Guidelines for the inclusion of low wind speed conditions into risk assessments

Although they are not often considered in risk assessments or safety cases, low wind speed conditions are likely to produce many of the worst case dispersion scenarios, especially for situations where dense vapour clouds would form close to the ground. The results of previous review and validation studies, undertaken by the authors for the UK Health and Safety Executive, have been drawn together in this paper to provide guidelines for the practical application of appropriate modelling of scenarios involving low wind speeds within quantified risk assessments. The production of these guidelines has been achieved by using example risk assessments covering the storage of chlorine, bromine, LPG and Liquid Oxygen, for each of which sensitivity studies were also undertaken. These demonstrated that the inclusion of low wind speeds has varying effects, depending on the material considered, which could be as much as 1-2 orders of magnitude. Most importantly, it also showed that, when low wind speeds are included, it is not only their dispersion effects but also their effects on source term and impact on the population, which need to be considered to ensure that the calculated risks are neither overly conservative nor optimistic.