This paper describes and assesses: (i) the use of a new instrument for
the determination of three-dimensional flow velocities in natural
rivers, the acoustic Doppler velocimeter (ADV); and (ii) a method for
positioning and orienting such measurements relative to a single local
coordinate system to relate flow velocity vectors with the bed and
water surface. The ADV uses the Doppler shift principle to measure the
velocity of small particles, assumed to be moving at velocities similar
to the fluid. Velocity is resolved into three orthogonal components,
and measured in a volume 5 cm below the sensor head, minimizing
interference of the flow field, and allowing measurements to be made
close to the bed. A simple method for positioning and orienting the
instrument using digital tacheometry is described, and is used to
obtain velocity measurements concurrently with measurements of both bed
and water surface topography. The paper includes a preliminary field
assessment of the ADV by comparing velocity profiles with those
generated from Marsh McBirney electromagnetic current meters, and a
full field assessment of the position and orientation methodology.
These results suggest that the recommended methods in combination with
an ADV are able to provide reliable mean three-dimensional velocity
field information and accurate bed and surface topography.