6.321 Carolina Beach, North Carolina . A protective beach was part of the 

 project at Carolina Beach, and is used to illustrate the planning of such 

 a beach. (See Figures 6-20 and 6-21.) The project included hurricane pro- 

 tection, but the illustration of protective beach planning will include 

 only the feature which would have been provided for beach erosion control 

 alone. The report on which the project is based was completed in 1961 

 (U.S. Army Engineer District, Wilmington, 1961), and the project was partly 

 constructed in 1965. 



The predominant direction of longshore transport is from north to 

 south. This conclusion was based on southerly growth of an offshore bar 

 at Carolina Beach Inlet, and shoaling at Cape Fear, 12 miles south of 

 Carolina Beach. Subsequent erosion south of Carolina Beach Inlet and 

 accretion north of a jetty at Masonboro Inlet, about 9 miles north of 

 Carolina Beach, have confirmed the direction. The long-term average 

 annual deficiency in material supply for the area was estimated in the 

 basic report at about 4 cubic yards per linear foot of beach. This esti- 

 mate was based on the rate of loss from 1938 to 1957, from the dune line 

 to the 24-foot depth contour. Carolina Beach Inlet, opened in 1952, appar- 

 ently had little effect on the shore of Carolina Beach before 1957; there- 

 fore, that deficiency in supply was considered the normal deficiency with- 

 out regard to the new inlet. 



For planning, it was estimated that 60 percent of the material in the 

 proposed borrow area in Myrtle Sound (behind Carolina Beach) would be com- 

 patible with the native material on the beach and nearshore bottom, and 

 would be suitable for beach fill. This estimate assumed that 40 percent 

 of the borrow material was finer in size characteristics than the existing 

 beach material and therefore would be winnowed due to its incompatibility 

 with the wave climate. The method of Krumbein and James (1965), was con- 

 sidered for determining the amount of fill to be placed. However, insuffi- 

 cient samples were taken from the foreshore and nearshore slopes to develop 

 characteristics of grain-size distribution for the native beach sand. 



Although samples taken from the beach after construction may not be 

 entirely indicative of the characteristics of the native sand, they do 

 represent to some extent the borrow material after it has been subjected 

 to wave action, presumably typical of the wave climate associated with 

 sorting on the natural beach. Samples taken from the original borrow 

 material and from the active beach profile in May 1967 were therefore used 

 to estimate the amount of material lost from the original fill as a result 

 of sorting action. 



The estimate was made by computing the ovitioat ratio (R^crit^ » 

 defined as the ratio of the volume required to be placed to the volume 

 retained on the beach in equilibrium with shore processes assuming the 

 specific gravities of the borrow and native materials are the same. 



Hcrit = ^- e 2(a!J^ -O^y) (6-1) 



6-17 



