derived for the pre -seawall condition and may be determined by the commonly 

 used method presented in the SPM (1984) . When the location of the toe of the 

 seawall coincides with the location of mean sea level, X s — 1. The following 

 empirical equation was proposed for prediction of maximum scour depth: 



"max 



= 1.60 (1 - X s ) 2/S 



(51) 



Figure 14. Definition sketch for Jones' method 



A major problem with the Jones equation is that zero scour is predicted when 

 the seawall is located at X s = 1 (at the shoreline) . This is contradicted in 

 every study examined; in fact, some have found that this seawall location 

 corresponds to the greatest scour condition. This suggests that use of this 

 equation should be limited to conditions where X s < 1.0. 



Example 2. For the following given initial design conditions, calculate 

 maximum scour depth. 



d g = 0.25 mm 



m = beach slope in front of seawall = 1:20 = 0.05 



H = 6.0 ft 



T =7 sec 



h„ = depth at base of wall = 2 . f t 



Solution: Initial calculation to be made is determination of X s as 



x - JL 



s IT 



5.0 

 10.0 



= 0.5 



The first step is to determine H b and h b , the depth of water at the point of 

 wave breaking, so that X b can be determined. The SPM (1984) provides a method 

 for determining H b and h b provided H , T , and the beach slope are known. The 

 method uses Goda's (1970) empirically derived relationships between H b /H and 

 H /L for a given beach slope. From linear wave theory, 



38 



