positions relative to the drift lines. The lack of constraining foredunes 

 allows overwash to take place for several years (5 to 10), augmenting this 

 sand supply. Drift-line dunes are usually not eroded during overwash since 

 they are located in landward positions. During the final stages of dune 

 recovery, washover passages through the foredunes periodically coalesce during 

 windy, interstorm periods. 



Eventually, the dune line becomes continuous and the back barrier de- 

 flates the intertidal elevations at which moist sand will not saltate (bounce 

 along by the impact of dislodged sand grains) . The net result of large-scale 

 overwash is that after many years (10 to 20), all barrier features are dis- 

 placed landward. New dunes, resulting from sand accumulation around vegeta- 

 tion initiated in drift lines, coalesce with vegetation expanding by rhizome 

 extension from remnant dunes. New salt marsh forms in the lee of these dunes, 

 and the barrier beach as a whole is displaced landward with the establishment 

 of the same general physiographic features and vegetative composition. 



Engineering implications. All sections of the Nauset Spit system are 

 subject to dramatic changes either by inlet activity or overwash. Southern 

 portions of the Nauset Spit system are eroding more rapidly than the northern 

 portions, which are nearer to the glacial cliffs, the major source of sediment 

 along Outer Cape Cod. Increased erosion rates lead to more rapid landward 

 migration and more unstable conditions. The outer shoreline appears to be 

 readjusting toward a slightly more southwest to northeast orientation. Due to 

 this shoreline movement, man-made structures along all sections of the spit 

 system will be subject to destruction during storms. The most stable unit, 

 Nauset Spit-Eastham, appears to be undergoing a longer migration cycle than 

 other sections of the spit system. 



Artificial creation and maintenance of dunes and salt marshes can be used 

 to extend various periods of the migration cycle but will not alter the basic 

 biogeological process. Extensive dune stabilisation can reduce overwash 

 activity for a period of time, resulting in calm back-barrier conditions nec- 

 essary for the establishment of salt-marsh vegetation. However, artificially 

 established dunes will continue to narrow in the absence of washover sediment 

 in upwind positions, and these foredunes will eventually be eroded. 



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