Hydro- and Morpho-dynamics Induced by a Vertical Slender Pile under Regular and Random Waves

Abstract

This paper proposes an experimental study of the hydro- and morpho-dynamics induced by a slender vertical pile fitted into a sandy seabed and forced by sea waves in finite waters. The scour depth at the pile and the seabed morphology around it were measured and analyzed for both regular and random wave forcing. Scour depths due to regular waves were compared to those predicted in a previous study, revealing good agreement. The maximum scour depth occurred at the lateral side of the pile for waves characterized by a Keulegan-Carpenter (KC) number smaller than 7–8 and behind the pile for waves with KC > 7–8. Such a result revealed an asymmetric effect on the scour depth because of the weaker reversal flow. This behavior was more evident for nonlinear waves being crests larger than troughs and, as a consequence, vortices, responsible for the scour processes, became even more energetic around crest stages. To better understand the dynamics in front of, around, and behind the cylinder, pressure measurements were obtained to evaluate the generation and evolution of vortices. The pressure data showed that the adverse pressure gradient around the pile was higher than on the upstream of the pile. The analyses of the pressure data seemed to confirm that, if the forcing was only due to waves, the effect of the lee-wake vortices on the scour depth around the cylinder was stronger than that due to the horseshoe vortices. Finally, the practical stochastic method proposed by previous authors to compute the maximum scour depth around a vertical pile exposed to random waves was verified. The best adaptation between measured and predicted scour depths was obtained by taking into account approximately half of the highest waves (instead of one-tenth) over the threshold; such a condition is still cautionary for a practical point of view. A simplification of the previously discussed methodology was proposed in which the characteristic wave that generates the scour depth is defined.