function of relative seawall distance X s and deepwater standing wave steepness 

 H s /L s : 



H„ 



1.94 + 0.57 ln(X s ) + 0.72 ln(H s /L s ) (53) 



One problem with this method is the potential for significant differences 

 between standing wave heights and deepwater progressive wave heights. For the 

 condition of unbroken waves impacting the seawall, the SPM (1984) indicates 

 that the maximum H s can potentially be as high as 2H . For the condition of 

 wave breaking prior to impacting the seawall, the difference between H s and H 

 is less significant and approaches zero when much of the incident wave energy 

 is lost to breaking. 



Example 3: For the following given initial design conditions, calculate 

 maximum scour depth. 



d s = 0.25 mm = 0.00082 ft 



m = beach slope in front of seawall = 1:20 



H = 6.0 ft 



T = 7 sec 



V^ = depth at base of wall = 2.0 ft 



Solution: Since initial conditions are similar to those in Example 2, the 

 depth of wave breaking is 7.68 ft. This indicates that deepwater parameters 

 can probably be used in Equation 53. X s was calculated in the previous 

 example and equals 0.74. The next step is to obtain the deepwater standing 

 wave steepness as 



H 





6 



.0 





L~o 





L 







= 





6.0 







5 



.12 T 2 







= 





6.0 







5 



.12 (7 . 



0) 2 





= 



0. 



0239 





Substituting into Equation 53 yields 

 S, 



max 



= 1.94 + 0.57 ln(0.74) + 0.72 ln(0.0238) 



= -0.9234 

 so that the maximum scoured depth would be 



S_ = -5.5 ft. 



40 



