THE RELATION BETWEEN DISCHARGE AND THE INTENSITY OF TURBULENCE ALONG AN EDDY FENCE
IN A POOL OF A ROCKY MOUNTAIN CHANNEL.
A field-based project was initiated to characterize the influence of varying discharge on the strength of turbulence generated by vortex shedding. Three-dimensional turbulence intensities were measured with an acoustic Doppler velocimeter at nine to 12 different profiles located downstream of a boulder constriction within a large pool on North Saint Vrain Creek, Colorado. Velocity profiles were located on either side of an eddy fence with the exact location varying slightly with discharge. Measurements were repeated for four different discharges varying from 20 to 50 percent of bankfull discharge. Because North Saint Vrain Creek is snowmelt-dominated, discharge conditions were relatively steady throughout the velocity measurements. Three-dimensional velocity profiles were also collected on three days for a series of similar discharges at an additional 66 locations throughout the pool and downstream riffle. Average velocities show evidence of vortex tilting along the eddy fence with bed velocities directed towards the bank and upper velocities directed towards the middle of the channel. Bed velocities, measured at 80 percent of the total flow depth, include maximum velocities in the z-direction of over 0.57 m/s with corresponding minimum values of under -0.64 m/s. At this same location, velocities in the x-direction ranged from at least -0.04 to 1.25 m/s, while velocities in the y-direction ranged from at least -0.57 to 0.45 m/s. These results show that instantaneous velocities are large enough to lift pebbles and carry them up the pool exit slope. The root mean square of x-, y- and z-components of bed velocity along the eddy fence were significantly related to both the distance from the boulder constriction and discharge. As expected, turbulence intensity decayed in the downstream direction. Turbulence also decreased at lower discharges. Based on these findings, it appears that instantaneous forces created by vortex shedding may play a more important role in scouring pools than average hydraulic conditions.
Melaine Gryboski ('01) and Jaime Goode (’02) setup the accoustic Doppler velocimeter on the North Fork Cache la Poudre River, CO (2001).
This material is based upon work supported by the National Science Foundation under Grant No. 9874751. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.
BACK