THE FLOW OF WATER IN IRRIGATION CHANNELS. 61 
may entirely change the character of the bottom of a smoothly lined 
canal. On the other hand, a water laden with fine silt flows far more 
freely after the silt has deposited in a slick coat over minor irregu- 
larities than in a new, though clean, canal. A canal carrying such a 
water may be designed for a far higher velocity through the same kind 
of soil than would be the case if the water were clear. It is necessary 
only to run but low heads in the new canal until a thick waxy deposit 
has been placed on the canal bed, after which the velocity may be 
nearly doubled over that which would have scoured the material in 
which the canal was originally excavated. (/) The prevailing wind 
direction may be given some consideration. A study of vertical 
velocity curves shows a marked change in form with change in wind 
condition. A downstream wind aids the flow of surface water to 
the extent that it has the maximum velocity in the vertical, while an 
upstream wind so shapes the velocity curve that the surface velocity 
is as slow as that near the bottom. 
(4) That there is a tendency toward a lower value of n as the ve- 
locity and hydraulic radius increase. Any experiments that are in- 
tended to bring out conclusively the extent of the variation and 
whether it is due to the change in velocity or the change in hydraulic 
radius must be conducted in very long, straight channels, in wood, 
or concrete, or steel, where the character of the wetted perimeter will 
remain unchanged as the water becomes deeper and deeper in the 
channel. Proper corrections must be applied for any slight changes 
in the mean velocity at the two ends of the reach tested. The reach 
must be far removed from all influence of curves and structures. 
(5) That a value of n must be chosen that will apply to the canal 
in question at the critical period of the season. For instance, most 
canals are cleaned once a year. A growth of moss may become very 
heavy by July or August, but the water supply or demand will prob- 
ably be much less than during the early days of June. If the canal 
is designed to carry its peak load on the basis of its being in good 
condition, there will still be sufficient carrying capacity for the smaller 
discharge when moss has appeared. 
(6) That in the design of earth channels having a trapezoidal form 
when constructed, the value of R should be computed on the basis 
that the canal takes an elliptical form within a short time and there- 
after maintains this shape unless altered artificially. 
ACKNOWLEDGMENTS. 
The writer and his associates desire to acknowledge indebtedness to 
the various engineers and managers of irrigation and power systems 
who permitted the use of canals under their charge for the tests and 
to the engineers of the United States Reclamation Service who al- 
lowed access to much original data on tests made by their engineers. 
