THE FLOW OF WATER IN IRRIGATION CHANNELS. 49 
few months' use than when new, and it is better to use a value of n 
that will hold for the greater part of the life of the flume. 
The above statements apply to the ordinary plank flume. Where 
staves are used which is quite rare since the advent of the steel flume, 
a lower value will probably hold throughout the life of the flume, for 
its very construction is such that it must stay in quite good condition 
or fail, due to the compression existing between the edges of the 
staves. In many cases the designer of a flume considers only the 
lumber, battens, and calking, while in actual practice the good 
influences of well-chosen materials are often completely lost through 
the accumulation of rock fragments, gravel, and so on. A very 
common location for a flume is down a bench cut in a rocky hillside. 
Whenever the cut is such that the fragments sloughing from the hill 
would fall into the flume, care should be exercised to keep the flume 
out from the side of the cut, or a guard wall should be constructed to 
divert debris so it will fall back of the flume. If high velocities are 
constantly to be maintained, ordinary dirt and small stones will roll 
on down the flume or accumulate on the inside of sharp curves. 
Short flumes are quite liable to take about the same value of n as the 
earth section above, as the same bottom becomes characteristic of 
both. 
I. 7i = 0.012 for well-constructed, clean flumes with surfaced lum- 
ber for both siding and battens. All lumber to run longitudinally. 
Alignment to consist of long tangents with gentle curves between. 
Construction to be such that the grade line will remain uniform, pre- 
venting sags and wavy alignment. A flume without battens may 
have a very slightly lower value of n, but this difference will be inap- 
preciable if the added length of the wetted perimeter due to the 
battens is considered in the design. Some very smooth grades of 
roofing materials used as linings also give a slightly lower value of n, 
but not enough to consider. If flumes are calked with oakum or 
other stuffing, care must be used that none projects into the water 
section if a high degree of efficiency is to be maintained. 
II. n = 0.013 for well-constructed, clean flumes of surfaced lumber 
and battens, following mountain contours, where the alignment will 
consist of about equal gentle curves and tangents. This value will 
also apply to flumes with alignment and grade as described in type I, 
but with vertical battens at intervals, with projecting calking or a 
slight amount of hardened asphalt or other waterproofing retarding 
the velocity. 
III. n = 0.014 for flumes of very smooth interior, but with many 
bends or sharp curves. This value also applies to those of type I 
with a location such that a slight amount of hillside debris is unavoid- 
able. Construction of type I, but with cracks poured with any water- 
79256°— Bull. 194—15 i 
