| 03 Nov 2025
Experimental Study of Time-Averaged Flow and Turbulence Structures over Low-Angle Tidal Dunes under Steady Bidirectional Flows
Abstract. Large-scale, high-resolution flume experiments were conducted under representative steady bidirectional flows in a large recirculating flume to investigate the time-averaged flow and turbulence structures over two-dimensional (2D) fixed tidal dunes. Two dune morphologies were considered for representing asymmetric dunes with low to intermediate-angle slopes. The dune morphologies are an idealised representation of natural tidal dunes. Specifically, the study aims to characterise in detail the influence of dune morphology on the properties of flow separation zones and turbulence structures above these dunes subjected to bidirectional flows, which are a representation of tidal flows. Results show that a smaller permanent flow separation zone is found over the tested intermediate-angle dune (mean lee slope of 12°, steep face of 22 compared to those found over angle-of-repose dunes detected close to the bed where a larger intermittent flow separation is found. The corresponding turbulent wake expands downwards to the trough before dissipating further downstream. Over the tested low-angle dune (mean lee slope of 10°, steep face of 15°), both small permanent and intermittent flow separations are observed. When the flow is opposed to the dune asymmetry and flows over a very gentle side (4°) with a short steep portion (10°), only a very small intermittent flow separation is detected. Over only a straight gentle side (4°), both flow separations are nonexistent. Neither intermediate- or low-angle dunes exhibit a distinct turbulent wake when the flow is opposed to the dune asymmetry, and a turbulence structure similar to that under a flatbed condition can be observed. Large-scale turbulence associated with both types of dunes is observed through the high occurrence of energy containing ejection and sweep events as revealed by quadrant analysis. Overall, our study demonstrates the significant impact of dune morphology, particularly the mean lee slope and steep face on the emerging flow and turbulence structures above intermediate- to low-angle tidal dunes.
This manuscript presents a high-quality and original study that successfully fills a relevant gap in the current research field. The experimental design is robust, and the data analysis is thorough and convincing. Overall, the scientific contribution is significant. Most of my feedback therefore concerns aspects of writing style, structure, and clarity rather than the scientific content itself. These revisions would enhance readability and ensure that the strength of the work is fully reflected in the text. Below I highlight major concerns, while in the attached pdf I added in-line questions and suggestions.
Title: The title currently includes the phrase “Steady bidirectional flow”, which is conceptually problematic, since a bidirectional flow, which fluctuates between +u_max and –u_max, cannot be steady by definition (as du/dt≠0). Upon reading further, it becomes clear that the authors refer to two separate experimental settings: a steady flow in one direction, and a steady flow in the opposite direction. While this design is valid, the term “steady bidirectional flow” may lead to initial confusion. I therefore recommend removing the word “steady” or rephrasing the title for improved clarity.
Abstract: The abstract contains all relevant information, but its organization could be improved. The current structure appears somewhat disordered, which obscures the central message of the paper. A clearer and more conventional scientific structure is recommended. The sections that I miss or would improve include:
The main findings—especially regarding non-steep dunes, irregular dune shapes, and the importance of leeside angle and steep-face morphology—should be emphasized more clearly. At present, the abstract does not adequately convey the strength and clarity of the paper’s contributions.
Introduction: The introduction reads somewhat unstructured and would benefit from clearer logical progression and stronger narrative coherence. The reader needs more guidance throughout the section: Why is each paragraph or subtopic relevant? How does each section build toward the research question? What is the main message or takeaway from each part?
Additionally, the introduction would be strengthened by including more recent literature, since many references are from 20 years ago, and much progress has been made in the field since then.
Methods and results: These sections are excellent. The analysis is detailed, systematic, and well-executed. The figures and interpretations convincingly support the conclusions. The only thing I would appreciate is a bit more explanation on the quandrant analysis and the interpretation of these results.
Discussion: The discussion is generally well written and organised. It could be expanded slightly to answer three of my remaining questions.
Firstly, it would be valuable to include a short section on implications for the field, addressing how these findings might influence or refine conclusions from previous field studies (e.g., de Lange et al, 2021 (https://doi.org/10.1029/2021WR030329), Prokocki et al., 2022 (https://doi.org/10.1002/ esp.5364), de Lange et al. 2024 (https://doi. org/10.1029/2023JF007340), and others).
Moreover, since the experimental design uses two opposing steady flows to approximate tidal dynamics, it would be useful to reflect on how an actual tidal flow—characterized by gradual acceleration and deceleration—might affect the observed results.
Finally, how would superimposed dunes affect your findings? Could you just add one or two sentence speculating on this? The reason I’m suggesting this is that superimposed dunes are recently found to be very important for sediment transport (I refer to the work of Judith Zomer), and clearly superimposed dunes impact the leeside and steep side angle of the primary dune.