Document: NRC Regulatory Guide
Document ID: f4c5fb1d-efb9-4168-9804-5ad3f6f64d06
Document Type: regulatory_guide
Title: Reporting Procedure for Mathematical Models Selected To Predict Heated Effluent Dispersion in Natural Water Bodies
Source: NRC Regulatory Guide Division 4
Source URL: https://www.nrc.gov/docs/ML0037/ML003739535.pdf
Revision Date: 2023-06
Chapter: 
Section ID: RG-4.4
CFR Part: 
CFR Title: 

Content:
esents the acceleration of gravity, dp/dz is the vertical density gradient, and dU/dz is the vertical gradient of the horizontal mean velocity. The quantity (l/p)(dp/dz) represents buoyancy or density stratification and is often referred to as the stability, while dU/dz can be used as a measure of turbulence. As Eq. (A-7) indicates, the Richardson number increases with increasing stability, implying that vertical eddy diffusion varies inversely with Richardson number. A number of formulations for vertical diffusion as a function of Richardson number have been proposed, but no single relationship has received universal acceptance. Natural turbulent diffusion occurs over all parts of a heated discharge. However, in the near-field region, mechanically induced turbulence due to jet momentum entrainment is the major source of heat dispersal; in the far-field, surface heat exchange actively transfers heat from surface plumes. Therefore, natural turbulence is often masked by other dispersion mechanisms except in submerged plumes or perhaps the transition regions of surface discharges. C. Surface Heat Exchange The third factor influencing the thermal dispersion of heated effluents is heat dissipation to the atmosphere from the surface of the water body. Surface cooling is an inherent property of air-water coupling and therefore acts over all portions of a surface effluent. However, since surface heat exchange varies directly with the surface area affected, the process becomes significant only over large plume areas. This condition is satisfied at some distance from the discharge point, after the effluent has been cooled appreciably by jet entrainment and natural turbulent diffusion. Hence, the effects of surface heat exchange need be considered only for the far-field. The processes that determine the amount of surface cooling from a heated plume are identical to those that prevail under typical ambient conditions. As a result, heating by a discharge can be regarded as a