Separation zone at open-channel junctions

The configuration of open-channel junctions controls local sedimentary processes, channel scour, and sidewall erosion through its influence on the flow patterns established as confluent streams compete for limited channel capacity. Entry of a tributary into a mainstream ensures detachment of flow from the channel sidewall immediately downstream from the junction, and flume experiments show well-defined relationships between the dimensions of the separation zone and discharge from the tributary. Other things equal, both the width and length of the separation zone increase systematically with an increase in confluence angle, though it is shown that values of width are much less than predicted by recent mathematical modelling. The zone grows at the expense of the proportion of the channel occupied by the immediate post-confluence flow with the consequence that near-bottom velocity increases by a factor of 1.3 as confluence angle is increased from 15°–90°. The implications for sediment entrainment are reviewed briefly.