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Major accident prevention and preparedness involve the determination of a toxic substance expected or actual release dispersion in the atmosphere, i.e., mathematical modeling of liquid mechanics phenomena. Nowadays, statistical mathematical models are usually used to model simulations of emergency situations in facilities in urban areas or in industrial complexes. Numerical CFD codes have been used mostly for specialized and detailed spatial analyses of physical and chemical phenomena and situations in enclosed spaces. With increasing computing power, these models are beginning to be applied also to complex problems in open spaces, including chemical accidents. Statistical and dynamic models give different results as the principles of the two methods, and the quantity and types of input parameters are different. The article directly compares the results of simulations of accidental gaseous ammonia releases from an ice arena into a complex urban area, obtained from ALOHA 5.4.3 statistical model and ANSYS Fluent 13.0 numerical CFD model. Real meteorological data were used for the simulations. It emerged that the results of statistical and CFD models may differ radically. The CFD model provided better quality data for addressing accidents.

When predicting the behaviour of a proposed capillary barrier, it is possible to utilize either the experimental measurements or the numerical modelling. Results of tipping trough modelling of a capillary barrier are used to study the reliability of numerical modelling. The required hydromechanical parameters of the capillary barrier layers are determined and used for the numerical repeating of the tipping trough experiments. The results are presented and discussed.Subsequently, the paper studies the problem of capillary barriers efficiency. A criterion is introduced that makes use of soil-moisture retention data in order to predict the efficiency of a capillary barrier. Hysteresis of retention curves of the applied materials is studied and its effect on the efficiency is discussed.