PREVENTION OF EROSION BY TERRACING. 



27 



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 variable 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 300 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 

 100 feet. 



From — 



To- 





 300 

 600 

 900 



300 



600 



900 



1,200 



0.05 

 .14 



.27 

 .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 



