208 



TRANSPORTATION OF DEBRIS BY RUNNING WATER. 



tinues unless the load is reduced considerably 

 below the normal. In practical operations 

 the first step toward the abatement of a clog 

 is te stop all feeding of load above the deposit, 

 that the stream may be able to take on load 

 and thereby reduce the deposit. 



CAPACITY AND SLOPE. 



The rate at which capacity for flume traction 

 is increased by increase of slope is contained 

 implicitly in the values of n and a assembled in 

 Table 68; for n gives the rate of variation of 

 capacity with 8 a, and the instantaneous rate 

 of variation of capacity with slope is given by 



ij = -q . All the tabulated values of n except 



two are greater than unity; and in each of the 

 cases where n is less than unity all values of \ 

 computed for the range of slopes covered by the 



experiments are greater than unity. The gen- 

 eral fact is thus indicated that, within the prac- 

 tical range of conditions, capacity increases 

 with slope in more than simple ratio. Effi- 

 ciency also increases as slope increases. 



The sensitiveness of capacity to changes in 

 slope varies with changes of condition; and this 

 variation might be illustrated, as in treating of 

 stream traction, by the tabulation and discus- 

 sion of values of the index of relative variation, 

 *!- It will suffice, however, in this case to 

 make comparisons by means of the synthetic 

 index, 7 t . Table 70 contains values of that in- 

 dex computed between the limits of S = 2.0 and 

 8 = 3.5. They represent 42 of the 51 series of 

 values given in Table 68, the other nine series 

 not having sufficient range for the computa- 

 tion. The arithmetical mean of the 42 values 

 of 7 t is 1.46, and the range is from 1.08 to 2.08. 



TABLE 70. Values of /, for flume traction, computed between the limits S2.0 and S=3.5. 



Inspection of these data shows, first, that 

 the values are always greater for Q = 0.363 

 than for Q = 0.734. The experiments deal 

 with no other discharges, but it is probably 

 true in general (as in case of stream traction) 

 that increase of discharge is accompanied by 

 decrease of the sensitiveness of capacity to 

 slope. 



If the index varies in a systematic way with 

 fineness of d6bris, its increase is connected 

 with decrease of fineness, but the finest debris 

 of the table, (C), carries large values of the 

 index. The apparent conflict of evidence has 

 its parallel in the fuller data for stream trac- 

 tion (see p. 108 and fig. 34), and it is possible 

 that the sensitiveness increases in two direc- 

 tions from a minimum value. Its variation 

 might in that case be connected with the law 

 relating capacity to fineness, as brought out in 

 a later section. 



The relation of the index to roughness of bed 

 does not follow a simple law. Its values are 

 in general least for the bed of rough lumber and 

 progressively greater for planed lumber, wood 

 blocks, and gravel pavement. 



The greater sensitiveness of capacity to slope 

 when the channel bed is a coarse pavement 

 may be connected with the fact that the mode 

 of transportation over such a bed is approxi- 

 mately stream traction; and this suggests that 

 in flume traction the sensitiveness may be less 

 than in stream traction. Direct comparison 

 can not be made with use of the values of 7, 

 in Table 70, because the slopes used in stream 

 traction experiments have less range; but spe- 

 cial computations were made, so far as the data 

 were found to overlap. The results are con- 

 tained in Table 71 and indicate that the sensi- 

 tiveness is greater for stream traction than for 

 flume traction over a smooth bed, in case of 

 grades (E) and (G), but less in case of grade (C). 



