154 



OLDHAM: SANDHILLS OF CLIFTON NEAR .KARACHI. 



The effect of an isolated, flexible, permeable obstacle, such as a 

 clump of grass, is different ; here the resistance to the wind is much 

 less, there is no heap formed in front, nor any great scour. On 

 the other hand, the wind which filters through the grass stems has its 

 velocity reduced and sand is deposited behind the grass clump ; as 

 shown in fig. 9 : — 





Fig. 9. — Diagram to illustrate the effect of a flexible permeable obstacle. 



The effect of a rigid and permeable obstacle, such as an open 

 fence, is a combination of the two ; in front no eddy is formed, there 

 is no scour, and the check in the velocity of the wind causes a deposit 

 of sand both in front of and behind the obstacle as indicated in 

 fig. 10: — 



Fig. 10.- Diagram to illustrate the effect of a rigid permeable obstacle. 



From a consideration of these three cases it will be seen that 

 there is a certain degree of permeability of the open fence which 

 will be most efficient, and it has been found that when the in- 

 tention is to cause an accumulation of sand, this is best attained 

 if the area blocked is about equal to that of the open spaces. 

 The materials generally used are twigs and branches, as small 

 as is compatible with sufficient strength to withstand the wind. 

 Where the fence has a height of 4 feet or so a supporting framework is 



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