55 



account the effect (if current on the movement of bf)ats, since the design of the struc- 

 tures must pro\icie for the imjiact of boats, either while they are docking or are moored 

 in the patii of tlie currents. In ivrry sH])s thi- conii}ination ol current action on the 

 boats and their inertia, under the necessity for speedy operation in and out of the 

 shps, requires a very strong and flexible structure to withstand the shocks without 

 damaging either the boats or the slips. 



Current action also accelerates leaching of chemical preservatives, either those 

 used for timber protection or as paints for metal protection. This partially accounts 

 for the shorter life of such protections in w.iler of strong current than in water of 

 less movement. 



Influences which increase current \elocity, or propeller action which agitates the 

 water, will tend to cause cutting and scouring of the mud bottom. While this ma\' ])v 

 desirable, at least to the extent of pre\'enting filling, and where adequate pro\ision 

 has been made for it in sul)structures, active scouring has often proved disastrous 

 where timber piles ha\e been protected only above the original mud line with the 

 protection not carried down far enough to allow for the scouring. In such cases the 

 luiprotected timber becomes exposed to borer attack and is destroyed. Scouring will 

 also in general lend to decrease the stability of structures indess they are carried 

 down an adequate distance below the mud line. 



The foregoing indicates that the most adverse condition is that where strong 

 currents prevail. 



S.M.INITY 



The condition of salinity may range from the negligible amounts in fresh river 

 water to ocean salinities of from 30 to ,^5 parts salt in 1, ()()(). This saline content serves 

 to accentuate many adverse marine influences. The rate of corrosion of metal is 

 greater in salt water than in fresh, which thus accelerates the deterioration in salt 

 water of cast iron, wrought iron and steel piling, sheet metal coverings for timber piles 

 and reinforcing steel in concrete. Iron and steel piling can be protected to a certain 

 extent by painting, but this eventually fails unless renewed. Yellow metal covering for 

 timber piles has been found impracticable because of corrosion; copper has given good 

 service, but must be thick enough to withstand a certain amount of surface corrosion. 

 Many failures of reinforced concrete have been caused by rusting of the reinforcing 

 steel, and protection against this corrosion constitutes one of the greatest problems in 

 respect to this type of material. Concrete work is likewise adversely affected by salt 

 water because of chemical action with the ingredients and the formation of laitance. 



Salinity is particularly important where marine borers are concerned. The danger 

 point is about 15 parts salt in 1,000. If this salinity, or greater, can occur and borers 

 are present, their attack on unprotected timber can be expected. As explained in the 

 Biological section. Teredo navalis is particularly destructive in the lower salinities, 

 from 10 to 20 parts in 1,000, a condition to be found in ri\er estuaries. River discharge 

 may vary so that the salinity is reduced below 15 in 1,000 for a period of time, but if 

 it rises again to this amount during the breeding season of Teredo and so remains for a 

 few succeeding months, unprotected timber will be destroyed. This borer will survive 

 in nearly fresh water if a salinity of 5 parts in 1,000 occurs every 30 days or oftener 

 (See p. 261). Limnoria and Bankia seem to require higher salinities, 20 parts in 1,000 

 or more (See pp. 292 and 327). Hence all borers are found acti\e in \\-ater of 20 parts 

 or more of salt in 1,000. 



Particular jeopardy is involved in river estuaries where fresh water conditions 

 have led to the use of unprotected timber in wharves, and where an unusual reduction 



