Intriguing velocity patterns near the stream edge

Posted on December 15, 2011

Complex flow patterns in streams affect channel form, the transport of sediment and contaminants, and the distribution of aquatic flora and fauna. Although hydraulic studies in artificial channels can shed light on these processes, most hydraulic measurements have been made under conditions of uniform flow in smooth-sided flume tanks. Until last year, no articles had described how velocity distributions are affected by the interaction between non-uniform (accelerating or decelerating) flow and the presence of bankside vegetation in gravel-bed channels. This deficit has now been remedied by water engineers using an experimental flume (8 m long, 0.4 m wide, 0.6 m high), the sides of which were covered by rice stems to simulate vegetated banks. The thickness of the gravel layer on the bottom was varied to give an upward slope of 2% along the flume. The maximum flow rate was 0.05-m3 / s and velocities were measured using a Doppler meter. In mid-stream, velocities were low very close to the bottom but rose at a smoothly accelerating rate as the height above the bed increased, approaching their maximum at about 0.2 times the total depth. In contrast, close to the edge of the channel the velocity profile was S-shaped: with increasing height above the bed, velocities at first rose, then fell, and then rose again at a decelerating rate. As a result, near the channel edge the maximum velocity was reached much higher up in the stream, at about 0.7 times the total depth. S-shaped velocity profiles, presumably caused by secondary currents near the vegetated banks, were not found in previous studies with smooth-sided flumes. The new study also found that levels of shear stress (turbulent fluctuations in momentum) were much higher near the edge than in the body of the channel, being greatest at mid-depths. The implications of such velocity patterns for material transport and microhabitat use in natural streams remain to be explored.

Reference: Afzalimehr, H., Najfabadi, E.F. & Singh, V.P. 2010. Effect of vegetation on banks on distributions of velocity and Reynolds stress under accelerating flow. Journal of Hydrologic Engineering 15 (9), September 1, 2010.