158 gilbert: transportation of debris 



energy, reduces velocity and thus reduces capacity for all parts 

 of the load. This principle affords a condition by which total 

 capacity is limited. Subject to this condition a stream's load 

 at any time is determined by the supply of debris and the fineness 

 of the available kinds. 



Flume transportation. In the experiments described above — 

 experiments illustrating stream transportation — the load trav- 

 ersed a plastic bed composed of its own material. Other 

 experiments were arranged in which the load traversed a rigid 

 bed, the bottom of a flume. Capacities are notably larger for 

 flume transportation than for stream transportation, and their 

 laws of variation are different. Rolling is an important mode 

 of progression. For rolled particles the capacity increases with 

 coarseness, for leaping particles with fineness. Capacity in- 

 creases with slope and usually with discharge also, but the rates 

 of increase are less than in stream transportation. Capacity is 

 reduced by roughness of bed. 



Vertical velocity curve. The vertical distribution of velocities 

 in a current is controlled by conditions. The level of maximum 

 velocity may have any position in the upper three-fourths of the 

 current. In loaded streams its position is higher as the load is 

 greater. In unloaded streams its position is higher as the slope 

 is steeper, as the discharge is greater, and as the bed is rougher. 



Pilot tube. The constant of the Pitot velocity gage — the 

 ratio between the head realized and the theoretic velocity head — 

 is not the same in all parts of a conduit, being less near the water 

 surface and greater near the bottom or side of the conduit. 



