46 BULLETIN 194, U. S. DEPARTMENT OF AGRICULTURE. 



as possible the correct value of n and then designing for whatever 

 overload is deemed conservative. Another method which is some- 

 times followed is to choose a relatively high value of n and then design 

 for the desired discharge. This method is not to be recommended and 

 in some cases may be even dangerous for the reason that the velocity 

 may so increase beyond a safe figure for the containing material that 

 the canal can be eventually operated at but a part of its designed 

 capacity. This fact is brought out in the case of a prominent canal 

 in western Nebraska. A value of n of 0.025 was chosen as applying 

 to such a canal in a moderate state of operative efficiency. As 

 determined by numerous experiments, the material cemented into 

 a smooth, hard canal bed with an efficiency almost equal to concrete; 

 that is, n is actually about 0.016, and when operated to about one- 

 half its depth the velocity becomes so high that there is great danger 

 of scouring the channel. Thus a large portion of the excavated 

 area of the canal can not be used unless fined with some material 

 which would withstand scouring or else checks installed to reduce the 

 grade. Had the correct value of n been chosen at the time of con- 

 struction, the slope might have been far less than it is and additional 

 territory made irrigable or a different-shaped cross section might 

 have been chosen which would have had about the same yardage 

 per given length and the same ultimate carrying capacity as was 

 desired for the present canal. 



In the second case cited above, where the engineer must estimate 

 the carrying capacity of a canal already constructed, there is room 

 for divergence in the estimates of two men equally competent and 

 using data collected under the same conditions. 



If the canal is carrying its capacity at the time of inspection the 

 results of two men should agree quite closely, as it is a mere matter of 

 making a discharge measurement. 



If the canal is about half fuU a careful measurement may be made 

 sinfilar to the ones for the experiments described in tliis pubfication. 

 From this experiment the value of n holding for the given condition 

 may be determined. If the canal is straight and there is no influence 

 from structures it may be safely assumed that the surface slope will 

 remain quite constant up to capacity. Any material change in the 

 value of n must be determined by the influence of the two banks 

 above the surface of the water flowing at the time of test. A clean 

 concrete, wood, or steel channel will have a sfightly lower value 

 of n. An earth channel rarely has the same character of material 

 throughout the length of the wetted perimeter, and due aUowance 

 must be made for changes. 



This involves the judgment of the engineer, and two men wiU 

 disagree more or less at this point: The general elevation of where the 

 water surface will be when the canal is full is another place where the 



