ISCST3 Tech Guide

Gaussian Plume Air Dispersion Model

7.1.5 The Dispersion Parameters

Point Source Dispersion Parameters.

See Section 1.1.5.1 for a discussion of the procedures use to calculate the standard deviation of the vertical concentration distribution Fz for point sources (sources without initial dimensions). Since the long term model assumes a uniform lateral distribution across the sector width, the model does not use the standard deviation of the lateral dispersion, Fy (except for use with the Schulman-Scire plume rise formulas described in Section 1.1.4.11).

Lateral and Vertical Virtual Distances.

See Section 1.1.5.2 for a discussion of the procedures used to calculate vertical virtual distances. The lateral virtual distance is given by:

Formula

where ro is the effective source radius in meters. For volume sources (see Section 2.2.2), the program sets ro equal to 2.15Fyo, where Fyo is the initial lateral dimension. For area sources (see Section 2.2.3), the program sets ro equal to xo/square root of piewhere xo is the length of the side of the area source. For plumes affected by building wakes (see Section 1.1.5.2), the program sets ro equal to 2.15 Fy´ where Fy´ is given for squat buildings by Equation (1-41), (1-42), or (1-43) for downwind distances between 3 and 10 building heights and for tall buildings by Equation (1-44) for downwind distances between 3 and 10 building widths. At downwind distances greater than 10 building heights for Equation (1-41), (1-42), or (1-43), Fy´ is held constant at the value of Fy´ calculated at a downwind distance of 10 building heights. Similarly, at downwind distances greater than 10 building widths for Equation (1-44), Fy´ is held constant at the value of Fy´ calculated at a downwind distance of 10 building widths.

Procedures Used to Account for the Effects of Building Wakes on Effluent Dispersion.

With the exception of the equations used to calculate the lateral virtual distance, the procedures used to account for the effects of building wake effects on effluent dispersion are the same as those outlined in Section 1.1.5.3 for the short-term model. The calculation of lateral virtual distances by the long-term model is discussed in Section 2.1.5.2 above.

Procedures Used to Account for Buoyancy-Induced Dispersion.

See the discussion given in Section 1.1.5.4.