DBAINAGE OF IKEIGATED 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 hnes 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 mil be cut away while silt will be deposited at 



% *>, 



Fig. 6. — Concrete automatic regulating check for preventing erosion during periods of lieavy 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 

 wUl be deposited in the latter, due to the reduction in velocity, and 

 the channel will become obstnictcd. Where the slope of a canal is 

 such that the velocity will cause erosion, "drops" should bo installed 

 to low(!r the water from one level to another without injury to the 

 channel. 



CHECKS. 



In some canals the usual flow is not sufnciont to cause damage by 

 erosion, but occasiouul floods increase tlu; discliiirgc to sue! i au e.xtont 

 that the velocity is dc^stnictivc!. in such canals checks, designed to 

 operate as spillways rluring high wat(T but having an opening at the 

 level of the canal }>ott<>m of sufliciciiit siz(! to })ass tlu^ ordinal y (low, 



