4 BULLETIN 190, U. S. DEPARTMENT OF AGRICULTURE. 
the average dangerous rise throughout the season and the approxi- 
mate percentage of void spaces in the soil. From these data the daily 
run-off that must be provided for may be computed. 
Another method of determining the required capacity is to ascer- 
tain the maximum height of the water table above the required 
drainage depth, and to determine the approximate percentage of 
void spaces in the soil. Then, assuming the number of days in which 
it is required to remove the calculated volume of water, the daily 
run-off may be determined. If the ground water be always at the 
surface, the same system may be employed, but the evaporation and 
any surface run-off that occurs must be taken into consideration. 
TOPOGRAPHY AND SOIL. 
The required capacity of a drainage system can not be reliably 
determined without an accurate survey and a careful study of the 
soil and subsoil. The survey should be made to ascertain the 
amounts and directions of slope, the locations of natural and artificial 
features that may influence the design of the system, and the dimen- 
sions of the tract as a whole and as to its various classified portions. 
The subsurface examinations should yield information as to the 
nature of the soil, its stratification, its water-carrying capacity, and 
its capillary power. 
From the data thus obtained, the proper location, frequency, and 
depth of drains may be determined, after which the sizes of the vari- 
ous units may be ascertained by reference to the required capacity. 
OPEN CANALS. 
In the design of an open canal the important points to be considered 
are the effectiveness of the drain, its carrying capacity, its mechanical 
construction, and its maintenance in good condition. 
Experience has taught the necessity of providing considerable depth. 
This should never be less than 6 feet, presuming that the maximum 
depth of flow will be 1 foot, and 8 feet would be a better minimum. 
Thus, as is shown in figure 1, the cross section of stream flow is small 
as compared with the cross section of the canal itself. The side slopes 
are quite flat; they never should be steeper than 1 horizontal to 1 
vertical, and it is sometimes necessary to make them as flat as 3 hori- 
zontal to 1 vertical. 
A berm of not less than 6 feet should be left on either side of the 
canal and the spoil should be banked up on one or both sides. The 
spoil should not be scattered over the adjacent farms for a number of 
years, at least, as it is "dead" material and will do injury to the soil. 
The spoil banks are useful in keeping waste water and storm water 
from entering the drain directly and injuring the channel. All of the 
spoil may be placed on one side if it is desired that a roadway run 
parallel with the drain. 
