Which technique is used to account for ground reflection in Gaussian plume calculations?

Which technique is used to account for ground reflection in Gaussian plume calculations?

• A. Direct numerical integration of boundary layer equations.
• B. Method of images; mirrored plume about ground plane.
• C. Neglecting ground effects altogether.
• D. Doubling the emission rate.

Answer: (B)

Ground reflection in Gaussian plume calculations is modeled using the method of images: you treat the ground as a mirror and place a fictitious source at the symmetric position below the ground plane. The real plume and this mirrored plume together create a concentration field that satisfies the boundary condition at the ground (no normal flux into the ground for a non-depositing surface). In practice, the result is a sum of two Gaussian plumes—one from the actual source at height H and one from the mirrored source at height -H—so concentrations near the ground are enhanced in a way that reflects the presence of the ground. Other approaches don’t capture the boundary behavior properly. Direct numerical integration of boundary-layer equations is more complex than the standard Gaussian approach. Ignoring ground effects misses a major part of the dispersion physics, and simply doubling the emission rate ignores how the reflected plume distributes vertically, leading to incorrect concentration profiles.

Ground reflection in Gaussian plume calculations is modeled using the method of images: you treat the ground as a mirror and place a fictitious source at the symmetric position below the ground plane. The real plume and this mirrored plume together create a concentration field that satisfies the boundary condition at the ground (no normal flux into the ground for a non-depositing surface). In practice, the result is a sum of two Gaussian plumes—one from the actual source at height H and one from the mirrored source at height -H—so concentrations near the ground are enhanced in a way that reflects the presence of the ground.

Other approaches don’t capture the boundary behavior properly. Direct numerical integration of boundary-layer equations is more complex than the standard Gaussian approach. Ignoring ground effects misses a major part of the dispersion physics, and simply doubling the emission rate ignores how the reflected plume distributes vertically, leading to incorrect concentration profiles.