Following overwash, organic debris is left on Che washover surface In 

 large clumps. Organized drift lines are also deposited along the outer mar- 

 gins of washover flats by spring high tides. The surface of the salt marsh 

 is generally increased to elevations above the natural range of salt-marsh 

 species. Fragments of dune plants present in drift lines regenerate and 

 seeds germinate leading to the establishment of dune vegetation. As with the 

 smaller washovers along Nauset Spit, the westerly winds deflate these wash- 

 overs (Fig. 133, d). Most of the deflated sediment is returned to the ocean 

 beach because extensive sections of the dune line have been leveled. Rhizome 

 extension from surrounding dunes plays a smaller role in the stabilization and 

 revegetation of large washovers than it does on smaller fans due to the large 

 ratio of fan area to vegetative perimeter. 



Dunes begin to develop in the location of drift material. In contrast 

 to the case of small washovers, these dunes continue to build as overwash 

 continues to add sediment to the back of the washover in upwind positions 

 relative to the drift lines. The lack of constraining foredunes allows over- 

 wash 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 deflates 

 to intertidal elevations at which moist sand will not saltate (Fig. 133). 

 The net result of large-scale overwash is that after many years (10 to 20), 

 all barrier features are displaced landward. New dunes, resulting from sand 

 accumulation around vegetation initiated in drift lines, coalesce with vege- 

 tation expanding by rhizome extension from remnant dunes. New salt marsh 

 forns 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. 



4. Engineering Implications . 



All sections of the Nauset Spit system are subject to draraatic changes 

 either by Inlet activity or overwash. The four units designated during this 

 study are eroding at progressively faster rates with distance from the major 

 source of sediment along Outer Cape Cod, the glacial cliffs. Increased ero- 

 sion 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. Mannsade 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. With the initiation of a new inlet through North 

 Beach, the town of Chatham will be subject to wave assault and severe erosion. 

 Dune stabilization will not prevent the eventual formation of an inlet through 

 this section cf North Beach, which is eroding at a rate of 5.8 meters per year 

 and is only 110 meters wide in soee areas. Salt marshes cannot serve aa an 

 inlet deterrent because of the length of titre required to establish a thick 

 peat layer. 



221 



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