Excerpt from Storm Evolution of Directional Seas in Shallow Water

Ocean wind waves are among the dominant forcing mechanisms for near shore dynamic processes. Of the broad suite of possible wave regimes, those associated with high energy are most important for engineering design. High energy waves produce the most rapid changes in natural or renourished beach morphologies and are most likely, by definition, to exceed the strength or elevation of a coastal protective structure, resulting in costly damage or execs sive overtopping. Complete knowledge of the types of extremely energetic natural wind wave fields that are incident on the nearshore is thus rather critical for efficient design of coastal engineering projects.

A complete wave field definition describes the way in which wave energy is distributed in both frequency and direction. Because of the difficulty in making high-resolution measurements of wave energy distributions, early engineering guidance treated a given sea state as consisting of a train of waves at a single frequency traveling in a single direction. While most wave fields are more complicated than this, there persists much guidance based on unidi rectional, monochromatic waves, as evidenced in the Shore Protection Manual (sfm Advances in time series measurement and efficient analysis in the last three decades have provided considerable knowledge of the frequency distributions of wave energy, enabling the development of good theoretical models and improved physical models of wave processes with a consequent improvement in engineering guidance.

Detailed measurements of wave direction are more difficult. Only within the last decade have practical methods of directional wave measurement been available, and these are still expensive and logistically demanding. Until very recently, there were no long-term observations from which to extract the directional character of nearshore seas with any statistical confidence. To alleviate this lack of knowledge, the Field Research Facility (frf) of the u.s. Army Engineer Waterways Experiment Station, Coastal Engineering Research Center, deployed a high-resolution directional wave gauge at its site near Duck, nc, in September 1986. Measurements covering a 5-year period, lasting through August 1991, enable considerable clarification of the direc tional nature of incident wind waves at this site.

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