101. Island narrowing must have begun before 1850. It will, of course, 

 end when the islands disappear or when one or both shorelines begin to pro- 

 grade. At the present average rate of island narrowing (0.9 m/year) , it will 

 take almost 1700 years for a 1500-m-wide island to narrow to nothing. Before 

 that happens, though, overwash, if allowed, will likely begin to transport 

 sand to the sound shoreline and island migration will commence. A reasonable 

 forecast based on past behavior is that narrowing will probably continue in 

 the foreseeable future. 

 Alongshore sediment transport reversal 



102. Waves approaching shore at acute angles and winds with a shore- 

 parallel component create alongshore currents. Sediment mobilized by wave 

 activity is moved by these currents. Over the period of a year the amount of 

 sand moved one way is rarely balanced by that which is moved the other way; the 

 difference is the net volume of littoral sand which moved preferentially in one 

 direction. This net volume and the direction it is moved may change from year 

 to year and over longer time periods as the wave and wind climate changes. 



103. Study results tell us little about the net volume moved; however, 



they provide some indication of the direction of net sediment transport. Other 



5 

 studies have suggested that, on the long-term average, 2 x 10 cu m/year of 



sediment moves north at Rudee Inlet" and 5 x 10 cu m/year moves south at Ore- 

 gon Inlet (U. S. Army Engineer District, Wilmington 1980). This net transport 

 indicates that a change in the net alongshore sediment transport direction-- 

 i.e., a transport reversal—occurs somewhere between the two inlets. Evidence 

 from this study suggests that the reversal occurs near latitude 36°4l' (Fig- 

 ure 28 shows a very uniform decrease in the shoreline retreat rate north and 

 south of that site (the north end of Back Bay)) to create a divergent long- 

 shore sediment transport nodal zone; i.e., a place where sand moves alongshore 

 to both the north and the south away from the site. Losses north and south 

 of latitude 36°41' are nearly equal and decrease progressively with distance. 

 Shoreline retreat rates are expected to decrease if a divergent nodal zone 

 exists because sediment moving away from the node will reach adjacent beaches 

 and thereby reduce the loss rates there. 



104. The large shoal complex east of the Chesapeake Bay entrance 



Personal Communication, James Melchor, 1981, Oceanographer, U. S. Army 

 Engineer District, Norfolk, Va. 



96 



