{"id":"c2e71504-8381-4941-be92-97efc0b19b72","arxiv_id":"2608.05593","paper_version":1,"verdict":"CONDITIONAL","confidence":"HIGH","novelty_score":6.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":2,"one_line_summary":"Continuous flume measurements show alternate-bar height grows faster at lower discharge on a bedload-based timescale, but with weaker discharge sensitivity than linear stability theory predicts.","lead":"Flume experiments that mapped riverbed elevation continuously under flowing water show that alternate bars grow faster at lower discharge on a sediment-transport timescale, but with weaker discharge sensitivity than linear stability theory predicts. The dataset provides benchmark constraints for refining bar-growth theories used in flood-risk and river-management predictions.","discovery_kind":"new_application","skeptic_critique":{"model":"deepseek-v4-flash","headline":"Landau-form substitution of H_MB for alpha_11 in Eq. (5) is the central unvalidated step; all discharge-sensitivity inferences inherit this functional-form assumption.","rationale":"The reader's weakest_assumption identifies exactly the step on which the central quantitative claim rests: the growth rate Omega_HMB is not observed directly but is obtained by fitting Eq. (5) to H_MB(t). The paper itself flags the absence of a theoretical guarantee in Section 3.3.1, and the Fourier analysis in Appendix B provides concrete evidence that H_MB is a composite of several modes, not the single amplitude alpha_11 of weakly nonlinear theory. Because Figure 8 compares the discharge sensitivity of a fitted quantity with the theoretical sensitivity of a different quantity, an unvalidated functional form can bias the headline result. The R^2 values in Appendix A1 show only that a sigmoid fits the data, not that the fitted parameter is the Landau growth rate. This concern is testable: fitting alternative saturation forms and using a model-free growth-rate estimator would show whether the empirical trend is robust. The proposed test addresses the concern directly without requiring new experiments. The reader's CONDITIONAL verdict already reflects this issue together with the lack of confidence intervals on a and post hoc exclusions, so my read does not change that verdict; it only sharpens the reason for it.","tokens_in":23041,"tokens_out":3379,"duration_ms":38122,"concrete_test":"Reanalyze all retained H_MB(t) series with a generalized saturation model, e.g., Richards dH/dt = Omega H (1 - (H/H_e)^n), and with a model-free slope estimate such as the median value of (dH_MB/dt*) / H_MB during the quasi-linear growth window; then recompute the power-law exponent a in Figure 8 for each estimator. If the exponent and its sign relative to the theoretical prediction are stable across estimators, the Landau-form assumption is not load-bearing; if the exponent changes materially or reverses, the headline claim depends on Eq. (5) and would need to be re-expressed or heavily qualified.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The load-bearing step is the identification of measured bar height H_MB with the single-mode amplitude alpha_11 in Eq. (5). Section 3.3.1 states 'there is no theoretical guarantee that bar height follows a Landau equation'; the substitution is justified only by the visual sigmoid shape and by R^2 values in Appendix A1. H_MB is a max-minus-min composite over a half-wavelength, so it contains contributions from alpha_20 and other modes; Appendix B reports alpha_20/alpha_11 up to about 0.8. Consequently, the fitted Omega_HMB is a lumped, model-dependent growth rate, not the linear growth rate Omega_11 of the most amplified single mode. Figure 8 then compares the discharge sensitivity of this fitted quantity with the theoretical sensitivity of Omega_11. If a different saturation form, or no Landau form, describes the data, the fitted Omega_HMB and the power-law exponent a change, and the central claim about weaker-than-linear sensitivity could weaken or reverse. High R^2 does not resolve this, because many sigmoidal functions fit bar-growth curves; the paper's own caveat makes this the central unresolved assumption.","agreement_with_reader":"agree"},"referee_report":{"model":"deepseek-v4-flash","summary":"The paper reports a laboratory flume study of alternate-bar development under four discharges and four channel slopes (16 conditions, at least five replicates each), using the non-intrusive Stream Tomography technique to measure bed topography continuously without interrupting flow. Bar height H_MB is defined as the max-minus-min bed elevation within a tracked half-wavelength, and its temporal evolution is fitted to the general solution of the Landau equation (Eq. 6) to extract a growth rate Omega_HMB and equilibrium height H_MBe. The dimensionless growth rate is then compared against the linear stability theory of Colombini et al. (1987) for the single alternate-bar mode alpha_11, using two sediment-transport formulas (Meyer-Peter and Mueller, Parker). The central empirical claim is that the growth rate of bar height increases as discharge decreases on a bedload-based timescale, but its sensitivity to discharge is weaker than the theoretical sensitivity of the linear growth rate, with the discrepancy growing at lower discharge. Fixed-bed Nays2D simulations over measured bed topographies are used to propose a mechanism: lower discharge produces larger height-to-depth ratios, stronger flow deflection, expanded non-sediment-transport zones, reduced bar migration, and concentration of transport in scour zones, promoting bar-height growth. Case 16 and some replicates are excluded from the analysis on criteria that are only partly specified in advance.","tokens_in":23236,"tokens_out":3297,"duration_ms":35523,"significance":"If upheld, the paper would provide the first continuous experimental estimates of the nonlinear growth coefficient of the Landau equation for alternate bars and a benchmark for bar stability theories, which is a meaningful contribution to fluvial geomorphology. The study has notable strengths: a multi-replicate design that addresses inherent variability in bar formation; a non-intrusive measurement technique that avoids flow interruption; direct data availability via a Zenodo dataset; and an explicit statement of the main modeling assumption, including its caveat. The hydraulic calculations over measured bed topography add a physically plausible mechanistic interpretation. However, the central claim depends on the unvalidated substitution of bar height H_MB for the single-mode amplitude alpha_11 in the Landau equation, and the headline discharge-sensitivity comparison lacks confidence intervals or significance testing. These issues are load-bearing for the main conclusion and require additional analysis before the result can be considered established.","major_comments":[{"comment":"The identification of the measured bar height H_MB with the single-mode amplitude alpha_11 in Eq. (5) is the load-bearing step of the paper. The authors themselves state that 'there is no theoretical guarantee that bar height follows a Landau equation.' H_MB is a composite quantity defined over a half-wavelength and contains contributions from alpha_20 and other modes; Appendix B reports alpha_20/alpha_11 up to about 0.8. Consequently, the fitted Omega_HMB is a lumped, model-dependent growth rate, not the linear growth rate Omega_11 of the most amplified single bar mode. Figure 8 then compares the discharge sensitivity of this fitted quantity with the theoretical sensitivity of Omega_11. If a different saturation form, or no Landau form, describes the data, the fitted Omega_HMB and the power-law exponent a would change, and the central claim about weaker-than-linear sensitivity could weaken or reverse. The paper should provide a sensitivity analysis with alternative sigmoidal forms (e.g., logistic or Gompertz), or an explicit derivation of how modal superposition biases the fitted growth rate, to show that the headline conclusion does not rely solely on the Landau functional form.","section":"Section 3.3.1, Eq. (5)"},{"comment":"The headline claim that the experimental growth rate is 'less sensitive' to discharge than the theoretical prediction is based on power-law fits y = b Q^a plotted in Figure 8, but no confidence intervals, standard errors, or significance tests are reported for the exponent a or for the comparison against the theoretical exponents. With at least three replicates per condition after screening, a bootstrap or mixed-effects analysis is feasible and would directly address whether the observed weaker sensitivity is statistically robust across slopes. As written, the visual separation in Figure 8 is not quantified, and the statement that the discrepancy 'becomes more pronounced at lower discharge' also lacks a formal test. These statistical gaps undermine the paper's central quantitative conclusion.","section":"Section 3.3.2, Figure 8"},{"comment":"The exclusion of Case 16 (Q = 1.7 L/s, I = 1/200) and of replicates with 'substantially lower reproducibility than the others' is partly post-hoc and may bias the discharge-sensitivity comparison. Case 16 is precisely the lowest-discharge, lowest-slope condition, the regime where the discrepancy with theory is largest, so excluding it could strengthen the appearance of a monotonic discharge trend. The authors should either justify exclusions with a pre-specified protocol, or report how the Figure 8 results change when Case 16 and the excluded replicates are included. Without this, the robustness of the inferred power-law exponents to data-selection choices remains unclear.","section":"Section 3, exclusions of Case 16 and replicates"},{"comment":"The goodness of fit R^2 > 0.90 (and about 0.75 in the least favorable case) is cited as evidence that the Landau equation adequately represents bar-height evolution, but this does not validate the functional form: many sigmoidal curves fit such time series equally well. The statistical screening that removes coefficients with p > 0.05 assesses coefficient reliability, not model adequacy. A residual analysis, a comparison with alternative growth-and-saturation models, or a test of the implied relationship between growth rate and current height (e.g., dH_MB/dt divided by H_MB versus H_MB^2) would provide much stronger support for the use of Eq. (5).","section":"Appendix A1"}],"minor_comments":[{"comment":"There is an internal equation-numbering inconsistency: the text says 'Equation (6)' when referring to the Landau equation, but Eq. (5) is the Landau equation and Eq. (6) is its general solution; please correct the cross-references.","section":"Section 3.3.1"},{"comment":"The caption of Figure 3 does not note that panel (o) is absent for I = 1/200 due to the exclusion of Case 16; please state this in the caption or figure layout for clarity.","section":"Table 1 and Figure 3"},{"comment":"The critical discharge Q_c appearing in Table 1 is said to be 'calculated by the method described later,' but the method is not described in Section 2.1; please provide the formula or a forward reference to Section 3.3.2 where the stability analysis is presented.","section":"Section 2.1"},{"comment":"The legend entries '0.3' and '0.7' for the theoretical curves are not explained in the caption; they should be identified as the r values (transverse bedload direction coefficient) used in the stability analysis.","section":"Figure 8, legend"},{"comment":"The piecewise expression for H_MB/h_0 in Eq. (11) is introduced without derivation or definition of the symbols beyond the preceding amplitudes; please clarify how this composite expression arises from the two-mode evolution equations (9) and (10).","section":"Section 5.1.2, Eq. (11)"},{"comment":"Several typographical issues need correction: 'manuscript submitted toJGR' appears without the intended spacing; 'v/upales' in the Figure 9 caption should read 'v/u panels'; and 'T able 1' heading contains an extra space.","section":"Various"}],"recommendation":"major_revision","confidential_remarks":"This is a well-motivated and data-rich study that fits the scope of JGR: Earth Surface. The main concern is that the headline quantitative claim rests on an unvalidated substitution of a composite measured quantity (H_MB) for a single theoretical mode amplitude (alpha_11), a point the authors acknowledge but do not resolve. The absence of uncertainty quantification around the power-law exponent a is also a serious weakness for a central result. I would be inclined to accept a revision that provides either a robustness analysis of the Landau-form assumption, a modal-decomposition-based correction to the growth-rate estimate, or a substantial statistical treatment showing the discharge-sensitivity comparison survives alternative interpretations. Given the high quality of the experimental data and the clear presentation, I see this as fixable within revision rather than requiring rejection."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"Two things to know. This is a genuinely new experimental dataset: continuous, flow-through measurements of alternate-bar height growth across 16 conditions with replicates, so the growth-rate–discharge relation is measured rather than inferred from final bed states. That alone justifies referee time. The second thing is that the headline quantitative claim—growth rate rises as discharge falls, but less sensitively than single-mode linear stability theory predicts—rests on a functional-form assumption the authors themselves admit is unproven, and the comparison carries no error bars on the fitted exponent.\n\nWhat the paper does well: the Stream Tomography setup is a real advance over the interrupt-the-flow methods that dominated earlier experiments; they run at least five replicates per condition; they check equilibrium heights against Ikeda's empirical curve; they validate the MPM versus Parker transport formulas against measured bar celerity in Appendix A3; and they surface the alpha_20/alpha_11 mode-interaction issue rather than hiding it. The qualitative trend in Figure 3—faster growth at lower discharge on the bedload timescale—is visible in the raw curves and does not depend on the Landau fit.\n\nSoft spots, in proportion. The Landau substitution of H_MB for alpha_11 is the load-bearing step, and the authors' own caveat that there is no theoretical guarantee is accurate: H_MB is a max-minus-min over a half-wavelength, so Omega_HMB is a lumped rate that includes alpha_20 and other modes. High R-squared does not resolve this, since many sigmoids fit growth curves. That turns the headline comparison into a measured lumped quantity versus a theoretical single-mode quantity. To carry the claim, they need at least confidence intervals on the fitted exponent a, and ideally a robustness check with an alternative saturation form. The exclusions—Case 16 plus replicates with substantially lower reproducibility—are partly post hoc. The Case 16 reason is physically sensible, but the thresholds should have been pre-specified. These are fixable with modest effort, not fatal flaws. The Nays2D mechanism is plausible and consistent with the measured celerity, though partly definitional since non-transport zones are identified as regions where tau-star falls below the critical value.\n\nWho this is for: fluvial morphodynamics and river engineering readers who want a benchmark constraint for bar stability theories; the dataset is the lasting contribution. It deserves a serious referee, and the required revisions are quantitative: error bars on a, robustness of the Landau form, and pre-specified exclusion criteria. I would send it out.","headline":"Solid new bar-growth dataset with a plausible qualitative trend; the quantitative comparison to theory needs error bars and pre-specified exclusions before the headline claim is trusted.","tokens_in":23791,"tokens_out":4367,"would_cite":true,"duration_ms":43930,"reading_group":"maybe","serious_thinker":"yes","would_accept_peer_review":true},"rs_alignment":null,"lean_confirmation":null,"pith_extraction":{"msc":[],"pacs":[],"model":"deepseek-v4-flash","headline":"This paper reports flume experiments that, for the first time, continuously track alternate-bar growth without stopping the flow, and compares the measured growth rates with weakly nonlinear theory.","keywords":["alternate bars","bar growth rate","Landau equation","Stream Tomography","flume experiment","water discharge","bar stability theory","bedload transport"],"falsifier":"Recompute the bar-height derivative directly from the raw Stream Tomography time series and test whether $(dH_{\\mathrm{MB}}/dt)/H_{\\mathrm{MB}}$ is a linear function of $(H_{\\mathrm{MB}}/H_{\\mathrm{MB}e})^2$. A systematic departure from that line — particularly during the fast-growth phase, where the fit is least good — would show that the Landau equation is only an approximation and that the reported $\\Omega_{H_{\\mathrm{MB}}}$ values cannot be identified with the theoretical linear growth rate. The bar-celerity mechanism could be checked independently by measuring sediment flux directly and comparing it with the relation $C = q_s / (2(1-p)Z)$ used in the paper.","tokens_in":22774,"feed_emoji":"🏞️","tokens_out":7734,"duration_ms":76328,"temperature":0.7,"pith_summary":"Using a nonintrusive bed-topography scanner, this paper measures the full life cycle of alternate river bars in a laboratory flume without ever stopping the flow. Across four discharges and four slopes, bar height rises along a sigmoidal curve that the authors fit with the Landau equation of weakly nonlinear bar theory, obtaining a growth rate and an equilibrium height for each condition. The central result is that the dimensionless growth rate of bar height increases as discharge decreases, but less sharply than linear stability theory predicts for a single alternate-bar mode, with the gap widening at low discharge. The paper also ties this to a hydraulic mechanism: lower discharge makes bars tall relative to flow depth, which deflects the flow, cuts off sediment transport behind bars, and feeds scour. If correct, the findings provide continuous experimental benchmarks for bar-stability theories and a basis for predicting how bar height responds to changing river discharge.","feed_headline":"River bars grow faster at lower flow — less steeply than theory","feed_subtitle":"Continuous bed scans let flume experiments track bar growth directly and test how much river discharge changes bar height.","key_machinery":"The central device is Stream Tomography, a laser-sheet and camera cart that maps the bed at one-minute intervals and one-centimeter-squared resolution while the flow continues, supplying complete time series of bar height $H_{\\mathrm{MB}}(t)$. These series are fitted with the general solution of the Landau equation, which yields the growth rate $\\Omega_{H_{\\mathrm{MB}}}$ and equilibrium height $H_{\\mathrm{MB}e}$ as empirical coefficients, with time made dimensionless using the Exner bedload timescale $t^* = t\\,q_{s0}/((1-p)h_0 B/2)$. The hydraulic interpretation rests on fixed-bed, depth-averaged flow simulations over the measured topography, from which the authors compute the flow-deflection ratio $|v|/u$ and the area where the Shields stress falls below the critical value for sediment motion.","core_discovery":"On a dimensionless bedload timescale, alternate-bar height in the flume grows faster and to a higher equilibrium value when discharge is lower. The full growth curves, captured without stopping the flow, are well described by the Landau equation \n$$\\frac{dH_{\\mathrm{MB}}}{dt} = \\Omega_{H_{\\mathrm{MB}}} H_{\\mathrm{MB}} \\left(1 - \\left(\\frac{H_{\\mathrm{MB}}}{H_{\\mathrm{MB}e}}\\right)^2\\right),$$\nand the fitted dimensionless growth rate $\\Omega_{H_{\\mathrm{MB}}}$ increases as discharge decreases, but its sensitivity is weaker than the maximum linear growth rate of the single alternate-bar mode ($m=1$) predicted by linear stability theory; the discrepancy grows at lower discharge. Fixed-bed flow calculations over the measured topography explain the trend: at low discharge the bar is tall relative to flow depth, flow deflection is stronger, non-transport zones spread downstream of depositional areas, bar migration slows, and flow and sediment transport concentrate in the scour zone, deepening the pools that set bar height.","pith_inferences":["Inference: the fitted $\\Omega_{H_{\\mathrm{MB}}}$ values could be used to calibrate the intermodal coupling coefficients in the two-mode amplitude equations the paper sketches, turning the observed growth of $\\alpha_{20}$ into a quantitative correction to the single-mode theory.","Inference: if linear theory systematically overshoots the discharge sensitivity of bar growth, flood-frequency projections built on single-mode growth rates may overstate future bar-height change; checking this would require matching long bed-elevation records to discharge histories in real rivers.","Inference: the mechanism implies a testable crossover — the regime of bimodal flow deflection, large non-transport zones, and suppressed migration should begin near a particular bar height-to-depth ratio, which a future experiment could isolate by varying discharge at fixed slope."],"forward_implications":["Lower discharge on a bedload-based timescale makes alternate bars grow faster and reach a larger equilibrium height, so a sustained discharge reduction should make bars taller and less mobile.","The measured growth-rate-versus-discharge curve gives stability theories a quantitative target: they must reproduce a weaker-than-linear sensitivity, especially at low discharge.","Bar-height growth in these runs is dominated by deepening scour rather than rising deposition, so monitoring the minimum bed elevation alone can track height development.","Flow deflection, non-transport zones, and bar celerity respond together with bar height, meaning bar development is a self-reinforcing feedback rather than a one-way response to discharge."],"supporting_citations":[{"why":"Supplies the weakly nonlinear Landau-equation framework whose amplitude evolution the bar-height fit is modeled on.","marker":"Colombini et al. (1987)"},{"why":"Introduces Stream Tomography, the continuous flow-through bed-measurement method that makes the full growth curves possible.","marker":"Moteki et al. (2022)"},{"why":"Provides the prior laboratory finding that equilibrium bar height increases with decreasing discharge, and the morphometric and modal context the present results build on.","marker":"Redolfi et al. (2020)"},{"why":"Defines the bedload transport formula used to build the dimensionless timescale and to compute the theoretical linear growth rates.","marker":"Meyer-Peter and Müller (1948)"},{"why":"Provides the empirical equilibrium-bar-height relation against which the fitted equilibrium heights are validated.","marker":"Ikeda (1984)"},{"why":"Documents the scour-dominated bar-height growth and near-constant wavelength that the present observations reproduce.","marker":"Fujita and Muramoto (1985)"},{"why":"Alternative sediment transport formula used to show that the main findings do not depend on the chosen transport law.","marker":"Parker (1978)"}],"fun_headline_variants":["Flume test: river bars grow faster at low discharge","Low discharge accelerates bar growth, defying linear theory","Slow discharge grows bars faster than stability theory","Flume scans reveal low-discharge bar growth acceleration"],"cache_read_input_tokens":3200,"weakest_assumption_plain":"The load-bearing premise is that bar height $H_{\\mathrm{MB}}(t)$ — a composite quantity made of many Fourier modes — evolves according to the same Landau equation as the amplitude $\\alpha_{11}$ of a single alternate-bar mode; the paper itself notes there is no theoretical guarantee of this, and if the true evolution is not Landau-type, the fitted growth rates are not the quantities being compared with stability theory.","fun_headline_variants_meta":{"raw":{"variants":["Flume test: river bars grow faster at low discharge","Low discharge accelerates bar growth, defying linear theory","Slow discharge grows bars faster than stability theory","Flume scans reveal low-discharge bar growth acceleration"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.000677,"raw_usage":{"total_tokens":3115,"prompt_tokens":1021,"completion_tokens":2094,"prompt_tokens_details":{"cached_tokens":384},"prompt_cache_hit_tokens":384,"prompt_cache_miss_tokens":637,"completion_tokens_details":{"reasoning_tokens":2031}},"tokens_in":637,"tokens_out":2094,"duration_ms":17078,"temperature":1.0,"reasoning_tokens":2031,"cache_read_input_tokens":384,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-08T05:52:58.724664+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"Recompute the bar-height derivative directly from the raw Stream Tomography time series and test whether $(dH_{\\mathrm{MB}}/dt)/H_{\\mathrm{MB}}$ is a linear function of $(H_{\\mathrm{MB}}/H_{\\mathrm{MB}e})^2$. A systematic departure from that line — particularly during the fast-growth phase, where the fit is least good — would show that the Landau equation is only an approximation and that the reported $\\Omega_{H_{\\mathrm{MB}}}$ values cannot be identified with the theoretical linear growth rate. The bar-celerity mechanism could be checked independently by measuring sediment flux directly and comparing it with the relation $C = q_s / (2(1-p)Z)$ used in the paper.","supporting_citations":[{"cited_title":"and Seminara, G","cited_arxiv_id":null,"evidence_quote":"Supplies the weakly nonlinear Landau-equation framework whose amplitude evolution the bar-height fit is modeled on."},{"cited_title":"and Murai, T","cited_arxiv_id":null,"evidence_quote":"Introduces Stream Tomography, the continuous flow-through bed-measurement method that makes the full growth curves possible."},{"cited_title":"and Welber, M","cited_arxiv_id":null,"evidence_quote":"Provides the prior laboratory finding that equilibrium bar height increases with decreasing discharge, and the morphometric and modal context the present results build on."},{"cited_title":", title =","cited_arxiv_id":null,"evidence_quote":"Provides the empirical equilibrium-bar-height relation against which the fitted equilibrium heights are validated."},{"cited_title":"and Muramoto, Y","cited_arxiv_id":null,"evidence_quote":"Documents the scour-dominated bar-height growth and near-constant wavelength that the present observations reproduce."},{"cited_title":", title =","cited_arxiv_id":null,"evidence_quote":"Alternative sediment transport formula used to show that the main findings do not depend on the chosen transport law."}],"review_version":1}