Use of Advanced Techniques to Model the Dispersion of Chlorine in Complex Terrain

Abstract

The EU Seveso II Directive requires operators of major hazard facilities to prepare safety reports for sites storing quantities of dangerous substances in excess of specified levels. The safety report should include an assessment of the risk associated with the facility, which will include an evaluation of the effects of releases of dangerous substances to the environment. The models commonly used for assessing the dispersion of dense gases in the atmosphere are based on the ‘ideal’ of flat, unobstructed terrain. For ‘real’ situations such models may be unduly pessimistic in their predictions and, in certain circumstances, may even be optimistic. This paper describes work undertaken by Environmental Resources Management (ERM), in association with sub-consultants Rowan Williams Davies & Irwin Inc. (RWDI) and the Health and Safety Laboratory (HSL) to model the dispersion of chlorine in complex terrain. The work involved physical modelling of releases in a Boundary Layer Wind Tunnel (BLWT) and the use of Computational Fluid Dynamics (CFD). The paper focuses on the key findings of the study, which provide a dramatic insight into how terrain and buildings can fundamentally alter the dispersion behaviour of dense gases. The results show how flat terrain models may overestimate the chlorine hazard range by as much as a factor of 5, whilst the predicted direction of travel of the cloud may err by up to 90°. This has implications not only for the assessment of risks associated with major hazard facilities, but also for land-use planning in the vicinity of the site and emergency preparedness.