TOP Of HOCK 



Figure A-5. Typical damaged riprap sections (U.S. Army Engineer District, 

 Omaha, 1969) . 



s 



The upstream embankment slope is exposed to an effective fetch across 

 the reservoir of about 1.4 miles (2.25 'kilometers); the downstream slope 

 is exposed to a 1.5-mile (2.41 kilometers) effective fetch. Using the 

 deepwater wave forecasting curves in U.S. Army, Corps of Engineers, 

 Coastal Engineering Research Center (1975), a Hg = 3.3 feet can be gener- 

 ated by a wind of about 55 miles (89 kilometers) per hour blowing over 

 a 1.5-mile fetch. A 3.6-second wave period is associated with this 3.3- 

 foot-high significant wave height. Thus, a storm with gusts of 70 miles 

 per hour over a 1.5-mile fetch is capable of generating some waves as high 

 as 5 feet, the designated design height. 



Both the 1967 and 1968 damage was temporarily repaired by dumping 

 quarrystone into the notches from a barge. Ufhile these emergency repairs 

 provided temporary protection, a permanent solution to the inadequate 

 riprap stability problem was needed. U.S. Army Engineer District, Omaha. 

 (1969) evaluated the following methods of upgrading the riprap stability: 

 (a) Overlaying with wire mesh or chain-link fence anchored in place to 

 hold the stone and prevent movement; (b) placing grout by various methods 

 into the voids between the existing stone to bind smaller stone together 

 into either larger or continuous solid units that are more wave resistant; 

 (c) overlaying with grouted preplaced mesh-enclosed coarse aggregate as 

 suggested by Milwaukee railroad officials; (d) overlaying with manufac- 

 tured concrete armor units such as tetrapods or tribars; and (e) over- 

 laying with quarrystone or boulders. Method (e) was selected as the cost- 

 effective plan with a good chance of success. 



To assure success of the stone overlay design, the U.S. Army Engineer 

 District, Omaha requested a series of wave tank tests be performed at 

 the Coastal Engineering Research Center (CERC) . These tests were intended 

 to determine stability of various stone overlay coverages for both quarry- 

 stone and boulders. 



29 



