ENERGY SPECTRA IN SHALLOW U.S. COASTAL WATERS 



by 

 Edward F. Thompson 



I. INTRODUCTION 



Wind-generated gravity waves, with periods ranging from 1 to 25 sec- 

 onds, are a primary concern in most coastal engineering work. Wind waves 

 relate directly to such processes as longshore sand movement, onshore- 

 offshore sand movement, and forces exerted on coastal structures. Hence, 

 wind waves must be considered in virtually all coastal engineering en- 

 deavors, especially in design and maintenance of harbors and exposed 

 navigation channels and in shore protection efforts. Edmisten (1978) 

 presents an extensive list of major applications for coastal wind wave 

 data. 



Coastal engineers and researchers have wrestled for decades with the 

 difficult problem of how to reduce the complexity of ocean wind waves to 

 simple, useful approximations. The most common approach has been to rep- 

 resent the dominant waves in a particular sea state with three simple 

 parameters: significant wave height, significant wave period, and wave 

 direction. The latter parameter is normally unavailable, especially with 

 gage data; however, it can be estimated from wind and weather records for 

 days of special interest. 



Practical techniques are available for estimating significant height 

 and period from a wave record. All of the accepted techniques produce 

 reasonable estimates for significant height. However, estimates for 

 significant period are generally more variable and their significance 

 is often questionable. One of the best approaches to estimating sig- 

 nificant or dominant period is to compute the distribution of wave energy 

 as a function of frequency (frequency is the reciprocal of period) and 

 use the wave period corresponding to the frequency of greatest energy 

 density. This period has a clear dynamic significance. The distribution 

 of wave energy as a function of frequency is commonly referred to as the 

 wave energy speotrum. 



Although significant height, period, and direction provide a useful 

 approximation to many sea states, these simple parameters omit details 

 of the sea state which can be important in some applications. For ex- 

 ample, the simultaneous occurrence of several prominent wave trains with 

 different wave periods and directions is an important consideration in 

 relating coastal wave conditions to longshore sediment movement. Design 

 of ships and flexible coastal structures, such as floating breakwaters 

 and offshore platforms, requires special consideration of how much wave 

 energy can be expected at frequencies near the resonant frequencies of 

 the structure. Similarly, effective harbor design requires that resonant 

 oscillations in the harbor be minimized. Since the effectiveness of 

 floating breakwaters in attenuating wave energy depends on the wave fre- 

 quency, floating breakwater designs should be formulated and evaluated 

 with a knowledge of how wave energy is distributed with frequency. 



