28 



BULLETIN 512, U. S. 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 



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300 600 900 1200 1500 1800 



Length of Terrace /n Feef 



Fig. 13. — Variable-graded terraces. Curves showing required grades for different land 

 slopes, terrace lengths, and vertical spacings. 



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. Wliere 

 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. 



