156 SYLVICULTURE IX THE TROPICS r.n 



When cutting drains it is useful to remember not to 

 allow two drains debouching from two sides into a 

 larger drain to have their opening opposite to one 

 another, as the opposite currents of water on meeting 

 may be checked and proper drainage may not be 

 effected. This is particularly the case when there is 

 much water coming from the drains, and especially so 

 when the fall of the drain is slight. As an example of 

 this I may quote the Blue Nile, which joins the White 

 Nile between Khartoum and Omdurman. When the 

 former comes down in flood it holds up the waters of 

 the latter to such an extent that its effects are felt up 

 the White Nile for two or three hundred miles. 



In making drains on swampy soil it may become 

 desirable to consider the effect of this drainage on 



1 2 3 4 5 6 



Fig. 49. 



forest vegetation growing on land above this area and 

 outside of the drainage scheme. The drainage may 

 have the effect of depriving the trees of the moisture 

 which they have become used to receiving, and of 

 injuring them. 1 



The depth to which the soil will have to be drained 

 will depend on the species which will be put in ; as a 

 rule it will not be much more or much less than 1 metre 

 (3 ft.). 



As regards the distance to be left between the 

 collecting drains, the following method may be adopted 

 for gauging it : 2 



The soil having to be drained to a depth indicated 

 by the line bb in Fig. 49, an experimental drain a is cut, 

 and, in a line at right angles to its course, a number of 



1 Sohlich, Man. of Forestry, vol. ii. 



Beyer, Waldbau, p. 77. 



