tests appeared to be more blocky than the quarrystone overlay in the small- 

 scale tests (compare Figs. 2 and 6). However, the scale effects shown in 

 Table 2 are consistent with the findings of Thomsenj Wohlt, and Harrison 

 (1972) who found sizable scale effects on riprap stability in small-scale 

 model tests. 



To determine how general the findings of this study are, it would be 

 valuable to compare the results with a study of conventional riprap sta- 

 bility. Ahrens (1975) is a convenient and useful comparison since the 

 stone gradations are similar and a wide range of wave and slope conditions 

 were tested at prototype scale. A comparison of stone gradations used in 

 Ahrens (1975) and this study is given in Table 3. The tests by Ahrens 

 were conducted in the CERC large wave tank and most used a riprap layer 

 between 1.5- and 2-median-stone diameters thick. A riprap layer thickness 

 between these diameters is considered a conventional two-layer riprap. 

 Revetment slopes of 1 on 2.5, 1 on 3.5, and 1 on 5 were tested at prototype 

 scale for wave periods between 2.8 arid 11.3 seconds. The conditions and 

 corresponding results which most closely matched these stone overlay tests 

 are: 



Slope 



Period 

 (s) 



4.2 

 4.2 



of 



No. 

 tests 



Avg. Ng 



1 on 2.5 

 1 on 3.5 





4 

 4 



1.99 



2,42 



Estimated Ng for 1 on 3 slope = 2.21 



The interpolated Ng = 2.21 for a conventional 1 on 3 riprap is approxi- 

 mately equal to the Ng = 2.26 for a 1 on 3, 100-percent stone overlay 

 armor (Table 1). The similarity of the zero-damage stability between the 

 stone overlay and conventional riprap suggests that the stability equa- 

 tion for conventional riprap design, developed by Ahrens and McCartney 

 (1975), can be used with equation (2) to estimate stable stone overlay 

 weights. This equation is: 



Ng = 1.46 (cot 6)^'^^ , (4) 



where 6 is the angle between the embankment face and the horizontal. 

 Equation (4) is intended for design use, and was made conservative enough 

 to account for the worst wave conditions and scatter in the test results. 

 No allowance is made in equation (4) for uncertainty in predicting the 

 design significant wave height. If equation (4) is solved for cot 9=3, 

 the stability number is 1.86, which is about 20 percent lower than the 

 large wave tank overlay test results. The lower stability number is 

 indicative of the conservatism included in equation (4) . 



The limit of tolerable damage is the maximum wave height a structure 

 can withstand without some loss of structural integrity. The ratio of 

 tolerable-damage wave height to zero-damage wave height is a measure of 



19 



