Maximum influence of wind on wave overtopping at rubble mound breakwaters under oblique wave attack

Abstract

Especially for coastal structures with a protruding crest wall, wind affects wave overtopping because the crest wall can cause a vertical wave motion above the crest that is susceptible to wind. Earlier studies indicate that wind can increase wave overtopping discharges up to about a factor 5 for coastal structures under perpendicular wave attack. This study investigates the maximum influence of wind on wave overtopping at rubble mound structures under oblique wave attack. To analyse the influence of the wave angle on the maximum influence of wind on wave overtopping, physical model tests in a wave basin have been performed.
The maximum influence of wind is determined by the assumption that all water that exceeds the top of the crest wall would be blown over the crest by onshore wind. The ratio of the total overtopping volume with onshore wind and the total overtopping volume for the same wave condition without wind, provides the maximum influence factor of wind. To obtain the volume of water that exceeds the top of the crest wall, a rotating paddle wheel is employed that transports all water reaching the top of the crest wall to the leeside of the structure.
In addition to earlier studies on the influence of wind on wave overtopping not only the effect of oblique waves on wave overtopping discharges is investigated, also the effect of wind on overtopping volumes in individual overtopping events is analysed.
Results show that also for oblique wave incidence, the effect of onshore wind can increase the amount of overtopping significantly. It is confirmed that the influence of wind is more pronounced in conditions with low overtopping discharges. This trend has shown to be accurate for the description not only of the mean overtopping discharge, but also of individual overtopping events.
Based on the present tests, the method to account for the maximum influence of wind on wave overtopping discharges has been extended by incorporating the effects of oblique waves. Using this in combination with existing methods to estimate wave overtopping in absence of wind, enables estimates of the actual wave overtopping in real life situations.