58 



TRANSPORTATION OF DEBRIS BY RUNNING WATER. 



15; EGB is the profile of one side wall. The 

 walls converge from F to G (see fig. 3), pro- 

 ducing the contraction at the outfall. It was 

 found that the bed of debris, instead of run- 

 ning to a feather edge at D (compare fig. 16), 

 held its thickness to C' and ended in a steep 

 incline. 



The device of contraction accomplished its 

 purpose of avoiding terminal difficulties, but it 

 was found to aggravate certain other difficul- 

 ties, next to be described. 



RHYTHM. 



Whenever the profile of a current was deter- 

 mined by a series of measurements applied to 

 the water surface, that surface was found to be 

 in a state of unrest. Its position in any ver- 

 tical fluctuated upward and downward rhyth- 

 mically. The amplitude of oscillation, which 

 might be great or small, was not constant: 



that is, the rhythm was not simple, but com- 

 pound. It consisted apparently of many 

 rhythmic elements differing one from another 

 in period and amplitude. 



The rhythmic quality, thus easily appre- 

 ciated by watching the play of the surface in 

 relation to a fixed point, permeated every 

 function of the current the slope of its profile, 

 both local and general, the slope of its bed, the 

 quantity of debris transported, the mode of its 

 transportation. The rhythm of the dune has 

 already been described, but associated with the 

 dunes were greater debris waves, also traveling 

 downstream and each involving the volume of 

 many dunes. In the bed of the long trough a 

 series of them could be seen; in the shorter 

 trough one or two might be made out, or the 

 effect might be only an alternate temporary 

 steepening and flattening of the general slope. 

 The rhythm of the antidune was accompanied 



FIGURE 18. Profiles ol channel bed, illustrating fractional rhythms associated with dunes of greater magnitude. Scales: Each horizontal 

 space =20 feet; each vertical space =0.4 foot. For Nos. 1 to 5 the average slope is 0.2 per cent, and the average load (of grade (C)) per foot 

 of channel width is 7 gm./sec. For Nos. 6 to 9 the average slope is 0.6 per cent and the average load 37 gm./sec. 



by rhythmic paroxysms and doubtless by 

 other rhythms which escaped recognition 

 because the steep slopes with which the anti- 

 dune was associated were not studied in the 

 long trough. Figure 18 shows a few channel- 

 bed profiles in which rhythmic features appeal 

 to the eye, but the greater number of such pro- 

 files merely show an irregularity in which 

 periodicity is not conspicuous. It is probable 

 that the currents were affected by numerous 

 coexistent rhythms, which served to confuse 

 one another and thus masked periodicity except 

 when some one rhythm was stronger than the 

 rest. 



The condition of relatively smooth channel 

 bed which intervened between the conditions 

 characterized severally by dunes and anti- 

 dunes was also a condition of relative uni- 

 formity in all the activities of the current, and 

 when it prevailed the rhythmic variations were 

 at a minimum. 



The rhythms of the transporting stream 

 manifestly constitute a group of phenomena 

 worthy of systematic study, but the Berkeley 

 laboratory, having a definite and different 

 theme, treated them only as difficulties inter- 

 fering with its work. It sought the capacity 

 for load inhering in the average of all the 

 diverse slopes presented by the rhythms, and 

 it necessarily treated the deviations of slope 

 measurements from that average as accidental 

 errors. 



The rhythms affected the determinations of 

 loads as well as slopes. The variations of 

 profile were effected by erosion and deposi- 

 tion, and a current which was eroding or 

 depositing carried more load at one point 

 than at another. As the loads were largely 

 determined by weighing the debris delivered 

 at the trough end in a limited time, the amount 

 obtained would depend in part on the phase of 

 slope variation near the point of delivery. 



