and offshore deposits are normally transported back to the beach by swells 

 after the storm. Onshore winds transport the sand from the beach toward the 

 foredune area, and another natural cycle of dune building proceeds. This dune 

 building, however, is generally at a very slow rate unless supplemented by 

 fences or vegetation. 



2. Positioning . 



The location of a barrier dune can have a major influence on its durabil- 

 ity and function. Well-vegetated dunes are effective against storm surge and 

 can withstand moderate degrees of overtopping, but they are highly vulnerable 

 to erosion if the beach berm is either overtopped or recedes due to persistent 

 wave attack. In the positioning of a new barrier dune, an allowance should be 

 made for the normal shoreline fluctuations that are characteristic of the 

 site. Serious problems of dune maintenance may often be avoided or minimized 

 by positioning the foredune far enough back from the high water line to allow 

 a reasonable amount of seasonal fluctuations. A minimum distance of 200 

 meters (650 feet) is suggested between the toe of the dune (sand fence) and 

 the high water line (Blumenthal, 1964). 



The process of dune growth is an important consideration in locating a 

 barrier dune. Fully vegetated dunes expand only toward the sand source, which 

 is usually the beach, and a relatively narrow strip of vegetation will, in 

 most cases, stop all wind-transported sand. This means that, where possible, 

 an allowance should be made for the seaward expansion of the dune with time. 

 Also, when two dunes are desired, the first must be developed landward and 

 have enough space left between it and the sea for the second or frontal dune. 



On many low-lying coasts the crest of the storm berm is the highest point 

 in the beach-dune area with the surface sloping back from the berm crest. 

 This places the base of a new barrier dune below the elevation of the storm 

 berm, making it more susceptible to overtopping during the early stages. It 

 may also encourage ponding of the water overtopping the storm berm, resulting 

 in water pressure, salt buildup, and destruction of vegetation along the toe 

 of the dune. Where this problem exists, the dune location will often repre- 

 sent a compromise. 



V. SAND BYPASSING 



1. General. 



An inlet is a short, narrow waterway connecting the sea or major lake with 

 interior waters. Inlets, which are either natural or improved to meet naviga- 

 tion requirements, interrupt sediment transport along the shore. A natural 

 inlet has a well-defined bar formation on its seaward side. A part of the 

 sand transported alongshore ordinarily moves across the inlet by way of this 

 outer bar — natural sand bypassing. However, the supply reaching the downdrift 

 shore is usually intermittent rather than regular, and the downdrift shore is 

 usually unstable for a considerable distance. If the tidal flow through the 

 inlet into the interior body of water is strong, part of the material moving 

 alongshore is carried into and permanently stored in the interior body of 

 water as a middle-ground shoal, reducing the supply available to nourish down- 

 drift shores. The outer bar normally migrates with a migrating inlet, but the 



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