A simple regression analysis of average lake level and bluff recession rate 

 for the 19 data values resulted in a correlation coefficient of 0.69, explaining 

 about 50 percent of the variance. The intercept of the trend line shifts to the 

 right and the slope is steeper than that found by Seibel (1972). The shift is 

 due primarily to the high rates of recession reported by Tanner (1976) for two 

 relatively low lake levels. 



These two points are important to the overall lake level bluff recession 

 relationship and may be explained by the lake levels shown in Figure 2. A 

 rate of 2.5 meters per year was recorded between 1956 and 1964, a period of 

 generally falling lake levels except for a sharp rise of 0.46 meter between 

 1959 and 1960. It is speculated that much of the recession occurred during 

 this period. The other point w^as a 4.5-meter per year recession rate between 

 1964 and 1970, a period of low average but rapidly rising lake levels. 



The two data points are important because they indicate that high rates of 

 recession can occur during low lake levels and that other factors than average 

 lake level need to be considered. 



4. Explanation of Seawall's Effect . 



A seawall protects the shoreline by separating land and water areas with 

 a fixed boundary. Because of high wave reflectivity off vertical and sloping 

 seawalls, the rate of erosion tends to increase in front of the seawalls and 

 it is difficult to maintain a fronting beach. According to the Shore Protec- 

 tion Manual (SPM) (U.S. Army, Corps of Engineers, Coastal Engineering Research 

 Center, 1977) the ground at the toe of a seawall, bulkhead, or revetment can be 

 expected to scour below the natural bed to a depth equal to the height of the 

 maximum unbroken wave which can be supported in the original water depth. 



Similar guidelines are not available for the effects on the shore adjacent 

 to the seawall, although the SPM cautions that when a seawall is built on a 

 receding shoreline, the recession on adjacent shores will continue and may be 

 accelerated. The three-dimensional aspect of seawalls has been discussed by 

 Silvester (1972; 1974; 1977). Silvester (1977) describes a seawall as a means 

 of protecting a shoreline which is receding due to an imbalance between the 

 supply of sediment and the sediment-carrying capacity of the incoming waves. 

 While the construction of a seawall will protect the land behind it, it only 

 accentuates the original problem by further reducing the supply of littoral 

 material to the unprotected region of the beach. This is actually what is 

 occurring downdrift of the long seawall in reach B. 



Silvester (1977) also theorizes that the interference of incident and re- 

 flected waves produces a short-crested wave system which increases the trans- 

 port of material in front of and immediately downdrift of a seawall over what 

 would have normally occurred without the wall. Farther downcoast, this excess 

 sediment can no longer be carried and it settles out as a shoal. 



In the specific case of the reach B seawall, the shoal did not appear on 

 any of the air photos, but the lack of a beach within the downdrift cut and 

 the occurrence of a beach farther downcoast offer some support to the Silvester 

 theory. 



58 



