Severe wave run-ups on fixed surface-piercing square column under focused waves

Severe wave run-ups on a surface-piercing column leading to strong uprush water jets may cause unexpected impact loads on offshore structures. To reveal the underlying mechanism related to the complicated wave-column interaction, this paper investigates the occurrence and evolution of wave run-ups on a fixed surface-piercing square column under focused waves experimentally and numerically by comparing the wave run-up profiles, wave loads on the column, and velocity and pressure fields in the cases of different peak periods and steepnesses. The results manifested that the wave run-up under a very steep focused wave is significantly influenced by the localized nonlinear interaction between the wave crest and the uplifted water mound in front of the column and experiences a different regime from those primarily due to nonlinear wave diffraction. In the cases of breaking or nearly breaking focused waves, the sudden change in the fluid velocity on the wave crest when impacting on the uplifted water mound dramatically increases the peak value and gradient of dynamic pressure in the adjacent region and hence results in critical impact load on the column and strong accelerating effect of the uprush flow. Additionally, a larger peak period can further increase the thickness (or volume) of the uprush flow, potentially increasing the damage risk of offshore structures.