Figure 6-27. Snow-type sand fencing filled to capacity, Padre Island, Texas. 



single-fence lifts as each fence fills (Fig. 6-28); or (2) by installing 

 double-fence rows with the individual fences spaced about 4 times the 

 fence height (4h) apart and following these with succeeding double-row 

 lifts as each fills (Fig. 6-29). Single rows of fencing are usually the 

 most cost-effective, particularly at the lower windspeeds, but double 

 fences may trap sand faster at the higher windspeeds. 



(g) Dune height is increased most effectively by positioning the 

 succeeding lifts near the crest of an existing dune (see Fig. 6-30). 

 However, under this system, the effective height of succeeding fences 

 decreases and difficulties may arise in supporting the fence nearest the 

 dune crest as the dune becomes higher and steeper. 



(h) Dune width is increased by installing succeeding lifts parallel 

 to and about 4h away from the existing fence (Fig. 6-31). The dune may 

 be widened either landward or seaward in this way if the dune is 

 unvegetated. 



(i) Accumulation of sand by fences is not constant and varies widely 

 with the location, the season of the year, and from year to year. Fences 

 may remain empty for months following installation, only to fill within a 

 few days by a single period of high winds. In order to take full 

 advantage of the available sand, fences must be observed regularly, 

 repaired if necessary, and new fences installed as existing fences fill. 

 Usually where appreciable sand is moving, a single, 1.2-meter fence will 

 fill within 1 year. 



6-40 



