CHAPTER XXIX 

 REPORTS ON HYDRAULIC MINING DEBRIS 



TRANSPORTATION OF DETRITUS 



The preface to Professional Paper No. 85 explains the relation of Gilbert's experimental 

 work in California to his earlier studies by an allusion to the third part of the Henry Mountains 

 report: 



Thirty-five years ago the writer made a study of the work of streams in shaping the face of the land. The 

 study included a qualitative and partly deductive investigation of the laws of) transportation of debris by 

 running water; and the limitations of such methods inspired a desire for quantitative data, such as could be 

 obtained only by experimentation with determinate conditions. The gratification of this desire was long deferred 

 but opportunity for experimentation finally came in connection with an investigation of problems occasioned 

 by the overloading of certain California rivers with waste from hydraulic mines. 



A primary purpose of the investigation was then stated to be the determination of the 

 relation which the load that is swept along the beds of actual rivers bears to various important 

 factors, such as river volume, velocity, slope, etc., but the complications of the natural problem 

 were found to be such that this purpose was not attained. A secondary purpose was to study 

 the mode of propulsion of the load and the laws connecting the total load with each of the 

 chief factors taken separately; and in this direction a greater success was achieved. 



The experiments, conducted at Berkeley chiefly by E. C. Murphy under Gilbert's direc- 

 tion, were made in horizontal troughs, 31.5 and 150 feet long, respectively, through which a 

 controlled current of water flowed. Sand of measured fineness was fed into the current at a 

 controlled rate until the slope of the sanded trough bed automatically came^to be just sufficient 

 to enable a specified current to transport a specified quantity of a specified kind of sand sup- 

 plied to it. By varying the different factors involved many combinations resulted, of which 

 130 were especially studied, an average of 10 determinations being made for each combination; 

 thus quantitative relations were obtained between width, depth, volume, slope, and velocity of 

 current, and texture and quantity of load. The problem was one in which Gilbert's exceptional 

 powers of mental and mathematical analysis were called into play, as all the experimental 

 data were submitted to computation in a critical fashion. As a measure of the complexity 

 of the computations, it may be noted that over 80 letter symbols were employed in the mathe- 

 matical expression of the relations studied. A characteristic indication of the nature of the 

 considerations which entered the problems treated is given in the following extracts from the 

 chapter on the relation of transportation capacity to "form ratio"; or ratio of stream 

 depth to width. 



When identical discharges are passed through troughs of different width and are loaded with d6bris of the 

 same grade [texture] and the loads are adjusted so as to establish the same slope, it is usually found, not only 

 that the capacity varies with the width, but that some intermediate width determines a greater capacity than 

 do the extreme widths. That is, the curve of capacity in relation to width exhibits a maximum. . . . The 

 explanation of the maximum, so far as its main elements are concerned, is not difficult. . . . Conceive a stream 

 of constant discharge and flowing down a constant slope but of variable width. The field of traction is deter- 

 mined by the width, and the evident tendency of this factor is to make the capacity increase as the width 

 increases. The rate of traction for each unit of width is determined by the bed velocity in that unit and the bed 

 velocity is intimately associated with the mean velocity. Velocity varies directly with depth, and, inasmuch 

 as increase of width causes (in a stream of constant discharge) decrease of depth, the tendency of this factor 

 is to make capacity decrease as width increases. Velocity is also affected by lateral resistance, the retarding 

 influence of the side walls of the channel. The retardation is greater as the wall surface is greater, therefore 

 as the depth is greater, and therefore as the width is less. As capacity varies inversel} 7 with the retardation, 

 and the retardation varies inversely with width, it follows that the tendency of this factor is to make capacity 

 increase as width increases. Thus the influence of width on capacity is threefold: Its increase (1) enlarges 

 capacity by broadening the field of traction, (2) reduces capacity by reducing depth, and (3) enlarges capacity 

 by reducing the field of the side-wall resistance. Now, without inquiring as to the laws which affect the several 

 factors, it is evident that when the width is greatly increased a condition is inevitably reached in which the 

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