GENERAL DRAINAGE INFORMATION 



TABLE VII 



89 



NUMBER OF ACRES FROM WHICH ONE FOURTH INCH OF WATER 

 WILL BE REMOVED IN TWENTY-FOUR HOURS BY OUTLET 

 TILE DRAINS OF DIFFERENT DIAMETERS AND DIFFERENT 

 LENGTHS WITH DIFFERENT GRADES 



119. Uniformity of grade. It is desirable to have the 

 grade uniform throughout the length of each line of tile. 

 This is not always possible for reasons which will appear 

 later. When changes in grade must be made, it is still 

 desirable to make them as few as possible, and to keep 

 the grade uniform in as large sections as possible. 



There is no objection whatever to changing the grade 

 from any rate of fall to a greater grade, but care must be 

 observed. The water moving through the drain carries 

 with it more or less fine material which has worked its 

 way through the joints of the tile. This material is spoken 

 of as silt. The particles of silt are sometimes so large as 

 to be moved but slowly by the running water in the tile 

 so slowly indeed that if the rate of flow of the water should 

 be decreased by ever so little, its force will then be insuffi- 



