the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 18 Sep 2025
New Outdoor Experimental River Facility to Study River Dynamics
Abstract. The Outdoor Experimental River Facility (OERF) is a new large-scale, semi-natural research facility designed to study river dynamics at scales that bridge small laboratory models and natural rivers. The facility comprises a 50 m long, 20 m wide floodplain corridor and is designed to sustain discharges up to 800 L s-1, allowing subcritical, fully rough flow with field-like Reynolds numbers approaching 105 – beyond values typical of small-scale planform experiments constrained by Froude similarity. In an initial 338 h (~14 days) straight-channel run without upstream sediment feed, a bi-modal gravel–sand bed (initial D50 = 10 mm) progressively armoured to ~22 mm, and reach-scale planform change remained modest despite a width-to-depth ratio of ~12 and a transport stage τ0/τc ~ 1.2. A three-phase, mathematically designed inlet bar–pool perturbation increased local velocities by 8–27 % and produced limited lateral bank erosion (~2.5–7.5 cm) but did not initiate meandering. The results delineate a narrow operational window for sustained bar growth and migration, long adjustment times, practical constraints of outdoor operation, and the moderating role of bank-material strength and toe armouring. Together, these findings show that field-like hydraulics are achievable within the facility while clarifying what limits mobility at this scale; they also motivate future experiments that couple hydrodynamic similarity with controlled sediment recirculation or feed and refined boundary controls to advance understanding of the controls on planform evolution.
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Status: open (until 01 Nov 2025)
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RC1: 'Comment on egusphere-2025-4352', Maarten Kleinhans, 20 Sep 2025
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AC1: 'Authors’ preliminary reply to RC1 (Maarten G. Kleinhans)', Basem Mahmoud, 29 Sep 2025
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We are grateful to Prof. M.G. Kleinhans (Referee 1) for these thoughtful and constructive comments and suggestions, and for pointing us to key literature. We recognize the referee’s leadership in experimental and theoretical fluvial geomorphology and are very appreciative for the guidance. We are considering a major revision that clarifies scope, corrects framing, and adds the specific analyses and citations requested. Below we respond to the main points and associate changes which will be undertaken to the manuscript. (We will post a full line-by-line response, with changes to the revised manuscript, following closure of the discussion.)
MAIN POINTS
- You are absolutely right regarding the paper’s framing/scope. The paper's strength lies not in forcing a narrative about meandering but in presenting the new facility through the clear lens of its initial results. We will reframe the manuscript to highlight the facility's capabilities for studying sediment sorting dynamics and the initiation of bank erosion. The discussion framing of our initial results will demonstrate how the OERF can capture the interplay between sedimentological (armouring) and morphological (bar development), a point you rightly note we underdeveloped. We will also incorporate your suggested key references (Lanzoni, 2000; Kleinhans et al., 2012) to contextualize our observations of a rapidly armouring, supply-limited bed and the implications for channel mobility. This paper reports results of the first of three studies in a unified experimental program: first, initial straight channel with no sediment recirculation and no vegetation, which investigates bank erosion initiation due to forced bar/pool development; second, initial sinuous channel with sediment recirculation but no vegetation, which investigates controls on bank erosion rate and patterns of channel evolution; third, initial sinuous channel with both sediment recirculation and floodplain vegetation, which compare rates and patterns of morphologic change to previous experiments without vegetation. Investigations of meandering processes will take place in the later experiments.
- We appreciate the push to move beyond generalized statements about Reynolds numbers and provide a more nuanced, process-based justification. Nuancing why high-Reynolds number flows, achievable in large experiments, are critical, will be argued for the OERF's value based on the concept of "unreasonable effectiveness" (Paola et al., 2009) and its capacity for process similarity. This perspective aligns with our view, and is well-supported in literature, that large-scale experimental facilities like the OERF complements smaller flume experiments, and are particularly beneficial for investigating certain processes where scale effects are critical, for example: sediment sorting dynamics (Hassan et al., 2024), near-bank turbulent processes governing erosion (Das and Debnath, 2025), and the complex interactions between flow and biota (Nikora, 2008). By focusing our revised manuscript on the first two areas, which are prominently featured in our initial experiment data, we can more effectively demonstrate the utility of the OERF and lay a solid foundation for future work that incorporates the third. We are confident that this reframing will produce a much stronger and more coherent paper, and we thank you again for your guidance in helping us achieve this.
We also would like to elaborate to the broader community on the scale effect regarding sediment mobility. To fit a river into a lab flume, everything is scaled down. Researchers proportionally shrink the gravel and the sand. So, 20 mm gravel might become 2 mm coarse sand, and 0.5 mm sand might become 0.05 mm silt. At this small scale, it is suggested that viscosity plays an important roll in the transport (the viscous effect), while the the natural river is dominated by different, hydraulically rough conditions. A gravel bed with a significant amount of very fine sand can still experience this viscous effect.
REFERENCES
Das, V. K., & Debnath, K. (2025). Understanding riverbank erosion through the Lens of Turbulence: A review. Journal of Hydrology, 649, 132484.
Hassan, M. A., Parker, G., Hassan, Y., An, C., Fu, X., & Venditti, J. G. (2024). The roles of geometry and viscosity in the mobilization of coarse sediment by finer sediment. Proceedings of the National Academy of Sciences, 121(38), e2409436121.
Nikora, V. (2008). 3 Hydrodynamics of gravel-bed rivers: scale issues. Developments in Earth Surface Processes, 11, 61-81.
Citation: https://doi.org/10.5194/egusphere-2025-4352-AC1 -
RC2: 'Reply on AC1', Maarten Kleinhans, 29 Sep 2025
reply
Thank you very much for your very constructive response. I agree that the proposed changes in the manuscript will make it much stronger, and a focus on processes where turbulence matters, such as the sediment sorting effects and vegetation-flow-sediment interactions that you mention, will also clearly argue for a large flume with high Reynolds numbers.
Citation: https://doi.org/10.5194/egusphere-2025-4352-RC2 -
EC1: 'Reply on RC2', Tom Coulthard, 30 Sep 2025
reply
Thanks both - great to see the discussion part being used so constructively.
Citation: https://doi.org/10.5194/egusphere-2025-4352-EC1
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EC1: 'Reply on RC2', Tom Coulthard, 30 Sep 2025
reply
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AC1: 'Authors’ preliminary reply to RC1 (Maarten G. Kleinhans)', Basem Mahmoud, 29 Sep 2025
reply
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Review of: New Outdoor Experimental River Facility to Study River Dynamics
Authors: Mahmoud, Dickson, Renault, Trudel, Biron, Sklar and Lacey
Reviewer: Maarten G Kleinhans, d.d. 20 Sept 2025
This is a well-written manuscript presenting a pilot experiment in a new and promising outdoor facility. However, there are two major issues and one minor issue with the manuscript related to the design for meandering and the underlying assumptions on the meandering process and related to the interaction between sediment sorting and morphological response. The less major issue is a poor development of reasons to do this kind of experiment, which is biased towards the outdated dogmas of engineering scale models. As such, the necessary recommendation is reject. I strongly suggest the authors to reconstruct a paper with much less focus on the meandering and much more focus on the armouring processes.
MAIN POINTS
* I congratulate the authors on obtaining a great experimental facility. In my opinion these experiments remain necessary complements to numerical modelling especially for the channel-bank interactions and for sediment sorting dynamics that are the focus of this paper.
* The arguments for doing physical modelling are poorly developed and need nuance.
* The experimental conditions conducive to meandering, which is apparently the purpose of the experiment, are not met. Literature suggestions are given.
* The interesting interaction between sediment sorting and morphological development is in part presented, but the essential point (the different timescales for armouring and for morphological response) is underdeveloped. Literature suggestions are given.
DETAILED POINTS (referring to line numbers)
8 the abstract would be more readable in words and without unexplained symbols
18-19 a recent review is found in my paper Kleinhans et al. 2024
24 a more appropriate reference is Schumm's ten ways to be wrong
26 this is far too simplistic and needs more nuance. Of course no physical model can ever produce the full complexity of natural river systems, because all models are simplified, namely on the controlled initial and boundary conditions, in the processes and mechanisms that are allowed by the operators, and possibly in certain scale effects (also see my 2024 paper for development of a complex systems view). Physical and numerical models are representations of a target system, in which the aim is to include relevant respects and to obtain a sufficient degree of similarity. The whole point of these models is to simplify otherwise science would be impossible for humans.
35 "full spectrum" likewise oversimplifies the matter.
38 other references are needed here, including the (in)famous Paola et al 2009 paper and my 2014 paper, both in ESR. Besides, some indoor facilities are quite large (for instance the USACE wave tank and the BAW river scale model in Germany), so the 'indoor' is irrelevant here.
41 Again simplistic. Why not representative? why not representative to a certain degree? Such representation serves a purpose, so if you want to argue that having low Re is not representative then a specific process is needed here for which that is relevant. As Paola and Metivier and I and many others have demonstrated, the Reynolds number is not relevant for many phenomena. The authors here provide no arguments but basically repeat the dogma from the engineering scale model world, which has been outdated in geomorphology for about a quarter of a century. To put it as clear as possible without the intention to be blunt: the fact that our meandering and estuarine and deltaic and debris flow experiments violate these rules is the reason that we obtained spectacular results where the classic scale models (including those shown in the recent Crosato and Mosselman paper) fail. This point was well put by Paola et al in 2009 in ESR.
44 "Inaccurate predictions" implies a certain purpose, such as engineering scale modelling of a certain target system where, for instance, measured water levels need to approximate those in the target within a predetermined accuracy range. However, for the study of many phenomena such a scaling is not needed and instead other aspects of the experiments need to be realized.
46 This depends very much on the dimensions of a natural system that one can have in mind. For example, the Ganges river is about 1000 times wider than the channel in this facility, so from the engineering dogma the scale numbers are off the chart. However, the van Dijk et al. 2012 paper cited by the authors, was the first to produce sustained dynamic meandering with chute cutoffs in the lab, and the occurrence and mechanisms of the chute cutoffs are arguably adequate representations of the same phenomenon in small and large rivers regardless of the geometric scale. By the implicit standards of the authors, however, their own experiment is a terrible representation of large meandering rivers. There are plenty of reasons to disagree with this position: this new facility is suitable and a great addition for a large number of phenomena as argued in Paola et al. 2009 and Kleinhans et al. 2014.
135 This reference is unfortunately not an acceptable reference unless the Dickson thesis is published open online for the forseeable future. Alternatively, the authors provide these descriptions or a summary thereof in an online supplement.
144 I don't understand how the W/D=12 can be reconciled with the objective of the authors to obtain incipient meandering. The formation of alternate bars requires at least double that W/D plus a dynamic perturbation on the upstream boundary, especially since this flume has about the same length/W ratio as that in the van Dijk et al experiments. This necessary W/D follows from bar theory and from empirical and experimental evidence. The Crosato and Mosselman paper cited by the authors does not provide this number of W/D=10. Our application of the theory (Kleinhans and van den Berg 2011) indicates that W/D>25 at least.
156 this history of perturbations is not correct (see Kleinhans et al 2024 for the full story). In the first place, Christian Braudrick based his static perturbation on Friedkin 1945. In the second place, van Dijk et al did not build on this idea of static topographic forcing. Instead, the idea was that in a dynamic meandering river, the topographic dynamics come from upstream, AND nature is full of perturbations in the inflow, sediment influx and directions thereof. We also discovered after conducting the experiments that Lanzoni and Seminara had developed some theory and a numerical meandering model showing the need of such instabilities. This was later mathematically proven by Weiss and Higdon (2022). Critically for the author's paper, this is solid evidence that dynamic meandering in a relatively short flume such as this one requires a large and dynamic upstream perturbation.
258 Given the tendency to armouring, the low mobility and the low W/D, and the initial flume runs to create a water-worked bed, it does not come as a surprise that no bars developed. This was also a result in Lanzoni/ 2000 experiments in a 1.5 m wide 50 m long flume, and he needed to increase the mobility to obtain bars, even though he used sediment recirculation rather than the lack of sediment supply in these experiments which inevitably leads to a static state. The extremely interesting behaviour of sediment mixtures is that an initially flat bed responds in two different manners, each with their own timescale: a sedimentological response, in this case armouring, and a morphological response, in this case alternate bars (but similarly in the context of bifurcations in bends that the Dutch did a lot of work on, see review on the effects of armouring in Kleinhans et al. 2012). So I agree with 284 that the sediment mobility needs to be tuned.
287 If the purpose of these experiments is to represent small streams, then the bank strength is indeed one way to avoid braiding and promote meandering. However, as we demonstrated in our experiments and models, starting with van Dijk et al 2012 and reviewed extensively in the aforementioned 2024 paper, the more likely and more important mechanism needed for meandering is surface cover by vegetation or cohesive sediment on the inner bend.
292 I agree with the suggestion to reconsider the sediment recirculation issue. A sediment feed setup has greatly different results and at low sediment mobility the system progressively develops towards a static stable state of armoured bed and possibly supply-limited bedforms. See Parker and Wilcock 1993 and Kleinhans 2005.
307 I agree that LSPIV is excellent for this kind of experiments and an appropriate reference is needed here (perhaps work by Wim Uijttewaal from whom we learned it?)
310 I suggest to attempt structure for motion applied to a set of fixed cameras, which worked well in our past experiments.
312 I suggest to use biological pest controls such as presented in our papers (Weisscher et al 2022, protocol in the supplementary material)
REFERENCES
Kleinhans, M. G. (2005), Upstream sediment input effects on experimental dune trough scour in sediment mixtures, J. Geophys. Res., 110, F04S06, doi:10.1029/2004JF000169
Kleinhans, M. G., McMahon, W. J., & Davies, N. S. (2024). What even is a meandering river? A philosophy-enhanced synthesis of multilevel causes and systemic interactions contributing to river meandering. Geological Society Special Publication, 540(1), 43-74, https://doi.org/10.1144/SP540-2022-138, open access copy here: https://research-portal.uu.nl/ws/files/243459455/What_even_is_a_meandering_river_A_philosophy-enhanced_synthesis_of_multilevel_causes_and_systemic_interactions_contributing_to_river_meandering.pdf
Kleinhans, M. G., & van den Berg, J. H. (2011). River channel and bar patterns explained and predicted by an empirical and a physics-based method. Earth Surface Processes and Landforms, 36, 721-738 doi:10.1002/esp.2090
Kleinhans, M. G., Ferguson, R. I., Lane, S. N., & Hardy, R. J. (2012). Splitting rivers at their seams: bifurcations and avulsion. Earth Surface Processes and Landforms, 38(1), 47-61, https://doi.org/10.1002/esp.3268
Lanzoni, S. (2000). Experiments on bar formation in a straight flume: 2. Graded sediment. Water Resources Research, 36(11), 3351-3363.
Parker, G. P., and P. R. Wilcock (1993), Sediment feed and recirculating flumes: Fundamental difference, J. Hydraul. Eng., 119, 1192–1204.
Weiss, S.F. and Higdon, J.J.L. 2022. Dynamics of meandering rivers in finite-length channels: linear theory. Journal of Fluid Mechanics, 938, A11, https://doi.org/10.1017/jfm.2022.131
Weisscher, S. A. H., Van den Hoven, K., Pierik, H. J., & Kleinhans, M. (2022). Building and raising land: mud and vegetation effects in infilling estuaries. Journal of Geophysical Research: Earth Surface, 127(1), 1-24. Article e2021JF006298, https://doi.org/10.1029/2021JF006298