50 



species, and foot and vehicular traffic can alter the morphologic and ecologic 

 conditions of dunes. 



Because dunes help protect inland areas from storm damage, many 

 communities promote dune stabilization and protection. For dune 

 construction, beach nourishment is often practiced in conjunction with mea- 

 sures to stabilize and trap sediments, (e.g., fencing, vegetative planting, or the 

 placing of other obstacles). These measures, as well as the construction of 

 walkways to reduce trampling, are also used for dune stabilization and 

 protection. 



Lithified or fossil dunes occur in some places where there are calcium 

 carbonate particles in the dune material. Climatic factors promote leaching 

 and reprecipitation of calcium carbonate, which may allow dunes to lithify 

 under favorable conditions. Modern coastal dunes that have become lithified 

 occur largely in tropical climates where there is a high level of calcium 

 carbonate in the coastal sediments and alternating wet and dry periods. 



Back Barrier and Lagoons 



During storms and high water levels, the sea may breach a dune ridge, 

 bringing sediment from the coastal area further inland. Some scientists call 

 the phenomenon overwash, and the product is called washover (Carter 1988). 

 Overwashing may result in a distinctive set of breach-throat-fan landforms, 

 forming a washover channel through the dune and a washover fan. This 

 washover fan develops on the landward side of a barrier and spreads over 

 parts of the backshore, back barrier flat, back barrier marsh, or shallow 

 lagoon (Figure 20). Storm frequency, overwash volume, dune susceptibility, 

 and barrier height are important factors in the development of washovers. 

 Low-lying or low-profile barriers in areas of frequent, severe storms have 

 numerous washovers. Storm surge ebb-residual flow, which occurs more 

 infrequently than overwash, may result in seaward currents and landforms. 



Washover channels are distinguished from inlets in that their elevation is 

 above mean sea level. The patterns of breaches may be related to a combina- 

 tion of marine or non-marine processes. Washover deposits provide an 

 important mechanism by which marine transgression may take place, not just 

 for sand-dominated, but also for gravel-dominated barriers (Carter and Orford 

 1984). Overwash features are particularly common in the Gulf of Mexico 

 coast and eastern seaboard of the United States. These areas experience 

 severe extratropical and tropical cyclones with some frequency. 



Coastal lagoons are shallow water bodies, often running parallel to the 

 coast and cormecting to the open sea with an inlet. Lagoons differ from tidal 

 flats in that they remain water-filled even at low tide and are separated from 

 the open sea by sand bars, barrier islands, or reefs. Lagoons have been 



Chapter 3 Variable Coastal Features 



