Velocity Field Measurements at an Overfall

The velocities and circulation patterns were measured and characterized for flows in the vicinity of an aeratedstraight drop overfall. The velocity vectors were determined for multiple tailwater levels at a constant flow rate and formultiple flow rates at a constant tailwater level. An acoustic Doppler velocimeter (ADV) was used to acquire three-dimensionalcomponent flow velocities along the channel centerline. The ADV probe performed well in areas of relativelylow flow velocity and low air entrainment, but the probe did not perform well in areas with high flow velocity and high airentrainment. The velocity measurements are presented and the near boundary flow velocities agreed well with previouslymeasured boundary stresses. The velocities near the vertical wall and horizontal floor were observed to be similar inmagnitude for a range of tailwater elevations and discharges. These velocity field measurements provide fundamentalinformation about the velocities and circulation patterns in the vicinity of an overfall. This information enhances ourunderstanding of the forces that cause headcut erosion.