Comparison of existing equations for local scour at bridge piers: parameter influence and validation

Local scour caused by the erosion or removal of sediment from bridge piers is a potential hazard to the safety of bridges. Numerous equations predicting local scour depth at bridge piers have been developed and compared over the past decades. However, little attention has been paid to the two widely used official equations in China. This study compared three types of commonly used equations, including the Chinese equations, HEC-18 equations, and Melville equations. Parameter influence analyses were firstly conducted to explore the effects of flow velocity, water depth, pier width, and sediment size on the calculated scour depth for each equation. The equations were then validated with 126 laboratory and 408 field data which were carefully screened. The analyses show that the effect degrees of each parameter on scour depths for the considered equations are significantly different, although the general trends are similar. The validation results show that the Chinese equations perform better with the field data than the laboratory data for which the equilibrium scour depths are significantly under-predicted by the equations. The HEC-18 equations predict the scour depths best for the laboratory data. The Melville equations over-predict the scour depth for both the laboratory data and the field data and are generally conservative.