up on the fathometer surveys. In general, the 1957 soundings and the 

 pile driving data agree within 1 or 2 feet, but at the two slight irregu- 

 larities described the difference is closer to 3 and 4 feet. It should be 

 noted that the causeway area of the 1957 soundings was uncertain at that 

 time. The 1973 fathometer survey is judged more reliable than the 1957 

 soundings because the tidal correction data are more reliable. On both 

 the 1957 and 1973 soundings, position control was poorest along the cause- 

 way. Also, the 1957 soundings were primarily restricted to the actual 

 island site. The same slight rise at about midlength of the causeway 

 that was observed from the pile driving records is shown in the 1973 sur- 

 vey. A trough projects northeastward from the island, approximately on 

 the causeway line. A ridge parallels the trough to the east. Other slight 

 irregularities occur near the island itself. 



The 1973 fathometer soundings also provided data from which subbottom 

 contours, presumably of the top of the siltstone underlying the bottom 

 sediments, were drawn. As noted in Figure 14, the overburden depths are 

 based on the arbitrary assumption that sound velocity in the overburden is 

 the same as sound velocity in the seawater. 



b. Evaluation - Some mismatch between the various hydrographic 

 surveys is to be expected considering the impossibility of maintaining a 

 continuous reference elevation in a surface craft with both wind waves 

 and tidal variations occurring during the surveys. The small scale of the 

 original hydrographic charts and the general smoothing of depth data from 

 these and other earlier surveys tends to mask irregularities of the type 

 disclosed by the pile driving records. Although all differences between 

 the various soundings could be explained as random errors to be expected 

 with the technique used, certain features discussed below are consistent 

 and appear reliable. 



The area west of the island shows erosion of 1 or 2 feet. Considering 

 the mussel shell deposits of up to 2.5 feet thickness at the toe of the 

 west face, an estimated maximum erosion of about 3 'feet appears reasonable, 

 probably as a result of wave-induced turbulence on the weather side of the 

 island. 



The lee side of the island shows a long tail of deposition 1 to 2 

 feet thick. Such a deposition is consistent with the reduced wave action 

 on the lee side. Observation of the construction during the early winter 

 period of^ 1957-1958 strongly suggests that a part of this deposit occurred 

 during construction when the core fill had only partial protection. An 

 estimated 23 percent of the core fill was lost during construction. 



A smaller eroded area seems to exist along the south side of the 

 island, tapering to about no change at the east end. Such behavior is 

 consistent with the westerly and easterly areas. 



On the north side, some deposition is indicated. This could also be 

 a construction-period change, since most barges were unloaded by crane in 

 this area and a thin blanket of rock protection was placed over the end 

 of the pipeway here. 



41 



