# GIS-Based Flood Assessment Using Hydraulic Modeling and Open Source Data: An Example of Application
## 2 Materials and Methods
### Research Methodology  
In the case study presented in this article, a segment of the Cigher River was analyzed using one-dimensional hydraulic modeling based on steady flow analysis, where it is assumed that the discharge and flow conditions of a river remain constant over time at any point in the system. In other words, in steady flow analysis, water discharge, depth,and velocity remain unchanged at each cross-section of the river during the analyzed time period. The following steps and phases (Figure 2) were undertaken to obtain the results:  
1. Flow and level analysis for the study area focused on:  
a. Seasonal and annual variation analysis;  
b. Extreme value analysis (EVA); the Weibull method was applied to calculate the non-probability and probability of occurrence, as well as the return period of maximum values (recurrence period). The maximum recorded discharges (Q) and water levels (H) at the Chier hydrometric station over a 5-year period were taken into account. Using the Weibull method, the probability of occurrence (P) was calculated using Equation (1), while the recurrence period (T) was calculated using Equation (2). \[\mathrm{P}=\frac{\mathrm{m}}{\mathrm{N}+1} \tag{1}\]  
where P is the probability that the maximum discharge/level will be reached or exceeded in a given year; m is the rank of the event (the discharge/level values are arranged in descending order, with the highest discharge receiving rank 1); N is the total number of observed years:  
\[\mathrm{T}=\frac{1}{\mathrm{P}}=\frac{\mathrm{N}+1}{\mathrm{m}} \tag{2}\]  
where T is the recurrence period (in years); P is the previously calculated exceedance probability. The estimation of maximum flow and water level for different recurrence periods (1 year, 5 years, 10 years, and 15 years) was carried out based on the linear equation (relation 3) and the logarithmic equation (relation 4). This method allows for describing the relationship between the observed maximum flow/level and the return period. \[\mathrm{Y}=3.9676\mathrm{x}+4.1007 \tag{3}\]  
Figure 2: Research methodology. where y is the estimated maximum flow rate (m\({}^{3}\)/s); six the recurrence period (years); 3.9676 and 4.1007 are coefficients obtained by adjusting the regression line to the available dataset  
\[\mathrm{Y}=0.3733\mathrm{ln}\ (\mathrm{x})+2.8766 \tag{4}\]  
where y is the estimated maximum level (m); x is the recurrence period (years); 0.3733 and 2.8766 are coefficients obtained by fitting the regression line to the available dataset. c. Establishing scenarios for modeling: four scenarios/simulations (S1, S2, S3, and S4) were defined under different discharge rates and water levels for 1, 5, 10, and 15 years, respectively: S1--Q = 8 m\({}^{3}\)/s, H = 3 m; S2--Q = 23 m\({}^{3}\)/s, H = 3.2 m; S3--Q = 41 m\({}^{3}\)/s, H = 3.35 m; S4--Q = 60 m\({}^{3}\)/s, and H = 3.5 m.  
2. DEM Processing. For the hydraulic analysis, a DEM with a 1-m resolution was used, generated from LiDAR data and processed using ArcGIS 10.8 software. The DEM processing included extracting the area of interest, applying a buffer to optimize the data, and validating it with control points measured in the field. The results of the DEM processing are detailed in the Section 3.1. 3. Geometry creation in HEC-RAS involved building the hydraulic model geometry essential for simulating water flow:  
a. tracing the river thalweg (centerline);  
b. marking riverbanks to define channel boundaries;  
c. delineating the floodplain, identifying areas at risk of flooding during high water levels;  
d. drawing cross-sections to detail variations in the riverbed profile; 32 cross-sections were established (numbered upstream to downstream, from ST 32 to ST 1), spaced approximately 500 m apart;  
e. correcting geometry errors to ensure model accuracy. For each cross-section, the Manning coefficient, which describes resistance to flow caused by the riverbed surface, was applied. A value of 0.035 was used for the riverbed and 0.06 for the floodplain. 4.