Numerical investigation on the mitigation of harbor oscillations by periodic undulating topography

Abstract

Mitigation of harbor oscillations is a crucial concern in coastal engineering. It is more economical and accessible for an established harbor to reduce the intensity of oscillations by changing the incident wave conditions rather than modifying the harbor layout. Periodic sandbars and sand ripples, which are common in nearshore zones, can be modified into submerged breakwaters. When the wavelength of the bottom undulations is approximately half of the incident wavelength, the resonant reflection (i.e., Bragg reflection) appears, leading to significant enhancements in wave reflection, which further results in the reduction of the incident wave energy.The present study evaluates the effectiveness of Bragg reflection in inhibiting harbor oscillations using an extended mild-slope equation. The results show that undulating seabed topography outside the harbor can significantly weaken the intensity of the harbor oscillations. Subsequently, a possible mitigation mechanism is proposed. The influences of the bar wavelength, bar number, bar height, and the slope of seabed on the mitigation effect of harbor oscillations are studied. Furthermore, it is found that the optimal mitigation effect for different resonant modes is not constant, and the reason for this is explained.