How does sediment move through the junctions of rivers? Sites where rivers join (known as junctions or confluences) are often sites where flood risk can be elevated, and also of ecological importance. This project addresses how sediment moves through them, something that we understand surprisingly poorly.

Lay summary

Rivers join at what are known as "junctions" or "confluences". We know that when rivers join, there is a change in the geometry of the channel downstream. We also know that the junction of rivers causes the formation of three dimensional flow processes that help to explain this geometry. But, it is not just river flows that mix at junctions. Sediment is also delivered. Sometimes, the delivery rate in the smaller river may be much bigger than that which can be transported in the main river, especially if the latter is less steep. This can lead to sedimentation problems. Similarly, we know that the junctions of rivers may create a complex mosaic of habitat, of ecological importance. To understand these processes, we need a much better understanding of how sediment moves through junctions. In this study, we will be studying a large number of river junctions with the Rhône system. We plan to apply a novel technique that allows us to measure, simultaneously, fine sediment and coarse sediment (moving at the bed) so that we can describe how sediment moves through the junction. We will explain our observations, considering how sediment delivery changes with the seasons. We will apply sophisticated numerical-mathematical models to further quantify sediment movement through the junctions and to establish those conditions when this becomes less effective. Our results will be of value for managing flood risk and also for river restoration.