Umeda, S., 2013. Scour process around monopiles during various phases of sea storms
An understanding of scour processes during the passage of sea storms is particularly important for the design of structures and the development of effective scour protection works. This study investigates the scour process at a slender monopile during various phases of a storm to give a better understanding of scour and backfilling mechanisms. Laboratory experiments were carried out to simulate the scouring effect at a pile by a storm consisting of a succession of seven phases of irregular waves. Detailed examinations were conducted to explore variations in the scour depth and bed topography. It turns out that scour patterns resulting from irregular waves can be characterized using the Keulegan-Carpenter number and the Shields parameter (KC1/3, θ1/3), which require significant near-bottom velocities. The scour process during a heavy storm consists of four main phases: (1) no bed deformation, (2) accretion, (3) erosion, and (4) backfilling. The amount of scouring increases as the order of the phase increased during a storm. The scour during phase (2), accretion, is in the transient condition between clear-water scour and live-bed scour. The scour in phase (3), erosion, is distinguished by live-bed scour, horseshoe vortices, vortex shedding and cone-shaped scour holes with vortex ripples. The influence of backfilling, phase (4), as a storm decreases in intensity, has a significant effect on the scour depth around the perimeter of the pile. The scour depth at the end of the event is much lower than the maximum scour depth during the event. However, backfilling of the scour hole makes only a weak recovery in total sediment volume around the pile, because the backfilling is confined almost exclusively to the neighborhood of the pile.