This manuscript examines the validity of an “artificial-shrinkage” method used in grain-resolved bedload transport simulations to mimic Navier-slip boundary conditions. The authors combine analytical boundary-layer estimates with independent DNS-DEM settling simulations to show that the imposed slip lengths are not small relative to the boundary-layer thickness, meaning the method cannot reproduce true Navier-slip behavior. Their results demonstrate that the resulting drag coefficients are substantially overestimated and do not correspond to any physically realistic fluid–particle interaction. They further present a null-hypothesis model showing that the trends in Zhang et al. (2025) can be reproduced without invoking grain-shape corrections, but simply by accounting for the virtual grain size used in the artificial shrinkage. Overall, the manuscript argues convincingly that the artificial-shrinkage method is inappropriate for testing grain-shape corrections in bedload transport equations. This reviewer has nothing to add to the arguments provided here.

Review of manuscript: egusphere-2025-4932

Title:  On the testing of grain shape corrections to bedload transport equations with grain-resolved numerical simulations

Authors: Yulan Chen, Orencio Durán, and Thomas Pähtz.

Deal et al. investigated the effect of grain shape on sediment transport. To address certain limitations in their experimental setup, Zhang et al. conducted numerical simulations, which largely corroborated the findings of Deal et al. In the present manuscript, the Authors aim to challenge the methodology of Zhang et al. and, consequently, the conclusions of Deal et al. Their argument is structured around three components: (1) an analytical critique, (2) a critique based on independent DNS–DEM simulations, and (3) an alternative interpretation of the results presented by Zhang et al.

I would like to congratulate the authors on their work, as the three arguments they present are particularly convincing. First, they point out that the shrinking length in Zhang et al. is comparable to the boundary-layer thickness, even though it should be much smaller. Second, based on independent simulations, they show that for a given l/d, imposing a Navier slip condition at l and a no-slip condition on a smaller sphere produces different results. Third, they offer an alternative explanation for the findings of Zhang et al.

I must confess that I found the manuscript difficult to follow, primarily because the overall structure of the argument is unclear. The paper does not follow the conventional Introduction–Methods–Results–Discussion format typically expected in this field. The Introduction is unusually dense, and in my opinion, the extensive use of equations at this stage is not ideal. A dedicated Discussion section is also missing. Instead, Methods and Results are interwoven and organized around the three proposed “falsification” steps. I would encourage the Authors to reorganize the manuscript for greater clarity. For instance, the numerical simulation description (currently in Section 2.2) and the derivation of equations in transformed and non-transformed coordinates (Sections 3.1.1 and 3.1.2) would be more appropriately placed in a consolidated Methods section. The Results section could then present the core findings (e.g., Figures 1 and 2), followed by a Discussion that interprets these findings in light of the analytical and numerical critiques (Sections 2.1 and 3.2, Figure 3). Of course, different reorganizations are possible according to the authors’ personal taste.

That being said, I recommend that the present manuscript be rejected with major revisions, not because the scientific arguments are unconvincing (indeed, they are sound and compelling) but because the current structure significantly impedes readability. Clear and accessible presentation of results is, in my view, essential for Earth Surface Dynamics. Nevertheless, I leave the final decision to the Associate Editor: if the AE considers the current organization to be compatible with the journal’s editorial standards, then acceptance in its present form would, of course, be appropriate.