For example, assume that a prediction of inlet velocities and the 

 amplitude of bay level fluctuations at Pentwater is desired if (ja.) the 

 inlet was deepened to 7.3 meters (24 feet), and (b) the inlet was allowed 

 to shoal to a depth of 1 . 8 meters (6 feet) . 



The monochromatic response of the inlet-bay system for these two inlet 

 modifications is predicted by changing the inlet geometry in the cali- 

 brated model of Pentwater. Each model was run with sinusoidal wave periods 

 between 0.5 and 5 hours and an amplitude of 3 centimeters to predict am- 

 plification of the wave in the harbor and maximum velocity in the inlet. 

 Figures 30 and 31 show the amplification and maximum velocities, respec- 

 tively, for the 1967 inlet geometry, and for inlet depths of 1.8 and 7.3 

 meters. The results are summarized in Table 8. 



Table 8. Summary of Pentwater hydraulic characteristics for 

 selected inlet depths. 







1967 







D=6 ft 



(11 ft<D<20 ft) 



D=24 ft 



^^^^o^max 



1.06 



1.7 



2.9 



\ (hr) 



5.0 



1.8 



1.0 



v„^^ Cft/s)^ 



max ^ ' ■" 



1.0 



2.0 



2.7 



'^vmax (^^)^ 



1.6 



1.4 



0.95 



^For ao = 0-1 foot. 



These predictions show that deepening the Pentwater channel from 1.8 

 to 7.3 meters causes the peak amplification and inlet velocity to in- 

 crease, and the Helmholtz period and period of maximum velocity to de- 

 crease. Comparison of the modes of oscillation of Lake Michigan (Table 3) 

 with the predicted velocities (Fig. 31) suggests that the 0.85-, 0.97-, 

 1.1-, 1.25-, and 1.44-hour waves will generate the highest reversing 

 currents at Pentwater if the inlet was deepened to 7.3 meters. 



The Lake Michigan water levels recorded on 18 August 1967 (Fig. 6) 

 were used to force the lumped parameter model for the selected depths 

 (Fig. 32). The model predicts that for these Lake Michigan level fluctu- 

 ations, the maximtim velocity for an inlet 1.8 meters deep would be 46 

 centimeters per second (1.5 feet per second); a 7.3-meter-deep inlet 

 would have a velocity of 107 centimeters per second (3.5 feet per second). 



VI. SUMMARY AND CONCLUSIONS 



1. Meteorologically generated seiches cause most of the significant 

 reversing currents at Great Lakes inlets. Seiche periods and node-anti- 

 node patterns can be predicted numerically. However, water levels must 



63 



