Step 3 - Estimate of lateral mixing coefficient

This approach for calculating required dilution is not applicable to turbidity

(an optical property of water), which is reduced in a nonlinear fashion by

dilution. A correlation curve for total suspended solids (TSS) versus turbidity

may be used to define the TSS concentration corresponding to the WQS for

turbidity. Such correlation curves will need to be empirically determined for

each discharge.

Step 3 - Estimate of lateral mixing coefficient.

a. Step 3.1. The depth of a simplified rectangular cross section for the

receiving water body should be calculated as follows:

(E-1)

where

d = average depth of the receiving water body channel, m

A = cross-sectional area of the channel, m2

W = surface width of the channel, m

Check to ensure that W is equal to or greater than 10 times the average depth

d. If not, the estimate of a lateral mixing coefficient is likely to be inadequate.

b. Step 3.2. Estimate the shear velocity by one of the following methods.

In rivers where the mean channel slope is known, use:

u * = gds

(E-2)

In rivers where the channel slope is not known, use:

u * = 0.1 u

(E-3)

where

u* = shear velocity in receiving water, m/sec-1

g = gravitational acceleration, 9.81 m/sec-2

d = average channel depth, m

S = slope of river bed (dimensionless)

= average of instantaneous velocities across the channel cross section,

m/sec-1.

If the flow rate of the receiving water is known, can be calculated as the

flow rate divided by the channel cross-sectional area. If the receiving-water flow

Appendix E Evaluation of Mixing in Surface Waters