3 



w H 



W = (7-117) 



50 3 



K (S - 1) cot e 



RH r 



The symbols are the same as defined for equation (7-116). Wcq is the weight 

 of the 50 percent size in the gradation. The maximum weight or graded rock, is 

 4.0 (Wcq) ; the minimum is 0.125 (WcQ^ . Additional information on riprap 

 gradation for exposure to wave forces is given by Ahrens (1981b). Knn is a 

 stability coefficient for angular, graded riprap, similar to K^ . Values 

 of Kdd are shown in Table 7-8. These values allow for 5 percent damage 

 (Hudson and Jackson, 1962). 



Use of graded riprap cover layers is generally more applicable to revet- 

 ments than to breakwaters or jetties. A limitation for the use of graded 

 riprap is that the design wave height should be less than about 1.5 m (5 

 ft). For waves higher than 1.5 m (5 ft), it is usually more economical to use 

 uniform-size armor units as specified by equation (7-116). 



Values of K^ and Knn are obtained from laboratory tests by first 

 determining values of the stability number N„ where 



w 1/3 H w 1/3 H 



N = -f-~ or ^ , .„ (7-118) 



^ W^ [S^-1] W 50^/^ [S^-1] 



The stability number is plotted as a function of cot 9 on log-log paper, 

 and a straight line is fitted as a bottom envelope to the data such that 



Ng = (Kp cot 6)^^^ or (k^^ cot Q]^^^ (7-119) 



Powers of cot 9 other than 1/3 often give a better fit to the data. Ng 

 can be used for armor design by replacing K^, cot 9 in equation (7-116) or 



3 

 K^^ cot 9 in equation (7-117) with Ng , where Ng is a function of some 



power of cot 9 . 



d. Selection of Stability Coefficient . The dimensionless stability 

 coefficient Y. ^ in equation (7-116) accounts for all variables other than 

 structure slope, wave height, and the specific gravity of water at the site 

 (i.e., fresh or salt water). These variables include: 



(1) Shape of armor units 



(2) Number of units comprising the thickness of armor layer 



(3) Manner of placing armor units 



(4) Surface roughness and sharpness of edges of armor units (degree of 

 interlocking of armor units) 



(5) Type of wave attacking structure (breaking or nonbreaking) 



7-207 



