考虑表层土厚度影响的双层河床桥墩局部冲刷特征分析OA

Local scour at bridge piers over multi-layered riverbeds evolves more complicatedly than over single-layer beds.The thickness of the surficial soil layer directly affects the maximum scour depth and final scour-hole morphology,yet traditional predictors often ignore the influence of surface soil thickness on pier foundation risks.To address this gap,we conducted clear-water flume experiments on single-layer and dual-layer beds(coarse upper layer over a fine lower layer).A total of 34 tests were conducted:nine on a single-layer uniform sand bed and the remainder on a double-layer uniform sand bed.The surface-layer thickness hb ranged from 6.07 to 11.50 cm,and the approach discharge ranged from Q=32.5 to 52.5 L/s.By systematically varying approach flow velocity and surficial-layer thickness,we documented the temporal evolution and equilibrium morphology of scour pits.The results indicate that,under double-layer bed conditions,a pronounced rate jump occurs as the scour front approaches the interlayer interface,leading to a stepped scour-pit morphology characterized by a gentle upper slope and a steep lower drop.Increasing surficial-layer thickness attenuates energy transmission to the interface and fosters a secondary armor layer at the pit bottom,markedly suppressing erosion of the underlying sediment.Based on these experiments,we introduce a surface-thickness correction coefficient Kw into the HEC-18(CSU)equation.The modified formulation exhibits significantly improved predictive accuracy over the original,providing a reliable tool for erosion-control design and risk assessment under complex geological conditions.