Mixing Zones
The mixing zone is the length of the river in which it is not homogeneous after
a point discharge. A river is seldom found homogeneous in nature, but for modeling
purposes the river or stream is considered homogeneous for simplicity. The reason one would
calculate the mixing zone would be for permitting and waste allocation laws. There are several
assumptions that must first be made.
- Upper stream is considered homogeneous with respect to water quality
variables
in the lateral and vertical directions
- No longitudinal mixing in the river
- No mixing of one parcel of water with another due to dispersion or
velocity gradients
The river, therfore, can be assumed as plug flow. This is a good assumption for streams
at steady state. For a point discharge on the bank of a river, the mixing zone can be
determined as follows (as derived by Yotsukura, 1968):
For the modeling of a midstream discharge, the mixing zone is defined as:
where for both equations:
- Lm = distance from the sourse to the zone where the discharge has been well mixed laterally in ft
- U = average stream velocity in fps
- B = average stream width in ft
- H = average stream depth in ft