38 



BULLETIN 126, U. S. DEPARTMENT OF AGRICULTURE. 



FACTORS AFFECTING SEEPAGE. 



It is not within the scope of this publication to include a detailed 

 discussion of the various factors influencing seepage, but in order to 

 form a reliable estimate of the loss by seepage from a proposed 

 canal, the principal factors should be carefully considered. Briefly 

 these are: 



(1) Size and shape of grains and general character of materials. 



(2) The gradual deposition of silt. • 



(3) Depth of water over the wetted perimeter. 



(4) The relation which the wetted perimeter of the canal bears to 

 other hydraulic elements. 



(5) Velocity of water in canal. 



(6) Inflow of seepage water. 



(7) Temperature of the soil and the water. 



A study of the results of the measurements secured brings out the 

 close relation existing between the unit loss as expressed in percent- 

 age of flow and the size of the canal. Three hundred and twenty- 

 three separate and distinct sets of measurements are grouped in 

 Table II according to the capacity of the channels. It is interesting 

 to note in this table the fairly constant decrease in the average loss 

 in per cent per mile as the capacity increases. 



Table II. — Summary of seepage measurements expressed in terms of per cent of total 

 flow lost per mile of channel for various sized canals. 



Capacity of canal (second- 

 feet). 



Number 

 of tests. 



Average 



loss per 



mile. 



Capacity of canal (second- 

 feet). 



Number 

 of tests. 



Average 



loss per 



mile. 





16 

 37 

 30 

 49 

 . 48 



Per cent . 

 25.7 

 20.2 

 11.7 

 12.1 

 5.5 



50 to 75 



31 

 26 

 45 

 27 

 14 



Per cent. 

 4 3 



1 to 5 . . 



75 to 100 



2 7 



5 to 10 



100 to 200 



1.8 



10 to 25. . 



200 to 800 



1.2 



25 to 50 



800 and over 



1.0 









THE CARRYING CAPACITIES OF CONCRETE-LINED CANALS. 



The laws governing the flow of water in concrete-lined channels do 

 not differ from those for any other waterway. The force of gravity 

 which produces motion in the water of a canal on a given grade is 

 usually quite evenly counterbalanced by the various conditions which 

 retard flow. These retarding influences are: (1) The frictional re- 

 sistance of the wetted perimeter of the channel; (2) the influence of 

 air in motion ; (3) the existence of sharp curves, projecting objects, 

 and irregularities of cross section, alignment, and grade; (4) the 

 presence of sand, gravel, stones, or other shifting material; and (5) 

 the presence of aquatic vegetation in the water or any rough coating 

 on the perimeter of the canal. 



