Engineering Geological Characterization and Microtopographic Controls on Slide–Pipeline Interaction in Steep Submarine Canyons, Qiongdongnan Basin

Steep submarine canyons pose challenges to deep-water pipeline routing because of rugged terrain and slide-related hazards. This study integrates 3D seismic data, shipborne and autonomous underwater vehicle bathymetry, sub-bottom profiles, sediment testing, and numerical simulations to characterize engineering geological conditions and examine local terrain effects on slide–pipeline interaction in the Qiongdongnan Basin. Canyons, landslides/slumps, and scarps were identified as the principal engineering geological units, while high-resolution AUV data constrained representative depressions, ridges, and terrain asymmetry along the planned pipeline corridor. A post-failure fluidized mass was modeled using a Herschel–Bulkley relationship fitted to fully remolded site sediment. Thirty independent simulations compared five alternative fixed, non-buried pipeline locations under six field-informed terrain scenarios. The numerical framework was assessed through grid-sensitivity analysis and quantitative comparison with a published flume-derived drag relationship. Under fixed-pipeline and quiescent-water conditions, depressions confined moving material near the terrain base, whereas ridges produced more variable flow and loading patterns. Pipeline loading also varied with terrain asymmetry, approach direction, and relative pipeline position. The results provide screening-level guidance for identifying terrain-sensitive route sections requiring site-specific pipeline–soil–structure assessment.