Temporal Evolution of Clear-Water Local Scour at Aligned and Skewed Complex Bridge Piers

Scour at bridge piers is time-dependent. In this paper, temporal evolution of clear-water scour at complex bridge piers is studied experimentally. The pier model has a typical form comprising three components, namely a rectangular column, a rectangular pile-cap, and a group of vertical piles underneath. Various relative pile-cap positions and skew angles ($\alpha$) from 0° to 45° were used to investigate their influence on scour evolution. New functions are proposed to fit temporal data and determine the equilibrium scour depth with better accuracy. The results show that the locations for scour initiation and the maximum scour depth may be different, and their relationship varies with pile-cap position and pier skew angle. Highly skewed piers tend to overcome the influence of width ratio of column to pile-cap ($D_c/D_{pc}$) on scour evolution, as the column itself becomes dominant. The sensitivity of scour evolution to pier skew angle decreases with higher pile-cap position, especially when it is entirely above the original bed. Four scour development stages were identified for complex piers, including initiation, stagnation, a developing stage, and equilibrium, with each stage being highly dependent on the degree of exposure of each of the pier components. The description of each development stage for different situations is given. The equilibrium time scale $t^{*}$ and the equilibrium scour depth $d_{se}$ for complex piers have similar dependence on flow shallowness ratio ($y_0/D_e$) and sediment coarseness ratio ($D_e/d_{50}$), as per the equation proposed by an authors’ previous study (Yang et al. 2018). A new equation is proposed to correct the percentage rate of scour development. The correction is especially useful for aligned complex piers, for which the rate of scour time development may be much lower than that for single-column piers. In general, we recommend using the modified Sheppard-Melville method and the corrected time-scale equation in this paper to predict clear-water equilibrium scour depth and scour evolution at complex bridge piers.