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1.5 ISC COMPLEX TERRAIN SCREENING ALGORITHMS

The Short Term model uses a steady‑state, sector-averaged Gaussian plume equation for applications in complex terrain (i.e., terrain above stack or release height).  Terrain below release height is referred to as simple terrain;  receptors located in simple terrain are modeled with the point source model described in Section 1.1.  The sector average approach used in complex terrain implies that the lateral (crosswind) distribution of concentrations is uniform across a 22.5 degree sector.  The complex terrain screening algorithms apply only to point source and volume source emissions;  area source and open pit emission sources are excluded.  The complex terrain point source model, which is based on the COMPLEX1 model, is described below.  The description parallels the discussion for the simple terrain algorithm in Section 1.1, and includes the basic Gaussian sector-average equation, the plume rise formulas, and the formulas used for determining dispersion parameters.

1.5.1 The Gaussian Sector Average Equation

The Short Term complex terrain screening algorithm for stacks uses the steady‑state, sector-averaged Gaussian plume equation for a continuous elevated source.  As with the simple terrain algorithm described in Section 1.1, the origin of the source's coordinate system is placed at the ground surface at the base of the stack for each source and each hour.  The x axis is positive in the downwind direction, the y axis is crosswind (normal) to the x axis and the z axis extends vertically.  The fixed receptor locations are converted to each source's coordinate system for each hourly concentration calculation.  Since the concentrations are uniform across a 22.5 degree sector, the complex terrain algorithms use the radial distance between source and receptor instead of downwind distance.  The calculation of the downwind, crosswind and radial distances is described in Section 1.5.2.  The hourly concentrations calculated for each source at each receptor are summed to obtain the total concentration produced at each receptor by the combined source emissions.

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