I ^ 



a. 



5 2 



o 



I 



*4^^^^^^s;fe 



•- 



1.0 



0.8 



0.6 



0.4 



0.2 







Locotion 



A Ludington 

 Muskegon 

 Worren Dunes 



Observations 



513 

 840 

 713 



Jon. Feb. Mar. Apr. Moy June July Aug. Sept. Oct. Nov. Dec 



Figure 8. Variation in average monthly breaking wave 

 statistics for three eastern Lake Michigan 

 locations, November 1971 to October 1975. 



hindcasted design wave data for the study area. They computed design wave 

 heights and periods for waves from three directions, for each season, and for 

 return periods of 5, 10, 20, 50, and 100 years. The highest calculated waves 

 were found to occur during the winter season (January, February, and March). 

 This indicates that the March data, shown in Figure 8, probably underestimate 

 the actual wave heights. It also underscores the combined importance of storm 

 occurrence and ice breakup on bluff recession. 



5. Slope Failure. 



Bluff erosion is a two-step process — erosion of the base of the bluff by 

 wave action, followed by gravity failure of the bluff slope. This process 

 results in new material being deposited at the base of the bluff continuing 

 the cycle. 



The basic mechanisms for slope failure are falls, rotational slumps, and 

 soil flows (Chieruzzi, and Baker, 1958). Falls occur when rocks or blocks of 

 bluff material are undercut enough to drop on the beach. This type of action 

 occurred at profile line 13. Rotational slumps are the result of shear fail- 

 ure along a "slip circle" (Edil and Vallejo, 1976), causing a major movement 

 of the bluff face or some section of it. This type of failure was important 

 at profile line 17 (see photo in App. B). Soil flow generally occurs when 

 ground water saturates a clay bluff, increasing the specific weight and reduc- 

 ing the internal shear stress (Carter, 1976). This mechanism may also be 

 important at profile line 17. 



17 



