o£ the bay is typically larger than the length of the inlet) and the 

 surface area of the bay is much smaller than the surface area of the 

 sea. 



a. Causes of Reversing Inlet Currents . Mortimer (1965) and Freeman, 

 Hamblin, and Murty (1974) show that significant reversing inlet current 

 velocities are caused by water level fluctuations in the Great Lakes which 

 generate a hydraulic response in the inlet and bay. The most important 

 Great Lakes water level fluctuations are due to the resonant seiching or 

 oscillation of the particular lake at its fundamental and harmonic periods. 



Seiches are initiated by storm pressure and wind forces on the sea 

 which redistribute water in the lake to cause a higher elevation than 

 normal in some areas and lower levels in other areas. When gravity tries 

 to restore the water level, seiches are generated. These seiches usually 

 continue for a number of cycles which may extend over a few days after 

 the storm has passed. 



When a seiche is generated in one of the lakes, the water level 

 fluctuations outside an inlet cause a head difference across the inlet, 

 which, in turn, generates a current in the inlet. Water discharge 

 through the inlet results in water added to or removed from the bay so 

 the bay level rises and falls in a pumping fashion for most seiching 

 periods of the lake. 



Astronomical tides and other nonseiching long waves cause water level 

 fluctuations of the Great Lakes; however, these fluctuations generally 

 have insufficient amplitudes or are at periods that usually do not sig- 

 nificantly influence inlet hydraulics. Storm surge, particularly on 

 shallow Lake Erie, may occasionally generate strong inlet currents. 



b. Mathematical Description of Inlet-Bay Hydraulics . The response 

 of an inlet-bay system may be described in terms of the one -dimensional 

 equation of water motion in the inlet and the continuity equation relat- 

 ing the rate of bay level change to the inlet discharge. 



The one-dimensional equation of motion along the inlet channel axis 

 can be written: 



8h 1 r / X , 3h . 3V 



•9x 



1 r / \ J ^ 9h ^ 3V ,-. 



a 

 Xl 



where 



X = distance along the channel 

 h = water surface level 



A^ = inlet cross-sectional area 



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