Conventional Approach 



3. It is important to be able to characterize wave directionality, but 

 the ability to do so has evolved very slowly. Coastal engineers consider wind 

 wave forces in one main sense: how they act at the ocean boundaries. Within 

 this realm, wave forces can be considered in two broad categories: how they 

 act on solid structures and how they induce sediment transport. Solid struc- 

 tures include seawalls, groins, jetties, breakwaters, and the hulls of operat- 

 ing dredges. Sediment transport is important in beach erosion, inlet migra- 

 tion, silting of navigation channels, foundation failure under some solid 

 structures, the fate of disposed dredged material, and the success of beach 

 nourishment projects. In any of these applications, the detailed mechanics is 

 extremely complex. Nonetheless, the gross behavior of wave-boundary interac- 

 tions can be estimated adequately with simple models which characterize their 

 dominant behavior. Obviously, if wind waves are used as input to a system 

 response model, then the results are, at best, only as good as the wave field 

 description. 



4. For computational efficiency, the simplest models are preferred. 

 Where knowledge is lacking, the simplest assumptions are usually made about 

 the behavior of real systems. These two concepts were employed (in the days 

 before computers and advanced gaging techniques) to characterize seas of 

 various complexities as single wave trains. An entire sea would be considered 

 to have a single characteristic height, frequency (monochromatic), and direc- 

 tion (unidirectional) , with most natural wave data being estimated from 

 observations. A broad base of engineering design guidance was established by 

 using these three parameters to characterize wind waves as forcing mechanisms. 

 Much of the guidance in the Shore Protection Manual (SPM)(1984) reflects this 

 simple approach to sea state description. 



5. With the advent of computers and advanced computation and gaging 

 techniques, it became possible to record and analyze the time series of sea 

 surface displacements at a point in space. Typical wave gages consisted of 

 wave staffs, pressure gages, or heave (vertical displacement) buoys. Analysis 

 of these data showed how wave energy was distributed in frequency; that is, at 

 any given time, a wave field could be considered to contain a number of wave 

 trains, each with its own amplitude and frequency. This type of (real) wave 



