CHAPTER 3 



WAVE AND WATER LEVEL PREDICTIONS 



I. INTRODUCTION 



Chapter 2 treated phenomena associated with surface waves as though each 

 phenomenon could be considered separately without regard to other phenomena. 

 Surface waves were discussed from the standpoint of motions and transforma- 

 tions without regard to wave generation. Furthermore, the water level, 

 Stillwater level (SWL), on which the waves propagated was assumed known. 



In this chapter, wave observations are presented to show characteristics 

 of surface waves in nature. The characteristics of real waves are much less 

 regular than those implied by theory. Also presented are procedures for 

 representing the complexity of the real sea by a small number of parameters. 

 Deviations between the actual waves and the parameter values are discussed. 



Theory for wave generation is reviewed to show progress in explaining and 

 predicting the actual complexity of the sea. Wave prediction is called 

 hindaasting when based on past meteorological conditions and forecasting when 

 based on predicted conditions. The same procedures are used for hindcasting 

 and forecasting; the only difference is the source of meteorological data. 

 The most advanced prediction techniques currently available can be used only 

 in a few laboratories because of the need for computers, the sophistication of 

 the models, and the need for correct weather data. However, simplified wave 

 prediction techniques, suitable for use by field offices, are presented. 

 While simplified prediction systems will not solve all problems, they can be 

 used to indicate probable wave conditions for some design studies. 



Prediction theories are reviewed to give the reader more perspective for 

 the simplified prediction methods provided. This will justify confidence in 

 some applications of the simplified procedures, will aid in recognizing 

 unexpected difficulties when they occur, and will indicate some conditions in 

 which they are not adequate. The problem of obtaining wind information for 

 wave hindcasting is discussed, and specific instructions for estimating wind 

 parameters are given. 



Many factors govern water levels at a shore during a storm. Other than 

 the tide, the principal factor is the effect of wind blowing over water. Some 

 of the largest increases in water level are due to severe storms, such as 

 hurricanes, which can cause storm surges higher than 7.0 meters (22 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; the best approaches available for pre- 

 dicting these water levels are elaborate numerical models which require use of 

 a computer. 



II. CHARACTERISTICS OF OCEAN WAVES 



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

 tic waves. The pattern of waves on any body of water exposed to winds 



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