156 gilbert: transportation of debris 



estimates of capacity. The ratio of depth to width is a more 

 important factor. For any combination of slope, discharge, 

 and fineness it is possible to reduce capacity to zero by making 

 the stream very wide and shallow or very narrow and deep. 

 Between these extremes is a particular ratio of depth to width, 

 p, corresponding to a maximum capacity. The values of p 

 range, under laboratory conditions, from 0.5 to 0.04, being greater 

 as slope, discharge, and fineness are less. 



Velocity. The velocity which determines capacity for bed 

 load is that near the stream's bed, but attempts to measure bed 

 velocity were not successful. Mean velocity was measured 

 instead. To make a definite comparison between capacity and 

 mean velocity it is necessary to postulate constancy in some 

 accessory condition. If slope be the constant, in which case 

 velocity changes with discharge, capacity varies on the average 

 with the 3.2 power of velocity. If discharge be the constant, in 

 which case velocity changes with slope, capacity varies on the 

 average with the 4.0 power of velocity. If depth be the constant, 

 in which case velocity changes with simultaneous changes of 

 slope and discharge, capacity varies on the average with the 3.7 

 power of velocity. The power expressing the sensitiveness of 

 capacity to changes of mean velocHy has in each case a wide 

 range of values, being greater as slope, discharge, and fineness 

 are less. 



Mixtures. In general, debris composed of particles of a single 

 size is moved less freely than debris containing particles of many 

 sizes. If fine material be added to coarse, not only is the total 

 load increased but a greater quantity of the coarse material is 

 carried. 



Modes of transportation; movement of particles. Some par- 

 ticles of the bed load slide; many roll; the multitude make short 

 skips or leaps, the process being called saltation. Saltation 

 grades into suspension. When particles of many sizes are moved 

 together the larger ones are rolled. 



Modes of transportation; collective 7novem.ent. When the condi- 

 tions are such that the bed load is small, the bed is molded into hills, 

 called dunes, which travel downstream. Their mode of advance 



