Interestingly, the bluff continued to erode behind the seawall but at a 

 rate 40 percent lower than the estimated rate without the seawall. This fur- 

 ther recession was probably caused by wave and spray overtopping, by sliomping 

 of the bluff, and by waves flanking the ends of the seawall. The material 

 eroded in this manner either filled in behind the seawall or contributed to 

 the littoral drift supply. 



The reduction in loss to the dune area is intriguing since it sustained 

 extensive erosion in the early years of the study. One explanation is that 

 the two seawalls adjacent to the dune acted like artificial headlands while 

 the beach in between evolved into a stable, crescent-shaped embayment (Dean 

 and Maurmeyer, 1977). 



During the same period, the two sections downdrift of the seawall experi- 

 enced a 380-percent increase in the volume eroded compared to the volume change 

 expected based on the recession rate in this area before the seawall was con- 

 structed. The actual increase in volume equaled about 80 percent of the de- 

 crease in volume of the dxjne and seawall sections due to the seawall. This 

 one case study does not prove the theory that the additional amount of mate- 

 rial eroded from the shores adjacent to a seawall will approximately equal the 

 amount of material removed from the sediment supply by the seawall, but it does 

 indicate that such a relationship may exist. 



Because no new material is being added to the system, the downdrift erosion 

 can be expected to continue, though probably at a reduced rate depending on 

 storm frequency, lake level, and the effectiveness of measures to mitigate the 

 erosion. 



V. SUMMARY 



Results. 



This report has dealt specifically with the bluff recession which occurred 

 in Berrien County, Michigan, near the Donald C. Cook Nuclear Plant between 

 1970 and 1974. Though site specific, some of the findings and the analysis 

 procedures used are applicable to other areas and studies. A major difference 

 between this study and others which have used aerial photos to measure bluff 

 recession is the quality and large scale of the aerial photos. Errors were 

 minimized by the selection of the best sets from the monthly photos in terms of 

 flight path, vegetation, ice cover, waves, offshore bars, and shadows. Measure- 

 ment distances were kept short and interpretation errors were reduced by using 

 stereo images to define the bluff. 



Measurements to the bluff line, bluff toe, and shoreline were made every 

 30.5 meters and bluff changes were computed to an accuracy of ±1.4 meters (see 

 App.). An adequate measure of the bluff recession rate for a receding reach 

 of shoreline over a 1-year period could be obtained with as few as 20 equally 

 spaced measurements per 1.6 kilometers (App.). The number reduced to 10 for 

 measurements over a 4-year period. Even fewer may be adequate for longer 

 periods. 



The bluff and shorelines of all of the five reaches eroded significantly 

 during the study. The average rate of bluff recession was 3.8 meters per year 



60 



