Flash floods are a dangerous phenomenon that generally affects small drainage basins. They are primarily initiated in the upper parts of the slopes, but their damaging effects are manifested mostly in residential areas, where naturally flowing streams were removed from the surface to the underground artificial channels. Therefore, there are no precise data about stream water levels available and only using surface runoff modelling is possible to simulate what happened during flash floods. Karlova Ves (Bratislava City District), formerly a small viniculture village, was threatened by floods (most probably including pluvial type) in history. In this paper, we used GRASS GIS tool r.sim.water to simulate the surface runoff of a flash flood that occurred in summer 2014 in the catchment of Čierny potok. The flood on 23 August 2014 was reported to have the highest rainfall per hour ~40 mm during the time of local meteorological measurements. The current orthophotomap was used to classify the land cover classes, which were assigned the value of the Manning’s roughness coefficient and infiltration rate. The topography was expressed by DTM from high-resolution LiDAR data. Our preliminary results indicate that land cover and land use are the essential factors that influence the initiation of flash floods, although the main driver of lower infiltration and change in flow direction is caused by urbanisation and a high proportion of impervious areas. Simulation showed that during 60 minutes of extreme rainfall (40 mm/hr) a surface runoff can reach a depth of water up to two meters in terrain depressions by a maximum discharge of 25 cubic meters. The revitalisation of natural urban areas by increasing vegetation cover in areas prone to flash floods and accumulation of water during higher rainfalls helps to prevent the damage caused by floods. URL: https://www.gcass.science.upjs.sk/