shoreline, and lake level at Pentwater is a valid model for other areas of the 

 eastern shore of Lake Michigan. (Author). 



131 HANDS, E. B. 1976b. "Some Data Points on Shoreline Retreat 

 Attributable to Coastal Subsidence: International Association of Hydrological 

 Sciences," Proceedings of the Anaheim Symposium, pp 629-645. 



Coastal subsidence increases flooding in low lying coastal regions. 

 Moreover, it disturbs the equilibrium profile, and allows waves to erode 

 bluffs formerly above the reach of wave uprush. Ensuing adjustment of the 

 profile drives the shoreline farther landward. Guidance is needed for obtain- 

 ing quantitative estimates of the shore's response. 



The mean surface elevation of Lake Michigan rose 0.5 m during a recent 

 four year period. Concurrently, major elements of the submerged profile re- 

 sponded by building upward and migrating 26 m landward. Approximately 8 m of 

 beach were lost due to submergence beneath the elevated lake surface; and an 

 additional 6 to 7 m were lost due to erosion. The shoreline, however, lagged 

 behind the rest of the profile in adjusting to the higher water levels. 



Over a longer period, certain sections of Lake Michigan have undergone 

 relative subsidence as a consequence of broad regional tilting of the earth's 

 crust. It is estimated that during the last century the Lake Michigan basin 

 tilted 0.06 to 0.09 m per 100 kilometers along its axis. Shore recession over 

 the last century increased at a rate of 19 + 10 m per 100 km in the direction 

 of greater subsidence. 



Other coastal areas with similar geomorphology and wave exposure can be 

 expected to recede at rates similar to those indicated above if subjected to 

 the same subsidence. The initial response to rapid subsidence may be on the 

 order of 50 units of retreat for each unit of subsidence. Profile retreat is, 

 however, a non- linear, time -dependent function of subsidence, and for slower 

 subsidence, shore-eroded sediments become spread over a broader profile, pro- 

 ducing a larger ratio of share retreat per unit of subsidence. Estimates of 

 shore recession due to slow crustal motion of the Great Lakes basin indicate 

 response ratios between 120:1 and 390:1. Furthermore according to the concept 

 of mass balance, the long term response ratio should also depend on the volume 

 and size distribution of sediments being supplied to the nearshore profile by 

 shore erosion. The lakeshore response was found to increase several fold 

 where inadequate backshore deposits supplied less beach material per unit of 

 recession. (Author) . 



132 HANDS, E. B. 1979. "Changes in Rates of Shore Retreat, Lake Michigan, 

 1967-76," CERC (Coastal Engineering Research Center), Technical Paper 



No. 79-4, US Army Corp of Engineers, Fort Belvoir, VA, pp 71. 



Shorelines tend to retreat landward as water levels rise. Less than 

 20 percent of the shore, lost as Lake Michigan rose between 1967 and 1976, was 

 due to direct inundation; the remaining 80 percent was due to increased ero- 

 sion in response to the higher lake levels. A simple correlation of lake 

 level change and simultaneous shore retreat ignores the inevitable lag between 

 process and response, but still accounts for 50 percent of the variance in 

 shore retreat. A graphic summary of field data is presented to estimate 

 effects of future lake level changes in similar coastal environments. 



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