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CORMIX Frequently Asked Questions (FAQ)

Welcome to the Frequently Asked Questions (FAQ) page. This page is designed to be your first approach in finding answers to common questions on mixing zones and how CORMIX models them.

So there is no need to "flame-out" like the guy on the right- just keep cool and check out the links below.

Please don't hesitate - send us your ideas on how we might make this FAQ more responsive to your needs.

Fire breather demonstrating principles of Buoyant Jet Mixing
A fire-breather demonstrates the principles of Buoyant Jet Mixing
Q.1. Where can I obtain a copy/license of CORMIX?

A. CORMIX software is licensed and distributed solely by MixZon Inc

The evaluation/demo of the new windows version of CORMIX is available for internet download from MixZon Inc after completing a site registration.

The first step to licensing CORMIX on your computer is to install the free evaluation version. The latest software release contains new tools for mixing zone analysis, updated hydrodynamics, and mixing zone decision support.

The downloads page contains links to the free evaluation version CORMIX v12.0E. The evaluation version can then be activated/unlocked to commercial versions after buying appropriate license/s from MixZon Inc.

The feature comparison table between various versions of CORMIX v12.0 is published at:

The current licensing and pricing options are published at:

The hydrodynamic models of CORMIX v1.x, CORMIX v2.x CORMIX v3.x (DOS versions), CORMIX-GI v4.x, CORMIX v5.x, CORMIX v6.x, CORMIX v7.x, CORMIX v8.x, CORMIX v9.x, CORMIX v10.x, and CORMIX v11.0x have been extensively updated over the last 25 years.

Therefore, commercial engineering analysis of mixing zones using LEGACY versions (versions less than CORMIX v12.0) is not recommended nor supported.

Please refer to the website for a list of improvements, new features, updates, upgrades, and bug fixes at:

Q.2. Where can I obtain a copy of the latest CORMIX User Manual?

A. The complete CORMIX User Manual (in Adobe Acrobat - PDF format) is available for download, using your MixZon account information from the downloads site (approximately 25.0 Mb).

The User Manual is also part of the CORMIX installation. If you have installed CORMIX v12.0 on your system, the User Manual is located in the installation directory under the ..\docs sub-directory as the file "User_Manual.pdf".

The manual can also be accessed from within the CORMIX UI by clicking on the "User Manual" button.

Q.3. Where and how do I seek additional CORMIX Technical Assistance/Support?

A. Check the technical support page for additional details.

MixZon Inc also offers extended technical support (CorSupport Subscription) and consulting for CORMIX software, mixing zone analysis, and outfall design.

The general procedure is as follows. If you are using CORMIX you will need to locate the corresponding input data file(s) (fileid.cmx) for each case under consideration. You should send this file(s) as an email attachment (no need to compress or alter) with specific questions regarding each case to the contact listed below. When making inquiries, please refer to specific cases and flow module regions within your prediction file(s) (fileid.prd).

Please send help requests within email text or as Adobe Acrobat attachments. Attachments such as Word documents are not accepted because of security risks in opening these files.

Legacy DOS/Windows versions of CORMIX and versions less than the current version, CORMIX v12.0, are NOT supported.

Q.4. Is CORMIX applicable to my problem?

A. CORMIX applications include the most common wastewater discharges where initial mixing zone characteristics are desired. The User Manual has a good primer on initial mixing processes in general and how CORMIX models them in particular.

Also, the links on this website should be helpful. In particular, near-field, boundary interaction, and far-field mixing processes should be thoroughly explored. Pay attention to stable versus unstable flow processes, near-field dynamic attachments, and the schematization of data for CORMIX data entry. The sections on applications and the image gallery should also be examined. Furthermore, look at CORMIX1, CORMIX2, and CORMIX3 system descriptions.

In addition, you can download the evaluation version of CORMIX and determine if CORMIX applies to your case. The evaluation version software allows a no-risk evaluation/demonstration of program capabilities.

Q.5. What type of training/background do I need to understand and use the CORMIX model?

A. Most successful CORMIX model users have a background in physical sciences and/or engineering. Many people apply the model successfully after carefully reading the User Manual and closely following the case studies presented in Appendices B, C, D, and E. Also, the links on this website may be useful.

Please check the training page for scheduled CORMIX workshops.

Q.6. Troubleshoot CORMIX installation issues?

Please refer to the CORMIX installation Troubleshooting Guide, for help related to this issue.

Q.7. How do I get the print function to work?

The save and print features have been DISABLED in the evaluation/demo version of CORMIX. You will need a valid, licensed version of CORMIX to save and print CORMIX output under Windows reliably.

You can save and print the output from the visualization tools like CorVue and CorSpy, directly, from within their respective UIs, using the Save As and Print toolbar buttons.

CorVue, CorSpy visualizations can be directly saved in JPEG, GIF, BMP, TIFF, and PostScript (*.ps) formats, using the toolbar buttons in the UI.

You can also use the Copy (Ctrl + C) and Paste (Ctrl + V) clipboard functionality to copy and paste the current visualizations into any document editor like MS-Word.

Q.8. Have the hydrodynamic simulation models changed from previous versions?

A. Yes, updated hydrodynamic models are used in all software release versions. Specifically, CORMIX1 and CORMIX2 simulation algorithms were modified for terminal level behavior in density stratified crossflow and for representation of the following asymptotic flow regimes: pure jet, pure plume, pure wake, and advected line puff. Additional details about the asymptotic flow regimes can be found in a paper by Jirka, 1999. The CORMIX3 simulation models were extensively revised replacing many near-field analytic solutions with an integral model approach.

Also, CORMIX predictions are within +/- 50% of one standard deviation of available data. When comparing predictions, dilution S should be used for comparison, not concentration. Furthermore, when comparing dilution S values, use the Logarithm of dilution S. All Log10(S) are well within the scatter of associated field and laboratory data.

The CORMIX software is continuously updated with new and improved features, updated hydrodynamics, rule-bases, and fixes for reported bugs. These are documented as part of the CORMIX Quality Assurance - New Features, Updates & Fixed Bug List, published on the MixZon website at

Q.9. How do I interpret plume dimension values and how can I compute/plot plume concentration isoline profiles?

A. The CORMIX User Manual shows plume width and profile definitions in Figure 5.5, and additional advice on plotting isoline concentrations appears on page 76 (New User Manual - 2007 v5.4).

The FORTRAN prediction files (e.g. fileid.prd) list the plume profile definitions used within each module at the beginning of the output for each module.

Plume horizontal width values (BH) are reported as half-widths until bank contact, after which they are reported as full widths. After bank contact, the plume centerline coordinates switch to the contacted bank, which then becomes a plane of symmetry.

Also, in most mixing zone analysis, minimum centerline dilution is often the only acceptable parameter to determine regulatory compliance. The CorVue visualization tool available in CORMIX gives 3-D and 2-D visualizations of minimum centerline concentration, flow dimensions, and regulatory mixing zone compliance.

Q.10. How do I account for an ambient background concentration of a pollutant in the receiving water?

A. As a mixing zone model, CORMIX assumes that sufficient ambient water is available to dilute the effluent to levels that approach the background concentration. Thus it is assumed that the volume flux (m3/s) of the discharge Q0 is much less than the volume flux of the ambient Qa (i.e. Q0 << Qa).

If the above volume and mass flux assumptions hold, then advice for simulating cases with ambient background concentrations as excess concentrations appears in the CORMIX User Manual as an example on page 61 (New User Manual - 2007 v5.4).

Q.11. How do I calibrate parameters in the CORMIX model?

A. CORMIX does not have any user-adjustable parameters. However, it is suggested that you run a sensitivity analysis representing a range of discharge (velocity, density) and/or environmental conditions (depth, velocity, density stratification) likely to occur at your site.

The advanced post-processing tool CorSens available in CORMIX v12.0GT can automatically create input files and execute batch simulations for sensitivity study analysis and outfall design. This tool allows the analyst to quickly assess boundary interaction and stability as well as regulatory mixing zone properties including CCC compliance for a range of discharge and ambient conditions.

Q.12. Why does CORMIX stop predictions before the region in which I need a dilution value?

A. In cases with steady ambient flows, check the prediction file (fileid.prd) for warning messages about limiting dilutions and/or low ambient velocities.

If the ambient velocity is very small (or zero), unsteady ambient flow processes may cause pollutant build-up in the near-field. Therefore the flow behavior and thus dilution will be highly dependent on local ambient geometry. There are no general modeling approaches to determine far-field mixing zones in these low ambient velocity cases. However detailed numerical models may be applied, but the time and expense involved often make these numerical modeling approaches impractical.

In cases with unsteady ambient environments, CORMIX is a steady-state model, whereas tidal cycles are an inherently unsteady ambient environment. In tidal mode, CORMIX accounts for the re-entrainment of the historic plume remaining from the previous tidal cycle. Because it takes time for the historic plume to develop, there is a limited region/time over which this re-entrainment will occur.

Q.13. How does CORMIX simulate tidal re-entrainment?

A. CORMIX is a steady-state model, whereas tidal environments are inherently unsteady. Because most regulatory mixing zone analysis requires "worst-case" dilution analysis, analysts sometimes consider conditions at slack tide (often zero ambient velocity) as representative of the "worst-case". However, minimum initial dilution generally will not occur at slack tide, but shortly after slack tide when the plume re-entrains material remaining from the previous tidal cycle. In tidal mode, CORMIX considers the reduction in initial dilution due to the re-entrainment of material remaining from the previous cycle. It does not consider unsteady build-up of material over several tidal cycles, it assumes complete flushing of the historic plume in the near-field, will occur within a tidal cycle.

If unsteady build-up in the near-field or far-field over multiple tidal cycles is likely at your site, additional methods of analysis may be necessary. Section 8.6 in Technical Guidance Manual for Performing Waste Load Allocations Book III: Estuaries Part 3 Use of Mixing Zone Models in Estuarine Waste Load Allocations (EPA 823-R92-004) discusses the calculation of pollutant build-up over multiple tidal cycles.

Q.14. What do I do if the CORMIX application crashes?

A. If you are using CORMIX v12.0, a popup window will appear stating that you should e-mail your input file to If you get a message that says "traceback error", "what is the FLOAT value for GPO . .", or "what is the value of . . .". In such cases, please send the corresponding data input file (fileid.cmx) as an attached file to technical support with a note about the case.

A vast majority of simulations will execute within seconds, however, depending on the flow classification and your computer configuration, some simulations may take up several hours.

For the "INPDATA" error - please refer to our FAQ at:

Q.15. Where can I obtain related CORMIX technical documents?

A. The CORMIX1 and CORMIX2 Technical Reports are currently available in hard copy format only through NTIS.

Scanned PDF copies of the CORMIX1, CORMIX2, and CORMIX3 Technical reports are available from MixZon Inc. for immediate download.

However, a complete CORMIX documentation set is available from MixZon Inc in online hypertext format. This product - CorDocs- is available in CORMIX v12.0G/GT/GTH/GTS/GTD/GTR and contains the CORMIX1 and CORMIX2 USEPA technical reports as well as the CORMIX3 technical report from the DeFrees Hydraulics lab.

Please, check the CORMIX downloads page for additional information.

Q.16. How does CORMIX account for wind direction on plume trajectory and mixing?

A. In CORMIX, wind velocity specification is used for two purposes:

  1. Wind velocity is used to adjust the heat exchange transfer rates for positively buoyant surface density current mixing for heated discharges.
  2. Wind velocity is used to adjust the turbulence in the surface layer to account for increased mixing for positively buoyant surface density currents.

Therefore, in general, wind is not a directional quantity within CORMIX but is used to adjust density current mixing properties. The required CORMIX ambient schematization assumes a surface velocity field in only one direction. If winds affect the ambient velocity field direction, that information should be captured by your schematization, where the wind-induced ambient velocity direction is specified as the positive x-axis direction.

However, if you have a directionally non-uniform ambient velocity profile and a stable CORMIX flow classification, then CorJet may be applied for detailed near-field turbulent buoyant jet mixing in such a directionally non-uniform velocity field. This application is specified by using skewed velocity profiles. See Appendix E of the CORMIX User Manual for an example application of near-field mixing analysis with a directionally skewed ambient velocity profile.

Q.17. Does CORMIX really give a jump in dilution if I add one port to a multiport diffuser?

A. No. This simply reflects a misunderstanding about the mechanisms controlling multiport diffuser mixing.

Mixing in multiport diffusers after plume merging is primarily controlled by the flux of momentum and buoyancy per unit of diffuser length in relation to the local layer depth Hs. Thus, CORMIX uses the primary controlling variable of flux per length of the diffuser, not the number of ports to determine mixing behavior. Changing the number of ports will not affect CORMIX flow classification or dilution estimates of near-field mixing after plume merging occurs.

However, changing the diffuser length can sometimes have a strong influence on mixing behavior. For instance, in density-stratified ambients, changing the diffuser length can affect flow trapping behavior and density current terminal level formation. CORMIX will alert the analyst to a flow class change when altering diffuser length.

Q.18 Does a discontinuous CORMIX prediction mean that the model is not reliable?

A. No, most certainly not. CORMIX is just reflecting the fact that mixing processes do not always behave in a continuous manner. These phenomena are well supported by experimental data, and CORMIX simply reflects their occurrence.

Flow processes can be unsteady because mixing is a turbulent and often chaotic process, where small changes in initial conditions can affect mixing behavior. Since CORMIX (like all other models) assumes steady-state discharge conditions, some jumps in dilution prediction may occur when changing ambient or discharge variables that force a transition from one flow class to another flow class. Therefore, some jumps in dilution may occur in sensitivity analysis when a change in discharge or ambient conditions causes a change in flow classification, i.e. a flow class transition.

In reality, such a flow class transition in nature will tend to be an unsteady process. In such an unsteady transition, the observed flow process (e.g. a density current upstream intrusion) may appear, dissipate, and then reappear again over time in an unsteady manner.

However, within a given flow class simulation, CORMIX will always give continuous dilution predictions between flow module regions. However, sometimes flow width predictions will experience a jump between flow module regions. This is largely an artifact based upon the definition of the plume width (i.e. concentration profiles based upon maximum centerline versus flux averaged). The concentration profile definition is largely an arbitrary quantity selected for mathematical convenience rather than for the significance of some underlying physical effect.

The most important points are that i) changes in mixing behavior are supported by available data, and ii) CORMIX alerts the analyst to the change in mixing behavior with its extensive program documentation. Thus, the savvy analyst will view this as evidence of the robust nature of CORMIX and should consider it carefully in diffuser design.

Other available models would not even indicate the possibility of a change in physical mixing behavior when changing discharge or ambient variables (e.g. when changing diffuser length or discharge density), and therefore may give "comforting" but misleading continuous dilution predictions.

Q.19. What is the difference between CORMIX and the USEPA PLUMES (Visual Plumes) models?

A. There are a host of distinct differences in usability, range of application, physical processes simulation, and modeling philosophy. A few of them are listed below.

  1. CORMIX emphasizes boundary interaction as it affects mixing processes. Boundary interaction will control discharge stability in the discharge vicinity and is well documented in peer-reviewed scientific literature. Determination of stable versus unstable flow is particularly important for near-field mixing of riverine discharges, or wherever mixing zone boundary interaction information is desired.

    CORMIX accounts for both vertical and lateral boundaries through a process called schematization. Schematization allows the analyst to predict flow behavior on shorelines, benthic regions, and other biologically important and chemically reactive regions. The schematization process does not imply that CORMIX requires a rectangular cross-section (as one publication mistakenly suggests) for successful application. Schematization should encourage the analyst to consider the finite space into which a discharge typically occurs, and the boundaries the flow is likely to interact with.

    PLUMES does not address the effects of vertical or horizontal boundaries on mixing or on discharge stability. This is indicated by the absence of any schematization process within PLUMES. It simply assumes the ambient water body is infinite. The issues of flow stability and/or boundary interaction are never addressed.

  2. CORMIX simulates density current mixing in the far-field. Density currents are gravity-driven flows that collapse into thin horizontal layers and resist the transition to passive diffusion. Sometimes density currents intrude upstream. CORMIX uses length scale methods to simulate upstream buoyant intrusions while density current flows are simulated with an integral model approach. The transition from density current to passive diffusion mixing is computed by a local flux Richardson stability criteria.

    PLUMES does not even consider the existence of this important physical process. For instance, density currents with upstream intrusion stagnation points are not simulated. Instead, PLUMES incorrectly assumes passive diffusion always occurs after the completion of near-field mixing. Whereas a density current will vertically collapse within a thin layer, a passive diffusion process can only increase in the vertical plume dimension. Thus PLUMES incorrectly simulates the underlying physics.

  3. CORMIX simulates near-field wake and Coanda dynamic attachments. Such attachments can cause high pollutant concentrations on the bottom in the vicinity of the discharge. Because of the biological importance of benthic regions, such bottom impact assessment is required in most regulatory mixing zone analyses.

    PLUMES has no mechanisms to check for or simulate dynamic plume attachments in the near-field. Because there is no schematization, an infinite water body is assumed with no local boundary near the discharge port. Thus PLUMES has no mechanisms to determine or report near-field benthic impacts.

  4. CORMIX uses a collection of jet-integral, length scale, integral, and passive diffusion approaches to simulate mixing zones. The CORMIX system contains a collection of about 30 regional flow modules to simulate the physics of mixing zones. In application, CORMIX selects one of several hundred possible combinations of these regional flow modules in sequence to construct a simulation model (i.e. flow class) for a complete site-specific mixing zone analysis. Thus in practical application, CORMIX provides a rule-verified interface to several hundred mixing zone mathematical models. The resulting flow processes simulated and corresponding design considerations are extensively supported by program documentation.

    PLUMES uses jet-integral, length scale, and passive diffusion approaches. It contains three jet-integral models (UM, UDKHG, PDS) and one length scale model (RSB) for near-field mixing. These models should only be applied to a stable near-field without dynamic attachments. But there are no methods or guidance within PLUMES to assess discharge stability. Furthermore, there are no peer-reviewed methods within PLUMES to simulate unstable (where all jet-integral models do not apply) or attached discharge cases, or to model density currents before passive diffusion.

  5. CORMIX provides a rigorous flow classification to determine discharge stability and assure correct model application. CORMIX uses the techniques of artificial intelligence to enforce data consistency, provide an assessment of mixing zone characteristics, and guide in model application. CORMIX contains a rule-base with over 2000 rules to assess discharge and ambient conditions and to evaluate model selection. These rules are extensively documented by the system and are supported by scientific peer-reviewed literature. The CORMIX rules and mathematical models of mixing are developed from over 200 years of historical knowledge gained from laboratory and field experiments on mixing processes. Simulation model selection with CORMIX occurs only after all data is checked for consistency, flow parameters are calculated, and the flow classification has been determined and thoroughly documented.

    PLUMES does not have any documented procedure for model selection. There is absolutely no guidance within PLUMES in regards to what model (Um, UDKHG, RSB) is applicable, or even if the input data given is consistent within internal model assumptions. The very first question asked within the Visual Plumes interface is "What model do you want to run?"

    Astute analysts understand that only after a thorough evaluation of discharge and ambient conditions can the correct choice of a model be made. The selection of a particular model before one has evidence to support its applicability may be viewed as a risky engineering practice.

  6. CORMIX and PLUMES both use similar jet-integral approaches to simulate near-field mixing zones in a stable environment without dynamic attachments. In these cases, both methods will give similar near-field dilution estimates, well within each other and the scatter of available field and laboratory data. However, the only way to assure that both models can be compared and applied is to:

    1. Determine that discharge stability is assured.
    2. Determine that there is no near-field dynamic attachment.
    3. Determine that information desired is needed only in the near-field, and that density current mixing is not important within the regulatory mixing zone.

    In these cases, both CORMIX and PLUMES can be applied. CORMIX will use the jet-integral model CorJet for near-field predictions. In PLUMES, the models Um, UDKHG, or RSB may be applied.

    But the only way to ensure correct model application, is to use a methodology that explicitly evaluates A), B), and C) as shown above. Presently, only CORMIX gives the analyst this assurance, delivered by the system documentation and rule base.

  7. In unstable discharge cases, CORMIX uses length scale analytic techniques for near-field predictions. All jet integral models (including CorJet, Um, UDKHG) do not apply for unstable discharges. The issue of discharge stability and CORMIX techniques for modeling unstable discharges are thoroughly documented within CORMIX and in peer-reviewed scientific literature.

    PLUMES will provide predictions using jet-integral models (UM or UDKHG) for unstable discharge cases. However, there is absolutely no evidence in any scientific peer-reviewed journal to support this approach. Moreover, the application of these jet-integral models in unstable cases leads to unreliable results.

  8. CORMIX uses the CorJet jet-integral model for detailed predictions of stable near-field mixing. CorJet can be used for any arbitrary stable density profile. It can simulate variable ambient current speed and direction as a function of depth. It can even simulate the so-called "nascent density" effect. CorJet is fully 3-dimensional for both single port and multiport diffusers. Furthermore, CorJet correctly simulates the following 4 asymptotic flow regimes: a) Pure jet, b) pure plume, c) pure wake, and d) advected line puff.

    PLUMES contains two jet-integral models, Um and UDKHG. Um predicts 3-dimensional flows for single port discharges, but is only 2-D for multiport diffusers. Um cannot be used in cases with zero ambient velocity. UDKHG predicts 3-dimensional flows for single port and multiport diffuser discharges but assumes unidirectional ambient flow or zero ambient flow velocity.

In summation, situations where both CORMIX and PLUMES can be applied would typically be deep ocean outfalls where near-field mixing is of interest only, and there is no possibility of dynamic bottom attachments. However, there is no procedure within PLUMES to test for the possibility of bottom-attached flows. In addition, if mixing zone information after the near-field is desired, then the possibility of a density current in the far-field must be considered. Again, there is no procedure within PLUMES to model density current mixing.

Only CORMIX contains rule-based documentation of procedures to test for bottom attached flows and density current mixing.

Q.20. Is CORMIX applicable for modeling oil spills in rivers and streams?

A. YES, CORMIX can be used to model river mixing of oil emulsions. However, CORMIX does not simulate the mixing of multiphasic liquids consisting of two or more immiscible liquid phases.

Please download the free evaluation version of CORMIX, CORMIX v12.0E, from the downloads page at MixZon Inc to determine if CORMIX applies to your case/study.

Q.21. Is CORMIX applicable to "submerged but elevated discharges"?

A. YES, CORMIX can be used to model "submerged but elevated discharges" where the port is NOT Deeply Submerged (H0 <= 1/3HD) nor is it Near Surface (H0 >= 2/3HD), i.e. the port elevation (H0) is such that 1/3HD < H0 < 2/3HD.

However, CORMIX1 and CORMIX2 apply to either a Deeply Submerged Discharge where, (H0 <= 1/3HD) or a Slightly Submerged, Near Surface Discharge (H0 >= 2/3HD) as shown in the figure.

When we try to enter a "submerged but elevated discharge" in CORMIX, we get a validation error indicating that the input data do not meet the CORMIX1/2 applicability criterion related to H0 and HD.

This model applicability criterion has been mistakenly construed as CORMIX not being applicable to model "submerged but elevated discharges".

Special advice on this limitation and modeling approach to work around this criterion is available in Section 7.4 of the CORMIX1 Technical Report (Doneker and Jirka, 1990) and Section 5.3 of the CORMIX2 Technical Report (Akar and Jirka, 1991)
CORMIX Definition Diagram

The above applicability criterion of port height (H0) and the depth at discharge (HD) in CORMIX is needed to assure a valid test for discharge stability (deep/stable or shallow/unstable flow conditions) in the flow classification scheme.

If the discharge is well submerged but 1/3HD < H0 < 2/3HD (i.e. a "submerged but elevated discharge"); CORMIX can still be used in the following iterative fashion by the process of proper analysis and schematization:

Case i) For a strongly, positively buoyant jet that tends to quickly rise to the surface, assume that the bottom lies higher so that the port elevation (H0) relative to the "reduced depth" meets the H0 <= 1/3 HD criterion. The CORMIX predictions with this new schematization will be valid if they indicate a stable flow class for this "reduced depth" condition without any unstable recirculation or bottom attachments. If the model predicts an attached flow class, try to re-schematize the bottom, while meeting the above criterion.

Case ii) For a strongly negatively buoyant jet that tends to rapidly sink towards the bottom, assume the water surface is "sufficiently higher" so that the port elevation (H0) meets the H0 <= 1/3 HD criterion. Evaluate the CORMIX predictions to check for stable discharge configurations that would not interact with the actual water surface. If the model predicts surface interaction with the schematized water surface, try to re-schematize the water surface, while meeting the above criterion.

Case iii) If unstable discharge conditions are expected or predicted (this would be indicated if the above assumptions are violated) then the actual port elevation (H0) is frequently of secondary importance, while water depth (HD) is the primary parameter. In this case, a reduced port elevation meeting the H0 <= 1/3HD limits can be specified for modeling purposes.

The experienced user will proceed with careful, iterative evaluation of such complex, and perhaps unusual, "submerged but elevated discharge" cases.

Q.22. The water quality of the effluent is better than that of the river. The discharge concentration input to CORMIX is the concentration above the ambient level. Is there a way to use CORMIX in this situation?

A. YES, CORMIX can be used in such scenarios.

This is the rare case where the effluent discharge would "improve" ambient water quality! In such scenarios:
Enter discharge concentration (C0) as the absolute value of the concentration deficit below ambient background. So if ambient concentration CA = 100 mg/l and effluent discharge concentration C0 = 10 mg/l; the concentration deficit would be 90 mg/l and this is the concentration entered into the CORMIX UI. Interpret the CORMIX results as the deficit below the ambient background.

Q.23. How do I convert CORMIX predicted plume centerline dilutions (Sc) to flux averaged (bulk) dilutions (Sf) and vice versa?

Please use the following conversions to convert between CORMIX predicted flux-averaged and plume centerline dilutions:

Sc = Minimum centerline dilution assumes a Gaussian profile
Sf = Bulk dilution (flux averaged), generally assumes a top-hat profile

Sf = Sc * 1.7 for point source/surface discharges;
Sf = Sc * 1.3 for line source discharges;

You can also refer to CORMIX User Manual, Section 5.2.2, page 74 for this information.

Q.24. Why does CORMIX data input validation check that the actual depth at the discharge location (HD) does not differ from the average ambient depth (HA) by more than +/- 30%?

CORMIX is a steady-state, near-field model that assumes a uniform ambient flow field within the schematized ambient cross-section.

The ambient depth at the discharge location (HD) is an important parameter that controls Near-Field Mixing. The average ambient depth (HA) is a parameter that controls Far-Field Mixing.

As a first step towards flow classification, CORMIX attempts to determine plume boundary interaction and discharge stability for the given ambient and discharge parameters.

However, CORMIX does not have any information a-priori to determine and establish a relationship between the source/discharge scale vis-à-vis the ambient flow scale needed to assess boundary interaction and discharge stability.

Additional information on discharge stability (also discussed in the online training sessions) are published at: &

The above requirement limits the ambient flow discontinuities (due to variations between HD and HA) that would be contrary to the CORMIX assumptions of a uniform flow field with the schematized ambient cross-section.

The above requirement facilitates CORMIX flow classification based on near-field stability checks and plume boundary interaction checks within the assumed ambient schematization.

One can usually get most cases to run in CORMIX with proper schematization.

It usually takes a few iterations to get the proper schematization for a specific case.

Q.25. Why does the CORMIX model require that the input value of multiport diffuser length (LD) be greater than the input local layer depth (HD)?

To have line source mixing behavior (multiport diffuser) in a uniform ambient, the multiport diffuser length LD must be equal to or greater than the depth HD at discharge. In a stratified ambient, LD must be greater than or equal to the applicable layer depth (HS).

CORMIX will output the following validation warnings if the inputs specified do not meet the above requirements:

"You have specified a diffuser length LD = xx m that is less than the local layer depth = yy m. Your input specification appears to be inconsistent with line source modeling assumptions which require the diffuser line to be greater than the local layer depth. Please carefully review line source modeling assumptions and your ambient and discharge schematization.

CORMIX can be used to model a majority of multiport diffuser discharges and maybe still used to model your specific diffuser configuration with appropriate input parameters.

If you need additional technical assistance to model this case, please contact MixZon Support at along with your CORMIX case file and a brief description of the conditions you are trying to model".

As a first step towards flow classification, CORMIX attempts to determine plume boundary interaction and discharge stability for the given ambient and discharge parameters.

However, CORMIX does not have any information a-priori to determine and establish a relationship between the source/discharge scale vis-à-vis the ambient flow scale needed to assess boundary interaction and discharge stability. Hence the CORMIX UI will force you to schematize your inputs so that LD > = HD initially.

Please review information on discharge stability published at:
Stable Discharge Conditions &
UnStable Discharge Conditions

Please review information on schematization at:

Proper schematization (usually an iterative process) of model inputs can help you model situations where the multiport diffuser length is less than the actual physical layer depth. Please carefully review CORMIX model predictions to check for plume boundary interactions with your schematized inputs.

Example for LD, HD/HS Relationship
An example to explain the rationale as to why CORMIX requires input LD >= HD (HS)
Q.26. What is an Internal Hydraulics Jump?

According to a Wikipedia article:
" A hydraulic jump is a phenomenon in the science of hydraulics which is frequently observed in open channel flow such as rivers and spillways. When liquid at high velocity discharges into a zone of lower velocity, a rather abrupt rise occurs in the liquid surface. The rapidly flowing liquid is abruptly slowed and increases in height, converting some of the flow's initial kinetic energy into an increase in potential energy, with some energy irreversibly lost through turbulence to heat. In an open channel flow, this manifests as the fast flow rapidly slowing and piling up on top of itself similar to how a shockwave forms.
Internal hydraulic jumps are hydraulic jumps that occur as internal waves in fluids of different density.

An internal hydraulic jump is a transition in the stratified flow where the flow undergoes a sudden transition from fast and thin flow to a slower and deeper flow. An internal hydraulic jump occurs when a super-critical stratified flow (Froude Number > 1) transitions to sub-critical flow (Froude Number < 1).

Example of internal hydraulics jump at boundary interaction
A Vertical Buoyant Jet, 3DLIF actual experiment by Dr. Philip Roberts and Ozeair Abessi, Georgia Institute of Technology.
The image shows internal hydraulics jumps after boundary interaction and during the transition to far-field.

In certain CORMIX simulations, the initial predicted plume width values from the end of the near-field are corrected in the next predicted far-field module to conserve the mass flux in the far-field. When the computed correction factor is quite large, it is usually due to the small ambient velocity relative to the strong mixing characteristics of the discharge. In such cases, CORMIX predicts localized RECIRCULATION REGIONS and INTERNAL HYDRAULICS JUMPS and concludes that the predicted flow appears to be highly UN-STEADY in the transition to the far-field and that the predicted far-field transition results MAY be unreliable. The end-user is requested to carefully evaluate the level of near-field lateral boundary interaction when reviewing simulation results.


For Bounded Ambient Cross-Sections, LIMITING DILUTION, SLIM = (QA/Q0) + 1 where
     QA is the ambient flow rate of the schematized input cross-section in CORMIX
     Q0 is the effluent discharge flow rate being modeled in CORMIX.

The LIMITING DILUTION is the maximum available physical dilution for the schematized input ambient cross-section. When the CORMIX model predicts a dilution value that is greater than SLIM, it provides a limiting dilution warning as follows:
" Mixing for this discharge configuration is constrained by the ambient flow. The previous module predictions MAY be UNRELIABLE since the limiting dilution cannot be exceeded for this unstable shallow discharge configuration. Carefully evaluate the degree of near-field lateral boundary interaction when reviewing simulation results. "

Q.28. Can CORMIX model the dilution in ammonia and/or pH?

The CORMIX model provides you with plume centerline trajectory and physical dilution S = C0/C where,
C0 is the initial concentration of the effluent and
C is the mixed concentration of the effluent downstream.

You can model the dilution of ammonia by specifying a "Non-Conservative" effluent with a 1st order Decay Rate K in CORMIX, (C = C0*e(-Kt)).
However, keep in mind that most physical dilution process in a regulatory mixing zone study generally have short time scales (a few minutes typically) so pollutant decay may not be a primary issue.

There is no direct way to model pH in CORMIX without knowledge of the buffering capacity of the receiving waters. If you have the pH titration curve associated with the effluent, then you should be able to calculate pH from physical dilution S predicted by CORMIX.

Q.29. Why does CORMIX require a minimum of 3 ports for multiport diffuser specification and modeling?

CORMIX input data validation requires that the number of ports (NOPEN) is a minimum of 3 when modeling multiport diffuser line sources. While two points in geometry define a “line”, two ports do not typically represent “line source” conditions for mixing zone modeling and outfall design modeling purposes. Diffusers with 3 ports or less are typically not recommended for practical and performance reasons. However, we do recommend diffuser line source assumption verification with CorHyd for all CORMIX mixing zone modeling projects. One should determine line source physical properties to evaluate discharge stability and near-field plume merging behavior.

Q.30. Can CORMIX be applied to model accidental release (like a slug or pulse discharge) of effluent into ambient water bodies?

CORMIX is a steady-state, continuous point source discharge model, i.e. dQ/dt = 0 for discharge and ambient flowrates. Accidental discharges (like a slug release or pulse discharge) are not steady-state by definition. One can compare the accidental pulse duration (PD) of the effluent discharge to the travel time (TT) to the location where a dilution prediction is desired. If TT < PD, then a steady-state model like CORMIX may offer reasonable dilution estimates. However, if TT > PD, then dilution predictions may be underestimated.

Q.31. In sediment discharge cases modeled in CORMIX, why is there a difference in the predicted plume sediment concentrations and sediment mass fluxes remaining in the plume, along the plume trajectory?

The decrease in sediment concentration in CORMIX model prediction is due to an increase in physical dilution (due to mixing) AND due to sediment particle settling (dependant on the particle density and fall velocities). However, the decrease in % of mass flux of sediment remaining in the plume is ONLY due to particle settling. That is, even though the sediment plume is more diluted (decrease in suspended sediment concentration), it still has the sediment particles that have NOT yet settled out.

Sediment particle separation and settling from the plume in the near-field is generally limited by the turbulent jet behavior of the plume. The CORMIX prediction methodology utilizes several simulation modules executed sequentially, corresponding to different flow processes and associated spatial regions (using the rule-based, flow classification mechanism). The first module, an “Initial Dilution Module”, assumes that no mass flux is lost in the jet stream due to particle settling. This is reflected in the CORMIX prediction file.

In reality, some mass will be discarded during initial dilution due to particle settling. To more appropriately represent this phenomenon, CorPlot (the plotting tool within CORMIX) will redistribute some of the sediment mass. Some of the total mass that is predicted to be deposited in the subsequent CORMIX module, the “Boundary Interaction Module”, is redistributed back to the discharge point. This redistribution is accomplished using a normal distribution. Conservation of mass is maintained.

Q.32. Can CORMIX be used to model "timed" discharges (i.e. the effluent discharge occurs only for a few minutes or hours in a day)?

CORMIX is a steady-state, regulatory mixing zone model for continuous point source discharges. It includes detailed near-field models with a simplified far-field modeling approach.

CORMIX predictions include the cumulative plume trajectory travel time (TT) at each output step. In general, any CORMIX prediction step where the plume travel times are close to the effluent discharge duration - should give reasonable estimates of steady-state mixing and dilution.

Please also refer to FAQ #30.

Q.33. Can CORMIX be used to model "multiple plumes that overlap or interact"?

CORMIX can directly model the effluent discharge from a multiport diffuser source (i.e., a single line source with multiple effluent discharge ports).

However, the CORMIX model does not model mixing from multiple interacting plumes from separate discharge sources.

One can use the CORMIX-DELFT3D Coupled Modeling Technique to model discharge plumes from multiple separate interacting discharge sources. Please refer to additional information at and
References (under the section titled CorTime - Times Series/Far-Field Model Link).

The first steps in using the coupled modeling technique are to understand the mixing behavior of the plumes from each source and to address the following questions:

  • Has the effluent plume dispersion from each discharge source been modeled separately? Have the effluent plume trajectories and characteristics from each source been analyzed using a sensitivity study for varying ambient and discharge conditions?
  • If the sources include multiple multiport diffusers located nearby, has a hydraulic analysis been completed on each source, and has uniform line source behavior been verified for each source?
  • What is the level of interaction of the predicted plumes from the multiple sources in relation to their source locations, and the space and time scales of this interaction vis-à-vis the regulatory mixing zone or region of interest? CORMIX can be used to model the plumes from each source separately. The resulting prediction from each source can be visualized together in CorVue by specifying x and y offsets. This can provide a basic visual representation of plume interaction from multiple sources. This may be helpful in the development of superposition techniques and image source analysis to model the mixing of interacting plumes in the far-field.
  • Is there detailed ambient time series field data (depths, velocities, densities) needed for the coupled modeling approach?
Reasonable answers to most of the above questions can be obtained by using the CORMIX model independently for each discharge source and in a relatively economical manner. This will facilitate the coupled modeling effort - which requires detailed field data for model setup and calibration and is a much greater undertaking.

Q.34. What is the justification for the use of the "equivalent slot diffuser" concept in CORMIX to model certain multiport diffuser line source designs?

Depending on the multiport diffuser design being modeled, the CORMIX predicted flow classification, and mixing complexity, the CORMIX2 model can simulate the discharge from a multiport diffuser line source as multiple individual plumes before and after merging or as a two-dimensional plume from an "equivalent slot diffuser".

The basic argument of the two-dimensional slot diffuser concept used in CORMIX is that after individual plumes merge the flow field of a multiport diffuser is equal to that of a slot discharge. In other words, the initial three-dimensional details of the individual jets from each port of the multiport diffuser have no distinguishable effects at longer distances from the diffuser and after the adjacent/neighboring plumes have merged.

This concept is physically reasonable, as the equivalent slot diffuser has the same kinetic energy input per unit length, which is ultimately expressed in turbulent jet mixing. This behavior of laterally limited jets is analogous to nozzle shape effects on free turbulent jets. After a certain characteristic distance, the initial nozzle shape effects are no longer felt and all jets approach an axisymmetric shape. The equivalent slot diffuser concept is useful for multiport diffuser classification as the number of governing parameters is reduced. As applied within CORMIX, it has been found to be of sufficient accuracy for multiport diffuser design and analysis over the past 30 years.

If more details for one of the individual jets/plumes from a single port of multiport diffuser in the initial region before merging is required, a subsequent application of CORMIX1 (single port discharge) modeling is recommended.

Justification and validations of the "equivalent slot diffuser" concept are provided in Jirka and Harleman, 1973 (MIT EL 73-014) and CORMIX2 references.

Q.35. What is a "Control Volume" and why are there only two sets of plume trjectory prediction points (one set for inflow and the other for outflow) in certain CORMIX predictions?

In fluid mechanics, a control volume is a mathematical abstraction used when developing numerical models of complex physical processes. Control volumes in CORMIX are used to represent plume boundary interactions with the surface, bottom, or a density stratified terminal layer. Control volumes are also employed to model unstable circulation regions. Generally, length scales are used in control volume techniques to simulate complex flows where more detailed methods are unavailable.

The boundary interaction region can have very complex hydrodynamics, characterized by turbulence such as eddies, vortices, internal hydraulic jumps, and other flow instabilities. For these regions, there are no mechanistically based mathematical models that predict mixing processes. CORMIX uses the conservation of mass and momentum to determine the output concentration for a control volume. Thus, the input (inflow) and output (outflow) concentrations and dilutions of the plume are known and conserve mass and momentum. The plume concentration profile in control volumes will change from gaussian (at the input/inflow) to a top-hat profile at the output/outflow.

If the specified regulatory mixing zone or the location where predicted plume properties are desired falls within a control volume, then CORMIX will generally report the outflow values at that location in the Session Report. Please carefully review the CORMIX prediction file in such instances.

Q.36. Why are the predicted plume (half) widths, BH, greater than the schematized ambient cross-section width, BS, in the near-field, in certain CORMIX simulations?

CORMIX is a regulatory mixing zone model and assumes that there is enough lateral ambient mixing water in the near-field for initial dilution to occur (in general, most rivers and streams where regulatory mixing zones are applied are much wider than they are deeper). Hence, within the near-field, the CORMIX model does not check for plume lateral boundary interaction. It does check for discharge plume stability and dynamic attachments due to vertical boundaries (bottom, surface, pycnoclines, and stratified ambient density profiles) within the near-field.

At the end of the near-field, the CORMIX model does check for plume lateral boundary interaction (in relation to the schematized lateral ambient boundaries). It provides the end-user several messages and warnings related to the same (if applicable to the case), and/or can make adjustments to the plume profiles, width, and dilutions going forward.

The end-user is requested to carefully review the near-field predictions, boundary interaction messages/warning, and only if required make the necessary adjustmens to the predicted plume near-field dilutons.

Q.37. Can CORMIX be used to model Total suspended solids (TSS)?

Yes, CORMIX may be used to model the physical mixing of TSS in effluent discharges in ambient receiving waters. The CORMIX model predictions of TSS assume that the TSS effluent concentrations get diluted by ambient entrainment just like any other dissolved pollutant, when TSS concentration in the effluent is entered as excess above the ambient background TSS concentration.

Depending on the TSS ambient and effluent discharge chemistry and that of the resulting admixture (and the possibility of coagulation/precipitation) - this assumption may or may not be valid for TSS in the resulting admixture.

We recommend a laboratory analysis of the effluent and ambient TSS properties be done first to determine if the chemistry affects the admixture TSS concentrations beyond that of just physical mixing.

If the laboratory analysis indicates any 1st order decay or growth in TSS in the admixture, such effects can be modeled in CORMIX using the "Non-Conservative" pollutant option and specifying the decay or growth value determined from the lab analysis.