(a) Suspension : Small or light grains are lifted into the airstream 

 and are blown appreciable distances. 



(b) Saltation ; Sand particles are carried by the wind in a series of 

 short jumps along the beach surface. 



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

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

 particles . 



These natural transportation methods effectively sort the original beach 

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

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

 beach. Although most sand particles move by saltation, surface creep may 

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



2 . Dune F ormation . 



Dune building begins when an obstruction on the beach lowers wind velocity 

 causing sand grains to deposit and accumulate. As the dune builds, it becomes 

 a major obstacle to the landward movement of windblown sand. In this manner, 

 the dune functions to conserve sand in close proximity to the beach system. 

 Foredunes are often created and maintained by the action of the beach grasses, 

 which trap and stabilize sand blown from the beach. 



Foredunes may be destroyed by the waves and high water levels associated 

 with severe storms or by beachgrass elimination (induced by drought, disease, 

 or overgrazing), which thereby permits local "blowouts." Foredune management 

 has two divisions — stabilization and maintenance of naturally occurring dunes, 

 and the creation and stabilization of protective dunes where they do not 

 already exist. Although dunes can be built by use of structures such as sand 

 fences, another effective procedure is to create a stabilized dune through the 

 use of vegetation. Current dune construction methodology is given by Knutson 

 (1977) and Woodhouse (1978). 



3 . Dune Cons t ruction Using Sand Fencing . 



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

 pickets driven into the sand, have been used to construct a foredune 

 (McLaughlin and Brown, 1942; Blumenthal, 1965; Jagschitz and Bell, 1966a; 

 Gage, 1970). Relatively inexpensive, readily available slat-type snow fenc- 

 ing (Fig. 6-26) is used almost exclusively in artificial, nonvegetative dune 

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

 (Savage and Woodhouse, 1969). Satisfactory, but short-term, results have been 

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



Field tests of dune building with sand fences under a variety of condi- 

 tions have been conducted at Cape Cod, Massachusetts, Core Banks, North 

 Carolina, and Padre Island, Texas. The following are guidelines and sugges- 

 tions based on these tests and observations recorded over the years: 



(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 5 



6-38 



