E.2.3 Recommended models and techniques

The rate of change of water velocity resulting from tidal effects can also

cause problems. The time taken for material to travel the length of the mixing

zone should be an order of magnitude smaller than the time taken for a

10-percent change in the mean water velocity. It may be possible to satisfy this

condition in a river, but it will probably not be possible to do so in most estuaries

during a significant portion of the tidal cycle.

Another potential difficulty in estuaries is the phenomenon of stratification.

Estuaries with low water velocities sometimes have a layer of relatively fresh

water near the surface with a much more saline denser layer of water near the

bottom and with quite a distinct interface between the two layers. The abrupt

change of density at the interface tends to inhibit vertical mixing through the

entire depth of the water column.

E.2.3 Recommended models and techniques

Several models and approaches for evaluation of initial mixing for CDF

discharges are provided in this appendix. Table E-1 provides a summary of the

characteristics of the various types of dredged material discharges,

hydrodynamic environments, and the models recommended for use in evaluation

of initial mixing for those conditions. Descriptions of each of the models and

details on applying the models are provided in the following sections of this

appendix.

Table E-1

Summary of Hydrodynamic Conditions and Applicable Models for

CDF Effluent and Surface Runoff Discharges

Applicable Model or

Technique

Model Hydrodynamics

Section

Conditions

Dilution Volume

Steady Uniform

General

MacIntyre

Steady Uniform

C4.0

Riverine

CORMIX1

Steady Uniform

C3.0

Tidally influenced Rivers

TABS2

Unsteady Nonuniform

C5.0

and Estuaries

CD-CORMIX has not been developed and verified for national application. However, the

fundamental processes contained in CD-CORMIX are applicable for continuous pipeline

discharges and this model is currently under investigation for future use.

TABS has not been developed and verified for national application for the indicated discharges.

However, the fundamental far-field processes contained in TABS are applicable for the indicated

discharges and this model can be adapted for use on a regional basis. Note that the TABS model

computes far-field effects only. Some independent near-field analysis is usually required.

E.3 Cornell Mixing Zone Expert System (CORMIX)

The Cornell Mixing Zone Expert System (CORMIX) is a steady state three-

dimensional (3-D) model (Donekar and Jirka 1990). CORMIX was developed to

predict the dilution and trajectory of a submerged single port discharge of

arbitrary density (positive, neutral, or negative) into a stratified or uniform-

density ambient environment with or without cross-flow. CORMIX is an

Appendix E Evaluation of Mixing in Surface Waters