CURRENT EFFECTS 55 



trary in such a case, the heights of the waves would be increased by 

 only about 1.06 times. And even in the rare cases when wind drifts 

 do flow at velocities so great that they would steepen the waves meas- 

 urably if they were contrary, it almost always happens that the waves 

 are running with the drift, because both the waves and the drift are 

 generated by the same wind system. The wind drifts with velocities 

 as high as 1.4 to 1.9 knots with 25 to 55-mile winds, that have been 

 reported at Diamond Shoal Lightship, about 15 miles out from the 

 land southeast of Cape Hatteras, are cases in point. 15 



Tidal currents, like wind drifts, are seldom strong enough out over 

 the ocean basins to cause any noticeable alterations in the shapes of 

 the waves; but they do often run so strongly in continental waters 

 that their effects are notorious for steepening any waves that may be 

 running against them there, and this is true even well out from the 

 land in many localities. Thus the tides run at velocities up to 1 to 

 1.6 knots on the shoaler parts of Georges Bank, which forms the off- 

 shore rim of the Gulf of Maine, while tidal velocities as high as 1.3 

 knots have also been recorded at Nantucket Lightship, which is sta- 

 tioned at the 30-fathom contour, 41 miles out from the land. And the 

 tides run even more strongly still around many a jutting headland, 

 as it also does in its passage up funnel-shaped bays or through narrow 

 channels and sounds. Tidal velocities at strength up to 1.8 knots 

 round the tip of Cape Cod, up to 2.5 to 3 knots in the Grand Manan 

 Channel at the entrance of the Bay of Fundy, up to 3 to 4 knots in the 

 Golden Gate (entrance to San Francisco Bay), up to 8 to 10 knots in 

 Seymour Narrows, British Columbia, and as high as 9 to 11 knots in 

 the narrow waters between Scotland and the Orkney Islands are 

 well known illustrations. 16 



In cases as extreme as these, a very moderate sea, indeed, may be 

 transformed very abruptly into one that is very dangerous to small 

 craft, upon the advance of the waves into the tideway, if the latter is 

 running against them. And many points, shoals, and bars in various 

 parts of the world owe their local names to this fact; "Pollock Rip" 

 at the entrance to Nantucket Sound, the "Rips" on Nantucket Shoals, 

 "Race Point" at the tip of Cape Cod are familiar examples on the east 

 coast of the United States. 



Steep, tumultuous waves also characterize the more swiftly flowing 

 parts of the major ocean currents, whenever the waves there are run- 



10 For tabulations of wind currents with different winds, at Diamond Shoal Lightship and 

 at other lightships along the Atlantic coast of the United States, see Haight, F. J. 1942. 

 Coastal currents along the Atlantic coast of the United States. Spec. pub. U. S. Coast 

 and geodetic survey, No. 230, pp. 55-70, tables 12-3 5. 



16 These velocities are cited from: Stevenson, 1874, p. 65; Kriimmel, 1911, p. 104; 

 Marmer, H. A., 1926, The Tide, New York. p. 99: Bigelow, H. B., 1927, Physical oceano- 

 graphy of the Gulf of Maine, Bull. U. S. Bur. Fish., vol. 40, pt. 2, p. 856 ; and from various 

 publications of the U. S. Coast and Geodetic Survey. 



