Location of a barrier dune can have a major influence on its dura- 

 bility and utility. Well-vegetated dunes are effective against storm 

 tides and are capable of withstanding moderate degrees of overtopping, 

 but they are highly vulnerable to undermining through beach recession 

 and persistent wave attack. In the placement of a new barrier dune, an 

 allowance should be made for the normal shoreline fluctuations character- 

 istic of the site. Serious problems of dune maintenance may often be 

 avoided or minimized by locating the foredune back from mean high water 

 (MHW) far enough to allow for a reasonable amount of seasonal fluctua- 

 tions. Dutch workers (Blumenthal, 1964) suggest that the minimum dis- 

 tance between the toe of the dune (sand fence) and MHW should be 200 

 meters. 



It is also important to consider the nature of dune growth in loca- 

 ting a barrier dune. Fully vegetated dunes expand only toward the sand 

 source, usually the beach, and a relatively narrow strip of vegetation 

 will, in most cases, stop all wind-transported sand. This means that, 

 where possible, allowance should be made for seaward expansion of the 

 dune with time. Also, if two dunes are desired, the first must be 

 developed landward and 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 it. 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 water coming over the storm berm, result- 

 ing in water pressure, salt buildup, and destruction of vegetation along 

 the toe of the dune. Where this problem exists, dune location must always 

 represent a compromise. 



2. Fences . 



Sand fences slow sand movement by reducing wind velocity in their 

 immediate vicinity (Phillips, 1975). These fences are widely used to 

 accumulate blowing sand. When properly designed and installed they can 

 be very efficient sand-trapping devices. Savage (1963) obtained accumu- 

 lations of 4.5 to 6 cubic meters of sand per meter of beach for single- 

 slat fences and up to 8.5 cubic meters of sand per meter for double fence 

 sections within a 7- to 8-month period. In a multiple fence experiment 

 (Savage and Woodhouse, 1968), 39 cubic meters of sand per meter was 

 trapped in 3 years using six fences starting with two 1.2-meter fences, 

 5 meters apart and adding fences in pairs as the earlier installations 

 filled. 



a. Types . Almost any material that forms a porous barrier to the 

 wind may be used to construct a sand fence. Porosity is essential — a 

 solid barrier will cause turbulence and scouring and may result in the 

 loss rather than the accumulation of sand. 



Earlier research involved driving pickets, composed of wooden sticks, 

 boards, bamboo, reeds, etc., into the sand along lines to form "fences" 



15 



