# Safety Assessment of Gas Pipelines Crossing River through Hydrodynamic Analysis
## 2 Materials and Methods
### Composition of Input Data  
#### 2.4.1 Assessment of Bed Material Composition and Sediment Load in the Hyunpung Stream  
According to the Development of a Guideline for the Selection of Sediment Transport Formulas [48], sediment load at measurement points can be estimated. The study area for the present research is the Hyunpung Third Bridge region in the Hyunpung Stream. The Hyunpung Stream Basic Plan provided the bed material composition, which is shown in Table 1. Table 1 presents the cumulative passing weight percentages of bed material composition in the Hyunpung Stream. The data indicate a dominance of coarse materials, such as gravel and cobble, particularly in the larger size fractions like LC (256 mm) and SC (128 mm). This suggests that the streambed is relatively resistant to scour, which could affect sediment transport dynamics and the risk of pipeline exposure due to scouring. The finer fractions, such as VFS (0.125 mm) and CM (0.0625 mm), make up a smaller percentage, indicating lower susceptibility to transport by normal river flow. However, these finer materials could become mobilized during flood events. This distribution of grain sizes plays a crucial role in determining the sediment transport capacity, and the higher proportion of coarser materials suggests a relatively stable riverbed with lower rates of sediment movement under normal flow conditions. The Ackers and White sediment transport equation, suitable for small to medium-sized rivers where the transport of sand and gravel is frequent, was applied. Consequently, the flow-sediment load results for the Hyunpung Stream Basin according to the sediment transport rate were derived, as shown in Table 2. Table 2 provides the flow duration analysis and sediment load results, highlighting the relationship between discharge and sediment transport across different flow conditions. The results show that the sediment load increases significantly during higher flow conditions, as seen during abundant flow (Q95), where the sediment transport rate reaches 8.21 tons/day. In contrast, under low flow (Q275), the sediment transport rate decreases to 1.24 tons/day. These findings underscore the importance of flow magnitude in sediment transport processes. The variation in sediment \begin{table}
\begin{tabular}{c c c c c c c c c c c c c} \hline \hline  & & & & & & & & & & & & & & \\ \hline
**LC** & **SC** & **VCG** & **CG** & **MG** & **FG** & **VFG** & **VCS** & **CS** & **MS** & **FS** & **VFS** & **CM** \\
**(256 mm)** & **(128 mm)** & **(64 mm)** & **(32 mm)** & **(16 mm)** & **(8 mm)** & **(4 mm)** & **(2 mm)** & **(1 mm)** & **(0.5 mm)** & **(0.25 mm)** & **(0.125 mm)** & **(0.0625 mm)** \\ \hline
100 & 94.27 & 74.17 & 50.84 & 34.88 & 29.01 & 27.50 & 26.10 & 9.56 & 6.10 & 3.60 & 1 & 0 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Passing weight percentage of bed material composition in the Hyunpung Stream. load across different flow conditions also emphasizes the need for dynamic management strategies that consider seasonal changes in discharge. Moreover, these results have direct implications for the stability of buried pipelines, as higher sediment loads during peak flows can lead to increased scouring and potential exposure of the pipeline. Flow duration represents the annual situation of daily flow at a reference point, providing an understanding of the scale of and variability in available river flows through flow duration analysis. However, there is no gauging station in the Hyunpung Stream. Therefore, the flow data from the Hyunpung gauging station near the confluence with the Nakdong River were used, and the flow duration in the Hyunpung Stream was estimated using the specific discharge method, which expresses the flow rate of water per unit area of the watershed to normalize discharge rates, making it easier to estimate the discharge for different watersheds. Abundant flow corresponds to the 95 th day, ordinary flow to the 185 th day, low flow to the 275 th day, and drought flow to the 355 th day on the flow duration curve.