averaged about 400,000 cubic meters per kilometer (900,000 cubic yards per 

 mile) of beach (Meyer and Chester, 1977). Thus, vdthin a few years, foredunes 

 can trap and store a volume of sand equivalent to the volumes eroded from 

 beaches during storms of moderate intensity. 



c. Long-Term Effects . Dolan (1972 and 1973) advances the concept that a 

 massive, unbroken foredune line restricts the landward edge of the surf zone 

 during storms, causing narrower beaches and thus increased turbulence in the 

 surf zone. The increased turbulence causes higher sand grain attrition and 

 winnowing rates and leads to accelerated losses of fine sand, an erosive pro- 

 cess that may be detrimental to the long-range stability of barrier islands. 

 However, as discussed in Section V,2,a, the effects of sediment size are 

 usually of secondary importance in littoral transport processes — processes 

 which are important in barrier island stability. In addition, geographical 

 location is probably more important in determining beach sand size than dune 

 effects, since both fine and coarse sand beaches front major foredune systems 

 in different geographical locations. For example, fine sand beaches front a 

 massive foredune system on Mustang Island, Texas, and coarse sand beaches 

 front dunes on the Cape Cod spits. 



Godfrey and Godfrey (1972) discuss the effect of a foredune system on the 

 long— term stability of the barrier islands of the Cape Hatteras and Cape 

 Lookout National Seashores, North Carolina. Important implicit assumptions of 

 the discussion are that no new supply or inadequate new supplies of sand are 

 available to the barrier island system and that rising sea level is, in 

 effect, creating a sand deficit by drowning some of the available island 

 volume. The point of the geomorphic discussion is that under such conditions 

 the islands must migrate landward to survive. A process called "oceanic 

 overwash" (the washing of sand from low foredunes or from the beach over the 

 island crest onto the deflation plain by overtopping waves) is described as an 

 important process in the landward migration of the islands. Since a foredune 

 system blocks overtopping and prevents oceanic overwash, foredunes are viewed 

 as a threat to barrier island stability. 



If the implicit assumptions and a geologic time frame are accepted, the 

 geomorphic concept presented has convincing logic and probably has merit. 

 However, the assumptions are not valid on all barrier islands or at all 

 locations in most barrier islands or at all locations in most barrier island 

 systems. Too, most coastal engineering projects are based on a useful life of 

 100 years or less. In such a short period, geologic processes, such as sea- 

 level rise, have a minor effect in comparison with the rapid changes caused by 

 wind and waves. Therefore, the island crest elevation and foredune system 

 will maintain their elevation relative to the mean water level on stable or 

 accreting shores over the life of most projects. On eroding shores, the 

 foredunes will eventually be eroded and overwash will result in shorevard 

 migration of the island profile; sand burial and wave and water damage will 

 occur behind the original duneline. Therefore, planning for and evaluation of 

 the probable success of a foredune system must consider the general level of 

 the area of the deflation plain to be protected, the rate of sea level rise, 

 and the rate of beach recession. 



4-112 



