that o£ wave exposure, which strongly affected the design of the island 

 revetment and causeway, and the plan shape of the island. The tetrapods 

 and armor rock were such significant contributors to the total cost that 

 wave exposure became a predominant design factor, justifying a detailed 

 evaluation of the economies of various possible design wave heights. 



A comparison of design assumptions to actual performance should be 

 based on wave exposure data. Because direct measurements of such data are 

 available, the evaluation has been made on indirect evidence, qualitative 

 in nature, and consequently unsatisfactory for a rigorous evaluation. 



Wave gages are needed to obtain the necessary quantitative informa- 

 tion on actual wave exposure. Three basic gage types are presently avail- 

 able: the surface-mounted staff gage, the pressure-sensitive gage which 

 is mounted on the bottom, and wave-rider buoys with telemetry systems. 

 Examples of the first two types are described by Williams (1969) . If the 

 pressure-sensitive bottom gage can be used in water depths of about 45 

 feet, then a desirable installation would consist of a set of three such 

 gages mounted in a triangular configuration at a depth of about 45 feet 

 southeast of the island, but far enough from it to eliminate any signifi- 

 cant reflection or refraction influences. This installation would monitor 

 the principal sector of wave exposure: from the southeast to the west. 



If bottom-mounted pressure gages cannot be used in more than 30 feet 

 of water, as Williams advises, then the installation should be moved to a 

 location due north of the island. At this location the principal exposure 

 of maximum wave attack from the west would still be monitored. Waves from 

 the south would be influenced by refraction and diffraction on reaching 

 this location, and would therefore not be characteristic of the unaffected 

 exposure of the island to the south. 



In addition to either of these monitoring systems, it would be desir- 

 able to mount a recording staff gage on a causeway pile at about midlength 

 along the causeway. From this location the westerly wave exposure could 

 be adequately monitored, and the data used both to substantiate that from 

 the bottom gages and to provide information on the smaller, sharper waves 

 which are attenuated by pressure gages. 



2. Sediment Gages 



One major consideration of the impact of the island on its environ- 

 ment is the effect on littoral transport. The influence of the island on 

 waves and currents might have had serious consequences with respect to 

 sedimentation or erosion in the coastal vicinity. However, it was 

 generally concluded that any such effects would not be serious, and the 

 comparison of aerial photos and sounding surveys substantiate that con- 

 clusion. However, sufficiently accurate quantitative data are not avail- 

 able to determine the precise effect of the island on these processes. 

 The island's effect on nearby bottom sediments appears to warrant further 

 study. A suggested technique would be to install a network of thin cali- 

 brated reference rods in the ocean floor adjacent to the island where the 



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