E.4.4 Example mixing-zone calculation

L = predicted mixing zone length, m

= cross-sectional average velocity (instantaneous or averaged over a few

minutes), m/sec-1

Tc = time taken for the observed value of to change by 10 percent, in

seconds

b. Step 5.2. The lateral dispersion of the effluent plume will not be

restricted by opposite bank of the receiving water body as long as:

8 Et L

W≥

(E-8)

where W is surface width of receiving water channel, m.

c. Step 6 - Estimate maximum width of mixing zone. The maximum width

of the mixing zone (measured perpendicular to the bank as shown in

Figure E-1 can be estimated as:

0.4840 Qe C e

(E-9)

u( C s - C b )d

where Y is maximum width of the mixing zone, m.

E.4.4 Example mixing-zone calculation

Following is a hypothetical mixing-zone calculation designed to illustrate the

use of the mixing-zone estimation equations. A proposed dredged material

containment area is expected to discharge into a river 480 ft (146.3 m) wide.

From a study of U.S. Geological Survey stream gage records, it is anticipated

that while effluent will be discharged, the lowest river flow will be about

7,600 ft3/sec (212.8 m3/sec) and that the river has a cross-sectional area of

4,000 ft2 (371.6 m2) at this flow rate. The local bed slope of the river is very

variable because of sediment transport. The containment area is expected to

have a peak discharge of 15 cfs. The only effluent contaminant that exceeds

WQS will be cadmium, which is expected to have an effluent concentration of

3.5 ug/L. The background concentration of cadmium in the river is below the

detection limit of 0.1 ug/L, and the applicable cadmium WQS is 0.25 ug/L. It

has been specified that the maximum acceptable mixing-zone size is a 750-ft

(228.6-m) radius centered on the effluent outfall.

Step 1 - Assumptions. Since the purpose of this hypothetical problem is to

demonstrate the use of the mixing-zone calculations, it has been defined so that

all the assumptions on which the calculations depend are valid. Decisions on

whether the assumptions are valid depend largely on the professional judgement

of personnel familiar with the disposal site.

E11

Appendix E Evaluation of Mixing in Surface Waters