downdrift. In this process of cyclic inlet breaching and spit regeneration, 

 the entire barrier structure has been effectively displaced landward. 



Overwash processes have proven to be of equal importance to the migra- 

 tion of the Nauset system. On Nauset Spit-Eastharn, a salt marsh existed 

 behind barrier dunes approximately 815 years before the present marsh. This 

 marsh was subsequently buried with overwash sand that was carried up to 250 m 

 beyond the bayward edge of the marsh. Sediment deposited along the bay 

 shoreline was colonized by salt-marsh vegetation, and sand subsequently 

 placed on top of th* salt marsh was colonized by dune vegetation. On North 

 Beach, core data revealed that washover sand had buried a salt marsh approx- 

 imately 200 years ago; peaty material was outcropping along the ocean beach 

 in 1978. Salt-marsh vegetation had colonized this washover surface and sur- 

 vived long enough to form a peat layer before being buried by overwash again. 

 Therefore, "barrier rollover" at this location has been determined to be less 

 than 250 yeirs and was accomplished by overwash processes. 



Barrier migration. Large-scale washovers play an important role in 

 barrier migration. Prior to overwash, the barrier beach may consist of a con- 

 tinuous dune line backed by salt marsh. During a major storm, the barrier 

 dune is eroded to a point where low-elevation dunes and blowouts are over- 

 topped by overwash surges. These surges erode an increasingly wide channel ■. 



through the dune line by lateral cutting until broad sections of the dunes 

 are entirely flattened. During overwash, large volumes of sand may be car- 

 ried from the beach and dune to the back, barrier. Some of this sediment may j- 

 be transported into the bay, resulting in landward extension of the barrier a 

 unit. f 



Following overwash, organic debris is left on the washover surface in rj 



large clumps. Drift lines are also deposited along the outer margins of w 



m 

 washover flats by spring high tides and overwash surge. The washover surface k 



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

 species. Therefore, fragments of dune plants present in drift lines regener- •> 

 ate and seeds germinate, leading to the establishment of dune vegetation. ■> 

 Rhizome extension from surrounding dunes plays a smaller role in the stabili- 

 zation and revegt. tation of large washovers than it does on smaller fans due V 

 to the large ratio of fan area to vegetative perimeter. V 

 Dunes develop in the location of drift material and continue to build as fej 

 overwash continues to add sediment to the back of the washover in upwind ;» 



