Limitations Of Our Model

Average Re-aeration Rates for Streams. The basic Streeter-Phelps equation used in this chapter only allows for an average re-aeration constant to be used. This is appropriate for many large streams, but smaller streams alternate between riffle areas (small rapids) and calm pools. In order to account for these varying re-aeration areas, one would need to use a numerical methods approach. However, by using an average re-aeration constant, adequate estimates of the DO curve for most streams can be obtained.

Sedimentation of BOD Particles. The basic Streeter-Phelps equation does not allow for sedimentation of raw sewage to the bottom of streams, which could occur in large slow-moving waters. Thus, if a total BOD is used, but only a portion of the BOD is actually in the stream water, consuming DO, the oxygen deficit (D0) would be overestimated. An extended version of the Street-Phelps equation is available to account for the sedimentation of sewage, but it requires measurement of additional (and less readily available) parameters (Metcalf and Eddy, 1991).

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