The characteristics of turbulence in a shear zone downstream of a channel constriction in a coarse-grained forced pool

A field-based project was initiated to characterize the influence of varying discharge and bed topography along a pool exit-slope on the strength of turbulence generated by vortex shedding. Velocities were measured with an ADV downstream of a boulder constriction within a shear zone of a large pool. Measurements were repeated for four flows that varied from 20% to 50% of bankfull discharge. An additional longitudinal profile was conducted along the pool thalweg. Plots of velocity demonstrate high near-bed velocities and turbulence levels in the pools. Turbulence levels were highest closer to the constriction in the shear zone. Turbulent kinetic energy decayed in the downstream direction in response to the bed topography and at lower discharges. Instantaneous velocities are large enough to temporarily lift pebbles at this depth. It appears that instantaneous forces created by vortex shedding may play an important role in scouring pools downstream of constrictions. The turbulence may also respond to changes in bed topography in a manner that encourages feedbacks among bed topography, characteristics of turbulence, and sediment transport.