Experimental investigation of the local scour characteristics of pipelines crossing mountainous rivers

Accurate prediction of the burial depth and suspended length for oil and gas pipelines crossing mountainous rivers is critical for ensuring structural integrity. In this study, systematic flume experiments are employed to examine the local scour characteristics under varying hydrodynamic conditions, with a particular emphasis on quantifying the relationships between the scour hole expansion rate and key parameters, including flow velocity, water depth, and pipe diameter. The experimental results demonstrate that riverbed evolution and scour hole development are predominantly governed by bedload transport. Notably, as the pipe diameter increases, the incidence of scour hole formation beneath the pipeline decreases significantly. The vertical expansion rate of scour holes peaks immediately upon initial erosion of the pipe bottom. The subsequent development of canalized flow leads to a progressive decline in the vertical scour rate, whereas the cumulative scour depth continues to increase. The vertical expansion dynamics at the pipe bottom conform to a first-order dynamic response equation, yielding a normalized time-dependent scour depth equation. Hydraulic parameters, pipe diameter, and sediment characteristics collectively influence the ultimate scour depth. Dimensionless correlations between the scour depth, relative sediment size, and Froude number ( Fr ) are established via Gauss–Seidel iteration. Distinct horizontal expansion regimes are identified: single-phase expansion dominates at Fr  > 0.6, whereas a secondary expansion phase emerges at Fr  ≤ 0.6. By integrating experimental data with empirical vertical expansion models, we propose a comprehensive horizontal scour expansion calculation model. These findings provide substantive insights into scour evolution mechanics and directly inform safety assessments for river-crossing pipelines in mountainous terrain.