r^= n k^(l -t^JItt^I (7-122) 



where N^ is the required number of individual armor units for a given 

 surface area, A is surface area, k is the layer coefficient, and P is 

 the average porosity of the cover layer in percent. Values of k^ and P 

 determined experimentally, are presented in Table 7-13. 



The thickness 

 following: 



A 



where Wcq is the weight of the 50 percent size in the gradation, or 



/W \l/3 

 r = 1.25 (^j (7-124) 



where W„ is the heaviest stone in the gradation, whichever of the three 



max 



is the greatest. The specified layer thickness should be increased by 50 

 percent for riprap placed underwater if conditions make placement to design 

 dimensions difficult. The placing density of riprap is calculated as the 

 weight of stone placed per unit area of structure slope, based on the measured 

 weight per unit volume of riprap. The placing density may be estimated as the 

 product of the layer thickness r , the unit weight of the rock w , and 



(^ 100 )• 



(4). Bottom Elevation of Primary Cover Layer . The armor units in the 

 cover layer (the weights are obtained by eq. 7-116) should be extended 

 downslope to an elevation below minimum SWL equal to the design wave height 

 H when the structure is in a depth >1.5H , as shown in Figure 7-116. When 

 the structure is in a depth <1.5H , armor units should be extended to the 

 bottom, as shown in Figure 7-117. 



On revetments located in shallow water, the primary cover layer should be 

 extended seaward of the structure toe on the natural bottom slope as scour 

 protection. 



The larger values of K^, for special-placement parallelepiped stone in 

 Table 7-8 can be obtained only if a toe mound is carefully placed to support 

 the quarrystones with their long axes perpendicular to the structure slope 

 (U.S. Army Corps of Engineers, 1979). For dolosse, it is recommended that the 

 bottom rows of units in the primary cover layer be "special placed" on top of 

 the secondary cover layer (Fig. 7-116), the toe berm (Fig. 7-117), or the bot- 

 tom itself, whenever wave conditions and water clarity permit. Site-specific 

 model studies have been performed with the bottom units placed with their 

 vertical flukes away from the slope and the second row of dolosse placed on or 

 overtopping the horizontal flukes of the lower units to assure that the units 

 interlock with the random-placed units farther up the slope (Carver, 1976; 



7-237 



