CHAPTER 3 

 WAVE AND WATER-LEVEL PREDICTIONS 



3.1 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 transformations without regard to wave generation. Furthermore, the 

 water level, stiltwater level (SWL) , on which the waves propagated was 

 assumed known. 



In this chapter, wave observations are presented to show characteris- 

 tics of surface waves in nature. The characteristics of real waves are 

 much less regular than those implied by theory. Also presented are pro- 

 cedures 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 fore- 

 casting 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 

 electronic computers, the sophistication of the models, and the need for 

 correct weather data. However, simplified wave prediction techniques, 

 suitable for use by field offices or a design group are presented. 



While simplified prediction systems will not solve all problems, they 

 can be used to indicate probable wave conditions for most design studies. 

 Simplified wave prediction can also be used to obtain statistical wave 

 data over several years. 



Review of prediction theories is presented to give the reader more 

 perspective for the simplified prediction methods that are presently 

 available. 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 graphs in Sections 3.5, Simplified Wave Prediction Models, and 

 3.6, Wave Forecasting for Shallow Water Areas, may be read with the pre- 

 cision justified by the underlying theory. The equations were derived 

 originally from graphs, and do not provide any physical understanding. 

 Calculations with the graphs should be carried out to tenths or hundredths 

 where this is feasible, and then rounded off in the final result. 



Predictions are compared with available observations wherever possible 

 to indicate their accuracy. Calibration of techniques applied to a spe- 

 cific geographic area by comparison with available observations is always 

 desirable. 



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