THE OBSEBVATIONS. 



35 



TERMS. 



Load. The quantity of debris transported 

 by a stream through any cross section iu a unit 

 of time is its load at that section. A part is 

 carried in suspension and a part by traction, 

 but as we are here concerned with traction 

 only, the fractional load is to be understood 

 when the word is used without specification. 

 Load is measured in grams per second, gm./sec. 

 For certain engineering purposes it is desirable 

 to consider load as volume, not as the sum of 

 the volumes of individual grains, but as the 

 gross space occupied by the debris as a natural 

 deposit. For the debris used in the experi- 

 ments in stream traction, with the mixture 

 of sizes ordinarily found in a river deposit, the 

 weight of 1 cubic foot is about 50,000 grams, or 

 110 pounds. The symbol for load is L. 



Capacity. The maximum load a stream 

 can carry is its capacity. It is measured in 

 grams per second, gm./sec. As the work of 

 the laboratory was largely to determine 

 capacity by measuring maximum load, the 

 two terms are to a large extent interchangeable 

 in the discussion of laboratory data, but the 

 distinction is nevertheless important. -The 

 symbol for capacity is 0. 



Capacity is a function of various conditions, 

 such as slope and discharge, and the chief 

 purpose of the laboratory investigation was to 

 discover the relations of capacity to conditions. 

 When a fully loaded stream undergoes some 

 change of condition affecting its capacity, 

 it becomes thereby overloaded or underloaded. 

 If overloaded, it drops part of its load, making 

 a deposit. If underloaded, it takes on more 

 load, thereby eroding its bed. Through these 

 reactions the profiles of stream beds are 

 adjusted, so far as stream beds are composed of 

 debris. If the bed is of rock in place, the under- 

 loaded stream can not obtain its complement 

 of debris, but nevertheless it attacks the bed. 

 By dragging debris over the rock it files or 

 corrades the bed of its channel. It is a general 

 fact that the loads of streams flowing on bed- 

 rock are less than their capacities. 



Competence. Under certain combinations of 

 controlling factors capacity is zero or negative. 

 If then some one factor be changed just enough 

 to render capacity positive, that factor in its 

 new condition is said to be competent, or else 



to be a measure of the stream's competence. 

 For example, a stream at its low stage can not 

 move the debris on its bed; with increase of 

 discharge a velocity is acquired such that 

 traction begins; and the discharge is then said 

 to be competent. A stream flowing over 

 a too gentle slope has no capacity, but coming 

 to a steeper slope it is just able to move debris; 

 the steeper slope is said to be competent. A 

 current flowing over debris of various sizes 

 transports the finer but can not move the 

 coarser; the fineness of the debris it can barely 

 move is the measure of its competence. 



Discharge. The quantity of water passing 

 through any cross section of a stream in a unit 

 of time is the discharge of the stream at that 

 point. It is measured in cubic feet per second, 

 ft. 3 /sec. The symbol is Q. 



Slope. The inclination of the water surface 

 in the direction of flow is known as the slope of 

 the stream. It is the ratio which fall, or loss 

 of head, bears to distance in the direction of 

 flow. Per cent slope, as explained on page 34, 

 is numerically 100 tunes as great, .being the 

 fall in a distance of 100 units. The terms 

 slope and per cent slope are also applied in 

 this report to the inclination of the bed of the 

 channel. 



Size of debris. The relative magnitude of 

 the debris particles making up the load may be 

 considered from two opposed viewpoints. 

 Thus we may say that the load varies inversely 

 with the coarseness of the debris, or that it 

 varies directly with the fineness. The second 

 viewpoint is here preferred because it conduces 

 to symmetry in formulation. Two very dif- 

 ferent measures of fineness have been con- 

 sidered, and on page 21 the material of the 

 laboratory is listed under both. One defines 

 fineness by the number of particles which, 

 placed side by side, occupy the linear space of 

 1 foot. The other defines it by the number of 

 particles required to fill the space of 1 cubic 

 foot. Linear fineness is the more readily 

 conceived, because it appeals to vision. Bulk 

 fineness is the more easily determined. The 

 symbol of linear fineness is F, of -bulk fine- 

 ness F t . 



Form ratio. Of the variable factors which 

 in combination produce the multifarious chan- 

 nel forms of natural streams, the laboratory 

 dealt extensively with but a single one, the 



