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Step 3 - Estimate lateral mixing coefficient |
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 Step 2 - Identify critical contaminant. Cadmium is the only effluent
contaminant that exceeds WQS for this example. It is therefore unnecessary to
determine the critical contaminant.
Step 3 - Estimate lateral mixing coefficient.
a. Step 3.1. From the problem statements,
(
)
A = 4,000 ft 2 371.6m 2
W = 480 ft (146.3m)
Calculate depth,
A
d=
W
371.6m 2
d=
= 2.54m
146.3m
Check that W ?is greater than or equal to 10 d . It is.
b. Step 3.2. Since the local bed slope can vary because of sediment
transport, the shear velocity should be estimated from the mean velocity.
Calculate the mean velocity by dividing the river flow of 7,600 ft3/sec
(212.8 m3/sec) by the cross-sectional area of 4,000 ft2 (371.6 m2):
(
)
7,600cfs
= 1.90 ft /sec -1 0.579m / sec -1
u=
2
4,000 ft
and calculate the shear velocity of the receiving waters as follows:
u* = 0.1 u
(
)
u* = 0.1 0.579m /sec-1 = 0.0579m /sec-1
c. Step 3.3. In rivers, the lateral mixing coefficient should be estimated as:
Et = 0.3du *
(
)
Et = 0.3 (2.54 m) 0.0579 m /sec-1
Et = 0.0441 m 2 /sec -1
Step 4 - Estimate mixing-zone length. Estimate using the problem
statements:
(
)
Qe = 15 cfs 0.425 m 3 /sec -1
E12
Appendix E Evaluation of Mixing in Surface Waters
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