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1.5.6 The Vertical Term

     The Vertical Term used in the complex terrain algorithm in ISC is the same as described in Section 1.1.6 for the simple terrain algorithm, except that the plume height and dispersion parameter input to the vertical term are based on the radial distance, as described above, and that the adjustment of plume height for terrain above stack base is different, as described in Section 1.5.6.1.

1.5.6.1 The Vertical Term in Complex Terrain.

     The ISC complex terrain algorithm makes the following assumption about plume behavior in complex terrain:

     ·The plume axis remains at the plume stabilization height above mean sea level as it passes over complex terrain for stable conditions (categories E and F), and uses a "half-height" correction factor for unstable and neutral conditions (categories A - D).

     ·The plume centerline height is never less than 10 m above the ground level in complex terrain.

     ·The mixing height is terrain following, i.e, the mixing height above ground at the receptor location is assumed to be the same as the height above ground at the source location.

     ·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).  The effective plume stabilization height at the point x,y is given by:

where:

     he= plume height at point x,y without terrain adjustment, as described in Section 1.5.4 (m)

 

     Ht

 

= z½(x,y) - zs = terrain height of the receptor location above the base of the stack (m)

  z½(x,y)

= height above mean sea level of terrain at the receptor location (x,y) (m)

 

     zs

 

= height above mean sea level of the base of the stack (m)

     FT

 

= terrain adjustment factor, which is 0.5 for stability categories A - D and 0.0 for stability categories E and F.

 

The effect of the terrain adjustment factor is that the plume height relative to stack base is deflected upwards by an amount equal to half of the terrain height as it passes over complex terrain during unstable and neutral conditions.  The plume height is not deflected by the terrain under stable conditions. 

 

 

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