SPECIAL TYPES OF DRAINS 263 



tures will permit. (10) Avoid throwing the tile out of 

 alignment in filling the ditch. 



The chief advantages of covered drains, especially 

 when constructed of tile, are: (1) Permanence. A well- 

 constructed system will last for many decades. (2) 

 Greater efficiency where they are suitable. (3) No waste 

 of land. (4) No interference with cultural operations. 

 (5) Require very little care for maintenance. (6) Less 

 cost over a period of years. 



125. Other types of drainage. Drainage may some- 

 times be accomplished by means of levees. Where land 

 is subject to overflow at either frequent or infrequent 

 intervals, such as river bottoms and tidal marshes, their 

 drainage consists largely in excluding these inundations. 

 Until this is accomplished, any other form of drainage 

 may be useless. Frequently direct drainage may advan- 

 tageously be combined with some form of levee, and 

 for tidal marshes is useful with the aid of the fresh 

 water derived from rainfall and upland drainage, in 

 removing its saltness. 



Wells or filter basins may be used to drain certain 

 sinks or flat areas having no other outlet. This is pos- 

 sible only where a very porous stratum occurs beneath 

 the soil within a reasonable depth. Usually this is 

 practicable where a clay stratum is underlain by sand 

 or gravel, as occurs in many sections of the country. 

 Wells are constructed through the clay to the porous 

 stratum, and this may be filled with stone or brush 

 as a filtering medium, and covered drains may be 

 emptied into these. 



126. Surface culture. Surface culture may be em- 



