48 BULLETIN 194, U. S. DEPAETMENT OF AGKICULTUEE. 



II. n=0.01'S for construction as in type I, but with curves as in tlie 

 usual mountain canyon. Same construction and alignment as in 

 type I, but mtli small amount of sand or debris in water. Construc- 

 tion as in type III, but in very favorable alignment or for water that 

 carries a small amount of fine silt that will eventually form a slick 

 coat. 



III. 72,=o.014 for hnings made by good construction under favor- 

 able conditions. The surface to be as left by smooth- jointed forms 

 or to be roughly troweled. Joints to be good, but causing some re- 

 tardation. Alignment about equal in curves and tangents, with no 

 spirals between. The bed to be clean and sides free from rough 

 deposits. In the opinion of the writer this is the value to use for 

 most linings on moderate-sized channels. 



IV. 71^0.015 for construction as in type III, but with sharp curves 

 and clean bottom or moderate curves and much debris on the bottom 

 but clean-cut sides. 



V. 71=0.016 for concrete as constructed by the average gang of 

 laborers, using forms that leave prominent hnes at the cracks, no 

 finish coat being appUed. Bed to have the usual small amount. of 

 rock fragments and patches of sand and gravel. Average amount of 

 curvature. In chmates where a rough deposit accumulates, as in 

 southern Cahfornia, a hning that originally had a value of n about 

 0.013 quickly assumes about this type. For this reason it appears 

 to the writer that labor and money spent in securing a very smooth 

 surface is lost where the deposit accumulates on smooth or rough ahke. 



VI. 71^=0.017 for roughl}^ coated hnings with uneven joints. This 

 value also is apphcable where rough deposits accumulate on the sides 

 and conditions of ahgnment are poor. 



VII. ri = 0.018 for very rough concrete with sharp curves and 

 deposits of gravel and moss. A broken gradient, irregular cross 

 section, and the like, contribute to such a high value of n. 



Where experiments show higher values of n than are given above 

 for concrete linings, the conditions are such, as a rule, that the con- 

 taining material has lost its identity as concrete or cement, and thick 

 coatings of sand, accumulations of moss, or deposits of sand change 

 the general classification of the channel. 



VALUE OF n FOR WOODEN FLUMES. 



While experimental flumes have been constructed that showed 

 values of n in the neighborhood of 0.009, yet in making tests on 

 flumes in commercial service values of n below 0.012 are so rare tliat 

 it is not thought advisable to recommend any value less than this. 

 Wood is subject to so many changes, due to the influences of climate, 

 wind, settling of earth, moss accumulations, warping, and so on, that 

 even when well constructed the value of n becomes greater after a 



