gilbert: transportation of debris 155 



load was fed to a stream of specific width and discharge, and 

 measurement was made of the slope to which the stream auto- 

 matically adjusted its bed so as to enable the current to transport 

 the load. 



The slope factor. For each combination of discharge, width, 

 and grade of debris there is a slope, called competent slope, 

 which limits transportation. With lower slopes there is no 

 load, or the stream has no capacity- for load. With higher 

 slopes capacity exists; and increase of slope gives increase of 

 capacity. The value of capacity is approximately proportional 

 to a power of the excess of slope above competent slope. If 

 S equal the stream's slope and a equal competent slope, then the 

 stream's capacity varies as {S — a)". This is not a deductive, 

 but an empiric law. The exponent n has not a fixed value, but 

 an indefinite series of values depending on conditions. Its range 

 of values in the experience of the laboratory is from 0.93 to 2.37, 

 the values being greater as the discharges are smaller or the 

 debris is coarser. 



The discharge factor. For each combination of width, slope, 

 and grade of debris there is a competent discharge, k. Calling 

 the stream's discharge Q, the stream's capacity varies as (Q — r/c)°. 

 The observed range of values for o is from 0.81 to 1.24, the values 

 being greater as the slopes are smaller or the debris is coarser. 

 Under like conditions o is less than n; or, in other words, ca- 

 pacity is less sensitive to changes of discharge than to changes of 

 slope. 



The fineness factor. For each combination of width, slope, 

 and discharge there is a limiting fineness of debris below which 

 no transportation takes place. Calling fineness (or degree of 

 comminution) F and competent fineness 0, the stream's capacity 

 varies with (F — 0)^'. The observed range of values for p is 

 from 0.50 to 0.62, the values being greater as slopes and dis- 

 charges are smaller. Capacity is less sensitive to changes in 

 fineness of debris than to changes in discharge or slope. 



The form factor. Most of the experiments were with straight 

 channels. A few with crooked channels yielded nearly the same 



^ Capacity is defined for the purposes of this paper as the maximum load of a 

 given kind of debris which a given stream can transport. 



