I 
PREVENTION OF EROSION BY TERRACING., ts 
with the absolute minimum and maximum values found in the 
surveys: 
Actual dimensions* of broad-base level-ridge terraces. 
Field averages. 
Absolute | Absolute 
Dimension. minimum. | maximum. 
Minimum. | Maximum. 
ASG WwAGthrOMmerrace. Wiesscee secs cee ese e ee cess ok feet _- 5 18 6.8 11.6 
HIE OF OUR oe) Laaec eek os sae aosasee SEB eaoeSE doses =i 1.6 8 1.4 
Vertical distance between terraces, v..........-.- dows 1.9 6.1 Moll 4.8 
Stone sland'suratcen. see sn ooo Sere ey per cent.. 1.4 Dies 7 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 14 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. 
Referring 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 
acba. 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 ac 0 a (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 dh? wh 600h?+-3hws 
[Daa We A se oe TOO 
where r=surface run-off depth, in inches; h=height of terrace, in feet; 
w=base width of terrace, in feet; v—vertical distance between terraces, in 
feet; d=horizontal distance between terraces, in feet; s=slope of land, in feet 
per hundred, =e ys 
