94 BULLETIN 512, U. S. DEPARTMENT OF AGRICULTURE. 
greater rates being used. Referring to the curve, the horizontal scale 
represents the durations of rainfall in minutes and the vertical scale 
the rates of rainfall in inches per hour. For example, from the curve 
the rate of rainfall for a rain lasting 30 minutes would be 4 inches 
per hour, and one of 5 minutes 9 inches per hour. The equation for 
this curve is: 
30 
Ue aie 
where 7 = rate of rainfall in inches per hour, and « = duration of 
rainfall in minutes. 
To determine the rate of discharge to be provided for in the design 
of a terrace system, the so-called rational method of computing run- 
_ off is employed. According to this method the maximum discharge 
will take place when water from the most remote point of the drain- 
age area above the terrace reaches the terrace outlet, provided the 
rate of rainfall continue uniform for a period equal to that required 
for water to travel from the upper to the lower end of the drainage - 
area. Hence if the length and grade of the terrace be known and 
the average velocity of flow be computed, the time interval can be 
obtained readily. For instance, if the time interval is found to be 
30 minutes, then the maximum rate of rainfall to be expected would, 
from the curve, be 4 inches pet hour. In computing this time 
interval the distance from the upper to the lower end of the drainage 
area is taken as being equal to the length of the terrace, the distance 
between terraces being disregarded. This practice results in the use 
of a little larger rate of run-off than would apply if the distance 
between terraces were included and therefore is on the side of safety. 
Furthermore, the velocity at the lower end of the terrace, instead of 
the average velocity along the terrace, was used in computing the 
time interval which likewise would result in the use of a little larger 
rate of run-off. 
Terraces with uniform grade—Field examinations of a great 
many graded terraces show that erosion of average soils takes place 
where the grade of the terrace exceeds 0.5 foot per 100 feet. Even 
at this grade some of the fine particles of soil are carried away in 
the run-off water. Many advocates of the graded terrace favor the 
use of a grade not to exceed 0.5 per cent. The following values were 
used in the computations for the curves discussed hereafter: Base 
width of terrace, 10 feet; height of terrace, 14 feet; depth of flow, 
8 foot; and value of “n” in Kutter’s formula, 0.035. Using the 
velocity computed by the formula, v=cyrs, where ¢ is the con- 
stant determined from Kutter’s formula, 7 the hydraulic radius, and 
s the slope, the time intervals were determined for terraces ranging 
in length from 300 to 1,800 feet. The corresponding rates of rainfall 
