propagate out of the generation area. When any of these events occurs, the 

 wave field begins to decay. Wave energy travels at a speed which increases 

 with the wave period, thus the energy packet leaving the generating area 

 spreads out over a larger area with increasing time. The apparent period at 

 the energy front increases and the wave height decreases. If the winds 

 subside before the sea is fully arisen, the longer waves may begin to decay 

 while the shorter waves are still growing. This possibility is recognized in 

 advanced wave prediction techniques. The hindcast spectra, computed by the 

 Inoue (1967) model and published by Guthrie (1971), show many examples of this 

 for low swell, as do the aerial photographs and spectra given by Harris 

 (1971). This swell is frequently overlooked in visual observations and even 

 in the subjective analysis of pen-and-ink records from coastal wave gages. 



Most coastal areas of the United States are situated so that most of the 

 waves reaching them are generated in water too deep for depth to affect wave 

 generation. In many of these areas, wave characteristics may be determined by 

 first analyzing meteorological data to find deepwater conditions. Then by 

 analyzing refraction (Chapter 2, Section 11,2, Refraction by Bathymetry), the 

 changes in wave characteristics as the wave moves through shallow water to the 

 shore may be estimated. In other areas, in particular along the North 

 Atlantic coast, where bathymetry is complex, refraction procedure results are 

 frequently difficult to interpret, and the conversion of deepwater wave data 

 to shallow-water and near-shore data becomes laborious and sometimes 

 inaccurate. 



Along the gulf coast and in many inland lakes, generation of waves by wind 

 is appreciably affected by water depth. In addition, the nature and extent of 

 transitional and shallow-water regions complicate ordinary refraction analysis 

 by introducing a bottom friction factor and associated wave energy 

 dissipation. 



IV. ESTIMATION OF SURFACE WINDS FOR WAVE PREDICTION 



Wind waves grow as a result of a flux of momentum and energy from the air 

 above the waves into the wave field. Most theories of wave growth consider 

 that this input of energy and momentum depends on the surface stress, which is 

 highly dependent upon windspeed and other factors that describe the atmos- 

 pheric boundary layer above the waves. Winds for wave prediction are normally 

 obtained either from direct observations over the fetch, by projection of 

 values over the fetch from observations over land, or by estimates based on 

 weather maps. Methods for estimating the windspeeds needed in Chapter 3, 

 Section V, to hindcast waves from these basic data types will be provided in 

 Chapter 3, Sections 2, 3, and 4. Prior to that, the following brief 

 discussion of the wind field above waves will be provided as background. 



1 . Winds Over Water . 



For discussion purposes, the wind will be considered to be driven by 

 large-scale pressure gradients in the atmosphere that have been in a near- 

 steady state. The winds above the wave field, then, can be considered as a 

 profile (Fig. 3-11). Some 1000 meters or so above the surface, the winds are 

 driven mainly by geostrophic balance between Coriolis and local pressure 

 gradient forces. Below this level, the frictional effects due to the presence 



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