rate of shore retreat seems to have decreased from 3 meters per year 

 during the 7 years before installation to zero after the device was in- 

 stalled. The question would then be whether the 2-year monitoring period 

 included representative conditions, or whether any other factors during 

 that period could have been responsible for the reduced erosion, and 

 whether the apparent benefits outweigh the known costs. 



4. Discussion . 



Figure A-1 can at best help the engineer to evaluate one factor in 

 what would probably be a multifaceted problem. If this important factor 

 is taken into account by extrapolation from actual measurements, the 

 other items can then be dealt with in their usual manner. 



The data in Figure A-1 are estimates of actual retreat during a period 

 of submergence. The actual retreat may be less than the ultimate retreat 

 necessary to reestablish equilibrium, both because of conditions under 

 which the data were collected and the simplistic manner in which the 

 data were analyzed. In the use of Figure A-1, lake level changes and 

 shore response should refer to net displacements over periods on the 

 order of 2 to 10 years. The applicability of Figure A-1 will also depend 

 on the degree of similarity between the problem site and the site where 

 data for Figure A-1 were collected. The environmental summary at the end 

 of this appendix will assist the engineer in comparing the problem area 

 to the present site. 



If a significant difference between sites exists, then the qualitative 

 effect this would have can be determined by considering sediment balance. 

 If the problem site has a deficiency of sand-sized material in the back- 

 shore, either because of low relief or the preponderance of very fine 

 grained material, then the retreat required to reestablish equilibrium, 

 with a unit increase in lake level, will be greater than Figure A-1 in- 

 dicates. The same will be true if there are longshore or offshore sedi- 

 ment sinks, or if the active profile is broader than in the study area. 

 More turbulence or lower nearshore gradients would increase the breadth 

 of the active profile and the anticipated retreat. Conversely, less re- 

 treat than predicted in Figure A-1 would be expected when the problem 

 area has a narrower active profile, higher or coarser backshore sand de- 

 posits, or a net inflex of sediment from external sources. 



5. A Summary of Environmental Conditions in the Study Area . 



The shore throughout the study area consists of unconsolidated de- 

 posits. As along most of the eastern shore of Lake Michigan, the shore 

 type alternates between sections of raorainal bluffs and dune-covered 

 plains. During the study, waves primarily attacked modern foredune 

 ridges which were present even where bluffs of glacial drift formed the 

 backshore. The presence of a shallow stiff clay was observed at a couple 

 of points of exposure both onshore at the base of bluffs and offshore 

 along the deeper sections of troughs between longshore bars. These 



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