Island crest elevation is determined by the nature of the sand form- 

 ing 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 +6 feet MSL; +4 feet MSL is typical for Padre Island, 

 Texas; +11 feet MSL is typical for Nauset Beach, Massachusetts. 



Landward of the upper limit of wave uprush or berm crest are the back- 

 shore and the deflation plain. This area is shaped by the wind, and in- 

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

 overtopped by waves. (e.g., Godfrey, 1972.) Obstructions which trap 

 wind-transported sand cause the formation of dunes in this area. (See 

 discussion in Section 6.4 Sand Dunes.) Beachgrasses which trap wind- 

 transported sand from the beach and the deflation plain are the major 

 agent in creating and maintaining foredunes. 



4.62 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. 



4.621 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 4-foot dune 

 shown in Figure 4-41 raises the maximum island elevation about 3 feet to 

 an elevation of 6 feet. On this beach of Padre Island, Texas, the water 

 levels and wave runup maintain an island crest elevation of +4 feet MSL 

 (about 2 feet above MHW) . This would imply that the limit of wave runup 

 in this area is 2 feet (the island crest elevation of 4 feet minus the 

 MHW of 2 feet). Assuming the wave runup to be the same for all water 

 levels, the 4-foot dune would prevent significant overtopping at water 

 levels up to 4 ft MSL (the 6-foot effective island height at the dune 

 crest minus 2 feet for wave runup). This water level occurs on the aver- 

 age once each 5 years along this section of coast. (See Figure 4-42.) 

 Thus, even a low dune, one which can be built with vegetation and sand 

 fences in this area in 1 year (Woodard et al . , 1971) provides consider- 

 able protection against wave overtopping. (See Section 5.3 and 6.3.) 



Foredunes or other continuous obstructions on barrier islands may 

 cause unacceptable ponding from the land side of the island when the la- 

 goon between the island and mainland is large enough to support the needed 

 wind setup. (See Section 3.8.) There is little danger of flooding from 



4-113 



