2 BULLETIN 854, U. S. DEPARTMENT OF AGRICULTURE. 



curing an enhanced degree of drainage, would not yield additional 

 benefit in proportion to the increase in cost, and might not be justified. 

 On the other hand, an inadequate installation might be so ineffective 

 as not to justify even the small expenditure it would require. It 

 is the engineer's high duty, therefore, in planning the general scheme 

 of improvement for any drainage undertaking, to determine just 

 what expenditure will yield a satisfactory return, and to so propor- 

 tion the details as to secure the maximum benefit from the invest- 

 ment. A deficiency at one point may reduce the effectiveness of 

 the whole system, while the elaboration of one part out of proportion 

 to the others might add materially to the expense without obtaining 

 any benefit. 



The formulae in general use for computing the velocity of flow 

 in tile drains were proposed years ago, when all drain tile were small 

 as compared with the larger sizes used to-day. Under the earlier 

 conditions, when other considerations had relatively large weight 

 in determining the size of tile to be used, accuracy in computing 

 carrying capacity was relatively unimportant; but nowadays drains 

 12 to 48 inches in diameter are common, and accurate knowledge 

 of the capacity is essential for economical design. 



Although many experiments have been made upon flow of water 

 in iron, steel, concrete, and wood-stave pipes, the results are not 

 directly applicable to tile drains. The tile usually are not nearly 

 so regular in size and shape as are the other pipes mentioned, and 

 specially noteworthy is the number and nature of the joints. While 

 the other conduits are either of continuous construction or in 10 to 

 20 foot lengths, drain tile are in lengths of only 1 to 3 feet. Fur- 

 thermore, with clay tile the nature of the materials used and the 

 methods of manufacture are the causes of some distortion in cross 

 section; this is particularly noticeable where two lengths abut. The 

 considerable unevenness at the joints, when multiplied by the. greater 

 number of joints, so greatly disturbs the flow of water as to make 

 formulas demised for other kinds of conduit inapplicable to tile drains. 



Realizing the need for accurate knowledge regarding the flow of 

 water in tile drains, plans for investigating this subject were made by 

 the drainage division of the Bureau of Public Roads, early in 1915. 

 The experiments so far made concern only the smaller sizes of tile, 

 and this report therefore should be considered as a progress report 

 of the investigation of the whole subject. 



SCOPE OF THE INVESTIGATION. 



Drain tile installed for agricultural improvement serve two some- 

 what distinct purposes — as collectors of excess water and as conduits 

 to convey the water to some more ?>r less distant outlet, but usually 

 both purposes are served coincidentally. The investigation herein 



