Time-averaged flow structure in the central region of a stream confluence

Previous process‐oriented field studies of stream confluences have focused mainly on fluvial dynamics at or immediately downstream of the location where the confluent flows enter the downstream channel. This study examines in detail the spatial evolution of the time‐averaged downstream velocity, cross‐stream velocity, and temperature fields between the junction apex, where the flows initially meet, and the entrance to the downstream channel. A well‐defined, vertically oriented mixing interface exists within this portion of the confluence, suggesting that lateral mixing of the incoming flows is limited. The downstream velocity field near the junction apex is characterized by two high‐velocity cores separated by an intervening region of low‐velocity or recirculating fluid. In the downstream direction, the high‐velocity cores move inwards towards the mixing interface and high‐velocity fluid progressively extends downwards into a zone of scour, resulting in an increase in flow velocity in the centre of the confluence. The cross‐stream velocity field is dominated by flow convergence, but also includes a component associated with a consistent pattern of secondary circulation. This pattern is characterized by two surface‐convergent helical cells, one on each side of the mixing interface. The helical cells appear to be the mechanism by which high‐momentum fluid near the surface is advected downwards into the zone of scour. For transport‐ineffective flows, the dimensions and intensities of the cells are controlled by the momentum ratio of the confluent streams and by the extant bed morphology within the confluence. Although the flow structure of formative events was not measured directly in this study, documented patterns of erosion and deposition within the central region of the confluence suggest that these events are dynamically similar to the measured flows, except for the fact that formative flows are not constrained by, but can reshape, the bed morphology. The results of this investigation are consistent with and augment previous findings on time‐averaged flow structure in the downstream portion of the confluence. © 1998 John Wiley & Sons, Ltd.