Numerical analysis has been performed for predicting the dispersion of heavy gas clouds released without initial momentum (i.e. dense gas puffs) within regions of complex topography. The three-dimensional non-steady differential equations governing transport are solved by means of the numerical finite volume method, using a collocated variable arrangement with a fully implicit integration over time. The turbulence effects on the flow properties are simulated by the two-equation k-epsilon turbulence model. Comparisons between calculated and measured data are presented, showing good agreement between them. The method is also used to predict a chlorine release within a fictitious industrial plant. The location of the release site, the atmospheric stability class and the wind direction are varied, in order to show the effect of topography on the cloud dispersion.