A Simple Technique for Stack Design in Complex Terrain

Abstract

The problem is addressed of determining the appropriate height of a stack in the vicinity of a terrain feature, such as a hill or a ridge, that is a few hundred meters high. The stably stratified atmosphere normally produces the poorest dispersion conditions for a site in rugged terrain and the required stack height should be determined for this type of atmosphere. A worst case analysis for dispersion in the air layer that crosses over a terrain obstacle in very stable, low Froude number flow is developed. It is suggested that a critical design condition for a stack would be given by the determination of the effective stack height such that the plume just reaches the air layer that flows over the terrain feature in the most stable atmosphere that is likely to occur. The most stable atmospheres normally occur during the winter with snow cover and clear skies. For this design condition, the plume would then be lifted clean of the terrain in the less stable atmospheres during the rest of the year when vegetation is active. Moreover, the second highest ground level concentration should then be within the regulatory limits.