Even in our days, the airflow patterns inside in urban areas presents an increased interest. Urban planners are trying to find ways to make cities safer places for citizens while increasing demand for new or existing areas. Based on these facts and considering the development of computational tools developed by engineers in the fields of fluid mechanics and fire mechanics, it is possible to predict the behavior of airflow patters during accidents in urban areas. In this current study, two different computational fluid dynamics techniques are combined to determine the structure of airflow within a street canyon. Initialy, Reynolds-Averaged Navier-Stokes (RANS) models were performed for various W/H ratios of a street canyon model, which are used for the Large Eddy Simulation (LES) method initialization. The numerical models were validated with a reduce scale model which was built and tested in a wind tunnel. HWA (Hot Wire Anemometer) techniques were used in order to define the initial boundary conditions of the kinetic energy and dissipation rate. Pressure measurements with pressure taps on the building’s surface are recorded, and the pressure coefficients are compared against the numerical simulations results. After the study of the airflow a risk assessment analysis was conducted. The examined accident referred to a natural gas pipe leakage at the center of the street canyon. A constant source of leakage and four different atmospheric boundary layer was studied. The flow and fire characteristics were defined for all different examined cases. The strong correlation between the flow phenomena and the thermodynamic state of the produced gases from the combustion kept under consideration. As a result, there was the influence from buoyancy forces in the airflow structure. The natural gas pool fire is transformed quickly into a fire jet creating a temperature distribution, which makes this kind of accidents more severe. Outcome of the risk assessment analysis was the distribution of the temperature field defining the fatal zones from the high temperatures inside in the street canyon.