STRUCTURES USED IN DRAINING AGRICULTURAL LAND 
13 
linings are employed to prevent sloughing in of soft material. These 
are usually constructed without a bottom. They should be designed 
as simply as possible. . Usually they are regarded as temporary 
structures which will not require replacement after the water table 
has been lowered and the slope of the banks has become stable. 
Unless a bottom is needed they should be made as wide and low as 
possible, so that after they have decayed little or no smoothing of the 
l3anks will be required. Careful placing is necessary to insure that 
such structures will remain in alignment, both vertical and hori- 
zontal. Although sometimes necessary, cross braces on the top 
B 
Ditch bottom- 
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Fig. 10. — Types of cunettes 
usually are objectionable, particularly where large weeds are apt 
to blow in. If such braces are used, they should be placed well above 
the surface of the normal flow. 
Three types are shown in Figure 10. (See also pi. 1, H.) The 
type shown in Figure 10, B does not require piles and may be used 
where the bottom is firm and there is no difficulty in maintaining 
horizontal alignment. The design in Figure 10, A calls for piles 
at intervals of 5 or 6 feet. In case these can not be driven into 
firm material, a top cross brace or a side brace as shown in Figure 
10, B will be required. The loading boards on the outside help to 
