phase lag (point A, Fig. 2). At values of T^'/T approaching 3 

 (short forcing periods), the inlet-bay system strongly dampens incident 

 waves and water level fluctuations in the sea have little influence on 

 inlet hydraulics (point B, Fig. 2). At values of T^'/T in the range 

 of 0.25<(Tg'/T) <2, the amount of frictional resistance in the channel 

 has a major influence on the response characteristics. Inlets with high 

 friction, e.g., tidal inlets on ocean coasts, typically have amplifications 

 of less than one. This amplification factor decreases as the forcing 

 wave period becomes shorter (point C, Fig. 2). At most tidal inlets, 

 the primary tidal period is large compared to the inlet-bay Helmholtz 

 period. Low-friction inlets, such as those on the Great Lakes, have 

 amplifications greater than one and phase lags of approximately 90° for 

 forcing waves with T^'/Tasl (point D, Fig. 2) which commonly occur. 



2. Prediction of Inlet Velocities. 



Prediction of inlet velocities requires (a) the time history of sea 

 or bay water levels, (b) the geometries of the inlet and bay, and (c) a 

 friction- calibrated model to relate water level fluctuations to inlet- 

 bay response. 



a. Great Lakes Water Level Fluctuations . In general, no methods are 

 presently available for inexpensively predicting all important amplitudes 

 and periods of water level fluctuations at any point in one of the Great 

 Lakes. Therefore, water levels generally must be measured. However, 

 inexpensive schemes are available for accurately predicting some periods 

 and relative amplitudes of seiches of the Great Lakes (Fee, 1968; Rao 

 and Schwab, 1974). Knowledge of the existing wave periods will aid in 

 the design of water level measuring systems, analysis procedures, and 

 preliminary inlet design. 



The basic method for estimating one-dimensional fundamental and 

 harmonic seiche periods, T]^, is to determine the time required for a 

 wave to travel twice the length of the basin: 



/ 



k^ 



T*= ; B. C3) 



where k is the mode of oscillation, L-u the length of the basin in 

 the direction of the seiche, and c the speed of the wave. Sample 

 predicted longitudinal seiches for Lake Michigan, using the Fee (1968) 

 computer program, are shown in Figure 3 for modes k-1, 2, and 3. 



b. Inlet and Bay Geometries . Hydrographic surveys, including Corps 

 of Engineers dredging records, can be used to determine inlet geometry; 

 i.e., length, width, and depth field. Maps and aerial photos can be 

 used to determine bay surface area. 



