2. Equipment . 



Small -amplitude sea level fluctuations with a period of approximately 

 the Helmholtz period of the inlet-bay system may generate relatively high 

 inlet water velocities as shown in numerical model calculations (see 

 Sec. II). Therefore, a water level recording system must be carefully 

 designed for each location to measure small amplitude, but potentially 

 important long waves. At the same time, this recording system should 

 eliminate any short -period, large -amplitude noise (e.g., wind waves) that 

 may mask the long waves in the record. For example, at Pentwater, records 

 should measure the low-amplitude waves with a period of 1 hour or longer, 

 and should exclude wind waves and other noise with periods of 1 minute 

 or less. 



One method of designing a stilling well to meet these requirements is 

 to use the linear damping well design (Noye, 1974). This stilling well 

 consists of a vertical cylinder with a sealed bottom and open to the 

 lake through a long, thin tube. Friction in the tube and the relative 

 cross-sectional areas of the tube and stilling well cause the system to 

 respond directly to long waves outside the well and to drastically dampen 

 the short-period noise. Design of stilling wells is discussed by Seelig 

 (1977). 



Fisher-Porter series 1500 digital float-type water level recorders 

 with a vertical resolution of 3 millimeters (0.01 foot) and sampling 

 intervals of 2 or 5 minutes were used to measure water levels in the 

 stilling wells. Data were collected on punched tape. Water levels were 

 measured for several months at each location (see Table 1) . 



Inlet velocities at Pentwater were measured during 1974 and 1975 

 using a Bendix current meter suspended by a cable approximately midway 

 along the channel, 4.5 meters (15 feet) from the north wall at mid- 

 depth. Velocity data were recorded on a strip chart and later digitized 

 for analysis at the same time interval as the water level data. 



3. Data Reduction and Analysis Techniques . 



Initially, the Helmholtz period of each inlet-bay system and the free 

 seiching mode periods. were calculated for the lakes and bays surveyed in 

 this study (see procedures in Sec. II). These calculations, in conjunc- 

 tion with a survey of published data on Great Lakes resonance character- 

 istics, gave an indication of the period and magnitude of important long 

 waves that could be expected at each location. The information was used 

 in the design of water level measurement equipment (discussed in previous 

 section) . 



When the field data collection program was completed, the digital 

 punched-tape water level records were mechanically converted to punchcards 

 for computer analysis. The first procedure for studying these data in- 

 cluded plotting the records for visual inspection. Then, a fast Fourier 



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