( 



The analyzed and summarized field wave spectra are discussed in 

 Section IV which includes samples of spectra representing the highest 

 measured wave conditions at each site, average spectra and siommaries 

 of parameters derived from the spectrum and the sea-surface elevation 

 distribution function. Conclusions from the results in Section IV are 

 provided in Section V. Major findings are summarized in Section VI. 

 Numerous individual high-energy spectral plots are contained in Appen- 

 dix A. Average spectra are included in Appendixes B and C. 



II. PHYSICAL CHARACTERISTICS OF SHALLOW-WATER OCEAN WAVES 



Wind waves change considerably when propagating from deep to shallow 

 water. Consider a train of uniform waves moving toward shore with crests 

 oriented parallel to the bottom contours. The wave height first decreases 

 slightly and then increases continually as the waves move into progres- 

 sively shallower water. The phase velocity, group velocity, and wave- 

 length all decrease with decreasing water depth. Wave energy is generally 

 reduced as waves propagate shoreward. 



It appears contradictory that the wave height in a train of uniform 

 waves moving shoreward increases while the energy decreases. Yet the 

 two effects occur together mainly because the wave profile changes. The 

 profile of a wave in deep water can usually be considered sinusoidal un- 

 less the waves are exceptionally high in relation to their wavelength. 

 However, the profile of a steep wave in very shallow water is decidedly 

 nonsinusoidal (Fig. 2). The troughs are broad and flat, and the crests 

 are narrow and high. Deviation of the wave profile from sinusoidal be- 

 comes important when the relative water depth (ratio of water depth to 

 wavelength) is less than 0.04 for wind waves with very low steepness 

 (ratio of wave height to wavelength) . Nonsinusoidal profiles also occur 

 for relative depths greater than 0.04 when the wave steepness is greater 

 than about 0.0015; i.e., for ratios of wave height to the product of 

 gravitational acceleration times wave period squared greater than 0.00006 

 (U.S. Army, Corps of Engineers, Coastal Engineering Research Center, 1977, 

 p. 2-35). For waves with very high steepness (greater than 0.0063 or 

 height divided by gT^ greater than 0.001), nonsinusoidal profiles occur 

 in all water depths. 



Since phase velocity of an ocean wave decreases with decreasing water 

 depth, a long wave crest approaching shore at an angle to the bottom con- 

 tours has a nonuniform phase velocity along the crest. The velocity dif- 

 ferences result in an apparent bending, or refracting, of the crest which 

 decreases the angle between the crest and the bottom contours. Refraction 

 can affect a wave when the relative water depth is less than one-half. 

 Relative depths for the gages in this report range typically from 0.04 to 

 1. Hence, refraction has potentially affected many of the wave measure- 

 ments considered in this study. Refraction can act to increase or de- 

 crease wave heights depending on wave period and direction and bottom 

 contour orientation. Since most of the gages are sufficiently nearshore 

 that large waves can move bottom sediment seaward of the gage, the bottom 

 contour orientation affecting refraction can change with time. 



M 



