then combining the two equations for y = 1.20 (embedded stone) yields, 



W = — ; (7-114) 



15.23 X 10' (w^-Wj^)3 (cos 0- sin 0)3 ' 



where 



W = minimum weight of the stable stone, pounds 



V = velocity of water acting directly on stone, feet per 

 second 



Wp = unit weight of stone, pounds per cubic foot 

 Wjj = unit weight of water, pounds per cubic foot 

 g = acceleration of gravity, 32.2 feet per second^ 



6 = angle of structure slope with the horizontal in the 

 direction of flow 



d^ = equivalent minimum stone diameter, feet 



3/w^ 



y = Isbash constant 1.20 and 0.86 for embedded and 

 nonembedded stone, respectively. 



A graphical solution of Equations 7-112 and 7-113 for the equivalent 

 stone diameter and stone weight is shown in Figure 7-100. For salt water, 

 the curve would be adjusted accordingly. The curves are considered appli- 

 cable to conditions where turbulence is not excessive, and the stones are 

 either embedded or nonembedded. 



7.5 IMPACT FORCES 



Impact forces are an important design consideration for shore struc- 

 tures, because of the increased use of thin flood walls and gated struc- 

 tures as part of hurricane protection barriers. High winds of a hurri- 

 cane propelling small pleasure craft, barges and floating debris can 

 cause great impact forces on a structure. Large floating ice masses also 

 cause large horizontal impact forces. If site and functional conditions 

 require the inclusion of impact forces in the design, other measures 

 should be taken either to limit the depth of water against the face of 

 the structure by providing a rubble-mound absorber against the face of 

 the wall, or a partly submerged structure seaward of the structure that 

 will ground floating masses, and eliminate the potential hazard and need 

 for impact design consideration. 



In many areas impact hazards may not occur, but where the potential 

 exists (as for harbor structures) , impact force should be evaluated from 

 impulse -momentum considerations. 



7-204 



