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# ISCST3 Tech Guide

Gaussian Plume Air Dispersion Model

## 6.6 ISC Treatment of Intermediate Terrain

In the ISC Short Term model, intermediate terrain is defined as terrain that exceeds the height of the release, but is below the plume centerline height. The plume centerline height used to define whether a given receptor is on intermediate terrain is the distance-dependent plume height calculated for the complex terrain algorithm, before the terrain adjustment (Section 1.5.6.2) is applied.

If the plume height is equal to or exceeds the terrain height, then that receptor is defined as complex terrain for that hour and that source, and the concentration is based on the complex terrain screening algorithm only. If the terrain height is below the plume height but exceeds the physical release height, then that receptor is defined as intermediate terrain for that hour and source. For intermediate terrain receptors, concentrations from both the simple terrain algorithm and the complex terrain algorithm are obtained and the higher of the two concentrations is used for that hour and that source. If the terrain height is less than or equal to the physical release height, then that receptor is defined as simple terrain, and the concentration is based on the simple terrain algorithm only.

For deposition calculations, the intermediate terrain analysis is first applied to the concentrations at a given receptor, and the algorithm (simple or complex) that gives the highest concentration at that receptor is used to calculate the deposition value.

Figure 1- Linear decay factor as a function of effective stack height, He it squat building is assumed for simplicity.

Figure 2 - Illustrations of two tiered building with different tiers dominating different wind directions.

Figure 3 - The method of multiple plume images used to simulate plume reflection in the ISC2 model.

Figure 4 - Schematic illustration of (a) urban and (b) rural mixing height interpolation procedures.

Figure 5 - Illustration of plume behavior in elevated terrain assumed by the ISC2 model.

Figure 6 - Illustration of the depletion factor FQ and the corresponding profile correction factor P(x,z)

Figure 7 - Vertical profile of concentration before and after applying P(x,z) shown in Figure 6.

Figure 8 - Exact and approximate representation of a line source by multiple volume sources.

Figure 9 - Representation of an irregularly shaped area source by 4 rectangular area sources.

Figure 10 - Effective area and along wind width for an open pit source.

Figure 11 - West Scavenging rate coefficient as a function of particle size (Jindal & Heinold, 1991)