THE FLOW OF WATER IN lEEIGATION CHANNELS. 51 



VALUES FOR n IN MASONRY-LINED CHANNELS. 



The writer has not sufficient tests on masonry-lined sections to 

 make suggestions as to specific values of n to use, but the following 

 general observations seem to be warranted: 



As a rule the proportion of the wetted perimeter extending across 

 the bottom far exceeds that on the two sides ; therefore the prepon- 

 derance of influence comes from the bottom. If this be of smooth 

 concrete and the sides are reasonably smooth, then the value of n 

 approximating that in average concrete may be used. If the sides 

 are unchinked, they will be rougher than the sides of the average 

 earth canal, but again the bottom must be given the most weight, 



VALUES OF n FOR EARTH CHANNELS. 



The value of n in earth channels extends over a far greater range 

 than in any other material. More complex conditions, more permu- 

 tations and combinations of conditions exist than are possible in a 

 channel that reasonable velocities can not erode. If kept clean, con- 

 crete, wood, or steel must maintain about the same cross section; as 

 a rule, uniform. Earth, on the other hand, may form a definite 

 boundary of a channel when new, but after a few years of operation 

 the character of the boundary has entirely changed. Grass, weeds, 

 and fibrous roots may form the material for nearly vertical sides, 

 while the bottom may silt up or scour deeper, be smooth, or deeply 

 pocketed. A distinct trapezoidal form changes to a segment of an 

 elUpse, silt depositing in the lower corners, while the middle of the bed 

 remains about the same or becomes strewn with rocks or gravel. 



The values of n given in the folio wmg list cover the standard con- 

 ditions, while a study of the descriptions of the channels under the 

 earth-channel headings will disclose the influence that changes the 

 value of n from one of these standards. 



I. n = 0.016 for excellent conditions of earth channels. The 

 velocity to be so low that a slick deposit of silt may accumulate, or 

 the natural material be such as to become smooth when wet. The 

 influence of vegetation at the edges to be a minimum. The water to 

 be free from moss and other aquatic growth. The alignment to be 

 free from b''nfls and sharp curves. 



II. 71 = 0.020 for well-constructed canals in firm earth or fine, 

 packed gravel where velocities are such that silt may fill the inter- 

 stices in the gravel. The bunks to be clean-cut and free from dis- 

 turbing vegetation. Tlu^ alignment to bo reasonably straight. 



III. 71 = 0.0225, although carried to one more significant figure, 

 hi given for the reason that it has long been used for tliis ty])e and 

 the tests do not disclose any reason for changing. This value for the 

 ttverago well-constructed canal in material which will (wentually 



