PREVENTION OP EROSION BY TERRACING. 



13 



with the absolute minimum and maximum values found in the 

 surveys : 



Actual dimensions ^ of troad-hase level-ridge terraces. 



Dimension. 



Absolute 

 minimum. 



Absolute 

 maximum. 



Field averages. 



Minimum. Maximum. 



Base width of terrace, w feet . 



Height of terrace, h do. . . 



Vertical distance between terraces, v do. . . 



Slope of land suraf ce per cent . 



5 



.5 

 1.9 

 1.4 



IS 

 l.G 

 0.1 



21.5 



0.8 



.8 



2.7 



2.7 



11.6 

 1.4 

 4.8 



11.8 



1 See fig. 5. 



From a study of the above data and observation of field conditions, 

 it is believed that a broad-base level-ridge terrace should be not less 

 than 1^ feet high and at least 10 feet broad at the base. Methods of 

 plowing and cultivation should be adopted which will tend to 

 increase the base width from year to year and thus virtually trans- 

 form the whole field into a series of terraces. (See fig. 2-D.) 



Since the stability of a broad-base level-ridge terrace with closed 

 ends depends upon its ability to retain the surface run-off water due 

 to rainfall over the area between it and the next terrace above, it is 

 apparent that the reservoir capacity above the terrace must be suffi- 

 cient to store this water. Upon this principle are based the following 

 remarks on the design of a system of broad-base level-ridge terraces. 



Eeferring to figure 5, it is seen that the cross-sectional area of the 

 water that can be stored above a terrace is represented by the area 

 a ch a. A plan view of the line to which water is backed up before 

 overtopping the terrace is shown in figure 6. A good idea of the 

 size of the reservoir area can be obtained from this plan. Assuming 

 that no water escapes around the ends of the terrace and that no 

 water is lost through percolation into the soil, it follows that for 

 the retention of all of the surface water the area acta (fig. 5) must 

 be made equal to the product of the depth of the rainfall and the 

 horizontal distance d. If percolation does take place, then the 

 amount of water lost should be deducted from the total amount of 

 rainfall. By equating the amount of surface run-off to the amount 

 of storage above the terrace per unit of length, the following expres- 

 sion is obtained: 



dr dW , wh 600^2^3^ws 



12=2^+^' «^^= lOOr 



where r=surface run-off depth, in inches; 7t=height of terrace, in feet; 

 «7=base width of terrace, in feet; t;=vertieal distance between terraces, in 

 feet ; (?=horizontal distance between terraces, in feet ; s=slope of land, in feet 



per hundred, =~^= 



