DRAINAGE OF IRRIGATED LAND. 
13 
No general rules can be given as to the arrangement of drains on 
an irrigated tract. The proper locations have been predetermined 
by nature and it is necessary to study the conditions well in order to 
avoid mistakes. However, since the damaging water in the irrigated 
section moves underground, it is the subsurface rather than the sur- 
face conditions that must be studied. Few of the lines will be parallel, 
but economical features of design must often be sacrificed to an ar- 
rangement that will give the best drainage results. 
PROTECTIVE DEVICES FOR OPEN CANALS. 
Changes in the direction of a canal should be made by easy curves; 
otherwise one bank will be cut away while silt will be deposited at 
»,. ""v. If. 
Fig. 6. — Concrete automatic regulating check for preventing erosion during periods of heavy discharge. 
the opposite side of the channel. If a section of canal having a slope 
which causes erosion discharge into a section having less slope, silt 
will be deposited in the latter, due to the reduction in velocity, and 
the channel will become obstructed. Where the slope of a canal is 
such that the velocity will cause erosion, " drops" should be installed 
to lower the water from one level to another without injury to the 
channel. 
CHECKS. 
In some canals the usual flow is not sufficient to cause damage by 
erosion, but occasional floods increase the discharge to such an extent 
that the velocity is destructive. In such canals checks, designed to 
operate as spillways during high water but having an opening at the 
level of the canal bottom of sufficient size to pass the ordinary flow, 
