Fences of these materials have certain advantages over the standard slat 

 fence. They are lighter in weight, less bulky, generally easier to handle, 

 and may be more durable. Installation is similar to that of slat fence. 

 The fabric is fastened to posts for support. These are usually 5.2- 

 centimeter by 10.4-centimeter by 1.8-meter wood, set 3 meters apart, 60 

 centimeters in the ground with diagonal 2.5- by 15-centimeter braces at 

 9- to 12-meter intervals. 



Fences made of several synthetics and covering a 40- to 86-percent 

 range of porosities, have been compared with the 50-percent porous slat 

 fence (Savage and Woodhouse, 1968). The results indicate that the fabric 

 fence is effective in trapping sand provided the porosity is within a 

 suitable range and the fence is adequately supported. 



The critical importance of porosity is shown in Figure 10. Very 

 little sand was trapped at porosities of 80 percent or above and the 

 voliime of sand trapped increased in an essentially linear fashion to the 

 highest density tested. Unfortimately, the range of fabric densities did 

 not reach the point beyond which increasing density appeared to decrease 

 effectiveness. However, in light of what is known generally regarding 

 the effect of porosity (Phillips, 1975), the maximum density tested 

 (60-percent closed, 40-percent open) probably approached that limit. 



Although fabric fences have some obvious advantages, mainly of conven- 

 ience, and when properly installed with suitable porosities, appear to be 

 roughly equal to slat fence, there are certain problems. All fabrics 

 tested had a pronounced tendency to sag (between posts) under sand accu- 

 mulation (Savage and Woodhouse, 1968). The sand appeared to adhere to 

 the fabric to a greater degree than to slats and as the sand settled, it 

 tended to take the fence down with it, greatly reducing the effective 

 height. This was successfully corrected by attaching the top of the fab- 

 ric to a tautly stretched wire which was in turn stapled to the posts. 

 This greatly improves functioning of the fabric but adds to both the 

 material and labor costs. Fabric fences are competitive with slat fences 

 in purchase price, but their installation costs are slightly higher. 



Fabric fences have another disadvantage which can become serious in 

 populated areas. They are susceptible to vandalism because of their net- 

 like appearance. This aspect definitely limits their usefulness in areas 

 frequented by the public, unless the fences can be effectively protected. 



b. Methods . Field tests of dune building with sand fences have 

 been conducted at Cape Cod, Massachusetts, Core Banks, North Carolina, 

 and Padre Island, Texas. The results of field and wind tunnel studies 

 by Bagnold (1941) and Manohar and Bruun (1970) have been reviewed and 

 summarized (Phillips, 1975). The following are guidelines and sugges- 

 tions based on these works plus some years of observation of sand fences 

 operating under a variety of conditions. 



(1) A fence porosity of about 40 to 50 percent appears to 

 be the most efficient in catching sand. 



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