(abstract from the Users' Manual)

The first criterion for selection of CE-QUAL-RIV1 for an applicaiton is whether the issues can be resolved with a one-dimensional cross-sectionally averaged model. Most riverine water quality issues can be resolved with a 1-D model; that is, in riverine systems, lateral and vertical gradients in water quality constituent concentrations are generally insignificant and unimportant relative to longitudinal gradients. This assumption implicitly means that vertical temperature, density, and chemical stratifications (which can play a dominant role in the water quality of lakes and reservoirs) are nonexistent or negligible for practical purposes. Thus, although this model can be used for run-of-the-river reservoirs, locks and dams, and reregulation pools, the user must first be sure that vertical stratification does not exist or is so minor as to not affect water quality conditions. In addition, where point source discharges are concerned the model should not be applied "near field" where mixing may not have occurred sufficiently to satisfy the 1-D assumption.

The second criterion for selection of CE-QUAL-RIV1 concerns the nature of the flow and the issues; although CE-QUAL-RIV1 was developed for water quality simulations of riverine systems with highly unsteady flow, it can be used for steady flow conditions. However, it may be easier and more economical to use another more simplistic formulation, such as the EPA model QUAL2E, which assumes steady flow. If the issues demand high resolution and the flows change substantially over a period of hours or days, then a fully dynamic model, such as CE-QUAL-RIV1 should be used.

Specific guidance has not been developed to determine at what condition a dynamic flow model should be used. The development of specific guidance would depend on the issues and the required resolution. For example, if daily average (or longer time averages, perhaps even steady-state) predictions are sufficient, then more simplistic modeling approaches may suffice, even for unsteady flow projects. However, if diel fluctuations (and peaks and troughs) are important and various interest groups are sensitive to such fluctuations, then a dynamic model would be necessary for an unsteady flow project. Transient flow conditions can produce substantially greater fluctuations in diel temperature and dissolved oxygen values than the natural diel effects.

Even when a study is highly sensitive and high resolution is required with diel fluctuations, a time-varying water quality model that assumes steady flow may suffice if the flows are relatively constant or change slowly during the simulation with respect to the travel time of the system. However, if the flows change substantially during a day and from day to day, then a dynamic flow model such as CE-QUAL-RIV1 should be considered. The study of riverine water quality resulting from the releases from peaking hydropower dams is a good example of the use of CE-QUAL-RIV1.

Nestler, J.M., Schneider, L.T., and Hall, B.R., 1993: Development of a simplified approach for assessing the effects of water release temperatures on tailwater habitat downstream of Fort Peck, Garrison, and Fort Randall Dams. Technical Report EL-93-23. U.S. Army Engineer Waterways Experiment Station, Vicksburg, MS.

Zimmerman, M.J., and Dortch, M.S., 1989: Modeling water quality of a reregulated stream below a peaking hydropower dam. Regulated Rivers: Research and Management 4: 235-247.

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