6. Reserve Stability . 



The ability of riprap to provide protection to an embankment when it is 

 exposed to waves greater than the zero-damage wave height is well known and 

 constitutes an important advantage in this type of revetment . This is re- 

 ferred to as vesevve stability . Reserve stability increases with the thick- 

 ness of the armor layer and the flatness of the embankment slope; these 

 characteristics are quantified in Figure 3 which is based on tests by Ahrens 

 (1975). The reserve stability in the figure is indicated by H/H z , the ratio 

 of the wave height to the zero-damage wave height. This ratio is equivalent 

 to the ratio of the stability number to the zero-damage stability number given 

 by equation (5) . Reserve stability is plotted in Figure 3 versus the parameter 



[(wso/v^Vs] (1 + 



cot 2 e) 1 /2 



where the quantity inside the bracket is the armor layer thickness in terms of 

 the typical stone dimension. In Figure 3, the zero-damage criterion (eq. 5) 

 is represented by the horizontal line where H/H z = 1.0; there is no damage 

 below this line. In the wedge-shaped region above this line, damage would be 

 expected but not failure. Failure, as used here, indicates that wave action 

 will remove filter stone from the damaged slope, but does not necessarily mean 

 the embankment will be destroyed . The dashline through the wedge-shaped 

 region is the 5-percent damage level given by equation (7) using the recom- 

 mended minimum armor layer thickness defined by [r/(W 5 g/w r ) /3] = 2.0. 



*************** EXAMPLE PROBLEM 2*************** 



This example, which is a continuation of example 1, illustrates the con- 

 cept of reserve stability and the use of Figure 3. 



GIVEN : 



cot 9 = 3.0 



H = 1.52 meters (5.0 feet) (design wave height) 



w r = 2,644 kilograms per cubic meter (165 pounds per cubic foot) 



w w = 1,000 kilograms per cubic meter (62.4 pounds per cubic foot) 



w 5 = 397 kilograms (875 pounds) (computed in example 1) 



In addition, it is specified that the armor be two layers thick, i.e., the 

 minimum thickness is given by equation (2) . 



x /3 

 w r 



This is required to determine the reserve stability parameter. 



W=2.0(Si)' 



14 



