So far as is known presently the mechanism of erosion and transport of 

 sand by wind, is analogous in all respects to that by water; the Reynolds 

 numbers of the process in the two media is similar, and observations of the 

 actual movement of sand grains shows the motion to be analogous in air and 

 water. An additional factor not yet investigated to the author's knowledge 

 is that of the role played by water content of the beach in causing a time 

 delay in the erosion-transportation sequence. The differences in character 

 of a given sand or beach mixture when dry or wet have been noted oftenj 

 generally J moist sand has a higher load bearin--' capacity than dry, while wet 

 sand often shows a much lower capacity than dj ,. „ Moist sand appears to ex- 

 hibit more cohesion than the dry, while wet s^.nd appears to be more fluid 

 even than the dry material. Yet the author has observed apparently dry sand 

 blown in considerable quantity from beach areas still wet from a receding 

 tide.. O'Brien and Rindlaub'' -' report that ".hi one occasion, sand was un- 

 pleasantly evident at 8 feet above the beach .:.uring a heavy rainstorm ac- 

 companied by wind with an intensity of 4-8 miles per hour-" They report 

 also an observation that wind of 15 miles per hour velocity moved sand in 

 the moist area below the last high water marko The evaporative effects of 

 the wind seem to be important to the problerri, probably entering chiefly by 

 changing the erodibility characteristics o2 the material, 



Bagnoldv2) has shown that sand movement in air occurs as a sand cloud 

 (sand in suspension) and as surface creep (bedload), in analogy to sand 

 movement by water. Similarly to sand movement in water the motion of the 

 sand grains occurs as rolling, saltation, and suspension. His experiments 

 show that the threshold velocity required to sustain sand movenBnt can be 

 expressed as; 



where CT and P are the densities of the sand grain and air respectively; 

 d is the grain diameter; and k' is the height above th^ surface at 

 which the tbj'eshold velocity occurs, related to the surface roughness 

 (parameter K in the Prandtl turbulent flow law)„ 



Through application of Prandtl 's rough-surface law for turbulent 

 flow and consideration of surface drag relations Bagnold suggests the 

 following expression for relating total sand flow to wind velocity 



Tp (Vh-Vt)^ 



e-cj^ I 



O S 5.75 IoqVk' 



where d is the mean diameter of any sand; D the mean diameter of a so-called 

 standard sand (0,024.cm); Vg the wind velocity measured at a height Z above 

 the sand surface; k' has the value 0.3 cm. for uniform sands; and C is a 

 coefficient varying from 1.5 for uniform grading of sand to 2.8 for sand 

 with a wide range of grain size . 



(1) The Transportation of Sand by YiTind - M. P. O'Brien and B. D. Rindlaub, 

 Civil Engineering, Vol. 6, No. 5$ 'Ma.y 1936. 



(2) The Transport of Sand by Wind, R„ A. Bagnold, The Geographical Journal 

 Vol. LXXIIX, No, 5 5 May 1937 « 



20 



