Effects of Relative Column Width and Pile-Cap Elevation on Local Scour Depth around Complex Piers

An experimental campaign of 25 long-duration (13–47 days) laboratory tests was carried out with three complex pier models under steady clear-water flow conditions. Each model, characterized by a different relation between the column and the pile-cap widths, $D_c/D_{pc}$, was tested for a variety of pile-cap positions relatively to the initial bed, $H_c$. The experimental data were used to describe the temporal evolution of the scour depth as a function of $H_c/h$ ($h$ = approach flow depth). The common criterion to stop experimental tests on complex piers was analyzed, and a new criterion was introduced. The equilibrium scour depth, $d_{se}$, was calculated by extrapolation of data series. The results are used to evaluate the effect of $D_c/D_{pc}$ and $H_c/h$ on $d_{se}$ when the pile cap is above the bed (Situation 1), partially buried in the bed (Situation 2), and completely buried in the bed (Situation 3). The analysis includes the definition of $H_c$ at which the maximum $d_{se}$ occurs through an equation that takes into account the $D_c/D_{pc}$ ratio, the relative pile-cap thickness, $T/h$, and the column and pile-cap shapes.