variations in that parameter. The slopes of profile lines nearest the two 

 inlet throats (profile lines 1 and 21) show the greatest slope of the study 

 area. The mean slopes at profile lines 2 and 3 have been artificially 

 altered by the beach fill operation. 



V. DISCUSSION 



1. Profile Changes , 



Holden Beach, exposed toward the south and partially protected from 

 large waves from the east by Frying Pan Shoals, is spared the severe ero- 

 sion caused by east and northeast storms, which arrive along the North 

 Carolina coast mainly during the autumn and winter months. These storms 

 remove large amounts of sand from beaches along the Outer Banks and those 

 shorelines exposed toward the east. 



The relative position of the annual mean, MSL intercept (Fig. 15) re- 

 flects the number of storm occurrences during the year of measurement 

 (Table 7). The erosion observed over 1971 took place during the year with 

 the largest number of identified storm events, while the increase in MSL 

 intercept over 1974 is correlated with the fewest storms. Changes in sand 

 volume and MSL intercept show extreme variability along the three profile 

 lines comprising each inlet reach, but both have resulted in considerable 

 increases. The limit of significant influence of the inlets, if such exists, 

 has not been determined by these studies. The selection of the inlet reaches, 

 however, provides a convenient separation point based on demonstrated vari- 

 ability. These two measurements are also quite variable along the central 

 reach, but regression analysis and evaluation of the annual change show the 

 MSL position to be extending seaward while the volume decreases. The total 

 annual loss in volume along the central reach (rate of volume change times 

 total length) is approximately balanced by the gains at the two inlet reaches. 

 A similar computation using the regression estimates indicates the volume 

 gains at the inlets are each 3 to 4 times greater than the loss along the cen- 

 tral reach. The island appears to be gaining sand volume at the ends while 

 losing volume along the center. The MSL intercept is also progressing sea- 

 ward more rapidly at the inlet areas. Plots of the actual beach profiles 

 were compared for 14 December 19 70 and 4 December 1974. Though possibly not 

 indicative of the entire 4-year span, each set of measurements was taken 

 after a storm (Birkemeier, 1979) so the general beach condition may be compar- 

 able. The earlier profiles were characterized by steep foreshore slopes and 

 a backshore area that was convex upward. This was backed by the coastal 

 dune, present in both surveys. The 4 December 1974 profiles showed an off- 

 shore bar along most of the central reach with a backshore concave upward, a 

 condition more typical of the storm profile. A considerable volume of sand 

 was removed above MSL and deposited in the offshore bar while the MSL inter- 

 cept was extended seaward. The actual volume change at the -0.9-meter MSL 

 datum appears to be very small, but the beach face was considerably lowered 

 and extended. If the long-term change in the central reach is toward a lower 

 backshore and extended foreshore, the island may be developing a greater sus- 

 ceptibility to dune erosion by direct wave attack during a storm accompanied 

 by high water and large waves. Future studies of beach volume changes should 

 extend farther into the offshore zone to measure the storage of sand in bars. 

 The rates of change of both MSL intercept and above MSL volume measured here 



53 



