conducted in the large wave tank, 15 feet (4.6 meters) wide, 20. feet 

 (6.1 meters) deep, and 635 feet (194 meters) long. The large wave tank 

 tests were used to verify the validity of the stone overlay concept at 

 prototype scale. Test conditions for both wave tanks are given in Table 

 1 (see Coastal Engineering Research Center, 1971, for a detailed descrip- 

 tion of the tanks) . 



2 . Small-Scale Tests . 



The small wave tank tests were run with a 1.5-foot water depth and 

 a wave period of 1.16 seconds which give a depth, d, to wavelength, 

 L, ratio of 0.24. This value (d/L = 0.24) coincides with a large number 

 of riprap stability tests previously run at CERC (Thomsen, Wohlt, and 

 Harrison, 1972). At the normal operating water depth of 15 feet in the 

 large tank, d/L = 0.24 gives a wave period of 3.67 seconds, the approxi- 

 mate design wave period at the Mobridge, South Dakota, railroad embankment 

 on the Oahe Reservoir. 



The core of the embankment in the small tank tests was composed of 

 packed sand with a median diameter of 0.2 millimeter. Between the core 

 and the riprap underlayer there was a 0.5-inch (12.7 millimeters) layer 

 of coarse filter sand with a median diameter of 1.2 millimeters. The 

 distinctive reddish-brown color of the filter sand made exposure of the 

 filter easy to observe. Crushed bluestone with a median diameter of 11 

 millimeters was used as a riprap layer. This layer was designated the 

 riprap underlayer for the tests using an overlay. The small wave tank 

 test setup is shown in Figure 1. 



Generotor 



Scale (ft) 



I 2 



Figure 1. Details of small wave tank test section. 



In 13 of the 14 small tank tests, waves were run in burst durations 

 of 20 seconds. The 20-second burst duration prevented wave energy reflec- 

 ted from the slope from being re-reflected from the wave blade while it 

 was still in motion. Such a condition would cause two distinct wave 

 trains to travel toward the embankment, making it more difficult to mea- 

 sure and describe the incident wave height. The 20-second burst duration 



