# Evaluation of Different Simulation Methods for Analyzing Flood Scenarios in the Danube Delta
## 4 Results  
The calibration of the hydraulic model was first performed upstream of the study area, more precisely in the Danube entrance sector in the Delta (Ceatal Chilia), and later downstream, at the mouth of the Sulina branch in the Black Sea. The calibration of the model in the HEC-RAS software was made by alternate values of the Manning roughness coefficients, between 0.025 and 0.03, and by applying the rating curve data from the Tulcea hydrometric station (Figure A5). In the next stage (unsteady flow module), in order to define the boundary conditions, an analysis of the hydrological data of the Danube was performed, which required the description of the water flow history for a period of time. Figures 6 and 7 show two cross-sections used for model calibration, which were made on both river parts. The absolute elevation value (Z) of the riverbed was determined by finding the difference between water surface value and a water depth value (H). The water level for the measured sections upstream and downstream of the hydrometric station was calculated using real-time kinematic positioning (RTK). To determine the water surface level, for each cross-section of the riverbed where bathymetric measurements were performed, two methods were used to determine the elevation values. The first method consists of measurements with RTK Rover (ROMPOS) Spectra precision SP80, and the second method consists of reading the values recorded at the hydrometric stations (at Tulcea, Sulina, Sfantu-Gheorghe, Chilia-Veche, Periprava). For georeferencing the data (spatial/global referencing) we used the geographic positioning data of each point in the riverbed profile and the data on the depth of each point (absolute elevation of the riverbed, referenced to the Black Sea reference system 75-BS75). The X, Y, Z data were thus georeferenced data obtained for each point measured by ADCP in the topo-bathymetric sections on the Danube branches. Figure 6: Profile S1 created with the ADCP-profile trajectory, measured parameters (total flow, boat speed, average speed, measured distance, depth) and water velocity graph (velocities distributed on cells, with a colour gradient of velocity intensity, on the water column). The profile was extracted from the River Surveyor Live software used in the processing of data collected by ADCP. Following the GIS pre-processing of topo-bathymetric data, a new digital model of the minor riverbed was obtained. The cumulation of the two models resulted in a complex digital model with updated data on the minor riverbed. The next step was the analysis of flood scenarios using the dynamic method. For the dynamic method, HEC-RAS 5.0.3 specialized software was used. In the next step, the complex digital terrain model was introduced into the software for defining and processing the geometric data. The geometric data used for hydrodynamic simulation were obtained using high-precision DTM, with a spatial resolution of 0.5 m \(\times\) 0.5 m. The next geometric elements were the channel centerline, bank lines, the flow-path lines above the banks, and 174 cross-section cut lines, perpendicular to the direction of the river flow. It is important to note that each cross-section covered the elevations of the stream bed to the banks, with a width of about 800 m to 1500 m. These geometric features were further used in the HEC-RAS hydraulic modeling step. An important step in hydraulic modeling is the assignment of the roughness values. The values of the Manning roughness coefficients were assigned according to the character of the riverbed, the relief, and the vegetation in the vicinity of the river. For the 2D flow area, the Manning roughness coefficient was calibrated in the range between 0.025 and 0.03. Regarding the calculation points of the 2D computational mesh, they were generated at steady intervals. The distance between the calculation points was set as a grid with a distance of 50 m on the x and y direction axes. In this way, finite elements were created for the 2D flow area.