ISCST3 Tech Guide
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 22.214.171.124) 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.