feet). Assuming the wave runup to be the same for all vater levels, the 1.3- 

 meter (4-foot) dune would prevent significant overtopping at water levels up 

 to 1.3 meters (4 feet) MSL (the 2-meter (6-foot) effective island height at 

 the dune crest minus 0.7 meter (2 feet) for wave runup). This water level 

 occurs on the average once each 5 years along this section of coast (see 

 Figure 4-42). Thus, even a low dune, which can be built with vegetation and 

 sand fences in this area in 1 year (Woodard et al. 1971) provides considerable 

 protection against wave overtopping (see Ch. 5 and 6). 



Foredunes or other continuous obstructions on barrier islands may cause 

 unacceptable ponding from the land side of the island when the lagoon between 

 the island and mainland is large enough to support the needed wind setup (see 

 Ch. 3, Sec. VIII). There is little danger of flooding from this source if the 

 lagoon is less than 8 kilometers (5 miles) wide. Where the lagoon is wider 

 (especially 16 kilometers (10 miles) or greater) flooding from the lagoon side 

 by wind setup should be investigated before large dune construction projects 

 are undertaken. 



b. Reservoir of Beach Sand . During storms, erosion of the beach occurs 



and the shoreline recedes. If the storm is severe, waves attack and erode the 



foredunes and supply sand to the beach; in later erosion stages, sand is 



supplied to the back of the island by overwash (Godfrey and Godfrey, 1972). 



Volumes of sand eroded from beaches during storms have been estimated in 

 recent beach investigations. Everts (1973) reported on two storms during 

 February 1972 which affected Jones Beach, New York, The first storm eroded an 

 average of 12,800 cubic meters per kilometer (27,000 cubic yards per mile) 

 above mean sea level for the 14.5-kilometer (9-mile) study area; the second 

 storm (2 weeks later) eroded an average of 16,600 cubic meters per kilometer 

 (35,000 cubic yards per mile) above mean sea level at the same site. Losses 

 at individual profiles ranged up to 57,000 cubic meters per kilometer (120,000 

 cubic yards per mile). Davis (1972) reported a beach erosion rate on Mustang 

 Island, Texas, following Hurricane Fern (September 1971), of 30.8 cubic meters 

 per meter (12.3 cubic yards per foot) of beach for a 460-meter (1,500-foot) 

 stretch of beach (about 31,000 cubic meters per kilometer (65,000 cubic yards 

 per mile) of beach). On Lake Michigan in July 1969, a storm eroded an average 

 of 9 cubic meters per linear meter (3.6 cubic yards per foot) of beach (about 

 13,800 cubic meters per kilometer (29,000 cubic yards per mile) from a 240- 

 meter (800-foot) beach near Stevensville, Michigan (Fox, 1970). Because much 

 of the eroded sand is usually returned to the beach by wave action soon after 

 the storm, these volumes are probably representative of temporary storm 

 losses. Birkemeier (1979) studied beach changes during a December 1977 storm 

 on Long Beach, New Jersey. He found that about one half of the material 

 eroded from the beach during the storm returned to the beach within 2 days 

 (see Sec. V,2,d). 



Volumes equivalent to those eroded during storms have been trapped and 

 stored in foredunes adjacent to the beach. Foredunes constructed along Padre 

 Island, Texas (Dahl et al. 1975), and Ocracoke Island, North Carolina 

 (Woodhouse, Seneca, and Browne, 1976), and Cape Cod, Massachusetts (Knutson, 

 1980), contain 120,000, 80,000, and 60,000 cubic meters of sand per kilometer 

 (275,000, 185,000, and 135,000 cubic yards per mile) of beach, respectively. 

 These volumes accumulated over periods of from 5 to 10 years. Sand volumes 

 trapped during a 30-year period by European beachgrass at Clatsup Spit, Oregon 



4-110 



