PREVENTION OF EROSION BY TERRACING. oT 
For this type of terrace it would not be necessary to maintain 
the same size of embankment throughout the length of the terrace, 
but the embankment could be reduced as the upper end is approached. 
The channel capacity required, which depends upon the drainage 
area above, decreases toward the upper end of a uniform graded 
terrace. 
Terraces with variable grade—Surveys were made of several 
fields with graded terraces where the grades were found to vary. 
These were in better condition than were any having uniform-graded 
terraces. The profiles of the grade lines of these terraces showed 
a tendency of the grade to increase toward the outlets, a short dis- 
tance at the upper end of the terrace being level. This practice 
possesses much merit. The grade is increased at intervals along the 
terrace to accommodate the continually augmented discharge from 
the increasing size of the drainage area. A lesser grade may be used 
at the lower end of a variable-graded terrace than is required for a 
uniform-graded terrace of the same length. This is due to the fact 
that a smaller rate of rainfall can be used, since with the lesser 
grade of the variable-graded terrace, the time required for the water 
to flow the length of the terrace is greater than for the uniform- 
graded terrace. 
In figure 13 are shown curves for terraces with siiishls grades, 
similar to the ones in figure 12 for terraces with uniform grades. 
It can be seen from the curves that the lengths of a variable-graded 
terrace that can be used, for a grade of 0.5 per cent at the lower end, 
are 1,570, 1,280, and 1,100 feet on slopes of 5, 10, and 15 per cent, 
respectively, as compared with lengths of 1,210, 970, and 820 feet 
for terraces with a uniform grade of 0.5 per cent. 
In laying off a terrace with variable grade, the grade should be 
increased at intervals of 200 or 800 feet and at all sharp bends where 
the terrace crosses a gully or depression in a field. For example, 
if it is desired to lay off a terrace on a 10 per cent slope, 1,200 feet 
long and with a vertical spacing of 4 feet, and the grade of the ter- 
race is to be changed every 300 feet, then from the curves in figure 13 
the grades would be as follows: 
Station. 
Grade in 
feet per 
Brom! Maps 100 feet. 
300 0. 05 
300 600 -l 
600 900 27 
900 1, 200 45 
It is seen from the above that the grade for the first 300 feet of 
terrace is almost negligible. This portion could well be laid off 
