The problem of obtaining wind information for wave hindcasting is 

 discussed, and specific instructions for estimating wind parameters are 

 given. 



Water levels continuously change. Changes due to astronomical tides 

 are predictable, and are well documented for many areas. Fluctuations due 

 to meteorological conditions are not as predictable, and are less well 

 documented. 



Many factors govern water levels at a shore during a storm. The 

 principle factor is the effect of wind blowing over water. Some of the 

 largest increases in water level are due to severe storms, such as hurri- 

 canes, which can cause storm surges higher than 25 feet at some locations 

 on the open coast and even higher water levels in bays and estuaries. 

 Estimating water levels caused by meteorological conditions is complex, 

 even for the simplest cases; and unfortunately, the best approaches avail- 

 able for predicting these water levels are elaborate computational tech- 

 niques which require the use of large digital computers. 



3.2 CHARACTERISTICS OF OCEAN WAVES 



The earlier discussion of waves was concerned with idealized, mono- 

 chromatic waves. The pattern of waves on any body of water exposed to 

 atmospheric winds generally contains waves of many periods. Typical 

 records from a recording gage during periods of steep waves (Fig. 3-1) 

 indicate that heights and periods of real waves are not as constant as is 

 assumed in theory. Wavelengths and directions of propagation are also 

 variable. (See Figure 3-2.) The prototype is so complex that some 

 idealization is required. 



3.21 SIGNIFICANT WAVE HEIGHT AND PERIOD 



An early idealized description of ocean waves postulated a significant 

 height and significant period, that would represent the characteristics of 

 the real sea in the form of monochromatic waves. 



The representation of a wave field by significant height and period 

 has the advantage of retaining much of the insight gained from the theo- 

 retical studies. Its value has been demonstrated in the solution of 

 engineering problems. For some problems this representation appears 

 adequate; for others it is useful, but not entirely satisfactory. 



To apply the significant wave concept it is necessary to define the 

 height and period parameters from wave observations. Munk (1944) defined 

 significant wave height, as the average height of the one-third highest 

 waves, and stated that it was about equal to the average height of the 

 waves as estimated by an experienced observer. This definition, while 

 useful, has some drawbacks in wave-record analysis. It is not always 

 clear which irregularities in the wave record should be counted to deter- 

 mine the total number of waves on which to compute the average height of 

 the one-third highest. The significant wave height is written as H1/3 or 

 simply Hg. 



3-2 



