Prediction of Equilibrium Local Scour Depth at Complex Bridge Piers

At present, bridges are frequently built with geometries that consist of a column founded on a pile cap and supported by an array of piles, referred to as common complex piers or most often (as also in this paper), briefly, as complex piers. This paper addresses prediction of local scour depth at those complex piers. An experimental campaign of 48 long-duration tests was performed with seven complex pier models under steady clear-water flow conditions to quantify the influence of the relative column width, $D_c/D_{\mathrm{pc}}$ ($D_c$ = column width, $D_{\mathrm{pc}}$ = pile cap width), the relative pile cap thickness, $T/h$ ($T$ = pile cap thickness, $h$ = approach flow depth), and the pile-group configuration on the equilibrium scour depth, $d_{\mathrm{se}}$. Several tests were carried out for the possible range of variations in the pile cap position relatively to the initial bed level. The values of $d_{\mathrm{se}}$ were estimated by extrapolation of the experimental data series. The performance of three methods, i.e., Auckland, HEC-18, and Florida Department of Transportation (FDOT), to predict $d_{\mathrm{se}}$ is analyzed and discussed. Based on the experimental results of the present study and on the conceptual approaches of Auckland and FDOT methods, an alternative formulation for a predictor of $d_{\mathrm{se}}$ at (common) complex piers is suggested and validated.