The influence of dune lee side shape on time-averaged velocities and turbulence

Underwater dunes are found in various environments with strong hydrodynamics and movable sediment such as rivers, estuaries and continental shelves. They have a diversity of morphology, ranging from low- to high-angle lee sides and sharp or rounded crests. Here, we investigate the influence of lee side morphology on flow properties (time-averaged velocities and turbulence). To do so, we carried out a large number of numerical simulations of flow over dunes with a variety of morphologies using Delft3D. Our results show that the value of the mean lee side angle in addition to the value and position of the maximum lee side angle have an influence on the flow properties investigated. We propose a classification with the following three types of dunes: (1) low-angle dunes (mean lee side < 10°), over which there is generally no flow separation and over which only little turbulence is created; (2) intermediate-angle dunes (mean lee side 10-17° ), over which an intermittent flow separation is likely over the trough; and (3) high-angle dunes (mean lee side > 17° ), over which the flow separates at the brink point and reattaches shortly after the trough and over which turbulence is high. The influence of maximum lee side slope value and position on flow characteristics depends on the dune type. We discuss the implications of the proposed dune classification on the interaction between dune morphology and flow.