SPATIAL AND TEMPORAL PATTERNS OF VELOCITY AND TURBULENCE IN A STEP-POOL CHANNEL

Three-dimensional measurements of time-averaged and turbulent velocity components were collected using a SonTek FlowTracker Handheld Acoustic Doppler Velocimeter (ADV) on a 40-m reach of a step-pool channel in the Colorado Rockies. This work was completed to examine the effects of morphologic position and discharge on flow structure in a steep (0.11 m/m) mountain channel and to evaluate the FlowTracker ADV’s performance in this environment. Thalweg velocity profiles were measured at positions associated with various bedform types (upstream of steps, at step lips, pools below steps, and cascades); and at 5 different discharges. The data collected here quantitatively illustrate spatial and temporal patterns of turbulence generation and energy dissipation in a steep channel. Analysis of variance indicated that bedform type significantly affected mean downstream and cross-stream velocities; downstream velocities were higher at locations upstream of steps and at step lips than in pools downstream of steps. Downstream velocities also decreased significantly with decreasing discharge, although discharge effects on cross-stream and vertical velocities were not significant. Discharge and bedform type also significantly affected turbulence intensities (represented here by root mean squares of velocity) for all flow components, with the greatest turbulence intensities occurring in pools and at high discharges. The FlowTracker ADV often produced questionable data in near-bed, near-surface, and highly aerated environments, necessitating extensive data filtering and complicating interpretation of velocity and turbulence characteristics in these locations. For comparison purposes with the FlowTracker ADV, velocity measurements were also collected during one field session using a one-dimensional electromagnetic current meter (ECM). In locations with highly aerated flow (e.g., at the base of steps), ADV-measured values for downstream velocity were substantially lower than those recorded by the ECM at the same positions, although in other locations similar values were obtained.