# Safety Assessment of Gas Pipelines Crossing River through Hydrodynamic Analysis
## 3 Results
### Short-Term Riverbed Change  
The short-term bed variations during the design flood corresponding to the 100-year return period were investigated by two-dimensional models (RMA2, SED2D). The hydrodynamic flow analysis must precede the sediment analysis, and the flow field describing the water level and velocity was obtained by RMA2. As shown in Figure 8, during the flood, the water level ranged from 22.66 m to 22.71 m, with the water level at the gas pipeline burial site measured at 22.69 m. As illustrated in Figure 8b, while passing through the bridge section, the velocity slightly decreased from 1.45 m/s to 0.95 m/s due to the expansion of the stream width. The analysis of short-term bed variations near Hyunpoong Bridge No. 3 revealed that deposition occurred throughout the river during the flood. The short-term bed variation analysis results over the flood duration are shown in Figure 9. Three hours after the flood onset, a deposition of 0.1 cm occurred, with minor changes in the riverbed and a slight scour observed along both banks. Six hours later, a deposition of 4 cm occurred along the  
Figure 8: Flow characteristics: (**a**) water level; (**b**) flow velocity. Figure 7: Changes in buried depth determined by long-term simulation. narrow right bank of Hyunpoong Bridge No. 3. Deposition was prominent in the middle section of the study area, with specific sections showing an increased bed height due to accumulated sediments. This indicates that sediment transport began to deposit as the flood velocity decreased. After 24 h, 2 cm of deposition occurred across the study area, indicating changes in the dynamic equilibrium state of sediment transport due to changes in flow velocity during floods. In the short-term riverbed variation simulation, it was confirmed that deposition gradually progressed over time in the cross-section where the gas pipeline is buried. As shown in Figure 10, fine deposition occurred in the central part of the riverbed during the flood period. Therefore, the two-dimensional short-term riverbed variation simulation demonstrates that the riverbed height changes over the course of the flood, with incoming sediments accumulating on the riverbed. The burial depth due to short-term bed variations is shown in Figure 11. Before the flood, the burial depth was EL 1.98 m. Three hours after the beginning of the flood, there was a deposition of 0.1 cm. As the flood progressed and the maximum flow occurred six hours later, the deposition further increased to 0.4 cm. By the end of the flood, the maximum deposition reached 2 cm, resulting in a burial depth of 2.00 m. This indicates that the impact of scour was negligible throughout the flood event. Specifically, from the beginning of the flood to its end, the prediction results showed a maximum cumulative variation of 2 cm. This variation demonstrates that deposition was the predominant process  
Figure 10: Prediction of short-term riverbed change. Figure 9: Riverbed change and deposition patterns during design flood: (a) 3 h; (b) 6 h; (c) 24 h.  
affecting the riverbed during the flood, and sediments were steadily depositing on the riverbed, thus maintaining the stability of the buried pipeline. The results suggest that despite the dynamic conditions during the flood, the pipeline's burial depth remained within safe limits, with negligible scour effects. This detailed analysis provides crucial insights into sediment dynamics and hydraulic behavior during flood events, ensuring that the infrastructure remains protected from potential scour-related damage. The findings emphasize the importance of considering short-term bed variations in hydraulic and sediment transport studies for effective river management and infrastructure safety. The results of the long-term and short-term simulations indicate that while the current burial depth standard ensures the safety of gas pipelines under normal conditions, there are potential challenges in reflecting the spatial characteristics of the watercourse.