APPLICATION TO NATURAL STREAMS. 



229 



barely competent to suspend debris of that 

 grade, and the stream is then loaded to its full 

 capacity. 



In a natural stream the suspended particles 

 are of many sizes, and all tend to reduce ve- 

 locity. Some of the particles are near the 

 limit of competence, and any increase of load 

 will so reduce velocity that these can no longer 

 be upheld. As to these coarsest particles the 

 stream is loaded to full capacity, but not as to 

 finer material. The addition of fine material 

 will cause the arrest of some of the coarser, 

 but will increase the total load. An exactly 

 parallel statement may be made as to the trac- 

 tional load. 



To recur to the laboratory determinations of 

 retardation by suspended material, it is of in- 

 terest to note that the loads tested in the ex- 

 periments, while greater than those ordinarily 

 found in rivers, are representative of flood 

 conditions in the more turbid streams. The 

 load of 2.15 per cent, which caused retardations 

 of 3 to 15 per cent, is equaled by ordinary floods 

 of the Colorado of the West and is exceeded 

 by ordinary floods of the Rio Grande and the 

 Pecos. For the Rio Grande there are several 

 records in the neighborhood of 10 per cent, 1 

 and small streams in arid lands are liable to 

 receive similar loads as a result of violent local 

 storms. On the other hand, the Mississippi 

 near its mouth carries an average load of only 

 0.07 per cent, with a recorded maximum of 

 0.8 per cent. 2 



An attempt was made to measure the re- 

 tardation of the current by tractional load. It 

 was assumed that the bed resistance of a loaded 

 stream has two parts, one due to the texture 

 of the bed, the other to the work of traction, 

 and that the reduction of velocity by traction 

 could be measured by comparing speeds of 

 loaded and unloaded streams on beds of the 

 same texture and slope. A series of experi- 

 ments were accordingly performed with un- 

 loaded streams flowing over beds composed of 

 fixed grains of debris, or d6bris pavements, 

 and it was thought tliat their mean velocities 

 would be materially higher than those observed 

 with streams otherwise similar but bearing 

 loads. When the comparison was made, how- 



1 U. 8. Geol. Surrey Water-Supply Paper 274, pp. 102-104, 1911. 



2 The average is on authority of Humphreys and Abbot (Physics and 

 hydraulics of the Mississippi); the maximum is from an observation 

 reported by 3. A. Seddon (Rept. Chief Eng. U. S. A., 1887, p. 3094) 

 "231.9 grams per cubic foot." 



ever, it was found that in 11 out of 16 experi- 

 ments the observed velocities were higher witli 

 loaded than' with unloaded streams; and the 

 average of the 16 results was of the same 

 tenor, ascribing a slight excess of velocity to 

 the loaded streams. As the estimates of mean 

 velocity had been based on observations of 

 depth, and as the observations of depth were 

 difficult in the case of loaded streams, it was 

 thought possible that the result was affected 

 by a systematic error in those depth measure- 

 ments which had been made by means of the 

 gage (p. 25). Examples were therefore sought 

 in which the depth had been obtained by com- 

 paring the average height of the water surface 

 during the run with the average height of the 

 debris bed after the water had been drawn off. 

 Thirteen instances were found in which such 

 better-conditioned measures could be com- 

 pared with measures of unloaded streams, and 

 in 11 of the 13 comparisons the unloaded 

 streams gave the higher mean velocities. The 

 average of the 13 velocities found for loaded 

 streams was 10 per cent less than the corre- 

 sponding average for unloaded streams. The 

 best of the available data, therefore, give evi- 

 dence of the consumption of stream energy by 

 traction, but the evidence is not so consistent 

 as to free the matter from doubt. In Table 80 

 V m i and V mu represent the mean velocities of 

 loaded and unloaded streams, and the various 

 data are arranged according to the magnitude 

 of the ratio of these velocities. This arrange- 

 ment brings out the apparent fact that the re- 

 duction of mean, velocity by traction is greater 

 for small loads than for large, for gentle slopes 

 than for steep, for low velocities than for high, 

 and for large depths than for small. While it 

 is entirely possible that such a result a result 

 opposed to my preconceptions has been occa- 

 sioned by a systematic error of observation, I 

 am disposed to regard it rather as the expres- 

 sion of some physical law which has escaped 

 my analysis. It is possibly connected with a 

 fact brought out in the following chapter in 

 the discussion of vertical velocity curves the 

 fact that addition of load hjas a pronounced 

 influence on the distribution of velocities, in- 

 creasing the contrast between velocities near 

 tne bed and the mean velocity. 



If the variations of the load effect be ignored 

 and attention given only to the means of quan- 

 tities compiled in the table, it appears that 10 



