6 



5 



^ 4 

 E 

 « 3 



-I 



Historic Recession Rote 

 Over 120-Yeor Biriod 



Recent Recession Rote 

 0>er 6-Ytar Period 



o] 50 ioO 150 200 I 250 300 | 350 



MICHIGAK CITY.INO. PENTWATER , MICH FRANKFORT , MICH. 



Distance Measured Northerly along the Eastern Shore of Lake Michigan (km) 



Figure 18. Comparison between rates of historic (1830 's to 1959' s) and 

 recent (1969-75) recession. Positions of survey stations on 

 the eastern shore of Lake Michigan are referenced to the 

 Porter-La Porte County boundary in Indiana. 



that recession rates during the period of rising water rose to six times 

 their longer term average. 



6. The Need to Adjust Recession Rates. 



The large temporal changes observed in recession rates make it diffi- 

 cult to determine a "natural erosion rate" to be expected for a given 

 beach on the Creat Lakes. Changes in the mean recession rate by a fac- 

 tor of 3 to 4 after some intervention by man (which may be related to 

 shore protection, etc.) can not definitely be ascribed to that interven- 

 tion, unless the water levels were essentially the same before and after 

 the action in question. An untested general rule for comparing recession 

 rates determined under different water level conditions would be to sub- 

 tract a factor presumably attributable to the difference in lake levels 

 from the "after-action" recession rate. Figure A-1 in Appendix A can 

 be used as a guide for determining this factor. If the resulting "ad- 

 justed estimate" does not differ substantially for the "before-action" 

 rate, then the conservative conclusion would be that there is insufficient 

 evidence to establish whether the action in question played any part in 

 increasing recession rates. 



This approach would be most directly applicable when the lake level 

 has changed from stable to rising. If the before-action survey interval 

 ends about the time the rise in lake levels ends, then it should be remem- 

 bered that the recession rates may remain high in response to that rise 

 for several years until erosion has brought the profile back into adjust- 

 ment. As the effects of high water may persist into the after-action 

 survey period, the full "correction factor" should not be applied unless 

 the shore has regained equilibrium. Examples are given in Appendix A. 



38 



