eastern shore of Lake Michigan can serve as a guide in the present sit- 

 uation. Figure A-1 suggests 14 to 19 meters of retreat occurs in response 

 to 0.4 meters of submergence. It is expected that individual measure- 

 ments from points up and down the problem beach would show greater vari- 

 ation as indicated by the scatter of points beyond the confidence band 

 for the mean in Figure A-1. However, at least 14 meters of retreat would 

 be a conservative estimate of the mean response due solely to the in- 

 creased lake level; it is the mean response which is of concern here. 

 Assuming the overall conditions of the problem site ar'6 similar to those 

 around Little Sable Point (as indicated by similarities in sand size, 

 nearshore bathymetry, wave exposure, etc.), the estimated lake level 

 effect is subtracted from the measured postproject retreat, and the re- 

 sulting adjusted rate becomes (24 minus 14 meters = 10 meters in 4 years) 

 2.5 meters per year. The adjusted recession rate is even less than the 

 historic average before the project (Table A-1). Thus, the increase in 

 lake levels is more than enough to explain the observed increase in shore 

 retreat. There is no evidence that the project itself resulted in any 

 increased shore retreat. In this example many variables that may have 

 influenced the rates were not measured, so it is still possible that the 

 project itself tended to increase erosion and that this tendency was 

 overshadowed by other factors. However, once adjusted, the available re- 

 cession rates are not sufficient to suggest the project has had any det- 

 rimental effect on the beach in question. 



b. Example 2 - Evaluating a Shore Protection Device . The second 

 hypothetical case involves the determination of how well a shore protec- 

 tion device has performed. The device was installed along a shore which 

 had experienced erosion during a recent period of high water. The average 

 beach width had decreased 40 meters in 7 years. Lake levels had risen 

 0,4 meter during the first 5 years, but had remained stable during the 

 2 years just before installation of the shore protection device. The 

 project was monitored for 2 years after installation, and no further shore 

 retreat was observed. Based on this information, how well did the device 

 seem to perform? 



Again, Figure A-1 suggests that the average distance the shore would 

 have receded in adjustment to the 0.4-meter increase in lake level is 

 from 14 to 19 meters. Subtracting this from the measured average retreat 

 leaves 21 to 26 meters unexplained (Table A-2). A conservative claim 

 would be that even after taking water level differences into account, the 



Table A-2. Data and adjustment for example 2. 



Earlier survey interval 



Retreat 40 m 



In _7_ yr 



Rate 5.7 m/yr 



Measured retreat 40 m 



Adjustment - 19 m 



Remaining retreat 21 m 



Adjusted rate 3 m/yr 



Later interval 







2 yr 

 stable 



No change in mean 



water level 

 No retreat 

 m/2yr 



48 



