from 1932 to 1970 is shown in Figure 13. This is the highest rate of 

 erosion in Brunswick County, averaging about 4.5 meters per year from 194 3 

 to 1970. Erosion rates over the rest of the island have been quite vari- 

 able in time (Fig. 14). The shoreline of the eastern reach exhibited a 

 recession rate of about 0.71 meter per year from 1942 to 1970. Langfelder, 

 Stafford, and Amein (1968) and Langfelder, et al. (1974) used aerial photos 

 to determine the recession of the high water line as well as the dune line. 

 The erosion rate of both lines has been nearly the same since 1957 and 

 approximately parallels the slope of the recession determined by U.S. Army 

 Engineer District, Wilmington (1973). All three studies indicate a marked 

 change in the rate of erosion after the early 1960 's. The positive slope 

 of the high water line during the latter years of the study indicate a sea- 

 ward growth of 0.66 meter per year (broken line, Langfelder, et al., 1974) 

 and 0.30 meter per year (solid line). A more recent study (Wahls, 1973) 

 estimated the composite erosion rate (from Shallotte Inlet to Lockwoods Folly 

 Inlet) of the dunes and shoreline as 0.6 and 1.5 meters per year from 19 38 

 to 1972. The interval from 1966 to 1972, however, shows accretion of the dune 

 and shoreline at annual rates of 1.71 and 0.15 meter per year, respectively. 



The long-term erosion rate determined by aerial photo analysis of the 

 southern North Carolina shoreline is presently being studied. Specific methods 

 and expected reliability of the estimates obtained by the analysis are explained 

 in Dolan, et al. (1979, 1980). 



The erosion rate during the BEP study period was estimated from measured 

 changes in above MSL volume and MSL shoreline position. Holden Beach was 

 divided into three reaches, each similar in response to processes and in 

 the degree of variability shown by the plots of volume and MSL intercept 

 change (Apps . C and D, respectively) . The three reaches are Lockwoods Folly 

 (profile lines 1, 2, and 3), central (profile lines 4 to 18), and Shallotte 

 (profile lines 19, 20, and 21). 



Plots of the change in MSL intercept and above MSL sand volume with each 

 successive measurement (Apps. C and D) give a qualitative indication of the 

 temporal variability of each profile line. Superposition of plots shows many 

 instances during which changes are of opposite sign, even at adjacent profile 

 lines, suggesting that spatial variability is also quite large. Linear regres- 

 sion analysis was used to evaluate the trends in shoreline position and volume 

 with slopes for each profile line given in Table 3. Though a trend could be 

 estabilished in each case, the coefficient of determination 



SS 

 _2 _ R _ sum of squares due to regression 



SS total sum of squares (corrected for mean) 



was significant at the 95-percent level in only six of the profile lines along 

 the central reach, indicated in Table 3. The proportion of total variation 

 about the mean explained by linear regression is not significant at the 95- 

 percent confidence level for the remaining profile lines. 



The annual rates of change predicted by the regression lines of MSL posi- 

 tion and unit volume are generally more extreme near the inlets than along the 

 central reach. Since profile lines are almost evenly spaced along the beach, 

 changes may be estimated by averaging the parameter of interest along the 

 selected reach. The generalizations developed by this method should be applied 



29 



