FIND: The maximum wave height above the design value which will not cause 

 riprap failure and the smallest median weight riprap which will not fail 

 for the design wave height. 



SOLUTION: The reserve stability parameter is 



(1 + cot 2 0) 1 / 2 = 2/10 = 6.32 



[(Wwr) 1 / 3 ] 



and using Figure 3 gives 



— = 1.31 

 H„ 



Therefore, H = 1.31 x 1.52 = 1.99 meters or 2.0 meters (6.5 feet). Thus, a 

 wave height as great as 2.0 meters will not cause failure; for wave heights 

 between 1.5 and 2.0 meters, some damage would be expected but not failure. 

 No damage would be expected below H = 1.5 meters; failure could occur for 

 H > 2.0 meters. 



From Figure 3 and recalling from example 1 that N sz = 1.74, gives 





N s N s 



— = rr-TT = 1-31 



N sz 1.74 



or 







N s = 1.31(1.74) = 2.28 



Then, 



using equation (4) 





1.52 





" S / W 50 \ 1/3 /2,644 \ 



= 2.28 



and solving for W50 gives, 



W 50 = ( 1 ' 52 / a?/ 644) \3 = 176 kil °g rams < 389 pounds) 



<"« 3 &§£-!)' 



Example 1 showed that W50 = 397 kilograms was necessary for no damage; for 

 VJ5 between 176 and 397 kilograms, damage could be. expected but no failure. 

 However, for W50 < 176 kilograms, failure could occur. 

 *************************************** 



7. Location of Damage . 



Damage to the armor layer can extend over a surprisingly large extent of 

 the revetment face. Generally, the worst damage is above the Stillwater level 

 (SWL) on steep slopes and below the SWL on flat slopes. Table 1 quantifies 

 the findings of Ahrens (1975) regarding the upper limit of damage, £ u , and the 

 lower limit of damage, 1%. In the table, & u and £& are divided by the wave 



15 



