656 SOILS: PllOPJSRTIES AND MANAGEMENT 



556. Storm channels. — Where large volumes of water 

 must be carried for a short time in addition to the normal 

 flow, a medium-sized tile drain may be combined with an 

 open surface channel for carrying away the flood water. 

 The open channel is located a little to one side of the 

 tile drain so that the latter may not be displaced by pos- 

 sible erosion. The open surface channel is made broad 

 and shallow in order to avoid interference with tillage 

 operations, and if erosion is likely to occur, it may be 

 kept in grass. 



557. Silt basins. — Silt basins are wells in the line 

 of tile drains, for collecting sediment that might other- 

 wise be deposited in the tile. The course of the drain 

 is intercepted and a small well is sunk two or more feet 

 below the bottom of the drain. The well extends to the 

 surface of the ground and has a cover. The inlet drains 

 come in at a slightly higher level than the outlet. The 

 heavy sediment drops to the bottom, whence it may be 

 removed from time to time. The end of the outlet tile 

 is finished with an elbow, turned down so as to prevent 

 the entrance of sticks or other floating material. The 

 walls of the well may be made of wood, concrete, or 

 DricK. 



558. Surface intakes. — The admission of surface water 

 into a tile drain should always be managed with great 

 care to_ remove the heavier sediment or other material 

 that might obstruct the tile. Screen boxes should be 

 used. The screen should incline to the intake at an angle 

 of fifty or sixty degrees, so that floating material, instead 

 of obstructing the flow, will be pushed upward out of the 

 course. 



659. Outlets. — As few outlets as is practicable should 

 be constructed for tile drains, and these should have a 



