Browsing by Author "Lefebvre, Alice"

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    Automated Bedform Identification-A Meta-Analysis of Current Methods and the Heterogeneity of Their Outputs
    Scheiber, Leon; Zomer, Judith; Wang, Li; Cisneros, Julia; Gutierrez, Ronald R.; Lefebvre, Alice (American Geophysical Union, 2024-03-25)
    Ongoing efforts to characterize underwater dunes have led to a considerable number of freely available tools that identify these bedforms in a (semi-)automated way. However, these tools differ with regard to their research focus and appear to produce results that are far from unequivocal. We scrutinize this assumption by comparing the results of five recently published dune identification tools in a comprehensive meta-analysis. Specifically, we analyze dune populations identified in three bathymetries under diverse flow conditions and compare the resulting dune characteristics in a quantitative manner. Besides the impact of underlying definitions, it is shown that the main heterogeneity arises from the consideration of a secondary dune scale, which has a significant influence on statistical distributions. Based on the quantitative results, we discuss the individual strengths and limitations of each algorithm, with the aim of outlining adequate fields of application. However, the concerted bedform analysis and subsequent combination of results have another benefit: the creation of a benchmarking data set which is inherently less biased by individual focus and therefore a valuable instrument for future validations. Nevertheless, it is apparent that the available tools are still very specific and that end-users would profit by their merging into a universal and modular toolbox.
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    The influence of dune lee side shape on time-averaged velocities and turbulence
    Lefebvre, Alice; Cisneros, Julia (Copernicus, 2023-07-11)
    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.