Journal of Loss Prevention in the Process Industries 16 (2003) 305–311 www.elsevier.com/locate/jlp Risk assessment of major hazards: the effect of ground slope Robin K.S. Hankin ∗ School of Geography and Environmental Science, The University of Auckland, Private Bag 92019, Glen Innes, Auckland, New Zealand Abstract Liquefied gases, such as chlorine and ammonia, are stored in large quantities at industrial sites. If released accidentally, they 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 [Nussey, Pantony, & Smallwood, 1992. HSE’s risk assessment tool, RISKAT. In: Major Hazards: Onshore and Offshore. pp. 607–638]. Little is understood about the effect of slope on risk. Here, the risk (probability) of being exposed to the gas cloud, given a release, is considered; probability language is needed because wind direction is assumed to be a random variable. This paper shows how the risk of being exposed to toxic gas released over a slope may be estimated using simple physical model- ling. The physical model used is that of Tickle [J. Hazard. Mater. 49 (1996) 29], who showed that a finite-volume instantaneous release on an inclined plane can form a stable wedge-shaped cloud that moves down the line of greatest slope. Nonzero windspeeds are accounted for by following Tickle’s suggestion of vectorially adding windspeed to the advection induced by the slope. A range of windspeeds and slopes are considered. The slopes substantially affect the risk in the sense that the predicted risk contours are far from circularly symmetric.  2003 Elsevier Science Ltd. All rights reserved. Keywords: Heavy gas dispersion; Risk assessment; Slopes 1. Introduction The assessment of off-site risk near major hazard sites is typically carried out using tools such as RISKAT (Nussey, Pantony, & Smallwood, 1992). These tools’ consequence analysis includes simulations of heavy gas dispersion, which are usually integral models (Britter, 1989; Nussey et al., 1992). Although the physics of heavy gas dispersion is known to be profoundly affected by topography (Britter, 1989; Hankin & Britter, 1999; Webber, Jones, & Martin, 1993), the effect of this on risk is less well understood. Here, risk is defined, for each ground position, as the probability of being passed over by a gas cloud, given a release. Probability language is used because the wind direction is assumed to be a random variable. In this work, a uniform wind rose is used. One might expect that high ground would reduce the ∗ Tel.: +64-9-373-7599; fax: +64-9-373-7042. E-mail address: r.hankin@auckland.ac.nz (R.K.S. Hankin). 0950-4230/03/$ - see front matter  2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0950-4230(03)00020-2 risk, as heavy gas typically flows downhill; but little quantification of this effect has been carried out to date. This paper shows how the risk of being exposed to toxic gas released over a slope may be estimated using simple physical modelling. 2. Heavy gas dispersion on a slope Heavy gas dispersion over flat ground is frequently modelled by assuming the cloud to form a simple cylin- drical shape which spreads downwards and outwards with a front Froude number of about 1 (Britter, 1979). In the absence of entrainment, the volume remains con- stant as the height decreases and the radius increases. The presence of a sloping lower boundary complicates matters. Kukkonen and Nikmo (1992) do present a model for heavy gas dispersion on a slope which pre- serves a cylindrical shape, but their model neglects the azimuthal dependence of leading edge height, which would break the circular symmetry. Webber et al. (1993) presented a model of the motion of a heavy gas cloud released on a uniform slope which