PREVENTION OF EROSION BY TERRACING. Te 
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. Asa 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 14 feet high and should be thrown up each year with a 
plow until they acquire gradually a cross section similar to those 
shown in figure 2-F, 
8 
per Hour 
D 
4 
i Raintal/ In inches 
me 
40 
Duration of Ratntall in Minurles 
Tic. 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 
