problem is demonstrated in Figure 26a where the difference between predicted 



K and observed K are plotted versus relative freeboard F/H . Fig- 

 r r mo b 



ure 26a shows that Equation 16 predicts K usually to within ±0.05 with lit- 

 tle systematic error except for high relative freeboards, i.e., F/H > 2.5 . 



mo 



Because of the possibility of systematic error for high relative freeboards, 

 it is recommended that if the relative freeboard exceeds 2.5, a value of 2.5 

 be used in Equation 16. When this procedure is applied to the data of this 

 study, it removes the systematic error as shown in Figure 26b. 



40. It is intended that the prediction equation for K , Equation 16, 

 be used with the wave transmission model (discussed in paragraph 34) in the 

 energy conservation relation given by Equation 15 to compute energy dissipated 

 by the reef. This approach was used to prepare Figure 27 which shows a scat- 

 ter plot of predicted energy dissipation versus "observed" energy dissipation 

 caused by the reef. Figure 27 shows that the procedure outlined above can 

 make good predictions of energy dissipation and the rather surprising fact 

 that, for some conditions, the reef can dissipate up to 90 percent of incident 

 wave energy. Generally, greatest energy dissipation was observed for short- 

 period waves on reefs which were high enough not to be overtopped. The lowest 

 observed energy dissipation of about 30 percent occurred for the few reefs 



with a relative crest height less than 0.7, i.e., h /d < 0.7 . For sub- 



c s 



merged reefs, energy dissipation increases with increasing steepness H /L 



mo p 



and with increasing relative reef width A /d L . Reefs with their crest 



t s p 



near the swl will dissipate between about 35 to 70 percent of incident wave 

 energy, and dissipation is strongly dependent on relative reef width as shown 

 in Figure 28. For reefs with moderate to heavy overtopping, i.e., 

 < F/H < 1.0 , energy dissipation is strongly dependent on the relative 

 reef width but not on wave steepness. 



41. Since wave energy dissipation characteristics of reef breakwaters 

 are so important, a special analysis was conducted to illustrate the influence 

 of the most important variables in a simple way that would still be consistent 

 with the data. This analysis used the most effective two variables in pre- 

 dicting K and the two most effective variables for predicting K with the 



t ° r 



provision that one of the variables be common to both K and K so that 



t r 



the predicted values could be plotted on a common axis. Fortunately, the re- 

 lative crest height h /d provides a good common variable. Good predictions 



are obtained for transmission using the variables h /d and B and for 



c s n 



39 



