64 



During the 3-year period (1991 to 1994) of the comparison at R14 (Fig- 

 ure 26), the volume of the beach/nearshore bar increased by 1 60 m /m, while 

 the offshore bar volume increased by 500 m /m. The bathymetry comparisons 

 revealed that R14 is located in the zone of most severe lake bed erosion, 

 exceeding 4 m from 1945 to 1995. However, during the period of the profile 

 volume comparison, R14 may have been a depositional sink for sediment from 

 the feeder beach (see Figure 26). If the annual rate of accumulation (approxi- 

 mately 200 m /m) at R14 had occurred over a 200-m section of the shore, the 

 depression in the lake bed near R14 could trap 40,000 m of sand per year 

 moving in an alongshore direction, which is approximately equivalent to the 

 annual volume of beach nourishment from 1991 to 1995. 



Profile R22, located 5 km south of the harbor jetties, also appears to be 

 directly influenced by the nourishment on the feeder beach (Figure 27). In the 

 fall of 1992, after 130,000 m"' of beach nourishment was placed on the feeder 

 beach in 1991 and early 1992, the beach/nearshore bar and offshore bar gained 

 significant quantities of sand, 375 m /m and 220 m /m, respectively (see Fig- 

 ure 27). From the fall of 1993 to the fall of 1994, the trend reversed and the 

 beach/nearshore bar and offshore bar eroded below the base volume of 

 August 30, 1991. This erosion trend may be explained by a break in the nour- 

 ishment program from the spring of 1992 to the fall of 1993, decreasing the 

 rate of sediment available for alongshore transport to the beaches south of the 

 harbor jetties. 



Exposure of the Cohesive Substrate 



Exposure and downcutting of the cohesive profile underneath the sand or 

 gravel lag at St. Joseph are the fundamental processes that determine at what 

 rate the shoreline retreats over time. Several factors can lead to the exposure, 

 or increase the potential for exposure, of the cohesive profile: (a) water level 

 fluctuations and associated bar migration in response to wave action, (b) reduc- 

 tion in the overlying sand/gravel cover, (c) increase in nearshore beach slopes, 

 and (d) changes in sediment grain size. The latter three factors have been 

 investigated in the previous sections of Chapter 4, and in general, they have 

 failed to fully explain the relatively even distribution of nearshore downcutting 

 evident from the bathymetry and profile comparisons. This even distribution 

 requires that the underlying till is exposed at all locations at some time. Vari- 

 ations in bar position with changing water levels appear to provide the missing 

 explanation. 



Monthly and yearly fluctuations in mean water levels for Lake Michigan 

 are described in Figure 28. During the period of the investigation, 1945 to 

 1995, there was extreme variability in lake levels, with a low yearly mean of 

 0.3 m below chart datum (IGLD '85) recorded in 1964 and the high of 1.3 m 

 above chart datum in 1986. On Great Lakes shores, rising lake levels together 

 with wave action move the bar formations onshore and conversely, during 

 falling lake levels, bars move offshore. Continuous migration of the bar and 



Chapter 4 Analyses of Coastal Processes and Geomorphology 



