17. Profile Site 17 . 



The sand terrace at this site (Fig. 22) was subjected to only 3 feet 

 of erosion the first year and 10 feet the second year. The erosion 

 occurred during the fall storm period with a small amount in late spring 

 of 1972. The beach is generally uniformly sloping and moderately wide. 

 There is a sand shortage in the inner nearshore area as evidenced by 

 incomplete ripples over a clay-till bottom (Davis, 1964). In addition, 

 water depth increases rapidly just beyond the plunge step. 



Erosion is largely the result of the general scarcity of sand which 

 inhibits beach and longshore bar growth, permitting more wave energy to 

 be imparted on the beach and sand terrace. Erosion in the second year is 

 unrepresentative because the profile is near a large tree on the 

 terrace which stabilized the terrace locally although more erosion 

 occurred a few tens of feet in either direction. 



VI. BEACH SEDIMENT CHARACTERISTICS 



The sediment characteristics for the profile sites and the range of 

 the sand sizes during the 2-year study are presented in Table 5. 



Beach sediments along eastern Lake Michigan are characterized by wide 

 ranges in textural character. Pebbles and cobbles are commonly scattered 

 or accumulated in narrow bands along the sand beaches. Size analyses were 

 made of the sand fraction only, and size data in this section refer only 

 to the sand fraction of a beach sample that might include a significant 

 amount of pebbles. Sand-size analysis consisted of computation (using 

 the RSA curve) of the mean, and the sorting and skewness of each sediment 

 sample by the graphic method (Folk, 1968). Mean is a measure of average 

 size; sorting refers to the uniformity of the sediment size; and skewness 

 measures the degree and direction of asymmetry. 



In general, beach sands of eastern Lake Michigan are well sorted, 

 positively skewed, and medium-coarse, with backshore sands generally finer 

 and more sorted than foreshore sands. Nearly all samples collected have 

 a mean grain size between 0.330 and 0.189 millimeters (1.60 and 2.40 phi), 

 although a few coarser sand samples occurred at sites 13 and 17 where 

 gravel is common. Coarse beach sands are typically adjacent to nearby 

 bluffs of glacial drift, i.e., sites 5, 12, 13, 15, and 17, and the finest 

 grain size is located on beaches adjacent to dune areas at Big Sable Point 

 (site 4) and Little Sable Point (site 6). Most beach sands are well 

 sorted with values between 0.20 and 0.25 phi units (Folk, 1968). Coarser 

 sands are generally less sorted than fine sands. Nearly all samples were 

 fine-skewed (0.00 to +0.30) with a few strongly fine-skewed and slightly 

 coarse-skewed types as exceptions. 



VII. VARIABLES AFFECTING PROFILE CHANGES 



The total bluff and terrace erosion for all 17 sites are shown by a 

 histogram (Fig. 23), with each year plotted separately. The total 



45 



