53 



conipli-'tfK' exposed abo\'e tlie nuul line ami borer attack on timber in\()l\'ed thus be 

 reduced. Ho\ve\'er, sufficient water usualK' stands on the mud surface to allow action 

 by Linniorla, so that it can destroy such material. While protections can be carefully 

 placed and maintained under such conditions, the adxantage applies only to a com- 

 parali\el\- limited pro])ortion of construction; in a majority' of cases there must be a 

 sufficient depth of water at low tide to permit boat operations. 



In general, the most adverse condition is that of sea water having wide changes in 

 level occurring with greatest frec|uency. Acherse factors, in their bearing on the use of 

 N'arious t\'pes of substructure, are discussed in the following pages. 



(\rki<:nts 



The movement of water in the form of currents creates numerous conditions having 

 a direct bearing on sub-structure problems. Of the many forms ol substructure which 

 may I'e built, ranging from solid hlls to piling, the latter is often reciuired in order that 

 the structure shall offer the least obstruction to currents. In tidal harbors the tidal 

 prism, or the \-olume of water lietween tidal le\-els, must not be decreased. Government 

 rulings estal)lish the "bulkhead" line, beyond which no solid embankment can be 

 built, and the "pierhead" line, beyond which no structure can be built; wharves built 

 between these two limits must be supported on piling. Thus, while it is often physically 

 possible to replace pile foundations with solid fills and ehminate the problems of main- 

 taining the piling, this can never be done in the case of structures outside Government 

 bulkhead lines. Pile structures will always be required where conditions demand that 

 current action shall have minimum obstruction. 



San Francisco Bay furnishes a striking examjile of tidal conditions with strong 

 currents prevailing. As explained in the H\clrographic section, this bay, over 50 miles 

 long and averaging several miles in width, is connected to the ocean by the Golden 

 Gate channel, less than a mile wide and over 300 feet deep at the center. The volume 

 of tidal water passing through the Gate is estimated to be over 2,000,000 acre feet. 

 The current created by this immense flow- is extremely strong, particularly along the 

 shores adjacent to the Gate channel, on the south side of which the San Francisco 

 waterfront is located. 



In 1886 it was reported by the Board of State Harbor Commissioners that marine 

 borer attack was more severe in the northern portion of the waterfront adjacent to the 

 Gate channel, where the current is stronger than elsewhere. This condition has pre- 

 vailed to the present time and, coupled with other similar observations, has led to the 

 general conclusion that marine borer attack is greater in strong currents than in water 

 of slight movement. In a similar wa\- it has been found that in a single pile the attack 

 is greatest on the side most exposed to the current. On the east shores of the Bay the 

 current action is much less than at San Francisco, which doubtlessly accounts in part 

 for the reduced borer attack there. 



Prevailing currents influence general w'aterfront plans. In the Historical section 

 mention is made of the trouble experienced with the original San Francisco waterfront 

 line as established by the State Legislature in 1851, this line following an irregular, 

 saw-toothed course along the rectangular city streets. Structures conforming to it 

 projected in various directions and interfered with the tidal flow by creating eddying 

 currents, which caused shoaling of wharf slips and required constant dredging. The 

 change in waterfront line and plans in 1878 was made so that the line might follow the 

 prevailing currents in a smooth curve and that the wharves would ofter least obstruc- 

 tion to the current, thus eliminating the eddying effect. 



The direction or orientation of wharves and ferry slips must often take into 



