# Safety Assessment of Gas Pipelines Crossing River through Hydrodynamic Analysis
## 2 Materials and Methods
### Composition of Input Data
#### 2.4.3 Boundary Conditions for Short-Term Riverbed Change Analysis  
To predict short-term bed changes, the two-dimensional models, the river flow analysis model (RMA2) and sediment transport and deposition model (SED2D), were used to analyze the patterns of scouring and deposition at specific river cross-sections for the design flood event near the study area. Since the SED2D model cannot calculate water levels and velocities on its own, RMA2 was used first before SED2D to calculate these parameters. The basic assumption in the model is that the sediment deposition or scour calculated within the model does not significantly affect the flow characteristics. The study area was modeled to cover 300 m upstream and 300 m downstream from the Hyunpung Third Bridge, which serves as the reference point for the gas pipeline, as shown in Figure 5. To ensure mesh-independent solutions while preparing the topographical data, the mesh quality check function provided in SMS was activated. This maintained the length-to-width ratio below 1:10, and corner angles were kept between 10 and 150 degrees. Sharp changes in slope, which could cause computational instability, were minimized. Additionally, areas where solution variables such as \(u\), \(v\), and \(h\) change rapidly, and critical areas of interest, were modeled with higher density, while areas with less interest and stable flow characteristics were modeled with lower density. The area ratio between elements was kept within a factor of two. To simulate short-term bed changes using the RMA2 model, it is necessary to input the flood discharge, water level boundary conditions, and roughness coefficients of the study area. For short-term simulations, specific flood events, such as those corresponding to a 100-year return period, were selected to assess the immediate impacts on the riverbed  
Figure 4: Upstream boundary conditions required for long-term simulations. Figure 5: Construction of topographical data for short-term riverbed change. during extreme hydrological events. These events represent critical conditions that could expose or damage the pipelines within a short time frame. The selected flood event, based on regional hydrological data, provides a reasonable and conservative estimate of the potential short-term risks to the buried pipelines. For the flood discharge, since no tributaries are flowing into the study area, the design flood discharge corresponding to the 100-year return period was adopted based on the Clark unit hydrograph at the Hyunpoong Third Bridge point. The flood discharge estimation methods include the Clark unit hydrograph, the NRCS dimensionless unit hydrograph, and the Snyder unit hydrograph. The Clark unit hydrograph method, which better reflects the characteristics of the watershed, was chosen, and the flood discharge estimation results are shown in Table 3. For the downstream boundary conditions, since the purpose of the study is to analyze bed changes rather than to review flood levels, the normal depth corresponding to the discharge was calculated and applied. According to the suggested values in the River Basic Plan, a channel slope of 0.0004 and roughness coefficient of 0.030 (see Table 4) were applied. For the SED2D model, the sediment load boundary conditions were prescribed based on Table 2, and the relationship between sediment load and discharge at the target point and the Hyunpoong Stream was expressed using the sediment rating curve as shown below. \[Q_{s}=aQ_{w}^{b} \tag{9}\]  
where  
\(Q_{s}\) = Sediment discharge;  
\(Q_{w}\) = Discharge;  
\(a\) and \(b\) = Empirical constants. Equation (9) was log-transformed to enable linear regression analysis. The values of \(Q_{s}\) and \(Q_{w}\) were log-transformed as well. Using the log-transformed data, a linear regression analysis was performed to determine the values of \(a\) and \(b\).