# Safety Assessment of Gas Pipelines Crossing River through Hydrodynamic Analysis
## 3 Results  
### Long-Term Riverbed Change  
According to the long-term riverbed change simulation using HEC-RAS, the cross-section of the gas pipeline buried in the river exhibited a gradual progression of sediment scour as time passed. This analysis is critical for ensuring the safety and stability of the gas pipeline over extended periods. As illustrated in Figure 6, which shows the long-term riverbed change along the A-A' section in Figures 3 and 5, there were no significant changes in the riverbed on the left and right banks of the river segment over time. However, scour was prominent in areas near the deepest parts of the riverbed. This finding underscores the importance of monitoring flow dynamics and sediment transport to prevent potential risks to the infrastructure. The simulation predicted that after 20 years, the maximum scour depth would reach up to 13 cm. This gradual increase in scour depth can be attributed to the increased flow velocity at the burial site and downstream, which enhances shear forces that lead to sediment removal around the pipeline. The increase in flow velocity can be linked to various hydrological events, including seasonal floods and changes in river discharge patterns, highlighting the dynamic nature of river systems.  
The burial depth of the gas pipeline at the location affected by long-term riverbed changes is detailed in Figure 7. At the evaluation site, the top height of the gas pipeline  
\begin{table}
\begin{tabular}{c c c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Measurement Point**} & \multirow{2}{*}{**Specific Gravity**} & \multicolumn{8}{c}{**Sieve Analysis Results (\%)**} \\ \cline{3-13}  & & **101.6** & **76.2** & **38.1** & **19.1** & **9.51** & **4.75** & **2.0** & **0.84** & **0.42** & **0.25** & **0.105** \\  & & **(mm)** & **(mm)** & **(mm)** & **(mm)** & **(mm)** & **(mm)** & **(mm)** & **(mm)** & **(mm)** & **(mm)** & **(mm)** & **(mm)** \\ \hline Upstream & 2.62 & & 100 & 78.2 & 54.7 & 40.8 & 22.0 & 17.6 & 10.3 & 6.3 & 3.7 & 1.9 \\ Downstream & 2.62 & & 100 & 82.1 & 57.4 & 37.1 & 30.1 & 26.1 & 14.5 & 8.4 & 6.1 & 3.2 \\ \hline \hline \end{tabular}
\end{table}
Table 5: Particle-size distribution analysis by study area.  
Figure 6: Prediction of long-term riverbed change.  
crossing the river is positioned at EL.15.48 m. The bed level at the pipeline's location is currently at EL.17.46 m, but the long-term riverbed change simulation analysis indicates that bed level changes are occurring. It is predicted that after 20 years, as scouring progresses, the burial depth will reach 1.85 m. This means that there is still a water depth margin of 0.65 m compared to the regulatory guidelines for narrow streams shown in Figure 1. Although the gas pipeline is buried in a narrow stream with a low flow discharge and velocities, it is necessary to establish efficient application and management strategies for regulations and standards, considering changes in the installation environment and the operating conditions of the pipelines buried in the river. While the hydraulic methods used for predicting burial depth emphasize the need for continuous monitoring and maintenance to mitigate risks related to riverbed changes, as highlighted by this study, it is necessary to develop a rationalized burial depth management plan that fits the specific environmental conditions of the river at the study site.