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Step 5 - Check length-dependent assumptions |
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 rate is not known, must be determined from velocity measurements taken at the
u
proposed site. It should be noted that should not be determined over a period
u
of time during which velocity changes occur as a result of changes in the
receiving-water flow rate.
c. Step 3.3. Estimate the lateral mixing coefficient by using one of the
following equations.
In rivers: Et = 0.3 du *
(E-4)
In estuaries: Et = 0.4 du *
(E-5)
where
Et = lateral mixing coefficient, m2/sec-1
d = average channel depth, m
u* = shear velocity, m/sec-1
The values of lateral mixing coefficient are derived from Fischer et al.
(1979) and are based on experimental studies of dispersion in various rivers.
Lateral mixing coefficients have been shown to vary widely from one location to
another, and the above equations give the lowest reasonable values so that
estimates of mixing zone size will be conservative.
Step 4 - Estimate mixing-zone length. If the assumptions presented earlier
are valid, the mixing zone will have a shape similar to the one shown in
Figure E-1. The length of the mixing zone (measured parallel to the bank) can be
estimated as:
2
1 Qe C e
π u ( - )d
L=
(E-6)
Et C s Cb
where
L = mixing zone length, m
Qe = effluent volumetric discharge rate, m3/sec-1
Step 5 - Check length-dependent assumptions.
a. Step 5.1. The flow in the water body near the mixing zone can be treated
as a steady-state flow as long as:
u
L ≤ Tc
(E-7)
10
where
E10
Appendix E Evaluation of Mixing in Surface Waters
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