VI. WAVE FORECASTING FOR SHALLOW WATER 



1 . 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 genera- 

 tion takes place in transitional or shallow water rather than in deep water. 

 Several forecasting approaches have been made, including the method given by 

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

 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 

 method presented here is based on successive approximations in which wave 

 energy is added due to wind stress and subtracted due to bottom friction and 

 percolation. This method uses deepwater forecasting relationships (Chapter 3, 

 Section V) to determine the energy added due to wind stress. Wave energy lost 

 due to bottom friction and percolation is determined from the relationships 

 developed by Bretschneider and Reid (1953). Resultant wave heights and 

 periods are obtained by combining the above relationships by numerical 

 methods. The basic assumptions applicable to development of deepwater wave 

 generation relationships as well as development of relationships for bottom 

 friction loss (Putnam and Johnson, 1949) and percolation loss (Putnam, 1949) 

 apply. The duration should be considered approximate. 



These shallow-water forecasting curves (Fig. 3-27 through 3-36) represent 

 an interim method for wave forecasting in shallow water. Modifications to the 

 shallow-water forecasting equations were made to provide a transition between 

 the revised deepwater forecasting equations and the shallow-water forecasting 

 model. Research is underway that may revise the shallow-water forecasting 

 model. Until the results of this new research are available, the curves 

 should be used. The curves are plotted from the following equations: 



SS = 



= 0.283 tanh 



Si 

 U 



0.530 



^- = 7.54 tanh 



0.833 



,3/4 1 



tanh 



3/8 1 



tanh< 



(3-39) 



(3-40) 



3-55 



