188.8.131.52 The Vertical Term in Elevated Simple Terrain.
The ISC models make the following assumption about plume behavior in elevated simple terrain (i.e., terrain that exceeds the stack base elevation but is below the release height):
·The plume axis remains at the plume stabilization height above mean sea level as it passes over elevated or depressed terrain.
·The mixing height is terrain following
·The wind speed is a function of height above the surface (see Equation (1‑6)).
Thus, a modified plume stabilization height he´ is substituted for the effective stack height he in the Vertical Term given by Equation (1‑50). For example, the effective plume stabilization height at the point x, y is given by:
= height above mean sea level of the base of the stack (m)
= height above mean sea level of terrain at the receptor location (x,y) (m)
It should also be noted that, as recommended by EPA, the ISC models "truncate" terrain at stack height as follows: if the terrain height z - zs exceeds the source release height, hs, the elevation of the receptor is automatically "chopped off" at the physical release height. The user is cautioned that concentrations at these complex terrain receptors are subject to considerable uncertainty. Figure 1‑5 illustrates the terrain‑adjustment procedures used by the ISC models for simple elevated terrain. The vertical term used with the complex terrain algorithms in ISC is described in Section 1.5.6.
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