DRAINAGE 299 



land lies along a stream in which the volume of water is 

 so large at flood time that it is impractical to attempt to 

 carry it in a channel, it is necessary to resort to artificial 

 banks or levees to prevent the water from spreading beyond 

 reasonable limits when it rises higher than the natural 





LltC 



After University of Minnesota 



Fig. 245. Typical cross section of a levee. The technical names of the 



various parts are indicated. 



banks. Large areas of bottom lands have been protected 

 from overflow by levees along the Illinois, Mississippi, and 

 other rivers. In reclaiming lands that are subject to over- 

 flow, it is essential that the levee system be planned and 

 located by an engineer experienced in levee work (Fig. 245). 



Well drainage. -.Drainage by sinking wells has been 

 given wide publicity, but without compensating results. 

 In rolling country deep depressions of small area are objec- 

 tionable from an agricultural standpoint, and to remove 

 them by ordinary methods of surface drainage is very 

 expensive. The sinking of wells to drain these small areas 

 has been attempted and occasionally gives satisfaction, but 

 experience indicates that even where results are obtained 

 for a short time, the capacity of the well for taking water 

 gradually decreases until it becomes ineffective. 



The principle of well drainage is the same as that of 

 surface drainage. Water flows by the action of gravity, 

 and there must be some channel through which it can 

 escape. If the lower soil strata are composed of open 

 seams or large fissures in the rock, such as occur in lime- 

 stone formations, or if there is a bed of gravel or sand which 

 has a drainage outlet in some hillside or stream bank, there 

 will be a passage for the water underground, and if the 



