VI. ROLE OF FOREDUNES IN SHORE PROCESSES 



1. Background . 



The cross section of a barrier island shaped solely by marine hydraulic 

 forces has three distinct subaerial features: beach, crest of island, and 

 deflation plain (see Fig. 4-41). The dimensions and shape of the beach change 

 in response to varying wave and tidal conditions (Section V,2,d), but usually 

 the beach face slopes upward to the island crest — the highest point on the 

 barrier island cross section. From the island crest, the back of the island 

 slopes gently across the deflation plain to the edge of the lagoon separating 

 the barrier island from the mainland. These three features are usually 

 present on duneless barrier island cross sections; however, their dimensions 

 may vary. 



Island crest elevation is determined by the nature of the sand forming the 

 beach and by the waves and water levels of the ocean. The beach and waves 

 interact to determine the elevation of the limit of wave runup — the primary 

 factor in determining island crest elevation. Normally the island crest 

 elevation is almost constant over long sections of beach. However, duneless 

 barrier island crest elevations vary with geographical area. For example, the 

 crest elevation typical of Core Banks, North Carolina, is about +2 meters (+6 

 feet) MSL; +1.3 meters (+4 feet) MSL is typical for Padre Island, Texas; +3.3 

 meters (+11 feet) MSL is typical for Nauset Beach, Massachusetts. 



Landward of the upper limit of wave uprush or berm crest are the backshore 

 and the deflation plain. This area is shaped by the wind and, infrequently, 

 by the flow of water down the plain when the island crest is overtopped by 

 waves (e.g., Godfrey and Godfrey, 1972). Obstructions which trap wind- 

 transported sand cause the formation of dunes in this area (see discussion in 

 Ch. 6, Sand Dunes). Beachgrasses which trap wind-transported sand from the 

 beach and the deflation plain are the major agent in creating and maintaining 

 foredunes. 



2. Role of Foredunes . 



Foredunes, the line of dunes just behind a beach, have two primary 

 functions in shore processes. First, they prevent overtopping of the island 

 during some abnormal sea conditions. Second, they serve as a reservoir for 

 beach sand. 



a. Prevention of Overtopping . By preventing water from overtopping, 

 foredunes prevent wave and water damage to installations landward of the 

 dune. They also block the water transport of sand from the beach area to the 

 back of the island and the flow (overwash) of overtopping sea water. 



Large reductions in water overtopping are effected by small increases in 

 foredune crest elevations. For example, the hypothetical 1.3-meter (4-foot) 

 dune shown in Figure 4-41 raises the maximum island elevation about 1 meter (3 

 feet) to an elevation of 2 meters (6 feet). On this beach of Padre Island, 

 Texas, the water levels and wave runup maintain an island crest elevation of 

 +1.3 meters (+4 feet) MSL (about 0.6 meter (2 feet) above MHW). This would 

 imply that the limit of wave runup in this area is 0.7 meter (2 feet) (the 

 island crest elevation of +1.3 meters (+4 feet) minus the MHW of 0.6 meter (2 



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