Scour at River-Crossing Cylindrical Structures in Degrading Channels

Channel degradation can expose river-crossing cylindrical structures (e.g., pipelines, canals, and tunnels) that are completely buried in the riverbed, altering their surrounding flow patterns to cause scour that threatens their structural safety. This paper reports a case study of scour at a river-crossing canal in a degrading river. Based on the historical topography and hydrological data, the cause, development process, and performance of countermeasures for scour at the river-crossing canal were analyzed, and recommendations for scour protection are proposed. The findings from 53 experiments of the scour at a river-crossing cylindrical structure in a degrading channel are presented. The experimental results indicate that there are three scour development modes at a river-crossing cylindrical structure in a degrading channel: (1) the exposure of the buried cylindrical structures induces downstream scour, with no tunnel erosion (i.e., scour below the bottom of the cylindrical structure) occurring at the cylindrical structure, (2) the riverbed degradation downstream of the cylindrical structure increases the water head over the cylindrical structure and seepage below the cylindrical structure, inducing tunnel erosion at the cylindrical structure, and (3) tunnel erosion is induced when a large-scale bedform trough approaches the cylindrical structure. Based on the experimental data, the critical conditions of each scour development mode were determined. The effects of the flow intensity, dimensionless water depth, and bed material roughness on the equilibrium scour depth were analyzed, and predictors for estimating the equilibrium scour depth at a river-crossing cylindrical structure in a degrading channel are proposed.