THE IKEIGATION AGE. 



247 



serve one-half the number of acres. If it is desired to 

 remove only one-eighth inch the sizes given in the 

 table will serve twice the number of acres indicated. 

 in this chapter because it is important to keep con- 

 stantly in mind the principles advocated in this book. 

 The author is not following any beaten track nor rely- 

 ing upon theory, but has taken experience as his guide, 

 and made assertions that can be easily verified in actual 

 experience. 



In the use of the table for determining the size 

 of drains, good judgment must be exercised in applying 

 it to the case in hand. The tract under consideration 

 may have such surface slopes that the underdrains may 

 be called upon to take the drainage of a much larger 



annual loss that will soon equal the amount that wat 

 apparently saved in their purchase. 



Figure 14. Large Tile Main in Place. 



The present tendency in drainage is to use large 

 tile mains in place of small open ditches where land is 



AREAS FROM WHICH ONE FOURTH INCH OF WATER WILL BE RHMOVBD IN TWENTY-FOUR HOURS BY OUTLET TILE DRAINS OF DIFFERENT DIAMETERS 



AND DIFFERENT LENGTHS WITH DIFFERENT GRADES, a 



Grade per 100 feet in Decimals of a foot (with * approximate equivalents in inches). 



a This table was computed frjm the formulas for determining the size for tile drains given in Elliott's Engineering for Land Drainage, which are: 



<3M 



<? 



.0105 



Where =velocity of flow in feet per second. 



rt sectional area of tile in square feet. 



rf=diameter of tile in feet. 



_/"=total fall in length of drain. 



/t depth of drain in feet at upper end. 



/total length of drain in feet. 



C?=discnarge of drain in cubic feet per second. 



A= acre drained. 



Constant 0.0105=quantity of water to be removed from 1 acre in 1 second of time. 



Computations are made lor two assumed lengths of drain 1,000 feet and 2.000 feet. Vik is 1.5 feet, that is one-half of depth of drain where the soil is 

 open and saturated with water, under which conditions the drain will discharge its maximum quantity. Where the soil is close no additional head will be 

 added by the free water of the soil , so that the factor }'ik should be omitted in computations. Three feet of soil above the top of the drain has been assum ed . 

 It will be be readily seen that the grade, length of drain, and openness of soil are important factors in the capacity of a tile drain for discharging soil water. 



area than if the land were nearly level. By reason of 

 the surface slope and drainage _a main may be required 

 to receive the drainage of twenty acres instead of ten, 

 as would appear at a casual glance. It is important to 

 take into account also all the facilities for natural 

 drainage when one undertakes to drain land by tiles. 

 Too large tiles involve an expense without adequate re- 

 turn, while those which are too small may entail an 



worth $200 an acre or more. It is not uncommon to 

 use tiles eighteen and twenty inches in diameter. Where 

 large tracts are drained some surface relief drains 

 should also be provided against excessive rainfall in 

 order to keep the expense of the main underdrains 

 within the limits of paying returns. 



DIGGING THE TRENCH. 



The trench should be started on the surface by a 



