PREVENTION OF EROSION BY TERRACING. 25 
were obtained for these time intervals from the rainfall curve in 
figure 10; these are the rates of run-off that would obtain at the lower 
ends of the terraces, assuming all the rainfall to run off. With the 
above data the required vertical distances between terraces of differ- 
ent lengths were computed and the curves in figure 11 plotted. Two 
factors of safety are included in the above computations, (1) the 
depth of flow is made only 2 foot, whereas the terrace is built 14 
feet high, and (2) all of the rainfall is assumed to run off, whereas 
a portion of it would percolate into the soil. Experience shows that 
a wide margin of safety is most desirable owing to the piling up of 
the water due to obstructions or abrupt bends, and to possible vari- 
ations in the height of the terrace and in the grade. 
To illustrate the use of the curves in figure 11, suppose it is desired 
to determine the proper vertical spacing on a field with a slope of 15 
Vertical Distance Belweer Terraces in Fee} 
Length of Terrace in feet 
Fig. 11.—Uniform-graded terraces, for grade of 0.5 per cent. Curves showing required 
vertical distances between terraces for different land slopes and terrace lengths. 
per cent for terraces 600, 700, 800, and 900 feet in length. Referring 
to the curve marked 15 per cent in figure 11, it is seen that the 
proper vertical spacings for terraces of these lengths are about 5.8, 
5.4, 5.1, and 4.7 feet, respectively. It will be seen from the curves 
that the longer the terrace the less must be the vertical spacing. 
Owing to this fact and to the greater likelihood of breaks in a long 
terrace, it is advisable to make the terraces as short as the governing 
conditions will permit. Where an adequate outlet is available at 
both ends of the terrace, they should be utilized by giving fall to the 
terrace from about the middle toward each end. For terraces less 
than 300 feet in length the same vertical spacing should be used as is 
given by the curves for 300 feet. 
If it is desired to maintain the same vertical spacing for all 
lengths of graded terraces, it becomes necessary to increase the grade 
