germinate at the upper edge of the intertidal zone. During the 3 years of 

 this study, Spavtina seedlings were not found on recent washovers but were 

 present in other upper-intertidal areas protected from overwash. 



3. Barrier Migration Model. 



Overwash and the vegetative response tc overwash have played important 

 roles in tha landward migration of Nauset Spit. Washovers along Nauset can bi 

 divided into two types based on their si^e and influence in barrier migration: 

 washover fans and flats. Washover fans form as breaches through the dune 

 line and result in small amounts of sand deposition to the lee of the dune 

 field (Fig. 132). Along the Nauset Spit system, these deposits are frequently 

 placed on salt-marsh vegetation; recovery from burial occurs only at the edges 

 of the washover where deposits are less than 34 centimeters for Spavtina 

 patens and 23 centimeters for Spavtina alt evni flora, tost marsh plants do not 

 recover from burial, leaving the washover fan barren and subject to wind 

 deflation. At Nauset Spit, the prevailing west winds carry washover sand to 

 the beach and to the back dunes adjacent to the washover throat (Fig. 132, c). 



The percentage of sand returned to the beach and lost from the system 

 depends on the general form of the washover, prevailing wind directions, and 

 the width of the washover throat. At the washover studied in detail along 

 Nauset Spit-Eastham, 52 percent of the sediment deflated from the fan was 

 added to the landward margin of the barrier dunes and 22 percent was returned 

 to the ocean beach. Although 62 percent of the original deposit was redis- 

 tributed by winds and tides, the entire washover remained above the general 

 elevation range of salt-marsh species. The eventual plant communities that 

 form on small washover fans are dependent on the elevation of the deflated 

 surface as it is stabilized by vegetation. 



Because the Nauset barrier system is oriented north to south and prevail- 

 ing sand-transporting winds are from tha west, there is little opportunity for 

 dune-building on small washover fans since there is only a limited amount of 

 sand in upwind positions. Low dunes may form on a washover fan, hut washover 

 sediment is reduced as dunes coalesce acrobP the washover thro<.-t, and new 

 dunes on washovers rapidly reach a maximum height and begin to deflate. After 

 several years (5 to 7), a small washover fan appears as a crescent-shaped rise 

 on the salt marsh adjacent to the dune line. The dune line may have migrated 

 slightly landward as sand deflated from the washover and accumulated in back- 

 bar-ier dunes (Fig. 132, d). Tine washover itself may he colonized by supra- 

 tidal vegetation, which has built very low dunes. Otherwise, these dunt3 may 

 have deflated and Anrnophila bveviligulata-domiv.^zed vegetation may be out- 

 coicpeted by species adapted to periodic saltwater flooding, such as Spavtina 

 patens var. rnonogyna and Agvopyvon punge.m. The net result of snall-scale 

 overwash is that the dune line is displaced slightly landward. 



In the second case, large-scale washovers play a very important role in 

 barrier migration (Fig. 133). Prior to overwash, the barrier beach may con- 

 sist of a continuous 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 

 overtopped by overwash surges. These surges erode an increasingly wide chan- 

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

 are entirely flattened (Fig. 133, c). During overwash, large volumes of sand 

 may be carried from the beach and dune to the back barrier. Some of this 

 sediment may be transported into the bay, resulting in landward extension of 

 the barrier unit. 



218 



