(a) Suspension . Small or light grains are lifted into the air 

 stream and carried appreciable distances; 



(b) Saltation . Individual particles are carried by the vd.nd in a 

 series of short jumps along the beach surface; and 



(c) Surface Creep . Particles are rolled or bounced along the 

 ground as a result of wind forces or impact of descending saltating 

 particles. 



These three ways of transportation effectively sort the original beach 

 material. Smaller particles are removed from the beach and dune area. 

 Larger particles remain on the beach. Medium-sized particles form the 

 foredunes. Although most sand particles move by saltation, surface creep 

 may account for 20 to 25 percent of the sand moved. (Bagnold, 1942.) 



6.42 DUNE FORMATION 



Dxine building begins when an obstruction causes deposition of sand 

 grains. As the dune builds, the seaward slope may become so steep that 

 saltating or creeping particles come to rest there. With higher wind 

 velocities, particles move up the face of the dune, settle in the lee of 

 the dune, and cause the dune to migrate in the direction the wind is blow- 

 ing. Foredunes are often created and maintained by the action of the 

 beachgrasses in trapping and stabilizing sand blown off the beach. 



Foredunes may be destroyed by the waves and tides of severe storms, 

 by drought or disease destroying the beachgrasses, or by overgrazing that 

 reduces the vegetative cover permitting local "blowouts." Foredune manage- 

 ment has two divisions--stabilization and maintenance of naturally occur- 

 ring dunes, and creation and stabilization of protective dunes where they 

 do not exist. Although dunes can be constructed by mechanical structures, 

 a preferred procedure is to create a stabilized, dune through the use of 

 vegetation. 



6.43 DUNE CONSTRUCTION— SAND FENCE 



Various mechanical methods, such as fencing made of brush or by 

 driving individual pickets into the sand, have been used to construct a 

 foredune. (Blumenthal, 1965, Jagschitz and Bell, 1966a, McLaughlin and 

 Brown, 1942, Gage, 1970.) Relatively inexpensive, readily available slat- 

 type snow fencing (Figure 6-29) is used almost exclusively in artificial 

 dune construction. Plastic fabrics have been investigated for use as sand 

 fences. (Savage and Woodhouse, 1969.) Although some preliminary results 

 were encouraging, these fabrics have not been tested sufficiently to pro- 

 vide an adequate evaluation. Satisfactory, but short-term results have 

 been obtained with jute-mesh fabric. (Barr, 1966.) 



Studies to develop techniques for constructing dunes of a desired size 

 and profile through use of sand fences have been conducted at Cape Cod, 



6-37 



