A visual comparison of the aerial photos revealed no detectable change 

 in the shoreline configuration adjacent to the shadow of Rincon Island 

 between 1947 and 1967. Small differences between the 1947 and the 1959 

 shorelines and between the 1959 and the 1967 shorelines can be seen, but 

 these appear to be caused by transient seasonal effects or those attribu- 

 table to shoreline freeway construction. Figure 12 shows 1947 and 1967 

 aerial vertical views of this region. 



7. Bottom Sediments 



a. Contour Data - The potential effect of the island's construction 

 on nearby bottom sediments was considered during its design. Such effects 

 could occur if currents, eddies, and wave-induced turbulence in the water 

 near the bottom were substantially altered by the island. Except for a 

 shallow surface layer of loose material, the bottom materials appeared 

 dense, and were considered stable against reported steady currents in the 

 area (less than 1 knot). Because water depths near the island range 

 roughly from 40 to 50 feet, near-bottom, wave-induced turbulence can only 

 arise from long-period waves. Severe storm conditions are relatively 

 infrequent, and it was concluded that little change in the bottom sediments 

 would be caused by the island's construction. To evaluate this conclusion, 

 comparisons have been made between preconstruction and postconstruction 

 bottom contours adjacent to the island. 



National Ocean Survey (formerly USC^GS) hydrographic charts of the 

 area provided initial data on water depths. Data from these charts, 

 supplemented by lead-line and fathometer soundings from surface craft, 

 will be termed the 1956 soundings. Fathometer surveys by the field engi- 

 neering staff will be termed the 1957 soundings. The most recent survey 

 was conducted in March 1973 with a fathometer and a subbottom sounder. 

 Scuba surveys show a deposit of mussel shells at the base of the west face 

 and smaller deposits at the base of the north and south faces. Such 

 deposits could compensate for bottom scour in these locations. Shallow- 

 drive samples at various distances from the north and south faces showed 

 no consistent differences. 



Figure 13 shows results of the several surveys. Contours of the 1956 

 soundings show a gentle, regular bottom slope, deepening gradually to the 

 southwest. The 40- foot-depth contour lies just northeast of the island, 

 the 45-foot contour bisects the island location, and the 50-foot contour 

 lies just southwest of the island. As a compromise of the various sources, 

 these are represented as straight and evenly spaced. The 1957 soundings, 

 shown plotted in 1-foot-depth increments, generally have the same orienta- 

 tion as the 1956 contours, and depths generally agree within 1 or 2 feet. 



Causeway pile driving records provided additional detail on depths in 

 the immediate vicinity of the causeway. They showed some variation from 

 the 1957 fathometer survey, and specifically disclosed the presence of a 

 slight rise in the bottom at about midlength of the causeway, which 

 closely agrees with the 1973 soundings. Nearer the island, a slight 

 depression was revealed by the pile driving records, but was not picked 



38 



