Major hazard risk assessment over non-flat terrain. Part I: continuous releases

Abstract

Liquefied gases, such as chlorine or ammonia, are stored in large quantities on industrial sites. If released accidentally, they can form a heavy gas cloud that has the potential to kill or injure large numbers of people. The dispersion of such a cloud is thus of interest to the risk assessment community. This is the first of a two-part paper. Here, the risk (probability) of being exposed to a continuously released toxic gas cloud, given a release, is considered; probability language is needed because wind direction is assumed to be a random variable. Part II considers instantaneous releases. Risk is considered in terms of R C , the probability at any point of receiving a non-zero dose given a continuous release. For flat ground, simple analytical techniques show that R C ≃ 3 2/3 2π F 2/3 ·r −1/3 ·u −1 ·Q 1/3 for a uniform windrose (windspeed u ) at radius r due to a continuous release of buoyancy flux Q . Here F is the front Froude number. This paper shows how the risk of being exposed to heavy gas from a steady continuous release is affected by non-flat terrain, using the established shallow layer model TWODEE (Journal of Hazardous Materials 66(3) (1999a) 211). Results from both simple slopes and real terrain are presented. A range of windspeeds and slopes are considered. For a steady 44 kg s −1 continuous release under a windspeed of 1 m s −1 , groundslopes as low as 2% can substantially affect the risk in the sense that the predicted risk contours are far from circularly symmetric. The real terrain data used is from Little Barrier Island, New Zealand (latitude 36°11′S, longitude 175°4′E), chosen on account of its being uninhabited, and having steep slopes and complex topography. As the windspeed increases from 1 to 10 m s −1 , risk contours become less affected by terrain in that their complexity is reduced.