visual estimated percentage (of the sand fraction) of heavy minerals was 35 

 percent on the backshore and 17 percent on the foreshore. There were no 

 obvious patterns of occurrence of heavy minerals either among profile lines or 

 seasonally. 



This summary of the sediment characteristics does not fully reveal the 

 complex nature of the beach sediments, particularly of individual samples and 

 between profile lines. To facilitate further analysis, the original data have 

 been compiled and placed in the CERC library (Birkemeier, 1981). 



VII. SUMMARY AND DISCUSSION 



This report has discussed the changes which occurred at 17 unique profile 

 lines located along the east coast of Lake Michigan. Although the report is 

 primarily a data report, the important factors affecting bluff recession, such 

 as lake levels, storms, shore protection structures, and composition, have 

 been analyzed. In general, the bluff line can be expected to respond to the 

 different processes (if these processes could be isolated) as follows: 



(a) Lake level — Increasing lake level increases bluff recession. 

 Decreasing lake level decreases bluff recession. Either trend should 

 affect all profile lines. Rate of change may be important. 



(b) Storms — High rates of bluff recession during short time inter- 

 vals, depending on storm duration and intensity, should affect all 

 lines. Expect great variation between lines due to different orienta- 

 tions, compositions, beach widths, and proximity to the storm path. 



(c) Shore protection structures — Varied but localized influence 

 which affects individual profile lines. 



(d) Bluff composition — Varied but localized influence which 

 affects individual profile lines. 



Although the available data are insufficient to isolate and quantify each 

 of these relationships, they do provide some insight into the complexity of 

 the bluff recession phenomena. For example, the dominant factor causing high 

 erosion at profile lines 4 and 16 was their proximity to shore protection 

 structures. The low recession recorded at profile line 13 appears to be due 

 to its till composition. Ground waterflow may be a controlling factor at 

 profile line 17. 



1. Summary. 



Long-term measurements like those of Powers (1958) and the Michigan 

 Department of Natural Resources (1975) report bluff recession rates of 0.5 to 

 2 meters (maximum) per year. These low values tend to obscure the fact that 

 the recession actually occurs in cycles of high and low recession rates. As 

 discussed in Section V, 2, the average annual rate of bluff recession per 

 profile between August 1970 and December 1974 was 2.5 meters per year, a value 

 exceeding the highest long-term rate. At individual profile lines, the dif- 

 ferences were even more dramatic. Profile line 4 retreated at an average rate 

 of 4.2 meters per year, more than twice the highest long-term average, and in 

 one instance retreated 6.1 meters between two surveys. 



36 



