10 BULLETIX 724, U. S. DEPAETMEXT OF AGRICULTURE. 
Velocity-Quantity Table Xo. 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. E is mean hydraulic radius in feet. C is coefficient taken fcrS equal to 0.001. Q is discharge 
in thousand cubic feet per hour. The pipe is assumed to run 0.95D full.l 
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 
.193 
.143 
58.6 
.485 
.841 
1.085 
1.534 
2.162 
1 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 
1 .6. 080 
6.990 
7.S13 
9.270 
11. 050 
12 
.771 
.287 
73.7 
3.452 
5.990 
7.730 
10. 925 
15. 410 
IS. 910 21.870 24.480 28.950 
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 40.520 18.240 23.550 
I 1 - 1 
33.300 .46.950 
57.600 66.600 74.600 SS. 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 flat slope. These indicate the transition in earth from 
the trapezoidal or dug ditch to the gutter type, 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 clays 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. 
