(Figure 51). The relationship between shoreline change and these features 

 appears reasonable and informative, but the relationship does not consider the 

 actual processes causing the changes. Extrapolation of future shoreline 

 changes using both past shoreline change data and the relationships between 

 those changes and local features improves the forecasts, but even these pre- 

 dictions must be treated with caution. Clearly, an effort to establish the 

 causes of the shoreline changes related to local features is warrented. 

 Barrier island migration and narrowing 



98. Barrier islands along the mid-Atlantic coast very likely formed on 

 the Continental Shelf considerably east of their present positions during a 

 period when sea level was much lower than it is today (Swift et al. 1972). 

 As sea level rose, the islands are thought to have migrated toward the con- 

 tinental land mass--or west in the study area. For this migration to have oc- 

 curred, the ocean side of the islands must have retreated and the sound side 

 must have prograded. During migration the islands likely had alternating 

 periods of net island narrowing and widening superimposed on the longer term 

 landward migration. Conditions favoring island migration are those that move 

 sand from the ocean side of the islands to the sound side. In the study area, 

 this would mean one or more of the following conditions: 



a. Overwash transport. The optimum conditions are a narrow island 

 (probably less than 1 km in width, and maybe quite a bit less); 

 a low island where dunes are absent, or low and discontinuous; 

 minimum vegetation, especially those shrubs and trees that 

 would hinder overwash; and storm surges of long duration in 

 which the water level exceeds the island elevation. 



b. Aeolian transport. The optimum condition is a strong onshore 

 wind that exceeds 25 km/hour (that necessary for sand transport) 

 for long periods of time; a wide, dry beach area that serves as 

 a source for wind-carried sand; and an absence of vegetation so 

 that the windblown sand can be carried to the sound side of 



the island. (A low, narrow island would probably allow a more 

 speedy trip for a sand grain from ocean to sound but is not 

 necessary for effective aeolian sand transport.) 



c. Inlet-related transport. Most important to island migration is 

 the presence of many large and relatively permanent inlets 

 which intercept sand moving in the littoral zone and move it 



in a net westward direction. An inlet is capable of removing 

 a large portion of the sand moving in an alongshore direction 

 and transferring it to shoals in the sound or to the sound 

 shoreline adjacent to the inlet. As the number, size, and 

 persistence of the inlets increase, the amount of sand moved 

 in a landward direction increases and the probability of island 

 migration increases. 



93 



