Service Command (1970) provides summaries of shipboard wave observations. 

 Summary of Synoptic Meteorological Observations (SSMO) for the hatched 

 areas indicated in Figure 3-17. Cumulative distribution functions for 

 wave heights as determined by both hindcasting techniques and the ship- 

 board observations are given in Figure 3-18. The average of the two 

 forecasting methods agrees reasonably well with the shipboard observations. 



3.54 ESTIMATING WAVE DECAY IN DEEP WATER 



Figures 3-19 and 3-20 are used to estimate wave characteristics after 



the waves have left the fetch area but are still travelling in deep water. 



With Figure 3-19, and given H^, T^, F^ and D (the decay distance), it is 

 possible to compute the ratios 



decayed wave height D decayed wave period D 



—z — = — , and — -: — — = — 



fetch wave height Hj, fetch wave period Tp 



With Figure 3-20, it is possible to compute wave travel time between 

 a fetch and a coast, knowing the decayed wave period T^ and the decay 

 distance D. 



This tvavet time t^ is determined by dividing the decay distance 

 by the deepwater group velocity for waves having a period equal to the 

 decayed period Ij^. These values enable the estimation of arrival times 

 for waves at the end of the decay distance. 



Waves, after leaving a generating area, will generally follow a great- 

 circle path toward a coast. However, sufficient accuracy is usually 

 obtained by assuming wave travel in a straight line on the synoptic chart. 

 Decay distance is found by measuring the straight line distance between 

 the front of a fetch and the point for which the forecast is being made. 

 If a forecast is being made for a coastal area, the effects of shoaling, 

 refraction, bottom friction and percolation will have to be considered in 

 translating the deepwater forecast to the shore. 



3.6 WAVE FORECASTING FOR SHALLOW WATER 



3.61 FORECASTING CURVES 



Water depth affects wave generation. For a given set of wind and 

 fetch conditions, wave heights will be smaller and wave periods shorter if 

 generation takes place in transitional or shallow water rather than in deep 

 water. Several forecasting approaches have been made; the method given by 

 Bretschneider as modified using the results of Ijima and Tang (1966) is 

 presented here. Bretschneider and Reid (1953) consider bottom friction 

 and percolation in the permeable sea bottom. 



There is no single theoretical development for determining the actual 

 growth of waves generated by winds blowing over relatively shallow water. 

 The numerical method presented here is based on successive approximations 



3-42 



