Modern flood risk assessment methods use multiple integrated concepts to perform analyses, taking into account multi-disciplinary expertise from different fields. A fundamental element for these analyses is the Digital Terrain Model (DTM), through which vulnerable areas can be identified based on hydraulic simulations. The overall process includes both analysis of meteorological conditions and local investigation of a specific section of the river system to predict the entire functional chain of the flood. For the needs of spatial analyses related to the territory of the Republic of Bulgaria, there are generally known freely available relief models such as: SRTM (30 × 30 m), EU-DEM (25 × 25 m), ALOS DEM (30 × 30 m). In addition to that free data, the available experimental DSM (4 × 4 m) at the MAF (Ministry of Agriculture and Food) can be used in flood risk studies, as well as the incoming high spatial resolution digital data (SfM, LiDAR) acquired through Unmanned Aerial Systems (UAS). There is no free LiDAR data available for Bulgaria's territory. In this paper, a comparative analysis of topographic conditions is made to identify potential floods in floodplains. This analysis evaluated different spatial resolutions of the terrain surface in a test area section of the Ogosta River. We used the FLO-2D Basic model for the simulation of the flows, freely available in its basic version. It consists of a quasi-2D hydrological–hydraulic dynamic flow model simulating channel flow, and unconfined overland flow over complex topography. Simulations were performed at different resolutions of DTMs obtained from UAS-SfM (structure from motion), which are acquired relatively quickly and at low cost. The data obtained by UASs are of high temporal and spatial resolution, allowing hydraulic models to be kept up-to-date for problematic river sections and potential high-risk flood areas.