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water in the estuary is mixed to the extent that the isohaline surfaces are nearly 

 vertical and the water density at the entrance is equal to that of the sea. In this 

 circumstance there is no stable mode of intrusion, such as the salt wedge, by 

 which sea water may enter with the flood tide; hence the inward flow tends to be 

 equally rapid at all levels except for the effects of bottom or side friction. In 

 this case, too, the average ebb volume should exceed the average flood volumes 

 by the river and ground water discharge. 



Under the conditions of complete vertical mixing that favor jet flow there 

 are opportunities for simple overturning motions to develop under wind stress. 

 The wind-driven circulation tends to overturn in the manner somewhat like a 

 belt on two pulleys turning on axles at right angles to the wind. In such simple 

 systems the surface layer moves with the wind at a more or less steady speed 

 but increases in thickness with fetch. Generally, the surface layer occupies 

 less than half the total depth so that the upwind return current moving over the 

 bottom is slower. 



In a stratified estuary the wind-driven motions may be layered. The 

 surface layer may have an overturning circulation of its own moving downwind 

 on the surface and upwind along an interface which rises upwind. The friction- 

 al stress on the interface may cause the bottom layer to develop a circulation 

 overturning in the reverse sense. It is possible that the interface may be so 

 steeply inclined that the heavier stratum of water reaches the free surface at the 

 upwind end of the embayment. In this case the circulation of bottom water may 

 overturn in the same sense as the surface layer provided the circulation due to 

 wind stress is stronger. The wind-driven and frictional driven parts of the bot- 

 tom circulation may also occur as two independent cells, in which case the line 

 along which the density interface cuts the free surface will also be a line of 

 horizontal convergence below which water sinks. 



If the wind stress produces waves with an amplitude equal to a major 

 fraction of the total depth of water, all traces of stratification may disappear. 

 This occurs in shallow water, sometimes to the extent that bottom se'diments 

 are caught up in the wave action. Due to the strong vertical transfer of momen- 

 tum, the overturning wind-driven circulation may actually slow down even though 

 the free surface is strongly inclined. Studies of the setup under very strong 

 winds have been made by Haurwitz (1951), Saville (1952), and under laboratory 

 conditions by Keulegan (1951). 



A study of the wind-driven circulation of the Scotian shelf has been made 

 recently by Longard and Banks (1952) and suggests the effect of wind on deeper, 

 less confined water along a relatively straight coastline. Due to the fact that 

 the circulation was less influenced by the land, the wind-induced slope of the 

 density interfaces gave rise to longshore currents and a helical circulation in 

 the surface layer. 



Current measurements in estuaries are usually made directly although 

 dynamic sections are also made particularly in connection with chemical inves- 

 tigations. Some of the instruments available for direct current measurements 

 in estuaries and coastal waters are listed by Adams (1942), Sverdrup and others 

 (1942), Romanovsky (1949) and von Arx (1950b). A critique of the ideal current 

 meter has been given by O'Brien and Folsom (1948). A valuable discussion of the 

 the use of the current pole in tidal waters has been written by Haight (1938). 

 There is, however, a great variety of methods: sonic methods, strain gauge, 

 capacitance and transducer methods, electromagnetic methods, dye tracers, 

 ionization tracers, radioactive tracers, wire angle techniques, and others, 

 which differ completely from the mechanical current meter methods that ordi- 

 narily come to mind. Many of the thermal and electrical methods were con- 



