212 



TRANSPORTATION OF DEBRIS BY RUNNING WATER. 



slope and discharge are so adjusted as to give 

 a moderate velocity, the progression of sand 

 may be by rolling, and in that case the capacity 

 for different sands will vary inversely with their 

 fineness. But if over the same smooth bed the 

 current runs swiftly, sand will be made to 

 travel by saltation, or by saltation and suspen- 

 sion, and then the capacity for different sands 

 will vary directly with their fineness. The par- 

 ticular velocity with which the function re- 

 verses will depend on the quality of the bed, 



being lower if the bed is somewhat rough, 

 because roughness changes rolling to saltation. 

 The critical velocity will be higher for gravel 

 than for sand, because higher velocity is 

 needed to make coarser debris leap. 



The experiments which have been made were 

 not sufficiently varied to afford test for these 

 inferences, and as there is no present oppor- 

 tunity for continuance of laboratory work the 

 inferences must be regarded as largely hypo- 

 thetic. 



TABLE 74. Capacities for flume traction of mixed grades and their component simple grades. 



MIXTURES. 



Three mixtures of simple grades of debris 

 were treated in the laboratory. The compo- 

 nents of one, (E^), were a medium sand and 

 a coarse sand. The others, (E 2 HjI 2 ) and 

 (E 3 H 2 I 3 J 2 ), combined medium sand with fine 

 gravel and coarser gravel. All three were 

 tested in relation to slope on the smooth chan- 

 nel bed and on the gravel pavement one on 

 the rough-sawn bed and two on the bed of 

 wooden blocks. One entered into the experi- 

 ments on capacity in relation to discharge. 

 The results are contained in Tables 67, 68, and 

 72. The data from those tables which pertain 

 to a channel width of 1 foot and a channel slope 

 of 3 per cent are assembled in Table 74. The 



table contains also the capacity quota for each 

 constituent, computed from the capacity for 

 the mixture; and beneath each of these quotas 

 is printed the capacity for the constituent when 

 the entire load is composed of it. 



A general fact brought out by this table is 

 that the current can transport more of a 

 mixture than it can of any one of the constit- 

 uent grades. The table records a single excep- 

 tion, the capacity for the mixture being 1,390 

 gm./sec., while that for its coarsest constituent 

 grade is 1,415 grams. 



Another general fact shown is that the ca- 

 pacity for each component as part of a mixture 

 is less than the capacity for the same compo- 

 nent if transported separately. To this also 

 there is a single apparent exception, but as it 



