Massachusetts; Core Banks, North Carolina; and Padre Island, Texas. Con- 

 clusions and recommendations based upon these studies are: 



(a) Fencing with a porosity (ratio of area of open space to total 

 projected area) of about 50 percent should be used. (Savage and Woodhouse, 

 1969.) Open and closed areas should be smaller than 2 inches in width. 



(b) Only straight fence alignment is recommended. (See Figure 

 6-30.) Fence configurations with side spurs or a zigzag alignment do not 

 increase the trapping effectiveness enough to be economical. (Savage, 1962.) 



(c) Placement of the fence at the proper distance shoreward of the 

 berm crest may be critical. The fence must be far enough back from the 

 berm crest to be away from frequent wave attack. Efforts have been most 

 successful when the selected fence line coincided with the natural vegeta- 

 tion line or foredune line prevalent in the area. This distance is usually 

 greater than 200 feet shoreward of the berm crest. 



(d) The fence should parallel the shoreline. It need not be per- 

 pendicular to the prevailing wind direction. The fence will fill if con- 

 structed with some angularity to sand-transporting winds. 



(e) If sand moves on the beach, sand fencing with 50 percent poros- 

 ity will usually fill to capacity within 1 year. (Savage and Woodhouse, 

 1969.) The dune will be about as high as the fence. The dune slopes will 

 range from about 1 on 4 to 1 on 7, depending on grain size and wind velocity. 



(f) More than one lift of fence can be filled within a year, if 

 the fill rate is closely observed. The rate of fill is not constant and 

 varies with local conditions. A fence may nearly fill during a short 

 period of high wind velocities. If the next lift of fence is erected 

 shortly after the filling of the existing fence, it also may be filled 

 within the same season. 



(g) Installation time and positioning of the succeeding lifts of 

 sand fence depend on the objectives, i.e. to increase the dune height, the 

 width of the dune base, or both. Dune height is increased by erecting 

 succeeding lifts of fence at the crest of the existing dune. (See Figure 

 6-31.) The effective height increase gained by positioning the fence at 

 the crest is nearly 3 feet with a 4-foot fence as shown by the second lift 

 in Figure 6-31. Note that the effective height gained from the third lift 

 is much less. The third lift was initially erected at the crest of the 

 dune shown by the 24-month profile. Dry, high winds occurred for several 

 days following installation. Turbulence at the dune crest eroded so much 

 sand from the base of the fence that support posts had to be reset to keep 

 the fence upright. Thus the effective height was reduced to 1 foot. 



Dune width is increased by erecting succeeding lifts of fence parallel 

 to and away from the existing fence. The second lift is placed shoreward 

 or landward of the existing fence depending on the direction the dune is to 

 be constructed. The offset distance between fence lines should be about 



6-39 



