are presented in Figure 58. The Dutch wide gradation is similar to well- 

 graded riprap and the Dutch narrow gradation is similar to very uniform 

 quarry stone. The two Dutch gradations can be used to provide upper and 

 lower bounds for stone gradations used in low-crested breakwater design. 



Bedding/filter layer considerations 



Usually reef breakwaters are built on a bedding/filter layer. The bedding 

 layer is designed to prevent excessive settlement of the structure due to armor 

 stone sinking into the underlying sediment. The ratio of median armor stone 

 size D 50 (A) to the median bedding stone size D 50 (B) provides a logical way to 

 characterize the bedding size. Two methods are available to select the size of 

 required bedding stone. Dutch guidance for revetment filter layers suggests 

 that D 50 (A)/D 50 (B) be approximately 4.5 or less (Van der Meer and Pilarczyk 

 1987). Ahrens (1975) suggests that Dj^AJ/D^fB) should not be greater than 

 4.0 to ensure that the underlayer is not pulled out through the armor layer by 

 wave action. Considering gradations used by Ahrens, a safe relation for 

 median stone dimensions would be D 50 (A)/D 50 (B) less than 6.8 (Ahrens and 

 Cox 1990). 



In low rubble-mound structures without a core, the bedding layer is often 

 extended across the entire width of the structure and beyond the toe of the 

 armor stone as shown in Figure 56. The bedding stone will often be subject 

 to direct wave attack during low water levels. Bedding stones at the toe of the 

 structure may not provide the desired stability or toe protection, resulting in 

 additional stones required along the toe as shown in Figure 55. Stability 

 against wave attack of exposed bedding stone is discussed in EM 1 1 10-2-2904 

 and detailed guidance on toe protection can be found in the Shore Protection 

 Manual (1984). 



Geotextiles can be used beneath the bedding layer to improve foundation 

 conditions or prevent the loss of sediment through the bedding layer if filter 

 criteria between the bedding layer and underlying soil are not met. Filter 

 criteria should be met between both the geotextile and bedding layer and the 

 geotextile and underlying soil. Geotextiles are discussed in the Shore 

 Protection Manual and by Moffatt and Nichol, Engineers (1983) and Eckert 

 and Callender (1984), who present detailed requirements for using geotextile 

 filters beneath quarrystone armor in coastal structures. 



98 



Other Construction Types 



Most U.S. and foreign nearshore breakwaters built for shore protection 

 have been rubble-mound structures. Rubble-mound construction of nearshore 

 breakwaters is advantageous because of the ability for rubble mounds to 

 dissipate wave energy effectively and provide lew reflection coefficients. An 

 extensive amount of research has been conducted for rubble-mound structures, 



Chapter 4 Structural Design Guidance 



