c. Methods of Analysis of Inlet-Bay Hydraulics . Inlet current 

 velocities and bay water surface oscillations may be measured to provide 

 necessary information on the hydraulic characteristics of an inlet-bay 

 system. Techniques used for the field measurements in this study are 

 discussed in Section III, 2. 



Several analytical methods are available for predicting inlet 

 hydraulics, depending on the type of information available and required 

 results. Three methods are discussed below. 



(1) Estimation of Seiche Periods Important to Inlet Hydraulics . 

 To estimate which of the Great Lakes seiche periods, Ty^, may be im- 

 portant, the frictionless inlet-bay Helmholtz period, T^', may be 

 determined from: 



^ g A^ 



(4) 



where L' is an added channel length determined from: 



L^^Tff'J 



L. =11 j^ I TiD 1 ^ (5-, 



L' accounts for the water masses in motion beyond the ends of the inlet 

 (Miles, 1948). Equations (4) and (5) may be iteratively solved to 

 obtain a value of T/^' . This approach proved to yield reasonably accu- 

 rate estimates for the inlets considered in this study. 



As a first approximation, seiche wave periods which are approximately 

 equal to the frictionless Helmholtz period (e.g., between 0.5 and 2 

 times Tjj'; see Fig. 2) will probably cause the highest inlet reversing 

 currents . 



The seiche node-antinode pattern in the Great Lakes will also influ- 

 ence the importance of the various seiche modes on an inlet -bay system. 

 Seiches with antinodes adjacent to the inlet will produce the largest 

 water level fluctuations. Since ends of the lakes are antinodes for all 

 modes of oscillation along that axis, bays at the end of a lake will 

 normally be subject to higher water level fluctuations than those at 

 other locations; e.g., midway along the longitudinal axis of Lake Michigan, 

 near Pentwater, the first longitudinal mode of oscillation has a node 

 (Fig. 3). Therefore, only small oscillations can be generated in the 

 lake at this point by this mode. The second longitudinal mode of oscil- 

 lation has an antinode adjacent to Pentwater, so large water level fluc- 

 tuations in Lake Michigan could be generated outside Pentwater by this 

 mode. 



(2) Estimation of Inlet Velocities from Bay Water Level Records . 

 A method of predicting inlet velocities, if high-quality bay water level 



18 



