LAND DRAINAGE 637 



erosion on high grades. Fine-textured clays are least 

 affected by erosion. The grades and rates of flow that 

 are permissible depend largely on the size of the ditch. 

 A velocity of three feet a second is usually the maximum 

 that is permissible. It may be a little higher in clay, 

 and should be a third lower in silt and fine sandy loam. 

 This rate of flow may be attained in ditches where the 

 water is several feet deep by a fall of only six inches to a 

 foot a mile. In small ditches where the water is a foot 

 or less in depth the grade may be from fifty to sixty feet 

 a mile, and in heavy clay, especially if it is compact and 

 stony, a still higher grade will not cause serious washing. 



These limits depend to a large extent on the amounts 

 of sediment that the water carries. Material in suspen- 

 sion greatly increases erosive action on the ditch walls. 



In constructing open ditches care should be taken to 

 deposit the earth several feet back from the edge of the 

 channel. This is desirable for two reasons : first, it re- 

 moves the weight from the unsupported bank, where 

 caving is very likely to occur when the soil is saturated ; 

 second, it provides a larger throat for the stream should 

 it be inclined to overflow. 



Another method of constructing an open ditch, es- 

 pecially in wet grass land, is to form a broad, shallow 

 channel by the use of a road scraper. The earth is 

 gradually worked back a rod or more, and the walls are 

 so flat, even with a ditch three feet deep, that crops grow 

 and may be collected in the bottom of the ditch. This 

 system reduces the loss of land and the interference with 

 farm operations. 



537. Construction of early types of underdrains. — 

 Any material or condition that affords an underground 

 passage for the flow of water measurably fulfills the func- 



