[1] Most past studies of river dune dynamics have concentrated on
two-dimensional (2-D) bed forms, with constant heights and straight
crest lines transverse to the flow, and their associated turbulent flow
structure. This morphological simplification imposes inherent
limitations on the interpretation and understanding of dune form and
flow dynamics in natural channels, where dune form is predominantly
three-dimensional. For example, studies over 2-D forms neglect the
significant influence that lateral flows and secondary circulation may
have on the flow structure and thus dune morphology. This paper details
a field study of a swath of 3-D dunes in the Rio Parana, Argentina. A
large (0.35 km wide, 1.2 km long) area of dunes was surveyed using a
multibeam echo sounder (MBES) that provided high-resolution 3-D detail
of the river bed. Simultaneous with the MBES survey, 3-D flow
information was obtained with an acoustic Doppler current profiler (
ADCP), revealing a complicated pattern of dune morphology and
associated flow structure within the swath. Dune three-dimensionality
appears intimately connected to the morphology of the upstream dune,
with changes in crest line curvature and crest line
bifurcations/junctions significantly influencing the downstream dune
form. Dunes with lobe or saddle-shaped crest lines were found to have
larger, more structured regions of vertical velocity with smaller
separation zones than more 2-D straight-crested dunes. These results
represent the first integrated study of 3-D dune form and mean flow
structure from the field and show several similarities to recent
laboratory models of flow over 3-D dunes.