THE FLOW OF WATER IN IREIGATION 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 

 Imnber, battens, and calking, while in actual practice the good 

 influences of weU-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 roU 

 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. 71=0.012 for well-constructed, clean flumes with surfaced lum- 

 ber for both siding and battens. AU lumber to run longitudmaUy. 

 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 arc 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 = 0.01.'3 for well-constructed, clean flumes of surfaced lumber 

 and battens, foUowing mountain contours, wliere the alignment wiU 

 consist of about equal gentle curves and tangents. This value will 

 also af>[)ly 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 hardcncid asphalt or other waterproofing retarding 

 the velocity. 



III. n = 0.014 for flumes of very smooth interior, but with many 

 bends or sliarp curves. This value also applies to those of type I 

 witli a location such tiiat a slight amount of liillsich; debris is unavoid- 

 able. Coastruction of type I, but with cracks poured wit h any watcr- 



V'/iVr— Hull. la4— 15 4 



