10 



BULLETIN 724^ U. S. DEPARTMENT OF AGRICULTURE. 



Velocity-Quantity Table No. 4. — For tile pipe with smooth joints. 



[Value of "n" used is 0.015. D is diameter of pipe in inches. A is wetted area of cross section of pipe in 

 square feet. R is mean hydraulic radius in feet. C is coefficient taken for S equal to 0.001. Q is discharge 

 in thousand cubic feet per hour. The pipe is assumed to rim 0.95D full.] 





A 



R 



C 



Slope, in feet, per foot. 



D 



0.001 



0.003 



0.005 



0.01 



0.02 



0.03 



0.04 



0.05 



0.07 



0.10 





Q 



Q 



Q 



Q 



Q 



Q 



Q 



Q 



Q 



Q 



4 



0.086 



0.095 



53.9 



0.162 



0.281 



0.363 



0.512 



0.722 



0.886 



1.025 



1.146 



1.356 



1.61 



6 



.19.3 



.143 



58.6 



.485 



.841 



1.085 



1.534 



2.162 



2.652 



3.070 



3. 438 



4.065 



4. 848 



8 



.343 



.191 



64.8 



1.104 



1.915 



2.470 



3.500 



4.928 



6. 080 



6.990 



7.813 



9.270 



11. 0.50 



12 



.771 



.287 



73.7 



3.452 



5.990 



7.730 



10. 925 



15.410 (18.910 



21. 870 



24. 480 



2S. 950 



34. 520 



15 



1.200 



.358 



77.9 



6. 360 



11. 030 



14. 230 



20. 130 



28. .380 34.820 



40. 260 



45.100 



53. 350 



63. 600 



18 



1.734 



.430 



81.4 



10. 520 



18. 240 



23. 550 



33. 300 



46. 950 ,57. 600 



66. 600 



74. 600 



88. 200 



105. 250 



It will be noted that the trapezoidal open ditch has been assumed 

 in computing capacity. This is by far the commonest type used in 

 cross-section designs, although the dimensions vary within rather 

 wide limits. This is supplemented by two ditches of the V type, one 

 having a very jflat slope. These indicate the transition in earth from 

 the trapezoidal or dug ditch to the gutter tjq^e, which often can be 

 made with the ordinary road grader. The common rectangular or 

 "square" ditch, so frequently seen in some localities, especially in 

 the southeastern part of the United States where sand claj^s abound, 

 is not calculated. This type of ditch is not recommended for several 

 reasons. In the first place there are very few soils that will stand 

 with a vertical face, particularly if that face be presented to flowing 

 water. Such materials as the loess of Adams County, Miss., and 

 certain grades of sand clay in the Carolinas and Georgia are rare. 

 Service tests indicate that such material will stand for years, but 

 ordinarily nothing but a service test will disclose that characteristic, 

 and no amount of examination appears to furnish sufficient data for 

 a decision as to whether or not an earthy material may be counted 

 upon to stand practically vertical. The safest assumption in design 

 is to rely on no unusual qualities in the soil at hand and to construct 

 a ditch on one of the approved designs used in the tables. 



The V-shaped ditches have several favorable characteristics. They 

 are not dangerous to traffic. They can be made with a grading 

 machine and can be cleaned and maintained very largely with a 

 grader, or for light work even with a drag. The highest velocity of 

 flow in the ditch cross section occurs toward the side away from the 

 road, and so the tendency to scour or wash, unless there be some dis- 

 turbing obstruction, occurs first on the side away from the road and 

 gives ample warning that attention is needed. These are the shapes 

 that most closely approach common gutter design, and thereby lend 

 themselves most readily to continued improvement by paving if the 

 surface be advanced in type to warrant such a change. 



