This paper reports combined results from field monitoring and numerical
modelling of mixing processes at river channel confluences. The field
results use an Acoustic Doppler Velocimeter to measure simultaneously
three-dimensional flow velocity and suspended sediment concentration in
a small measuring volume 5cm below the sensor head. The velocimeter was
deployed in a confluence of three channels each with different
suspended sediment concentrations. Data were collected from along a
mixing layer interface between two of the confluent channels. The
results illustrate that mixing occurs on a number of temporal scales,
but that the nature of the link between periodic flow field
fluctuations and suspended sediment transport varies with distance
through the confluence. There is a strong correlation between
cross-stream velocity variation and suspended sediment transfer at the
entry to the confluence, associated with the presence of a stagnation
zone. After rapid development of these instabilities, their further
evolution is associated with eddy stretching, such that whilst there
remain strong fluctuations in suspended sediment concentration further
downstream, these are no longer correlated with velocity variation.
Preliminary attempts to model these processes numerically are presented
based upon Large Eddy Simulation.