178 



TRANSPORTATION OF DEBRIS BY RUNNING WATER. 



ing to the coarser grade (D), testifies again to 

 the advantage for traction of a mixture as 

 compared to a grade of narrow range in fineness. 

 It will be observed also that each of the 

 curves from the natural grade resembles closely 

 its neighbors from artificial grades. So far as 

 their evidence goes the type is the same for 

 both, and the tendency of their evidence is to 

 show that the laws connecting capacity with 

 slope, as developed by the study of sorted 

 debris, apply also to unsorted stream alluvium. 



300- 



200 



100- 



fA) 



I 2 



Slope 



FIGURE 62. Capacity-slope curves for a natural grade of de'bris, com- 

 pared with curves for sieve-separated grades. 



Table 63 (p. 180) gives computed finenesses 

 for various mixtures and for this natural grade 

 of de'bris. The fineness of the natural grade is 

 nearly identical with that of grade (C), and the 

 two thus afford a direct comparison between the 

 capacities of a natural grade and a narrowly 

 limited grade. For the same discharge and 

 slope their tabulated capacities are respectively 

 168 and 143 gm./sec., the ratio of advantage to 

 the natural grade being 1.17. The computed 

 fineness of grade (CDEFG) does not corre- 

 spond to that of any simple grade, but a com- 

 parison made by means of interpolation gives 

 150 and 131 gm./sec. as corresponding capaci- 

 ties for the mixture and a simple grade of the 

 same mean fineness, and the ratio of the first 

 to the second is 1.15. These ratios are smaller 

 than those estimated from data for binary 

 mixtures (p. 174), but are coordinate in value. 

 The question of relative authority will be con- 

 sidered later. 



CAUSES OF SUPERIOR MOBILITY OF 

 MIXTURES. 



When a finer grade of debris is added to a 

 coarser the finer grains occupy interspaces 



among the coarser and thereby make the sur- 

 face of the stream bed smoother. This quality 

 of smoothness appealed to the eye during the 

 progress of the experiments. One of the 

 coarser grains, resting on a surface composed 

 of its fellows, may sink so far into a hollow as 

 not to be easily dislodged by the current, but 

 when such hollows are partly filled by the 

 smaller grains its position is higher and it can 

 withstand less force of current. In other 

 words, the larger particles are moved more 

 readily on the smoother bed, and this fact also 

 was a matter of direct visual observation. The 

 promotion of mobility applies not only to the 

 starting of the grain but to its continuance in 

 motion. It encounters less resistance as it 

 rolls or skips along the bed, and it is less apt to 

 be arrested. When a single large particle 

 travels along a bed composed wholly of grains 

 much smaller it rarely leaps, but rolls instead, 

 and it must in general be true that the larger 

 particles in mixtures roll more and skip less 

 than their smaller companions. 



The admixture of finer debris thus changes 

 the mode of traction for the coarser, and it is 

 believed that the enhanced capacity is due 

 mainly to this change. Capacity for the 

 coarser is increased because the new condition 

 reduces its resistance to the force of the 

 current. 



The fact that under some conditions the 

 capacity for fine material is slightly increased 

 by the addition of coarser is not so easily ex- 

 plained. The coarser grams do not make the 

 bed smoother but rougher. The rougher bed 

 retards the current. Even while rolling the 

 larger grains are holding back the water, and 

 the larger grains reduce the area of bed on 

 which the traction of the smaller takes place. 

 In these ways the presence of the coarse mate- 

 rial tends to reduce the capacity of the current 

 for the fine, and these factors certainly seem 

 adequate to explain the general fact that 

 capacity for the finer debris is reduced by ad- 

 mixture of the coarser. 



Two factors may be named with the opposite 

 tendency. The first is the impact of the 

 coarser particles. In rolling and leaping they 

 disturb the finer, tending thus to dislodge them 

 from their resting places and either start them 

 forward or else give them new positions from 

 which they may be more easily swept. The 

 second is the production of diversity in the 



