Influence of Cross Section on Wave Scour around Piles

The study extends the investigation of scour around circular vertical piles in waves, reported in 1992 in two works by Sumer et al. In the present paper, the influence of cross‐sectional shape of piles on scour is studied. An extensive series of tests was carried out with two kinds of cross sections: a circular cross section, and a square cross section tested at two orientations to the flow (namely, 90° orientation and 45° orientation), therefore giving three different test configurations. The equilibrium scour depth as well as the time scale of the scour process were investigated. Based on the present data, design equations are established relating the equilibrium scour depth, $S_0$, to the Keulegan‐Carpenter number $\left(\mathrm{KC}\right)$. For a given $\mathrm{KC}$ where $\mathrm{KC}⪝100$, the scour depth is the largest for the square pile with 45° orientation and smallest for the square pile with 90° orientation, the circular‐cylinder scour lying in between. In all three cases, the equilibrium scour depth normalized by the pile width $(S_0/D)$ increases with increasing $\mathrm{KC}$ number. For $\mathrm{KC}⪞100$, however, $S_0/D$ approaches a constant value, which is about 2 for the square piles and about 1.3 for the circular pile. Regarding the time scale of the scour process, this quantity is governed by two parameters, namely the Keulegan‐Carpenter number and the Shields parameter. Given the latter two parameters, the time scale was found to be somewhat smaller in the case of the square pile with a 90° orientation than those in the other two cases.