Measurements were tabulated and amounts of change were computed with the 

 aid of a con5)uter program. The estimated accuracy of a bluff position measure- 

 ment for a single station is +0.91 meter (see computations in App.). A change 

 in bluff position can be measured to an estimated accuracy of ±1.4 meters. The 

 accuracy of average changes and rates determined over all the stations in a 

 reach is dependent on the number of measurement points and the standard devi- 

 ation of the measured values. Since collecting data from photo imagery is 

 tedious, only as few measurements as possible should be made. The problem of 

 estimating how many measurements to take is discussed in the Appendix. 



Because of the increased problems with parallax (see App.), greater daily 

 variability, changing lake levels, and measurement point identification, meas- 

 urements of the shoreline and the bluff toe were less accurate than measure- 

 ments of the bluff crest. To reduce the effect of emergence and submergence, 

 shoreline measurements were corrected to an average lake level using the meas- 

 ured lake levels, for the day each photo was taken and an approximate foreshore 

 slope (generally 0.1; foreshore slope was estimated from monthly surveys of 

 profile lines near the powerplant and of CERC profile line 16 within reach B 

 (Davis, Fingleton, and Pritchett, 1975)). This correction was not made where 

 the shoreline was constrained by a vertical shore-parallel structure. 



Annual rates of change were computed for both the shore and bluff lines for 

 each period based on period length. Although this provides some comparison of 

 changes between periods of varying length (4 to 19 months), it can be misleading, 

 particularly for periods shorter than 1 year and for winter periods when actual 

 changes are restricted to ice-free periods (the effect of computing rates based 

 on the ice-free period is discussed in Section IV, 3). 



Any recession rate measurement taken out of context may be somewhat mis- 

 leading. For instance, if an area retreated 6 meters during one storm, with no 

 other recession during the year, the annual retreat rate is 6 meters per year. 

 If, however, monthly measurements were made, the annual retreat rate for the 

 storm month is 72 meters per year while the retreat rate for the remaining 11 

 months would be zero. The retreat rate over the full 12 months would, of course, 

 still be 6 meters per year. To avoid possible confusion, comparisons should be 

 made of rates determined for similar or nearly similar time periods. Because 

 of the seasonal nature of Great Lakes processes, annual rates (regardless of 

 ice-cover periods) are the most logical. However, the rates must be based on 

 data covering a period greater than 1 year to be useful. 



To minimize confusion in this study, the actual amounts of change for each 

 period along with the rates have been tabulated for each reach. Annual changes 

 have also been computed by combining 6-month periods. 



2, Reach A. 



Reach A (Fig. 5), extends for 1.71 kilometers and includes 57 measurement 

 stations. It covers a stretch of high, lightly developed bluff which has under- 

 gone extensive erosion. Only four shore protection structures are within the 

 reach, and all include seawalls built before 1970. Although the structures 

 offered some localized protection, the bluff continued to erode behind them. 

 Some of the most dramatic erosion occurred at the northern end of the reach 

 (Fig. 6) where at least one house toppled over the bluff. 



