The ultimate goal of this Project is to develop a computer model which provides the capability of predicting the behaviour of a heavier-than-air gas cloud during a spill in an urban area. Current mathematical models apply to smooth terrain only. This is a significant limitation because accidental spills are likely to have the most impact in suburban or urban settings. The novel feature of the model will be the ability of this model to account for the presence of obstacles in the path of the gas cloud and for certain types of non-uniform terrain. Toward this end, several heavy gas experiments were performed at model scale in the RWDI wind tunnel at the Guelph facility. The setup, wind tunnel calibration, instrument calibration, testing of uniform arrays and data analysis comprised Phase I. We report in this paper on Phase I. Flow visualization records and concentration profiles are presented which identify the concept of a threshold wind speed. Above this speed, the cloud behaviour is not significantly influenced by the obstacles. However, below this speed, the dose-time relationship is significantly increased.