## 3 Methodology
### Generation of Flood Models in HEC-RAS  
HEC-RAS 6.5 software developed by the USACE, which is freely available, was used to create flood scenarios. HEC-RAS 6.5 is capable of performing flood mapping with water surface profile results. Moreover, using the HEC-RAS 6.5 geometry and calculated water surface profiles, the flood depth and floodplain boundary are created through the RAS Mapper [64]. The DEM data (raster format) and river data (vector format) of the sub-basin created in the previous step were imported into the software. Then, the connections of the river system (river, bank lines, and flow paths) and cross-sections were determined. Bank lines delineate the primary channel boundaries within a cross-sectional profile. Flow paths are used to calculate the distances between sections in the left and right floodplain [64]. Flow paths were created downstream along the center of mass of the flow in the left and right floodplain. Cross-sections were generated based on the spatial arrangement of cross-section lines and the attributes of other layers, including the river, bank lines, and terrain layers. The cross-sections were positioned perpendicular to the anticipated flow direction in both the channel and overbank areas. The study area shows regular flow during heavy rains due to its topographical structure and defined boundaries along the channel and the river mouth structure that provides discharge in a certain period of time, and floods occur when the water height exceeds the bed boundaries. In this context, the steady flow analysis method, which requires constant flow data, was preferred since constant flow rates are used in the projections. The flow regime is determined as subcritical. Boundary conditions are needed to determine the initial water surface at the ends of the river system (upstream and downstream). An initial water surface is required for the program to start calculations. In a subcritical flow regime, boundary conditions are only necessary at the downstream ends of the river system [42]. At the downstream ends of the river system, the boundary condition is defined as the critical depth. Thus, the critical depth for each profile was automatically calculated by the software and used as a boundary condition. Then, Manning's roughness coefficients, which represent the resistance to flow in streams and floodplains and are represented in HEC-RAS 6.5 as \"n\" value, were calculated. Since the directional spreading of the overland flow depends on terrain roughness as well as topographic conditions, the Manning roughness coefficients are the most important parameters outside of geometry that conditions flow spreading [65]. Therefore, the geometry of the river (topography and bathymetry), boundary conditions (roughness, flow regime, etc. ), and Manning roughness coefficients are the factors that determine the outcomes of the hydrological model [66]. Manning's n value of the basin was created using actual data. Since there are no gauging stations in the basin, these data were prepared by selecting the data from the technical bulletin prepared by DSI for Bogacay as a result of land surveys (Table 1). To determine the Manning roughness coefficients of the rivers in Turkiye, a technical bulletin is prepared by the DSI [55] and the Manning coefficients of the rivers can be calculated by the Cowan [67] method (Equation (1)) [68]. \[n=(n_{0}+n_{1}+n_{2}+n_{3}+n_{4})m \tag{1}\]  
where \(n_{0}\): type of material in the riverbed, \(n_{1}\): degree of channel irregularity, \(n_{2}\): change in channel radius, \(n_{3}\): similar effects of barriers, \(n_{4}\): vegetation cover, and \(m\): represents the degree of channel curvature. In the technical bulletin prepared by the specialized flood commission of the DSI [55], the roughness table has been updated as follows (Table 1). A land survey was carried out in the study area to select the relevant values in the roughness table.