PREVENTION OF EEOSTON 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 safetj?^ are included in the above computations, (1) the 

 depth of flow is made only f foot, whereas the terrace is built 11 

 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 



300 600 900 1200 1500 



Length of Terrace in Feef 



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 



