II. DATA COLLECTION 



1. Profile Stations. 



Hydrographlc surveys were conducted near Pentwater Harbor on the eastern 

 shore of Lake Michigan In both the spring and fall of 1969 (Fig. 3). These 

 surveys revealed little variation in nearshore bathymetry beyond the first 50 

 meters offshore. The formation, migration, and eventual welding of an ephem- 

 eral coastal bar to the subaerial beach constituted the major change during 

 these periods of relatively limited wave action. However, when these profiles 

 were compared with profiles from several of the same stations 2 years earlier, 

 apparent changes in bathymetry were evident out to a depth of 5 meters. To 

 further investigate this apparent long-term profile evolution, profile changes 

 were monitored in 1971, 1975, and 1976 by resurveying the 10 original stations 

 (established in 1967 within a kilometer of the jettied entrance to Pentwater 

 Harbor) and 24 additional stations spread over an adjacent 50 kilometers (Fig. 

 4). 



2. Survey Periods and Earlier Reporting . 



Profiles were measured during six different survey periods over a span of 

 9 years. The survey periods and monthly mean lake levels are shown in Figure 

 5. Changes in bathymetry between 1967 and 1969 were reported in Saylor and 

 Hands (1970). Hands (1976a) provided a description of the cross-sectional and 

 areal geometry of the longshore bars throughout the 50-kllometer reach, as 

 well as information on grain-size variations and some speculation on the pro- 

 file adjustment between 1967 and 1971. Hands (1976b) compared profile devel- 

 opment through 1975 with a possible relationship between regional tilting of 

 the Great Lakes basin and variations in historic bluff recession around the 

 perimeter of Lake t-lichigan over a 120-year period. Hands (1979) Incorporated 

 results from the 1976 survey to describe the effects of water level changes on 

 the shore and on the inner parts of the profile (±100 meters from the shore- 

 line). The present report summarizes adjustments of the wider responding 

 profile and recommends procedures for estimating shore and nearshore changes 

 likely to occur in sandy regions of the Great Lakes as a result of future 

 variations in mean water level elevations. 



3. Profile Procedures. 



In 1967 the profiles were measured by winching ashore a four-wheel level- 

 ing cart, halted every 5 meters so that elevations could be determined, using 

 an engineer's level located on the shore. Upon reaching the shore, the cart 

 was pulled by Jeep down the beach to the next station and then towed offshore 

 by boat. This raeth'od limited coverage to depths less than 5 meters and 

 required a moderately wide, unobstructed beach for efficient operations. In 

 subsequent years, echo sounding was used to extend coverage on the outer part 

 of the profile but instrument leveling continued to be used in shallow water 

 to provide an overlap with the sounding record and extend the profile into the 

 dunes. Boat positioning was accomplished by an optical intersection using two 

 transits in 1969 and 1971 (Hands, 1976a) and by a range and azimuth microwave 

 system in 1975. In all years a transit was locked on the profile azimuth for 

 the individual station being sounded; radio contact between a transit man and 

 the boat operator ensured that the boat remained on line. 



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