26 



BULLETIN" 512, U. S. 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 



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300 



Fig. 12.- 



600 



1500 



1800 



900 1200 



Length of Terrace in Feet 



■Uniform-graded terraces. Curves showing required grades for different land 

 slopes, terrace lengths, and vertical spacings. 



ii, grade greater than 0.5 per cent the lengths of the terraces on the 

 5, 10, and 15 per cent slopes would be limited 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. 



