( 3 ) Stabilization of Washovers . 



(a) Washover T hr oats. Stabilization of washover throats is 

 dependent on the size of the throat, the frequency of overwash in the area, 

 and the orientation of the barrier. Small washover throats may quickly close 

 if overwash pressure is reraoved. Many washovers, like site 2 in 1976, begin 

 as meandering channels through the dune line. Dune vegetation can grow 

 rapidly into the channel trapping sand and repairing the breach. On Nauset 

 Spit, winds from the northwest and southwest often build wind-shadow dunes, 

 which help to close these breaches. Winds from the northeast and southeast 

 may be deflected by the dune line, so that sand is blown along the shoreline 

 and deposited in embayments and breaches, maintaining a smooth seaward contour 

 to the dune line. 



Large features remain open for longer periods of time. If a throat is 

 broad and oriented parallel to prevailing winds, a channel may be eroded by 

 the wind to an elevation below the threshold level for washover. An area may 

 remain susceptible to washover for many years, enlarging until peripheral 

 dunes are completely leveled. 



A washover throat may deflate to the proximity of the water table, result- 

 ing in wetter surface sands which are not easily moved by the wind. Large- 

 size material, such as cobbles and gravel, along with sand, are deposited in 

 the throat by washover surges; smaller material is carried farther onto the 

 washover tan. With the deflation of each successive washover, the lag surface 

 becomes more concentrated. At site 1 throat in 1977, a pavement of cobbles 

 and heavy minerals indicated considerable deflation. Once the surface has 

 stabilized, plant colonization can begin. 



(b) Washover Fa ns. Small washovers are cooanon along Nauset Spit 

 and are often rapidly stabilized unless continually affected by subsequent 

 overwashes or human impact, such as pedestrian trampling or off-road vehicle 

 trespassing. Ln sonse cases, a small fan will be enlarged by future over- 

 washes; this is particularly common when the barrier dune line has a low 

 elevation profile and is very narrow. 



Following overwash, the prevailing offshore winds deflate these fans. 

 Much of the washover sand is blown onto the beach face or deposited on the 

 back dunee adjacent to the fan. Lag layers of larger size material is&y fona, 

 but their occurrence is somewhat ainitalzed since material of this size range 

 is not frequently transported beyond the washover throat. 



If the depth of sediment burial is shallow (less than 30 centimeters), 

 salt-marsh plants may recover. Although existing salt-ssarsh plants may be 

 killed by extensive burial, subsequent wind-deflation Esy lower the fsn to 

 intertidal elevations favorable for colonisation by high taarsh pleat spades. 

 Recolonization may occur froia seeds or by rhizoaa extension froa adjacent, 

 undisturbed stands of salt-marsh vegetation. 



While small dunes (less than 1 meter high) may develop from drift-line 

 deposits on washovers of this scale, major additions to the dune line are not 

 possible unless the washover is significantly enlarged by subsequent overwash. 

 Sand supply for dune building, available from Che deflation of the washover, 

 is restricted by the small size of the fan. Without sufficient quantities of 

 sand, major new dunes do not form; overwash at this scale plays an insignifi- 

 cant role in overall barrier riynaalcs and landward migration. 



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