WATER DENSITY AND ITS APPLICATIONS 



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RIVER MERSEY 



Figure 4. 



usually south-flowing and relatively cold. Salinity is responsible for 

 the density gradient that sets up the current, since, from the stand- 

 point of temperature alone, the inshore water obviously would be 

 heavier rather than lighter. 



Along the east side of the oceans, coastal density currents 

 generally flow northward and are warmer than the offshore water. 

 Thus, both temperature and salinity contribute to the density gradi- 

 ent between inshore and offshore water. 



Exceptions to the above occur where the Continental Shelf is 

 narrow and where little river water is brought down to the sea. In 

 such cases local winds and seasonal variations in rainfall are impor- 

 tant in determining whether or not a typical coastal current will 

 develop. For example, on the east coast of the United States there 

 is a well-developed coastal current as far south as Fort Pierce, 

 Florida. Beyond that point the shelf narrows rapidly and the coastal 

 current, known as the Florida Countercurrent, becomes weak and 

 sometimes nearly nonexistent. The coastal water off Miami is 

 slightly cooler than the offshore north-flowing Florida Current. It 

 is also slightly fresher, enough so that the countercurrent generally 

 flows during the wetter months of the year. During the spring 

 and summer months, however, steady southeasterly breezes carry 

 the Florida Current close inshore and the coastal current is obliter- 

 ated entirely or driven below the surface where it can be detected 

 only by a subsurface method. 



The extent to which a current parallels a coast depends, in con- 

 siderable measure, on the difference in density between inshore and 

 offshore water. When the density difference is slight, the current 

 may frequently be set onshore or offshore. The directions that it 

 takes will be dependent to a large degree on the bottom topography. 

 Frictional retardation by bottom particles in contact with a current 

 not only affect its velocity but also its direction. The tendency is 



for the normal twisting to the right (in the Northern Hemisphere) 

 to be accentuated as the current moves over a shoaling bottom and 

 to be lessened over a deepening bottom. Thus, when the water is 

 relatively homogeneous, it is common to find an inshore set towards 

 reefs and islets. 



DENSITY AND SAFE CHANNEL DEPTHS 

 Density is an element which must be reckoned with when chan- 

 nel depths become a factor for entering port. The relation between 

 salt water loaded draft and required channel depth is a problem not 

 understood by all mariners. Several factors unite to increase the 

 channel depth requirements over the salt water loaded draft, 

 Figure 3. 



As shown in the table, the salt water loaded draft for a 28,000 

 DWT vessel is approximately 33 feet 5 inches. Upon entering fresh 

 water a ship of that tonnage will sink approximately one quarter 

 inch for every foot of draft in salt water, or about 8 additional inches, 

 due to water density variation. In addition, to improve steering 

 characteristics she will be trimmed with a "drag" of approximately 

 3 inches for each hundred feet of length, or 1 foot 8 inches. A ves- 

 sel sets at a depth determined by the relation between her average 

 density and the density of the water she displaces. When underway, 

 however, a "bow wave" is formed which starts moving away. The 

 vessel itself moves forward steadily into the space previously occu- 

 pied by the wave. Hence, a vessel underway moves in an artificial 

 wave trough created by her own bow. She therefore rides some- 

 what lower than when at rest. This effect of depression varies 

 with speed and form of the hull. The effect is accentuated in shal- 

 low water, and when the vessel occupies a considerable part of the 

 channel cross-section the effect is greatly increased. When this is 

 the case, the water velocities alongside and under the vessel are 



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