be measured near the point of interest to obtain detailed amplitudes and 

 periods of the active seiche modes. 



2. A simple numerical model developed at the Coastal Engineering 

 Research Center (CERC) can be used to predict inlet velocities, discharge, 

 and bay levels for Great Lakes inlets. The model was applied to evaluate 

 the hydraulic characteristics of several Great Lakes inlets, and examples 

 are given using the model for typical design computations. 



3. Numerical modeling of selected inlets showed that head, temporal 

 acceleration or inertia, convective acceleration, and friction may all 



be important in controlling the hydraulics of Great Lakes inlets. Temporal 

 acceleration may be especially important as it causes bay fluctuations to 

 be amplified and out of phase with the forcing wave. As a result, a 

 large head differential may be generated for waves with periods approx- 

 imately equal to the Helmholtz period of the inlet-bay system. For a 

 given amplitude, the highest reversing inlet currents will occur for wave 

 periods slightly smaller than the Helmholtz period. Since even a small- 

 amplitude seiche may generate significant reversing inlet velocities if 

 the wave period is near the inlet-bay Helmholtz period, water levels 

 should be carefully measured. 



4. Reversing inlet currents can also be predicted by the continuity 

 equation from high-quality bay water level records. Cumulative frequency 

 distributions of inlet velocity developed in this manner are presented 

 for several Great Lakes inlets. 



5. Reversing velocities at most inlets are generally small. 

 However, velocities may be high if the inlet is located where lake seiche 

 amplitudes are relatively large and have a period approximately equal to 

 the inlet-bay system Helmholtz period; e.g., at Duluth-Superior. 



67 



