twodee: the Health and Safety Laboratory's shallow layer model for heavy gas dispersion Part 1. Mathematical basis and physical assumptions

• Published in: Journal of hazardous materials Vol. 66; no. 3; pp. 211 - 226
• Main Authors: Hankin, R.K.S., Britter, R.E.
• Format: Journal Article
• Language: English
• Published: Amsterdam Elsevier B.V 14-05-1999; Elsevier
• Subjects: Accident prevention; Applied sciences; Atmospheric pollution; Computational fluid dynamics; Convection, turbulence, diffusion. Boundary layer structure and dynamics; Diffusion; Earth, ocean, space; Environmental Monitoring - methods; Exact sciences and technology; External geophysics; Gases - analysis; Gases - pharmacokinetics; Gravitation; Health and Safety Laboratory; Health risks; Humans; Land use; Land use planning; Mathematical models; Mathematics; Meteorology; Models, Molecular; Pollutants physicochemistry study: properties, effects, reactions, transport and distribution; Pollution; Predictive Value of Tests; Reproducibility of Results; Rheology; Risk assessment; Risk Assessment - methods; Thermodynamics; Toxic materials; twodee; Health and Safety Laboratory; Land use planning; twodee; Gas emission; Dense gas; Theoretical study; Air pollution; Air quality; Atmospheric dispersion; Mathematical model; Pollutant emission; Modeling
• ISSN: 0304-3894; 1873-3336
• DOI: 10.1016/S0304-3894(98)00269-6

The Major Hazard Assessment Unit of the Health and Safety Executive (HSE) provides advice to local planning authorities on land use planning in the vicinity of major hazard sites. For sites with the potential for large scale releases of toxic heavy gases such as chlorine this advice is based on risk levels and is informed by use of the computerised risk assessment tool
riskat [C. Nussey, M. Pantony, R. Smallwood, HSE's risk assessment tool
riskat, Major Hazards: Onshore and Offshore, October, 1992]. At present RISKAT uses consequence models for heavy gas dispersion that assume flat terrain. This paper is the first of a three part paper. Part 1 describes the mathematical basis of
twodee, the Health and Safety Laboratory's shallow layer model for heavy gas dispersion. The shallow layer approach used by
twodee is a compromise between the complexity of CFD models and the simpler integral models. Motivated by the low aspect ratio of typical heavy gas clouds, shallow layer models use depth-averaged variables to describe the flow behaviour. This approach is particularly well suited to assess the effect of complex terrain because the downslope buoyancy force is easily included. Entrainment may be incorporated into a shallow layer model by the use of empirical formulae. Part 2 of this paper presents the numerical scheme used to solve the
twodee mathematical model, and validated against theoretical results. Part 3 compares the results of the
twodee model with the experimental results taken at Thorney Island [J. McQuaid, B. Roebuck, The dispersion of heavier-than-air gas from a fenced enclosure. Final report to the US Coast Guard on contract with the Health and Safety Executive, Technical Report RPG 1185, Safety Engineering Laboratory, Research and Laboratory Services Division, Broad Lane, Sheffield S3 7HQ, UK, 1985].