On large-scale flow structures in a gravel-bed river

Previous studies have suggested the presence of large-scale now structures in gravel-bed rivers. These structures are pictured as intermittent high-sp eed wedges separated by regions of lower velocity. However, the characteris tics of these structures have not been examined in detail through either vi sualisation techniques or detailed field measurements. This paper confirms the presence of large-scale flow structures in gravel bed rivers, pictures their sequence and patterns and characterises their mean and individual pro perties. The analysis relies on a new technique for displaying velocity flu ctuations in a space-time matrix that allows one to see the structures as t hey pass an array of current meters. Streamwise and vertical velocities wer e measured simultaneously with an array of three electromagnetic current me ters. The sampling frequency was 20 Hz. Five velocity profiles of up to 13 1-min series of measurements and one profile of three 20-min measurements w ere sampled. These data suggest the presence of large wedges of faster flui d joined by regions of slower fluid. Space-time correlation analysis confir med the presence of vertical coherence of the flow. The average angle of th e front of the wedges is 36 degrees. Although individual structures are var iable in size and shape, a new detection technique using all three velocity signals simultaneously showed that their average frequency is nine events per minute and their duration is more than 2 s. The high-speed wedges displ ay a complex organisation and do not show a preferred sequence of events as was postulated by previous studies. Because of their duration and size, th ese high-speed wedges are likely to play a major role in bedload sediment t ransport. (C) 2000 Elsevier Science B.V. All rights reserved.