154 SYLVICULTURE IN THE TROPICS pt.h 



very slightly greater than that point. The drains 

 may be made either open or covered. For large works 

 covered drains may be used, as they are not apt to get 

 choked or silted up in the same degree as open drains, 

 into which no end of silt and refuse collects, and which 

 must be more constantly looked after. But the cost 

 of making them is much greater than that of open 

 drains, and for ordinary forest works the latter will 

 usually suffice. 



The drains are usually cut with sloping sides, the 

 gradient of the slope varying according to the nature 

 of the soil. In sandy soils the slope may be 1 in 3, 

 and it may be increased according to the hardness and 

 tenacious nature of the soil until the sides are almost 

 perpendicular. On tea estates in Ceylon the sides are 

 made perpendicular, but the soil is very tenacious, and 

 even there it would be preferable to slope them a little. 



In a small area it may be sufficient to have only 

 one size of collecting drains, but where operations cover 

 more ground there may be two or even three classes 

 of drains. These may be divided as follows : J 



The collecting drains, which take up the water 

 directly from the layer of soil to be drained ; the 

 secondary or receiving drains, which carry away the 

 waters passed into them by the collecting drains ; and 

 the main or outflow drains, which are only required 

 when more than one small basin is beino- drained. The 



O 



secondary drains then discharge themselves into one 

 channel which carries their accumulated volume of 

 water away. In the plan shown in Fig. 48 the collect- 

 ing drains are indicated at a, while the secondary 

 drains are denoted by b, and the main drains by c. 

 This discharges all the water collected in the basin 

 into a river which carries it away. 



The main and secondary drains are intended to 

 carry away the water as quickly as possible, and should 

 be made at right angles to the contour lines, i.e. straight 

 down the slope, unless the soil is too friable or the slope 



1 Fernandez, Indian Sylviculture, p. 22!. 



