he 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 assessm
ent of the risk associated with the facility, which will include an evaluat
ion of the effects of releases of dangerous substances to the environment.
The models commonly used fur assessing the dispersion of dense gases in the
atmosphere are based on the 'ideal' of flat, unobstructed terrain. For 're
al' situations such models may be unduly pessimistic in their predictions a
nd, 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 Hea
lth and Safety Laboratory (HSL) to model the dispersion of chlorine in comp
lex 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 drama
tic insight into how terrain and buildings can fundamentally alter the disp
ersion behaviour of dense gases. The results show how flat tel rain models
may overestimate the chlorine hazard range by as much as a factor of 5, whi
lst the predicted direction of travel of the cloud may err by up to 90 degr
ees. This has implications not only for the assessment of risks associated
with major hazard facilities, but also for land-use planning in the vicinit
y of the site and emergency preparedness.