Table 6-5. Shape-placing coefficient and porosity of armor units. 



Armor unit 



Placement method 



n 



Shape-placing 

 coefficient 



Porosity 

 P 



Tribar 



Random 



2 



1.02 



0.54 



Tetrapod 



Random 



2 



1.04 



0.50 



Trilong 



Random 



2 



0.94 



0.40 



Dolos 



Random 



2 



0.81 



0.56 



Cube 



Launched 



(---- 



- - - Not applicable - - 



--- ) 



a large percentage of the tests to determine the effects of storm waves 

 at low tide on the stability of the toe part of the repair section of 

 armor units. The water depth at the toe of the existing rubble-mound 

 part of the jetty head was 36 feet MLLW. Since the bottom slope seaward 

 of the toe in the immediate vicinity of the structure was about 1 on 10, 

 the critically breaking waves that could attack the structure correspond 

 to breaking depths somewhat greater than the depth at the toe. The sta- 

 tistical deepwater data and the results of the refraction study indicated 

 that the design waves for the structure at each water depth would be crit- 

 ically breaking waves (the largest wave that can attack a particular struc- 

 ture for a given depth and wave period without forming a cushion of water 

 between the in^iinging jet of water and the structure slope). The magni- 

 tude of such waves must be determined by use of scale models. Prelimi- 

 nary tests with several of the different types of repair sections under 

 consideration were conducted to determine the critically breaking waves, 

 since the waves would be used both in the testing program and in the de- 

 signing of the prototype repair section. This procedure is in contrast 

 to the situation where the structure under design is a rubble-mound break- 

 water trunk. In this case the design waves can be selected from available 

 statistical deepwater wave data and refraction studies, and from available 

 experimental data relating the largest waves that can attack the struc- 

 ture as a function of the breakwater slope, bottom slope seaward of the 

 toe, the depth of water at the toe, and the wave characteristics (Jackson, 

 1968). The results of the preliminary tests showed that a 16-second, 40- 

 foot-high wave was the largest and most damaging that would break on the 

 repair sections with a Stillwater level of +7 feet MLLW. The correspond- 

 ing wave for the 0.0-foot Stillwater level was a 16-second wave with a 

 height of 31 feet. Larger waves are believed to have occurred according 

 to the available statistical data and refraction study; however, such 

 waves would break before reaching the structure. In an attempt in 1963 

 to protect the toe of the concrete monolith on the south jetty, 100-ton 

 unlinked concrete cubes were cast on the edge of the monolith and launched 

 by jacking up the rear side of the bottom form and applying air pressure 

 to initiate movement of the cube from the form bottom. This method and 

 other approaches to the launching problem, such as sliding the cubes down 

 an inclined plane, were tested in the model. It was found that either of 

 the launching methods tested would be satisfactory. Since the original 

 proposal called for repairs to the jetty heads by linking clusters of 



39 



