The specified armor stone gradation called for riprap with a medium 

 weight of 125 pounds (57 kilograms). However, the actual median riprap 

 weight in place ranged between 10 and 75 pounds (4.5 and 34 kilograms), 

 which was considerably less than specified. The specified riprap grada- 

 tion and actual riprap gradations are shown in Figure A- 3. 



1 80 



ho 



2 40 

 30 

 10 













^:^ 













■~--., 





































'■V 



's 



V, 









'-- 



~« 





































N 



> 



^ 









N 



X 



Actual Gradotion 

 Within Arsnnf 





























\ 



X- 









^ ^^elatively Small Rock 































\ N -—Actual Gradotion 

 \ ^ J^ Within Area of 



N 





































^■^ "■1 Relulively Luiyo IMl 1 





































1^ 







N 



s 





16 pet 



>lin 



Size 

 \ 































1 



N 









N 



































'"^^ ^^Specified Gradation 

 ^**<^of Existing Riprap 





-i- 



































, T 



tt 



> 



:.. 









a& 



Figure A- 3. 



o o o 

 o o o 



Stan* Weight ( lb) Specific Gravity of Stone 2.65 



Riprap gradation for railroad embankment (U.S. Army Engineer 

 District, Omaha, 1969). 



From 1962 to 1967 the reservoir pool level fluctuated between eleva- 

 tions of 1,555 and 1,597 feet (474 and 487 meters). The first riprap 

 failure was noticed on the upstream face in June 1967. Damage consisted 

 of a 2,700-foot-long (823 meters) wave-cut notch similar to that shown in 

 the photos in Figure A-4. This damage was • attributed to storm waves from 

 the northeast on 30 April and 1 May 1967 during which the Mobridge Weather 

 Bureau Station reported gusts to 70 miles (113 kilometers) per hour. 



More riprap damage occurred on the upstream slope during 1968 at a 



higher reservoir pool level. This damage notch was not as extensive as 

 the 1967 damage although it was about 2,500 feet (762 meters) long. 



Typical sections of the damaged riprap revetment are shown in Figure A-5. 



U.S. Army Engineer District, Omaha (1969) estimated a 5-foot (1.52 

 meters) wave caused the 1967 and 1968 slope damage. This estimate was 

 based on the wave- cut notch and position of driftwood on the slope. 

 Further analysis by the District concluded that the embankment would be 

 exposed during a 100-year period to 250 waves with a height of 5 feet or 

 higher. This 5- foot wave height was selected as the design wave for the 

 permanent revetment repair. Assuming the 5-foot height approximately 

 represents the 1-percent exceedance wave height of a Rayleigh distribu- 

 tion, the significant height is 3.3 feet (1.01 meters). The 1-percent 

 exceedance wave height is the height exceeded by 1-percent of the waves. 

 Therefore, the design significant wave height, U„ , is 3.3 feet. 



27 



