The time scale of the scour process around a single, vertical pile is studied both in steady current and in waves. An extensive series of tests were carried out in a current flume and in a wave flume with various sizes of circular cross-section piles ranging from 30 mm to 510 mm in the steady current case and from 10 mm to 40 mm in the wave case, using sand. The study indicates that the time scale in the steady current case is governed by two main parameters, namely the Shields parameter e and the ratio o/D where D is the pile diameter and 0 is the boundary-layer thickness which is replaced by the flow depth in the present study. The range of the former parameter in the present experiments was θ = 0.06 – 0.29 and that of the latter parameter was δ/D = 0.04 – 10. In the wave case, on the other hand, the governing parameters are, the Shields parameter and the Keulegan-Carpenter number, KC. The range of e in the present tests was θ = 0.07 – 0.19 and that of KC was KC = 6 – 30.
When a pile is placed in an originally plane bed, scour will take place around the pile due to the action of waves and current. This process is for instance of importance in connection with the stability of platforms. Too large scour around a platform leg may reduce the stability of the structure leading to failure. The scour depth corresponding to the fully-developed stage of scour process has been studied very extensively for steady currents (see a comprehensive review of the subject by Breusers et al. (1977) and Hjort (1975)) and fairly extensively for combined waves and current (Wang & Herbich (1983), Herbich et al.
