Mixing processes at river confluences: Field informed numerical modelling

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.