Values of n were estimated for other Great Lakes inlets, based on 

 experience at Pentwater and Toronto. Then, the frictionless Helmholtz 

 period was estimated for each inlet-bay system using equations (4) and 

 (5), and the numerical model. Numerical model and frictionless , 

 Helmholtz period results are summarized in Table 5. 



The amplitude response curves and predicted maximum velocities are 

 shown for selected inlets on Lake Michigan (Fig. 16), Lake Superior 

 (Fig. 17), and Lakes Erie and Ontario (Fig. 18). A 3-centimeter mono- 

 chromatic forcing-wave amplitude was used in these models. For waves 

 of different amplitudes, the maximum inlet velocity is approximately 

 proportional to amplitude. Water level changes throughout the forcing 

 cycle cause nonlinear effects (i.e., aj,/a^ is slightly different at 

 high and low water) , so that the mean of ebb and flood conditions is 

 used in the response curves in this study. 



This analysis shows that all of the jettied inlet systems studied 

 have significant inertial effects because long waves at or near the 

 Helmholtz period of each system have higher amplitudes in the bay than 

 in the Great Lakes . 



The inlet-bay systems modeled have a wide variation in response 

 characteristics from one system to another because of the complicated 

 interactions between the four terms in the equation of motion of the 

 inlet and the response of the bay to the inlet. Pentwater, for example, 

 has a moderate amount of wave amplification and produces inlet velocities 

 greater than 30 centimeters per second (1 foot per second) for forcing 

 waves of 3-centimeter amplitude and periods ranging from 0.9 to 2.5 hours 

 (Fig. 16) . White Lake has less wave amplification, but the interaction 

 between the inlet and bay produces higher velocities over a wider range 

 of forcing periods (greater than 30 centimeters per second for periods 

 of 1 to 5.6 hours) (Fig. 16). Since Little Lake and Presque Isle have 

 the capacity to generate reversing currents in only a narrow window of 

 forcing periods, it is unlikely that significant reversing currents will 

 be frequently generated (Figs. 17 and 18). 



Duluth-Superior has the highest capacity for generating reversing 

 currents for a given wave amplitude with maximum velocities occurring at 

 a theoretical forcing period of 1.1 hours. The mean velocities in Duluth 

 (inlet 2), are approximately 1.5 times larger than in Superior (inlet 1) 

 (Fig. 17) . A unique feature of the Duluth-Superior system is that the 

 model predicts a net flow into the harbor through the Duluth inlet and a 

 net outflow through the Superior inlet when the forcing period is near 1 

 hour (Table 6). This asymmetry in flow throughout the forcing cycle will 

 generate a small counterclockwise net flow throughout the inlet-bay system 

 at Duluth-Superior. 



North Pond, in 1975, had two short natural inlets connecting a 

 relatively large bay to Lake Ontario. North Pond does not amplify long 

 waves because the mass of water in the inlets is small compared to bay 

 size, and friction in the inlets is high due to the shallow-water depths 

 (Fig. 18) . Since friction is high, North Pond behaves like a traditional 

 tidal inlet with a balance between head and friction in the inlets. 



42 



