During progress and development of methods for analysis of floods (more accurate topographic data, increased capacity of computer equipment, development of hydrodynamic modelling tools, etc.) a new approach to determining flood areas is becoming more and more popular, namely the simplified hydrological-hydraulic two-dimensional model with direct rainfall. When using rain-on-grid methods, the hydrological part is already integrated into the hydraulic model, as rainfall amount and spatial distribution is inserted directly on the mesh grid for the upstream boundary condition. The master's thesis presents two methods of determining floodplains, one using a traditional approach, with constructed separate hydrological and hydraulic models, and other using the direct rainfall method to the 2D modelling grid. For comparison, a small catchment area, called NC Polje in Bohinjska Bistrica, was selected, where flash and pluvial floods occur. We tested the sensitivity of the numerical model to changes in roughness coefficient, the size of the numerical cell and the distribution of rainfall. The change in cell size of the computational mesh showed the greatest impact on modelling results. In conclusion, the use of two-dimensional modelling with direct rainfall proved to be useful approach, especially in the areas of flash and pluvial floods, but carefully determining the roughness coefficient and professional assessment of the obtained results is essential.