46 



ENGINEERING FOR LAND DRAINAGE. 



tard the flow. But in locating mains in this way note 

 that unless the slough slopes but little toward the 

 centre Hne, one centre main of sufficient capacity gives 

 better results. There are cases where this system may 

 be followed advantageously both with respect to cost 

 and efficiency, while in others it would prove expensive 

 and faulty. See Fig. 7. 



Single-line System. — By this is meant the plan of 

 laying parallel lines in the direction of the greatest 

 slope, giving each a sepafate outlet into an open ditch 

 as shown in Fig. 8. This is adapted to large areas 

 which have very slight fall and where it is consequently 

 necessary to use every inch of it in grade and depth of 



Fig. 8. — Single-line System. 



drains, and also to avoid the use of excessively large 

 tile for mains which under such circumstances would 

 be required if a system involving mains and laterals 

 were used. A caution which should be emphasized in 

 locating drains on level land where grades must neces- 

 sarily be very light is to avoid overcharging mains, a 

 thing that is frequently done because the difference in 

 grades is so often overlooked. 



These different systems, or modifications of them, 

 may be used on different tracts, and in various localities 



