Search

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. In this paper, we present a comparison of several recently published bedform analysis tools that were created to measure the size and shape of underwater dunes. We compare these tools and how they performed on three different seabed elevation maps that detail dunes in a river, an estuary, and a laboratory flume. We focus on the resulting measurements of dune morphology, such as dune height and length. We show that the consideration of a secondary dune scale has a significant influence on the statistical distributions that describe the measured dune populations. With this knowledge, we offer a discussion of the strengths and limitations of each tool and examples of its proper application. We also offer the combination of the measurements from each tool as a benchmarking dataset that can be used for future tool creation and method validation. Finally, we recommend that each tool be used with specific needs in mind and a universal and modular toolbox should be created that incorporates all available options for dune identification. Five recent dune identification tools are compared in a meta‐analysis assessing three bathymetries from diverse flow environmentsResults differ in sampling size, computation time and dune characteristics, but mainly regarding the allowance of small‐scale bedformsThe combination of results can be used as benchmarking data in future validations because it is less biased by individual research focus Five recent dune identification tools are compared in a meta‐analysis assessing three bathymetries from diverse flow environments Results differ in sampling size, computation time and dune characteristics, but mainly regarding the allowance of small‐scale bedforms The combination of results can be used as benchmarking data in future validations because it is less biased by individual research focus

Dunes are ubiquitous features in alluvial channels, serve as major agents of sediment transport and contribute significantly to flow resistance. Research in the past decade has illustrated the complexity of dune geometry and widespread occurrence of dunes that have a low leeside angle. However, there is a debate concerning the occurrence of such dunes and their formative processes. This paper seeks to further our understanding of low‐angle dunes by utilizing data from a robust set of shallow flow laboratory experiments detailing equilibrium bedform morphology across a range of sediment transport conditions. Analysis of bedform morphology demonstrates that dunes with low‐angle leesides are generated in shallow laboratory flows and are not restricted to deep rivers. Of the possible processes that have been proposed to explain the formation of low‐angle dunes, this finding unequivocally shows that liquefied leeside avalanches, which rely on deep flows for their generation, are not a controlling mechanism. In addition, dunes formed under suspension‐dominated conditions possess lower leeside angles compared with those formed under bedload‐dominated conditions. However, where bedload transport dominates and sediment suspension is likely of lesser importance, low‐angle dunes are still present, and preliminary analysis shows that bedform superimposition can result in lowering of the dune leeside angle. Low and intermediate angle dunes formed under these various conditions also have a lower potential for large‐scale, permanent, leeside flow separation compared with angle‐of‐repose dunes, confirming the need to account for these differences in predictions of flow resistance associated with dune form roughness. Dunes that form on a riverbed have often been thought to possess high‐angle sloping downstream faces. However, in deep rivers, it has been shown that dunes overwhelmingly have shallower sloping, low‐angle, downstream faces—forming low‐angle dunes. This finding has raised questions regarding the occurrence of low‐angle dunes within different depth rivers and the processes controlling their formation. In this paper, we analyze data from shallow‐depth laboratory experiments that detail 3D maps of dunes in order to examine their shape and help understand the processes that lead to their formation. We show that liquefied leeside avalanches are not a control because low‐angle dunes can form in shallow flows where such avalanches cannot be generated. Instead, we find that low and intermediate angle dunes are common when sediment transport is dominated by particles in suspension rather than in bedload transport, suggesting that sediment suspension is vital to their generation. However, when sediment transport is dominated by particles rolling along the bed in bedload transport, smaller superimposed bedforms can lower the leeside angle of the dune over which they are climbing. These findings suggest a balance between sediment suspension and bedform superimposition as mechanisms controlling the formation of low and intermediate angle dunes. Such characterization of dunes is important for predicting flow and flood height within river channels. Low, intermediate, and high angle dunes form in shallow laboratory flowsSediment suspension and bedform superimposition are the key mechanisms controlling the formation of low‐angle dunesLiquefied leeside avalanches do not play a role in the formation of low‐angle dunes Low, intermediate, and high angle dunes form in shallow laboratory flows Sediment suspension and bedform superimposition are the key mechanisms controlling the formation of low‐angle dunes Liquefied leeside avalanches do not play a role in the formation of low‐angle dunes