Hydrodynamic (Wave and Current) Loading - CAESAR II - Help

CAESAR II Users Guide

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Ocean waves are generated by wind and propagate out of the generating area. Ocean wave generation is dependent on the wind speed, the duration of the wind, the water depth, and the distance over which the wind blows the fetch length. There are several two-dimensional wave theories, but the three most widely used are the Airy (linear) wave theory, Stokes 5th Order wave theory, and Dean's Stream Function wave theory. The latter two theories are non-linear wave theories and provide a better description of the near surface effects of the wave.

Of course, wave motion is a three-dimensional action, but it can be adequately represented by two dimensions. One dimension is the direction the wave travels, and the other dimension is vertical through the water column. Two dimensional waves are not found in the marine environment, but are somewhat easy to define and determine properties for. In actuality, waves undergo spreading, in the third dimension. To understand this concept, think about a stone dropped in a pond. As the wave spreads, the diameter of the circle increases. In addition to wave spreading, a real sea state includes waves of various periods, heights, and lengths. To address these actual conditions, you must use a sea spectrum that includes a spreading function.

Airy (linear) wave theory assumes the free surface is symmetric about the mean water level. Additionally, water particle motion is in a closed circular orbit, the diameter of which decays with depth. You should take the term circular loosely because, the orbit varies from circular to elliptical based on whether the wave is in shallow or deep water.

Additionally, for shallow water waves, the wave height to depth ratio (H/D) is limited to 0.78 to avoid breaking.

None of the wave theories address breaking waves.

The figure below shows a typical wave and associated hydrodynamic parameters.

  • SWL

The still water level.

  • L

The wave length or horizontal distance between successive crests or troughs.

  • H

The wave height or vertical distance between the crest and trough.

  • D

The water depth or vertical distance from the bottom to the still water level.

  • h

The surface elevation measured from the still water level.