26 BULLETIN 512, U. 8. DEPARTMENT OF AGRICULTURE. 
\ 
for the longer terraces, since‘ the drainage area increases with the 
length of the terrace while the cross-sectional area of the channel 
- remains constant. Field observations show that spacings of 3, 4, 
and 5 feet give the most satisfactory results on slopes of 5, 10, and 15 
per cent, respectively. Using these data, the three curves in figure 12 
were plotted. It can be seen from these curves that for a given 
vertical spacing and land slope, the grade required increases rapidly 
as the length of the terrace increases, and if it were not desired to use 
Grade of Terrace in Feet per Hundred 
rd 
Ne 
ert 
300 600 900 1200 1500 1800 
Length of Jerrace in Feet 
Fic. 12.—Uniform-graded terraces. Curves showing required grades for different land 
slopes, terrace lengths, and vertical spacings. 
a grade greater than 0.5 per cent the lengths of the terraces on the 
5, 10, and 15 per cent slopes would be lmited to 1,210, 970, and 820 
feet, respectively. The curves show also that terraces up to 300 feet 
in length require very little grade. A terrace of only 300 feet re- 
quires practically no grade on any type of soil, because a sufficient 
grade will be created by the distribution of water above the terrace to 
cause a flow toward the ends. Other things being equal, the most 
efficient terracing is found where comparatively short terraces and 
low grades exist, 
