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 
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. 
