CONCRETE LINING FOR IRRIGATION CANALS. 51 



the sacrifice of so much head might entail greater cost than the build- 

 ing of a larger canal on a lighter grade. 



Again, the extra cost and inconvenience in making suitable turnouts 

 to divert water from such a canal would serve to lessen the advan- 

 tages gained by having a high velocity. 



Lastly, pulsations are a common feature in all channels in which 

 the water flows at a high velocity. The water surface consists of 

 irregular waves which travel at various distances apart and an 

 extra height of lining is required to prevent the waves from over- 

 topping it. 



It is believed that a mean velocity of between 8 and 10 feet per 

 second is about as high as shbuld be adopted in lined canals under 

 ordinary conditions. 



ALIGNMENT OF LINED CANALS. 



In locating a new canal for concrete lining, sharp curves should be 

 avoided if possible. The reduction of curvature in a location over 

 a rough country with steep slopes may increase considerably the 

 amount and the cost of excavation, but this additional expense may 

 be more than compensated by the advantages gained in having flat 

 curves. 



In flowing around curves the surface of the water tends to rise on 

 the outer side due to centrifugal force. The height to which it will 

 rise in any given case will depend on the velocity of the water and 

 the sharpness of the curve. In order to maintain a uniform height 

 of lining above the water surface of the canal, the practice has been 

 to raise the outer lining. In the case of the Tieton main canal of 

 the United States Reclamation Service, Yakima project, Washing- 

 ton, on the sharpest curves, having a radius of 57.6 feet and a velocity 

 of 9 feet per second, the superelevation amounted to about 1 per cent 

 of the width. 



The presence of a large number of sharp curves likewise increases 

 the cost of both the earth trimming and the laying of the concrete. 



In railroad location it is customary to limit the curvature within a 

 certain fixed maximum regardless of expense, but in the construction 

 of irrigation canals, on account of the wide range permissible, it has 

 not been customary to fix any such limit. The expense of excavation 

 required to lessen curvature should be balanced against the disad- 

 vantages and extra cost of lining sharp curves. 



THE EFFECT OF ALKALI ON CONCRETE LINING 



Throughout the West are to be found here and there instances of 

 concrete construction having been disintegrated through the action 

 of alkali salts. While some uncertainties remain regarding the exact 

 nature of such action, there seems to be no question but that the prin- 



