Example 4: For the following given initial design conditions, calculate 

 maximum scour depth. 



d g - 0.25 mm - 0.00082 ft 



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



H - 6.0 ft 



T =7 sec 



h„ = depth at base of wall = 2.0 ft 



Solution: The first step is to determine h„/L and H /L : 



h„ 



2. 







= 0. 



008 



L 



250 



88 



and 











H o _ 



6 







= 



.024 



L n 



250 



.88 



Since these values fall within the limits of -0.011 < h w /L < 0.045 and 

 0.015 < ffo/L < 0.040, Equation 53 may be used to calculate S max as: 



— jS* = ^((22.72 2.0)/250.88) +.25 



= 0.66 



and therefore 



S ffiax = 3.94 ft 



41. The following equation was developed by Herbich and Ko (1968) using 

 limited 2-D laboratory data to predict ultimate depth of scour S max for an 

 initially flat slope where waves do not break prior to impacting the 

 structure: 



(h-a iz /2) f (l-JC r ) u. 3/4 C D p d g^Lj 



11/2 



d g (Y. - Y) 



(55) 



In the above . 



a ir = M i + H r (56) 



«r = 4r (57) 



The above method requires knowledge of a relationship between incident and 

 reflected wave heights, either through measurements made in the laboratory or 

 when available, through published values of K r . Although Equation 55 was 

 claimed to be in reasonable agreement with results from laboratory model 



42 



