Appendix E: Evaluation of Mixing in Surface Waters

Appendix E

Evaluation of Mixing in Surface

Waters

E.1 Introduction

This appendix presents a variety of techniques for evaluating the size of

mixing zones for effluent and surface runoff discharges from confined disposal

facilities (CDFs) to surface water. Discussions of the applicability and

limitations of the techniques and procedures for performing the required

calculations or applying the models are presented.

E.1.1 Background

Whenever contaminant concentrations in a dredged material discharge are

above WQS, there will be some limited initial mixing zone (or zone of dilution)

in the vicinity of the discharge point where receiving WQS (WQS) may be

exceeded. It is not possible to set universal standards for the acceptable size of

mixing zones since receiving water conditions vary so much from one location to

another. The 404(b)(1) Guidelines (U.S. Army Engineer Waterways Experiment

Statin (USAEWES 1976)) therefore instruct that, as part of the dredging permit

process, the size of any proposed mixing zone should be estimated and submitted

to the permitting authority. The permitting authority must then consider

receiving water conditions at the proposed site and decide if the proposed

mixing-zone size is acceptable.

Many state regulatory agencies may specify a limit to mixing-zone dimen-

sions as a condition in granting the State water quality certification. In this case,

the mixing zone necessary to meet applicable standards must not exceed the

specified limits.

The size of a mixing zone depends on a number of factors including the con-

taminant or dredged material concentrations in the discharge, concentrations in

the receiving water, the applicable WQS, discharge density and flow rate, receiv-

ing water flow rate and turbulence, and the geometry of the discharge vessel,

pipeline, or outlet structure and the receiving water boundaries. Since the maxi-

mum allowable mixing zone specified by regulatory agencies is usually on the

Appendix E Evaluation of Mixing in Surface Waters