STRUCTURES USED IN DRAINING AGRICULTURAL LAND 
11 
should be 2-inch material. One-inch lumber will do for bottoms 
except in fluid soils where an upward pressure is exerted. The top 
pieces should fit tightly together, but the bottom pieces should be 
separated from one-fourth to one-half inch to admit water. Such, 
boxes should not be over 24 inches in width nor over 14 inches in 
height. In larger sizes 3-inch lumber should be used for the top, 
with the sides made up of short pieces and arranged so that the 
joints interlock as shown in Figure 8, C. The larger sizes should 
be built in short sections and arranged so that joints can be fastened 
together. 
Trenching covers a large percentage of the total cost of deep 
drainage, and it will usually prove better economy in the long run to 
use durable material even at a greater first cost. 
STRUCTURES FOR OPEN DITCHES 
DROPS AND CHECKS 
To prevent injury to canals from excessive velocities, drops are 
installed to concentrate the excess fall at one or more points. Plate 
In design, drainage 
1, G shows a type used for small discharges. 
Fig.. 8. — Types of lumber box drains 
drops are not different from those used in irrigation practice with the 
exception that in most cases they should be able to withstand the 
destructive action of discharges considerably in excess of the normal 
flow of the canal. 
The primary purpose of many open drainage systems is to carry 
seepage water, and to provide drainage this flow may occupy only 
a small part of the cross-sectional area of the channel, which may 
have steep grades without danger of erosion. However, floods or 
breaks in irrigation systems may produce high velocities and con- 
sequent damage. This may be overcome by (1) reducing the grade 
and installing drops; (2) using checks to retard flood flows but 
also permitting the passage of the normal flow without checking; or 
(3) a combination of these two methods. Figure 9 and Plate 1, H 
show a combination check and drop which may be made to serve 
equally well for any one of the three methods by the varia- 
tion of certain dimensions. The breast-wall or overflow weir should 
be made as long as possible and the opening for normal flow as large 
as conditions will permit. 
Factors to be considered in the design of such a system are: (1) 
Quantity of normal flow, its depth, and minimum velocity permis- 
sible; (2) the volume and maximum allowable velocity of the flood 
