E.4.3 Calculation procedure

d. Step 4. Calculate mixing-zone length.

e. Step 5. Check assumptions that depend on mixing-zone length.

Step 6. Calculate the maximum width of the mixing zone.

Step 1 - Assumptions. In order to apply the analytical solution described in

this section, the following assumptions are required:

a. No major change in cross-sectional shape, sharp bends, major inflows or

outflows, or obstructions to flow exist in the receiving water body in

proximity to the mixing zone.

b. The receiving water body can be reasonably approximated by a shallow

rectangular cross section.

c. The confined disposal area effluent enters the receiving water as a point

source at the bank with negligible horizontal momentum.

d. Differences in density between the effluent and receiving water and in

settling rates of suspended particles within the boundary of the mixing

zone are negligible.

e. The flow condition in the vicinity of the mixing zone can be

approximated as a steady-state velocity flowing parallel to the bank of

the receiving water.

The major cause of dispersion in the receiving water body is the tur-

bulence and shear flow associated with the horizontal water flow.

g. The effluent plume is vertically well mixed, so that contaminant concen-

trations do not vary significantly with depth.

h. The width of the effluent plume is small enough that its lateral

dispersion is not restricted by the opposite bank of the receiving water

body.

Step 2 - Identify critical contaminant. It is necessary to calculate the

dilution required within the mixing zone in order to reach applicable WQS for all

contaminants of concern. This requires an estimate of the effluent

concentrations of regulated contaminants. The contaminant that requires the

greatest amount of dilution should be calculated as described in Chapter 4,

Section 4.3.1, of the main text.

The maximum boundary of the mixing zone will be defined as the isopleth

(line of constant concentration) where the concentration of the most critical

contaminant is reduced to the concentration specified by the appropriate WQS.

It should be noted that if background concentrations exceed the WQS, the

concept of a mixing zone is inapplicable.

Appendix E Evaluation of Mixing in Surface Waters