28 BULLETIN 512, U. 8. DEPARTMENT OF AGRICULTURE. 
level. If a terrace with a uniform grade were used, the curve in 
figure 12 shows that a grade of 0.77 per cent would be required. 
Both practice and theory show that the variable-graded terrace is 
superior to the uniform-graded type. 
Outlets—Wherever possible terraces should end at natural drain- 
age channels. The absence of a suitable drainage outlet within the 
limits of a field often necessitates ending the terraces at fence lines, 
depressions or draws. The volume of water which is discharged 
from the ends of a system of graded terraces often erodes unsightly 
and objectionable ditches along the ends of the terraces to the foot 
lA 
er 
Yl | 
| 
ol Ae 
Grade of Jerrace in Feet per Hundred 
gteeeus 
BW Ee cea 
ees 
900 1200 1500 1800 
Length of Terrace in Feet 
Fig. 13.—Variable-graded terraces. Curves showing required grades for different land 
slopes, terrace lengths, and vertical spacings. 
300 600 
of the slope. Erosion in such channels can be reduced greatly by 
lining them with stones or seeding them to grass (see Plate V, fig. 2). 
The channels and banks of graded terraces should not be cultivated 
for 20 to 30 feet from the outlet channel but should be permanently 
sodded. Breaks commonly occur and erosion is most active near the 
ends of graded terraces, owing to the usually large volume of water 
passing. Some sort of protective covering of stones, boards or other 
hard material should be employed to prevent this washing. Where 
a terrace discharges into a deep ditch a box trough is used sometimes 
to give the water a free overfall into the ditch, This prevents ero- 
sion in the terrace channel. 
