ii21» Wind blown sand accumulates in several distinctive ways and 

 characteristic forms. Any appreciable accrual is loosely termed a dune. 

 Strictly, according to the most generally accepted definition, a true 

 dune is one capable of moving freely as a unit and which can exist in- 

 dependently of any fixed surface structure. The loose definition is 

 used herein^ Sand accumulations caused directly by fixed obstructions 

 in the path of the wind, unlike true dunes, are dependent for their con- 

 tinued existence on the presence of the obstacle which causes and fixes 

 them so they cannot move away. 



14.22, When an object is placed so as to interrupt the wind flow, 

 the air path in front and behind the obstacle is divided into two parts 

 by a somewhat ill-defined surface of discontinuity. Outside this surface 

 the air stream flows smoothly by; but the volume within the vjind shadow 

 of the obstacle is filled with swirls and vortices of air whose average 

 forward velocity is less than that of the air stream outside. Downwind 

 from the obstacle, the forward velocity of the air inside the shadow 

 gradually increases and the shadow fades away to merge eventually with 

 the general flow of the wind. The sand grains which strike the obstacle 

 rebound from it and come to rest in the relatively stagnant air in front. 

 When the resulting heap has grown up so that its slopes stand at the 

 limiting angle of repose (about 3k°J all additional material slides down 

 the slope to join tlie sand stream passing along the side of the ob- 

 stacle, 



14.23, Dune Building , - Dunes can be caused to form by the use of 

 sand fenc3S or pentration oil, 



I1.2I1., Sand fences can be constructed in movable sections or made of 

 individual pickets driven into the sand» The width and length of the 

 pickets may vary but the spacing of the pickets is important, with no 

 more than 50 per cent cf the surface covered. For best results the 

 space between the pickets should equal the width of the pickets. In 

 order to widen the crest of the dune, andfecilitate establishment of 

 vegetation, two lines of fence about 30 feet apart should be used. The 

 use of a single fence tends to make dunes vjith a sharp crest unfavorable 

 to establishment of cover. As the dune builds up on the fence, the 

 fence can be raised until the desired height is attained. The b elt can 

 be broadened by shifting the second fence windward as the dune grows, 

 or by the addition of a third fence. 



U25<. The sand fences should be constructed normal to the pre- 

 vailing wind direction unless it is desired to cause the sand to move 

 longitudinally along the fence to fill in low gaps. In this case the 

 fences may be built slightly quartering to the general shifting direction 

 of the sand movement or panelling may be resorted to, 



I1.26, Panelling, to divert or stop sand, consists of curved or flat 

 barriers in a single slant or in S/ arrangement. As the line of the slant 

 or \/ approaches the normal to the wind direction, there is more and more 



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