Chapter 21 



PROBLEMS OF IRRIGATION CANALS 



Alfred R. Golze 



Director, Office of Programs and Finance 

 U. S. Bureau of Reclamation 

 Washington, D. C. 



CAUSE OF SILTING 



Irrigation canals are the waterways by which water is transported 

 from a source of supply, such as a reservoir or river, to the area on 

 which it will be used for irrigating the land for agricultural purposes. 

 Laterals are small irrigation canals that take the water from the 

 principal irrigation canals and carry it to the individual farms. Irri- 

 gation canals and laterals may be either open ditches or closed con- 

 duits, depending on local conditions. Open ditches may be lined or 

 unlined, depending on whether losses by seepage are sufficient to 

 justify lining with impervious materials. 



An unlined irrigation canal does not differ greatly from a river or 

 navigation channel as far as problems of sedimentation and erosion 

 are concerned. Like the navigation canal, an irrigation canal is de- 

 signed to have certain hydraulic properties which are obtained by 

 constructing the canal to a predetermined cross section and grade. 

 Just as a river is subject to aggradation or degradation, so a canal 

 is subject to aggradation or degradation if its hydraulic properties 

 are destroyed through sedimentation or erosion. 



Sedimentation of canals — and canals in this instance and hereafter 

 should be understood to mean both main or principal irrigation canals 

 and irrigation laterals — is defined as the uncontrolled deposition of 

 silt in the canals. This silt may be from sloughing of the canal banks ; 

 it may be waterborne, from erosion of sections of the watershed con- 

 tributing to the water supply; or it may be local erosion from lands 

 adjacent to canals. Problems of sedimentation in irrigation canals 

 are, therefore, similar to the sedimentation problems to be found in 

 other types of canals, reservoirs, and in natural waterways. 



Sedimentation of irrigation canals creates two problems: (1) how 



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