The Effect of Dune-Dune Interaction on Migration Speed of Subaqueous Bedfoms

Interfaces between granular matter and fluids are commonplace in nature, ranging from sand deserts to river beds. It is well known that sufficiently energetic fluid flow mobilises and transports grains. Moreover, dynamical coupling between topography and flow fields may lead to the formation of ripples and dunes. The bedforms migrate downstream at a rate controlled by their own shape and the fluid flow. It is generally observed that under fixed flow conditions small dunes migrate faster, but there is no consensus about the exact scaling law. In order to test the competing theories, we investigate experimentally the migration speed of subaqueous bedforms under turbulent forcing in a custom-built experimental set-up. The annular geometry provides a unique opportunity to track bedforms in a steady flow over long periods of time. Here we show that apart from the size, the streamwise separation length between neighbouring bedforms can significantly affect their migration speed. Our results reveal strong dune-dune interactions and challenge the universality of local scaling laws relating dune sizes to migration speed.