predicting recession at a single point as compared to predicting the 

 mean recession along some stretch of shore. The curves in Figure A-1 

 are intended to serve as guides for making a conservative estimate of the 

 mean response due solely to lake level changes, as illustrated in the 

 following examples. Because the assumptions required to make a strict 

 statistical inference may not be justified, the curves are not intended 

 to support probability-like statements as to exactly how reliable such 

 a correction would be. Reference to the upper or lower curve simply indi- 

 cates a safe or conservative interpretation. In some cases, as in example 

 1 below, this will be sufficient basis for a decision. 



3. Engineering Application . 



Two examples are given to illustrate applications of Figure A-1 to 

 field problems . 



a. Example 1 - lias a Coastal Project Increased Erosion on An Adja- 

 cent Beach? The effect of a project on shore erosion, and the extent of 

 its influence in an alongshore direction are uncertain. Assume that 

 during the 50-year period just before the project, the shore retreated 

 150 meters (for an average rate of 3 meters per year) , In the 4 years 

 since completion of the project, the same stretch of shore receded 24 

 meters (6 meters per year; Table A-1). Further assume these rates are 

 well established by measurements at a number of points along a particular 

 beach. Do these data provide clear evidence that the project accelerated 

 erosion on that beach? If so, is the difference in retreat (3 meters per 

 year X 4 years = 12 meters) a reasonable estimate of the beach loss 

 caused by the structure? 



Table A-1. Data and adjustment for example 1. 



Earlier survey interval 



Later interval 



Retreat 



150 



24 m 



In 

 Rate 



50 yr 

 3 m/yr 



4 yr 

 6 m/yr 



Measured retreat 



150 



24 m 



Adjustment 



-0 m 



-14 m 



Remaining retreat 

 Adjusted rate 



150 m 

 3 m/yr 



10 m 

 2.5 m/yr 



Suppose that lake levels began rising a few years before project com- 

 pletion, and during the 4-year survey period after construction the 

 annual mean lake level rose 0.4 meter. The preceding 50 years had been 

 marked by several cycles of rising and falling water levels with no sig- 

 nificant net change in lake level elevation. (Such conditions would not 

 be unusual on the Great Lakes.) The higher water levels certainly played 

 some role in increasing shore retreat during the latter period. Can the 

 amount of additional erosion due to high water be estimated? 



Assuming the response of the shore will be directly proportional to 

 the amount of water level change, and that the retreat measured on the 



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