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 where
t = required time for lateral spreading, sec
L = necessary width of the front edge of mixing zone, ft
r = one-half initial width of the plume at point of discharge (radius of
initial surface mixing), ft
? = turbulent dissipation parameter
Values for ? range from 0.00015 to 0.005 with a value of 0.005 being appro-
priate in a dynamic environment such as an estuary (Brandsma and Divoky
1976). As discussed earlier, values for r will be influenced by the method of
disposal and will be site specific.
The calculated time can then be used to determine the longitudinal distance
the discharge will travel as it is spreading to the required width. This distance
can be computed from:
X =V wt
(E-14)
where
X = longitudinal movement of discharge, ft
Vw = velocity of water at disposal site, ft/sec
t = necessary time of travel, sec
The results of the previous equations can then be combined to estimate the
projected surface area of the proposed discharge. This area can be computed as:
L + 2r
X
A=
(E-15)
2
where
A = surface area, ft2
L = width of front edge of mixing zone, ft
r = radius of initial surface mixing, ft
X = length of the mixing zone, ft
This approach will characterize a proposed discharge by defining the volume
of dilution water per unit time that will be required to achieve some acceptable
concentration at the edge of the mixing zone. Also, the length and width (and
hence the surface area) of the necessary mixing zone will be approximated.
E17
Appendix E Evaluation of Mixing in Surface Waters
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