Coastal Hydraulics

Wave Forces

 

Introduction of Wave Forces

 

An important application of water wave mechanics is the determination of the forces induced by waves on fixed and compliant structures and the motions of floating objects. All objects, whether floating on the sea or attached to the bottom, are subjected to wave forces, and therefore these forces are of central interest to the designer of these structures.

 

The investigation of wave forces has been under way for a considerable time and numerous studies have been carried out for the case of wave forces on a vertical pile, yet no wave force calculation procedure has been developed to date for this most simple case for which there is uniform agreement. Although for long-crested waves, with a single fundamental period, theories are available which accurately represent the water particle motions in the absence of a pile for a wide range of wave characteristics, at present there is no reliable procedure for calculating the wave interaction with a structure for all conditions of interest. Watching a wave impinge on a vertical pile, the complexity of the problem becomes immediately obvious. As the wave crest approaches the pile, a bow wave forms and run-up occurs on the front of the pile, while a wake develops at the rear. We know from fluid mechanics that the wake signifies separated flow, which is impossible to treat analytically. Moreover at Reynolds numbers of interest, the flow is generally turbulent. As the wave crest passes and the trough reaches the pile, the flow field reverses and the previously formed wake may wash back past the pile as a new wake is formed. All of these phenomena clearly violate our previous assumptions of irrotational flow with small-amplitude waves and small velocities. 

 

Later discussions will describe the wave forces as comprised of an inertia and drag force component. In the case of structures that are large relative to the wave length, the wake effects are not important; the inertia force dominates, and accurate calculation methods exist. For objects that are small, the wake plays a dominant role on both the drag and inertia force components, and the roughness characteristics of the object are also of significance. In the latter case, no reliable analytical approaches are available and experimental results provide the major design basis.