DISCHARGE MEASUREMENT
The total discharge for a running water body can be estimated by the float method with wind and
other surface effects, by die study, or by actual subsection flow measurement, depending on cost,
time, personnel, local conditions, etc. The discharge in a stream cross-section is typically measured
from a subsection using the following equation:
Q
=
Sum(mean depthwidth
×
×
mean velocity)
n
1
2
1
2
(
)
(
) ×
(
)
Q
=
hh ww
+
vv
+
(19.2)
n
n
1
n
n
1
n
n
1
n
=
1
where
Q = Discharge (cfs).
w n = n th distance from initial point 0 (ft).
h n = n th water depth (ft).
v n = n th velocity (ft/s).
A velocity meter measures velocity ( v ).
 
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