# Safety Assessment of Gas Pipelines Crossing River through Hydrodynamic Analysis
## 2 Materials and Methods  
### Hydrodynamic Safety Assessment of Buried Gas Pipelines in Riverbeds  
In Korea, when burying gas pipelines in rivers, the distance between the outer surface of the pipeline and the planned riverbed elevation varies based on the size of the water-course, such as rivers, small streams, and canals. Figure 1 illustrates the standard burial depths for different types of watercourses, with large rivers requiring a burial depth of over 4.0 m, medium-sized rivers requiring depths over 2.5 m, and narrow streams requiring a minimum depth of 1.2 m. These depth standards are crucial in ensuring that the pipelines remain safely buried even during events of deposition or scour. Similarly, other countries such as the United States, the United Kingdom, Canada, and Australia also regulate burial depths based on regional safety considerations ([PERSON] et al., 2016; [PERSON] et al., 2017; [PERSON] et al., 2018; [PERSON] et al., 2019; [PERSON] et al., 2020). For gas pipelines crossing rivers, maintaining the required burial depth becomes highly challenging due to the occurrence of deposition and scour. Figure 1 serves as a visual reference for the regulatory depth requirements, highlighting the importance of adhering to these standards to minimize the risk of pipeline exposure. Therefore, before any expected changes to the riverbed, a thorough hydraulic investigation should be conducted as part of the safety assessment. This requires periodic safety assessments and active management to prevent critical damage caused by riverbed changes. A hydrodynamic safety assessment for pipelines buried in rivers entails the consideration of several factors such as subsurface conditions, geological characteristics, riverbed change, scour, and deposition. The hydrodynamic analysis targets cases where pipelines are buried within sediment layers and when they are exposed due to scour or riverbed changes. The degree of sediment deposition and scour can be determined by sediment transport modeling, which is preceded by hydrodynamic modeling featuring the river width, flow rate, velocity, and geological conditions. Figure 2 illustrates the procedure for assessing the safety of buried gas pipelines, focusing on both long-term and short-term changes in riverbeds. The safety assessment is governed by defined safety standards. The  
Figure 1: Gas pipeline safety assessment guideline (Urban Gas Business Act, 2022). basic investigation stage includes a watershed survey, river status assessment, and review of flood damage history. Based on these initial investigations, the model construction stage involves collecting and constructing topographic data, analyzing riverbed materials, and measuring sediment levels and discharge rates. Following this, the investigation of the stability of the river channel is conducted using both empirical and analytical methods. The long-term riverbed change assessment stage focuses on how the riverbed may change over an extended period under normal discharge conditions, including ordinary discharge and long-term changes occurring over several years or decades. For the long-term riverbed change analysis, this study employs the one-dimensional model HEC-RAS. HEC-RAS was chosen because it is a widely accepted and validated model developed by the U.S. Army Corps of Engineers, specifically designed for simulating one-dimensional steady and unsteady flow, sediment transport, and riverbed changes over long periods. Its robust capabilities in handling complex river networks and sediment transport modules and the ease of integrating hydrological data make it suitable for modeling long-term riverbed evolution. Additionally, HEC-RAS offers extensive documentation and user support and has been successfully applied in numerous studies, ensuring reliability and credibility in the results. In contrast, the short-term riverbed change assessment stage evaluates the immediate impacts on the riverbed due to extreme flood discharge and short-term changes  
Figure 2: Hydrodynamic safety assessment process.