FLOW AROUND BENDS 115 



An important secondary effect of the changes in surface elevation at 

 the beginning of a curve is the disturbance of the velocity distribution 

 in the cross section. The relative surface elevation at the outer bank 

 is accompanied by a reduction in velocity in that region, while the sur- 

 face depression at the inner bank produces a local acceleration of 

 velocity. This condition may usually be observed at the beginning of a 

 sharp bend in a river channel, and produces an appearance as though 

 the rapidly moving water had moved over from the center of the channel 

 to a course near the inner bank. It should be remembered, however, 

 that this appearance is partly deceptive. The rapidly moving water 

 near the inner bank is in a large part the same water which was near 

 the bank farther up stream, moving slowly, and which has been acceler- 

 ated at the approach of the curve. 



At the lower end of the curve another adjustment of the transverse 

 surface profile must be effected in joining to the straight channel. As 

 before, it seems to be easier for the higher surface, now at the outer 

 bank, to be depressed, than for the lower surface, now at the inner bank, 

 to be raised, in order that the two may be adjusted to a common level. 

 Observation of actual channels shows that the longitudinal slope often 

 becomes nearly level at the inner bank for a short distance, and the 

 major part of the necessary surface adjustment is obtained by a rather 

 sudden drop of the water surface along the outer bank. Just as was 

 true at the beginning of the curve, the sudden drop of the water surface 

 is necessarily accompanied by a corresponding acceleration of velocity, 

 but with this important difference. At the upper end of the curve, the 

 water receiving the acceleration was in that part of the stream near the 

 bank which had previously possessed a relatively low velocity; there- 

 fore, even after being accelerated its velocity was not conspicuously 

 high. At the lower end of the curve, however — particularly on a long 

 curve of 90 degrees or more — the water receiving the sudden acceler- 

 ation near the outer bank is the water already possessing the highest 

 velocity of any in the cross section, having been thrown outward 

 towards the outer bank by centrifugal action during its progress around 

 the curve; therefore, after being accelerated, its velocity is conspicu- 

 ously high, often greater than at any other point within a considerable 

 distance either up or down stream. This high velocity close to the 

 outer bank often produces heavy erosion. Though erosion often takes 

 place near the inner bank at the beginning of the curve, it is generally 

 much less extensive than that to be seen near the outer bank at the 

 lower end of a curve. 



Curves in open channels are objectionable, first, because they tend 

 to increase frictional losses; and second, because they increase the 

 danger of serious local erosion. Where bank protection becomes neces- 



