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<br />Or'.'Q <br />u~~: . <br /> <br />. Equations available for modeling the diffusion of a plume released <br /> <br />from an elevated smokestack assume that the terrain downwind from the <br /> <br />stack is level. When the downwind terrain is not level, as in the Hunt- <br /> <br />ington Canyon site situation, interpretation of the results of modeling <br /> <br />computations becomes complex. A conservative assumption is that elevated <br /> <br />terrain receives a concentration on its surface equal to that which would <br /> <br />exist at the same coordinates within a diffusing plume were it flowing <br /> <br />over level terrain. Use of this assumption probably results in computed <br /> <br />concentrations which exceed what would be actually observed over the ele- <br /> <br />vated terrain. Observations of flow over models of mountain terrain in <br /> <br />low-speed wind tunnels suggest that streamlines are deformed upward by <br /> <br />elevated terrain and do not impact the terrain at their free air eleva- <br /> <br />tion. <br /> <br />In addition to the complications introduced by irregular terrain, <br /> <br />modeling results include other uncertainties. The final results are sub- <br /> <br />ject to wide-ranging variation, depending on what plume rise equation is <br /> <br />used and what assumptions are made concerning the stability of the atmos- <br /> <br />phere. <br /> <br />At the request of the Bureau of Reclamation, air quality experts of <br /> <br />the Tennessee Valley Authority made independent modeling studies of dif- <br /> <br />fusion to be expected southeast of the Huntington Canyon site. <br /> <br />Their estimates are shown below: <br /> <br />13 <br />