may be determined by first analyzing meteorological data to find deep- 

 water conditions. Then, by analyzing refraction (Section 2.32, General - 

 Refraction by Bathymetry.), the changes in wave characteristics as the 

 wave moves through shallow water to the shore may be found. In other 

 areas, in particular along the North- Atlantic coast, wKere the bathymetry 

 is complex, refraction procedure results are frequently difficult to inter- 

 pret, 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 re- 

 fraction analysis by introducing a bottom-friction factor and associated 

 wave energy dissipation. 



3.4 WIND INFORMATION NEEDED FOR WAVE PREDICTION 



Wave prediction from first principles, as described above, requires 

 very detailed specification of the wind field near the water surface. This 

 is generally developed in two steps: (1) Estimation of the mean fvee air 

 wind speed and direction, (This step may be omitted for reservoirs and small 

 lakes if surface wind observations are available.), and (2) Estimation of 

 the mean surfaae wind speed and direction. 



When the full wave generation process is considered, a large capacity 

 computer must be used for the calculations, and fairly complex procedures 

 may be used for determining the wind field. Engineers who require wave 

 hindcasts for only a few locations, and perhaps for only a few dates must 

 employ simpler techniques. A brief discussion of the processes involved 

 in determining the surface wind and techniques suitable for use in deter- 

 mining the characteristics of the wind field needed for the simplified wave 

 prediction model described in Section 3.5, Simplified Wave Prediction Models, 

 are given in this section. These procedures will be accurate (within 20 

 percent) about two-thirds of the time. The following discussion provides 

 guidance for recognizing cases in which the simplified procedures are not 

 appropriate. Errors resulting from disregarding the exceptional situations 

 tend to be random. Thus climatological summaries, based on hindcast data, 

 may be much more accurate than the individual values that go into them. 



Wind reports from ships at sea are generally estimates based on the 

 appearance of the waves, the drifting of smoke, or the flapping of flags - 

 although some are anemometer measurements. Actually, even if all ships 

 were equipped with several aneometers, the wind field over the sea would 

 still not be known in sufficient detail or precision to permit full 

 exploitation of modem theories for wave generation. 



Fortunately, estimates of the surface wind field that are usefully 

 accurate most of the time can be based on the isobaric pattern of synoptic 

 weather charts . 



3-20 



