the harbor's mean tide volume is usually sufficient to prevent water stagnation. Although 

 such exchanges penetrate only part of the harbor, sufficient diffusion occurs to maintain 

 adequate water quality. 



Where water level fluctuations are small, special arrangements may be necessary to ensure 

 adequate water exchange. Sometimes this can be done by providing two entrances to the 

 harbor so that wind-generated currents or tidal currents, feeble though they may be, move 

 continuously through the harbor. It is important, however, to make sure that the outside 

 source of exchange water is not already polluted. Complete exchange of water about every 

 10 days is usually adequate for proper control. If this is not done naturally, a 

 mechanical-flow generator may be required. Such devices are manufactured especially for 

 this purpose (Fig. 16) and may be installed to circulate the water within a single basin or to 

 move it from one basin to another through a large conduit. 



Water exchange does not always ensure good quality, especially in the back basins of a 

 multibasin harbor. A significant factor in water quality control in any harbor is elimination 

 of the direct sources of pollution. These sources may be: (a) discharges of industrial wastes 

 and sewage into the harbor, (b) local surface runoff, (c) discharge of water from storm 

 drains from tributary areas, (d) flushing of boats' sanitary facihties within the harbor, 

 (e) inadequate control of bilges, or (f) dumping of garbage and trash in the harbor waters. 

 Sanitary -sewer and industrial waste discharges into harbor waters must be eliminated in 



Figure 16. Mechanical current generator. 



40 



