24 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, _ 



where y = rate of rainfall in inches per hour, and x = 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 jDer 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 rim-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 Avere 

 used in the computations for the curves discussed hereafter: Base 

 width of terrace, 10 feet; height of terrace, l^- feet; depth of flow, 

 J foot ; and value of " n " in Kutter's formula, 0.035. Using the 

 velocity computed by the formula, v = c-yjrs, where c is the con- 

 stant determined from Kutter's formula, r 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 



