for toe berm stone stability against wave action (not scour protection) . The 

 guidance states that "unless site-specific model tests are conducted to 

 justify higher values of N s , stability number should be selected based on the 

 lower limit curves presented in Figures 11 and 12, and the individual toe berm 

 armor stone weights should range from a maximum of 1.3 W 50 to a mimimum of 

 0.7 W 50 ." For toe buttressing stone protection for wave stability only, 

 limited 2-D stability tests for toe buttressing a one-layer uniformly placed 

 tri-bar structure, a stability number N s equal to 1.5 should be used in a 

 wave -breaking environment. 



57. For smaller rubble-mound structures such as revetments, the 

 S /H < 1 rule of thumb, which was developed for use with vertical seawalls, 

 should be appropriate for determining ultimate scour depth. In such cases, 

 structures should be designed such that the seaward face of the structure is 

 extended downward to the expected scour depth, typically equal to the maximum 

 wave height carried in that depth of water. 



Vertical Piles and Similar Structures 



58. For scour prediction methods at vertical piles, the method dis- 

 cussed in Section IV by Herbich et al . (1984) should provide sufficient design 

 guidance . 



Vertical Wall Structures 



59. Results from Fowler (1992) and numerous field studies tend to 

 support the widely used rule of thumb which states that S max /H < 1. Another 

 rule-of- thumb method, Dean's approximate principle, appears to be supported by 

 numerous laboratory studies and limited field observations; however, a major 

 shortcoming of this method is that it requires determination of beach profiles 

 for given sediments and wave climate both prior to and subsequent to a design 

 event. At present, this is quite difficult to accomplish. When used with 

 various semi -empirical equations for prediction of S max , the equation of Song 

 and Schiller (1973) performed reasonably well within the limits of applicabil- 

 ity given by 0.5 < X/X b < 1. An empirical equation based solely on irregular 

 laboratory wave data also has been proposed subject to previously described 

 limitations and appears to predict scour depth observed by others quite well 

 (Chesnutt and Schiller 1971, Barnett 1987). 



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