PREVENTION OF EEOSION BY TERRACING. 



23 



The width and height of old terraces depend upon their size at 

 the time of construction and methods of plowing employed to main- 

 tain them. As a rule terraces that are tended properly grow broader 

 with age and diminish in effective height, so that what is lost in 

 height and diminished size of water channel is gained in broadness, 

 and generally in an increased absorptive capacity of the soil. Graded 

 terraces should be built originally about 10 feet broad at the base 

 and about 1^ feet high and should be thrown up each year with a 

 plow imtil they acquire gradually a cross section similar to those 

 shown in figure 2-F. 



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20 30 40 



Duraf/'on of Ram fa// in Minutes 



50 



60 



Fig. 10. — Rates of rainfall for short periods, for which graded terraces should be designed. 



The principle involved in the design of a graded terrace is that 

 the channel above the terrace be made of such a size and grade that 

 it will conduct the surface water slowly to a drainage outlet with- 

 out the possibility of the water overtopping the terrace. Hence, it is 

 necessary to know something of the rates and duration of the rainfall 

 which is the source of the surface water. 



A study was made of rainfall intensities for short periods, as 

 shown by the Weather Bureau records for the humid portions of 

 the United States, and a curve (fig. 10) was plotted which is 

 thought to represent closely the rates of rainfall for short periods 

 that should be provided for in the design of a system of graded 

 terraces. The records show that these rates sometimes will be ex- 

 ceeded, but not so frequently in any given locality as to warrant 



