EXPERIMENTS WITH CROOKED CHANNELS. 



197 



nate in value with those of debris slope. The 

 slopes computed from load measurements are 

 probably of less weight. 



TABLE 64. Comparison of slopes required for straight and 

 crooked channels, respectively, under identical conditions 

 of discharge, fineness, width, and load. 



[DeT>ris of grade (C); discharge, 0.363 fU/sec.; width, 1 foot.] 



NOTE. Values marked n are given greater weight than those marked 6. 



FORMS AND SLOPES. 



The combinations of bends in the experiment 

 channels are shown in figure 67. In three of 

 the channels the bends were angular; in two 

 curved. In channel I the angle of deflection 

 was 10.5; there 'was a single group of four 

 bends, returning the course to its original direc- 

 tion ; and the short reaches were approximately 

 5 feet long. In channel II the arrangement 

 was the same, -with reaches of about 2.5 feet 

 and deflection angles of 21.5. In channel III 

 were three groups of four deflections each, the 

 angles being of 40.9 and the length of reach 

 about 1.4 feet. Channel IV had the same pro- 

 portions as No. II, with the substitution of 

 curves for angles. The radius of curvature for 

 the medial line was 6.55 feet. Channel V con- 

 tained two groups of curves, each similar to the 

 group in No. IV. 



From the data (Table 64) connected with 

 channels I, II, and III, it appears that with 

 angular bends a greater slope is necessary to 

 transport the load than when the channel is 

 straight that is, the capacity is reduced by 

 angular bends. The reduction is greatest when 

 the angle of deflection is greatest, and it is so 



small for an angle of 10 as to bave doubt 

 whether it might not disappear altogether with 

 a somewhat smaller angle. 



The single group of curves (IV) appears to 

 reduce capacity slightly (increase of slope for 

 same load): but the double group (V) gives 

 slopes indicating an increase of capacity. 



As the bends of alluvial streams are curved, 

 the curved experiment channels may be as- 

 sumed to represent them better than do the 

 angular channels, and it is possible that mean- 

 dering channels have a greater capacity for 

 traction than straight channels of the same 

 length. There are, however, certain elements 

 of incompleteness in the representation which 

 make definite inference hazardous. The course 

 of a stream which shapes its own channel 

 through an alluvial plain is made up of bends 

 and reaches. In passing from reach to bend 

 there is a gradual increase of curvature until 

 the radius of curvature, for the medial line, is 

 between twice and three times the width of the 

 channel, and the change from bend to reach is 

 also gradual. The forms are automatically 

 adjusted to the system of accelerations and 

 velocities within the current. The angular 

 change of direction in the bend may be one of 

 a few degrees only, but in meandering streams 

 it is commonly from 90 to 180. In the arti- 

 ficial channels all curves were circular arcs with 

 a radius of 6.55 times the channel width; there 

 was no graduation in the radial acceleration 

 due to deflection; the change from right-hand 

 deflection to left-hand deflection was abrupt, 

 without the intervention of a reach, and the 

 changes of direction were through angles of 

 21.5 and 43. That such differences are com- 

 petent to affect transportation to a material 

 extent is indicated by the relations of deeps 

 and shoals (crossings) to bends. Fargue, 1 from 

 a discussion of an artificially adjusted portion 

 of the Garonne, reached the conclusion that the 

 distance downstream from the apex of a curve 

 to the deepest point of the associated deep and 

 the distance downstream from a point of inflec- 

 tion to the associated crossing are each nor- 

 mally one-fourth the length of a stream unit 

 defined as the portion between two points of 

 inflection. In our experiments (see fig. 68) 

 each of these distances is one-half the length 

 of the stream unit instead of one-fourth. 



1 Fargue, L., La forme du lit des rivieres a fond mobae, Paris, 1908. 



