12 
BULLETIN 1408, U. S. DEPARTMENT OF AGRICULTURE 
discharge; and (3) the cost of structures and excess excavation 
where drops are used. If the canal be deep, the flood discharge 
moderate, and the cost of structure low, then checks without drops 
should be used. When very large flood discharges are to be cared 
for the canal must be constructed on a grade flat enough to prevent 
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SECTION S-S 
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excessive velocities and the structure used only as a drop. For some 
combinations of the factors mentioned it will be found economical 
to design the system to use a combination check and drop. 
TIMBER LININGS IN OPEN DITCHES 
In saturated soil frequently it is impossible to dig a ditch to proper 
depth or to keep it open after excavation, in which event timber 
