I£ water level fluctuations just outside the inlet are known, the 

 numerical model can be used to predict the resulting channel velocities 

 and bay level fluctuations. Lake Michigan levels just outside of Pent- 

 water were measured by Duane and Saylor (1967) during August 1967 (Fig. 6) 

 Although the record appears confused, a spectral analysis shows that the 

 record is composed of a large number of clearly defined seiche modes of 

 Lake Michigan. Using this record to force the model of Pentwater, a bay 

 level time history is predicted which adequately agrees with measured 

 bay levels (Fig. 6). The inlet-bay system responds primarily to waves 

 with periods of 1 to 3 hours (near the Pentwater Helmholtz period) ; 

 shorter period waves are damped. This gives the bay level record a 

 smoother appearance than the Lake Michigan record. The maximum predicted 

 inlet current velocity during this episode is 60 centimeters (2 feet) 

 per second. 



Other models that neglect temporary acceleration, e.g., Keulegan 

 (1967), should not be used for inlets on the Great Lakes where temporal 

 acceleration, head difference, and friction are important during the 

 response cycle (see Fig. 4). 



III. THE FIELD DATA COLLECTION PROGRAM 



Measurements were made at a number of inlet-bay systems throughout 

 the Great Lakes during 1974 and 1975 (Fig. 7). These study sites were 

 chosen because the inlets are typical on the Great Lakes, are of special 

 economic importance, or have maintenance problems. 



1. Field Measurements. 



Pentwater, Michigan, located midway along the longitudinal axis of 

 Lake Michigan, was selected as the primary study location because the 

 inlet-bay system at this location has a simple, fairly common geometry. 

 Good historical field data are also available at Pentwater from Duane 

 and Saylor (1967) who simultaneously measured water levels in Pentwater 

 bay and Lake Michigan as well as inlet velocities during July and August 

 1967. The U.S. Army Engineer District, Detroit, provided inlet geometry 

 data of the inlet from hydrographic surveys taken twice a year. 



Field measurements during this study (Fig. 8) included water level 

 measurements at two locations (east and west ends of Pentwater bay) in 



1974, and at one other location in 1975. Current velocities in the 

 inlet were measured concurrently with water levels during both 1974 and 



1975. Hydrographic surveys were also taken at Pentwater. Other field 

 data collection sites were: Portage Lake, Ludington, White Lake, Muskegon, 

 and Holland on Lake Michigan (Fig. 8); Little Lake and Duluth-Superior on 

 Lake Siiperior (Fig. 9); Presque Isle on Lake Erie (Fig. 10); and North 

 Pond and Little Sodus on Lake Ontario (Fig. 11). 



Field measurement types, locations, dates, and data sources for all 

 the study sites are summarized in Table 1; approximate dimensions of these 

 inlet-bay systems are summarized in Table 2. 



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