1 Introduction 



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 exces- 

 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 

 (SPM 1984). 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. 



The purpose of this report is to elucidate, by illustration, the nature of the 

 frequency and directional distributions of wind wave energy during storm 

 events occurring during the 5-year observation period. These results are 



Chapter 1 Introduction 



