The importance of individual storms can be seen in Figure 13, which 

 illustrates with respect to time the variation in (a) total bluff recession 

 (for the 15 profile lines), (b) average and annual lake level, and (c) stormi- 

 ness. Storminess is defined as the sum of the average daily windspeed between 

 two surveys when the wind was onshore (wind direction <10° or >170°) and 

 greater than 29 kilometers per hour (recorded at Muskegon, Michigan). This 

 definition, though arbitrary, is based on the assumption that most bluff 

 recession occurs during stormy periods with high onshore winds. Note that 

 storminess appears somewhat insensitive to major storms such as the March 1973 

 storm. 



Figure 13 shows that peak amounts of bluff recession occur during periods 

 of seasonal minimum lake levels and maximum storminess. For the 40 ice-free 

 surveying periods (as shown in Table 3, April to January), the correlation 

 coefficient resulting from a simple linear regression between storminess and 

 bluff recession was 0.50 (significant at the 1-percent level). Although the 

 study period was too short to adequately evaluate the effects of long-term 

 lake level changes, the greatest shift in average lake level occurred between 

 1972 and 1973, which corresponded to the period of greatest bluff recession. 

 The storminess of this period was, however, not significantly different from 

 other storm periods. 



4. The Effect of Structures. 



As mentioned previously, the anomalously high recession measured at pro- 

 file lines 4 and 16 appeared to result from the effect of local shore protec- 

 tion structures. Six of the remaining 15 profile lines are also located near 

 structures though the effects were less apparent. 



Davis, Fingleton, and Pritchett (1975) attributed their 1972 measurements 

 of high recession (and volume loss) at profile line 4 to the sheet-pile sea- 

 wall located less than 200 meters to the north; however, this erosion unex- 

 plicably stopped in 1973 and the area remained stable through the end of the 

 study (see Fig. 9). 



Somewhat more interesting and better documented is the situation which 

 occurred at profile line 16. Until December 1973, this profile line had been 

 one of the most stable, receding only 2 meters since August 1970. Then in 

 1974, the line lost 9.8 meters of bluff, an amount equal to 28 percent of the 

 total recession recorded for all 17 profile lines during 1974. 



Birkeraeier (1980) attributed this dramatic increase in recession to the 

 579-meter seawall constructed 275 meters updrift (north) of profile line 16. 

 After the seawall was completed in November 1971, the rate of erosion in- 

 creased at the downdrift (south) end of the seawall, eventually forming a 

 crescentic-shaped cut. This cut lengthened, reaching profile line 16, in late 

 1973. The bluff receded during 8 of the last 12 months of study. 



The erosion at profile line 16 has continued. A field trip to the area in 

 October 1976 found two large precast concrete seawalls placed at the base of 

 the bluff across the profile line and a number of sandbag groins placed along 

 the shore farther to the south. Both are evidence of further erosion. 



28 



