Topographic sediment features arise from the complex interaction between flow and transport of sediment under the action of a current - unidirectional, oscillatory, combined or multidirectional. Depositional bedforms occur in a wide variety of environments, including deserts, rivers, estuaries, deltas, beaches, continental shelves, deep seas, volcanic regions, and sub- and pro-glacial environments. They are generated and affected by a wide range of flows and forcings, such as aeolian transport, waves, tidal currents, offshore storms, temperate glacier flows, turbidity currents and subaqueous mass flows, deep-sea currents, pyroclastic currents on Earth and other flows on extra-terrestrial bodies. Bedforms leave a unique signature in sedimentary records, allowing stratigraphic interpretation and reconstruction of contemporary and past climate and landscape evolution in terrestrial and other planetary environments.
This session aims to highlight many aspects of the complex interaction between flow, bedforms and sedimentary structures on Earth as well as on extra-terrestrial surfaces, from their description to interpretation, and from modelling to experiments and field quantification, with studies ranging across differing spatial and temporal scales, from large-scale organisation patterns down to the grain-scale, as well as the palaeo-dynamic and morphodynamic aspects of control and feedback between flow, sediment transport and bedform evolution.
The session welcomes contributions from field, laboratory, theoretical, and numerical approaches intended to advance our knowledge of how to decipher information contained in terrestrial and extra-terrestrial sedimentary bedforms, and help foster fruitful discussions between sedimentologists, geomorphologists, hydrologists, physicists and all researchers working on understanding bedform dynamics and their sedimentary signatures.
Our zoom session is still open for breakout discussions till 13:00 CET! Join the zoom link at session SSP 3.3 which follows our session and you can join us: https://meetingorganizer.copernicus.org/EGU21/session/39690#vPICO_presentations
vPICO presentations: Thu, 29 Apr
Unlike terrestrial sandy deserts, Mars hosts two scales of ripples in fine sand. Larger, meter-scale ripples are morphologically distinct from small, decimeter-scale ripples, and their size, in particular, decreases with increasing atmospheric density. As a result, it was recently proposed that the equilibrium size of the larger ripples is set by an aerodynamic process, which makes them larger under thinner atmospheres. Under this hypothesis, large martian ripples would be distinct from smaller, decimeter-scale impact ripples in a mechanistic sense. Several workers have followed up on these initial observations to either corroborate, counter, or expand upon that hypothesis. Notably, a mechanistic model that not only corroborates the hypothesis that the size of large martian ripples is set by an aerodynamic process but also suggests that they arise from an aerodynamic instability, distinct from the grain-impact instability thought to be responsible for the formation of impact ripples, was developed. Conversely, other workers proposed that large ripples can develop from small impact ripples in a numerical model due to Mars’ low atmospheric pressure. In the latter model, the ripples’ growth-limiting mechanism is consistent with an aerodynamic process, but the large ripples would not be a separate class of ripples – they would simply be a larger version of the small impact ripples. Here, we explore this debate by synthesizing recent advances in large-ripple formation and offer potential avenues to address outstanding questions. Although significant knowledge gaps remain, it is clear that large martian ripples are larger where the atmosphere is less dense. The size of large martian ripples thus remain a powerful paleoclimate indicator.
How to cite: Lapotre, M., Ewing, R., and Lamb, M.: An Evolving Understanding of Enigmatic Large Ripples on Mars, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-525, https://doi.org/10.5194/egusphere-egu21-525, 2021.
The ESA/ROSCOSMOS ExoMars 2022 will land in Oxia Planum an area that shows outcrops of clay-rich Noachian-aged phyllosilicates overlaid by an Early Amazonian volcanic dark resistant unit (Adru) . Using HiRISE images, we identified NE-SW (53.9 ± 13.2°) oriented TARs overlying an enigmatic ~EW (95.4 ± 10°) oriented ridge pattern that we interpreted as periodic bedrock ridges (PBRs) . Ridges (~ 38 m spaced) display Y-junctions, show cross-cutting fractures and share the same blocky texture of the bedrock they are associated with. Ridge crestlines are locally found in continuity outside and inside heavily eroded impact craters around the dark upstanding material (Adru) exposed in the center of many craters. These stratigraphic relationships suggest that the ridges (PBRs) formed after the event(s) that eroded the crater rims and thus after deposition of the Adru (2.6 Ga). Ridges are even visible in association with impact crater ejecta and are superimposed by 10-25 m craters and boulders, so they pre-date these impact events. When associated with crater ejecta, ridges locally show two different crests. Both crests are truncated by craters suggesting they were emplaced before the impacts. We interpret this double crest arrangement as megaripples detaching from PBRs. The ejecta deposited over the megaripple-PBRs favored the preservation of the megaripple crests from a subsequent episode/s of erosion that led to the complete exposure of the PBRs on the plain. Because the preserved megaripples are locally visible on the southern edges of the PBRs, the wind that formed the megaripple-PBR system should have blown from N-NNE because the megaripples are located at the downwind side of PBRs . To better understand the relative age of the ridges, we mapped their occurrence on 316 craters in the study area that we qualitatively classified as relatively degraded/old and pristine/young. Results show that ridges are only found in degraded/old craters but are never found inside pristine/young craters. Thus, the ridge forming process was only active in-between the formation of degraded/old and pristine/young craters. A major change in the wind regime occurred during or after the event that exposed the PBRs: N-NNE winds that shaped the PBRs changed into dominant SE winds that led to the deposition of the TARs above the PBR/megaripples. This work unveils a complex history of aeolian erosion and deposition in Oxia Planum during the Amazonian. By visiting PBRs for the first time, the ExoMars 2022 mission will provide further constraints on PBR formation and paleo-winds, shedding light on a past Amazonian environment.
This work is a summary of a manuscript that is currently in press on Geophysical Research Letters: Silvestro et al. 2021, Periodic Bedrock Ridges at the ExoMars 2022 Landing Site: Evidence for a Changing Wind Regime. DOI: 10.1029/2020GL091651.
 Quantin-Nataf C. et al. (2021). Astrobiology, 21, N.3.
 Silvestro S. et al. (2020). 6th Int. Planet. Dunes Work. 12-15 May, 2020. LPI No. 2188, id.3009.
 Hugenholtz C. H. et al. (2015). Aeolian Res. 18, 135–144.
How to cite: Silvestro, S., Tirsch, D., Pacifici, A., Salese, F., Vaz, D., Neesemann, A., Popa, C., Pajola, M., Franzese, G., Mongelluzzo, G., Ruggeri, A. C., Cozzolino, F., Porto, C., and Esposito, F.: Change in the Wind and Climate at the ExoMars 2022 Landing Site in Oxia Planum (Mars). , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3429, https://doi.org/10.5194/egusphere-egu21-3429, 2021.
Multiscale bedforms exist in diverse environments. Globally, trains of small secondary bedforms have been observed in fluvial systems, where they are superimposed on larger fluvial dunes. Yet, we understand little about the morphodynamics of these superimposed bedforms and their interaction with larger bedform scales. It is unclear what their impact is on the overall system functioning, for example in terms of sediment transport and (near-bed) flow dynamics.
Bed elevation data with a high spatiotemporal resolution, obtained during a dedicated field campaign in the river Waal, a main distributary of the river Rhine, have shed light on the morphodynamics of fluvial dunes and superimposed bedforms. Results from the study indicate that superimposed bedforms persist over low-angled lee sides, whereas they disintegrate over lee side angles steeper than . The transport of bed sediment associated with secondary bedform migration is significant. The small bedforms migrate with a celerity that is an order of magnitude larger, from which a transport rate can be inferred that equals and in some sections even exceeds the transport associated with primary river dunes. Where superimposed bedforms disintegrate at or downstream of the dune lee slope, superimposed bedforms fully contribute to the migration of the primary dune. Where they persist over the dune lee side however, the sediment transport inferred from superimposed bedforms over the dune crest might partly contribute to primary dune migration. A significant portion, however, will also be transported over the dune lee side and trough and form an additional transport component. Both the persistence of the superimposed bedforms on the primary dune lee and their size and shape, appear to depend on the primary dune morphology. This is likely related to the flow structure—i.e. the presence of flow separation and the properties of the downstream, turbulent wake—that depends on the primary lee slope angle and height.
In our current work, we build upon this study, and analyse the morphodynamics of these two bedform scales across a much larger spatial and temporal scale. Small-scale superimposed bedforms appear to be ubiquitous in the river Waal and can be observed across a range of discharge conditions. Our analysis quantifies and predicts when secondary bedform occur and persist over primary lee sides. We further aim to understand how secondary bedform morphology depends on primary dune characteristics as well as environmental conditions such as (changes in) discharge, and the bed sediment properties. In relation to that, we question to what extent superimposed bedforms in turn affect the primary dune morphology, their migration celerity and associated bedload transport.
How to cite: Zomer, J., Naqshband, S., and Hoitink, T.: Multiscale bedform interactions in a lowland river, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7814, https://doi.org/10.5194/egusphere-egu21-7814, 2021.
It is well known that barchan dunes are not isolated bedforms but are able to interact with one another both directly, through collisions and the emission/absorption of flux, and indirectly, due to the effects of turbulence in the wake of a dune. In recent years, wave-tank experiments, continuum simulations, and cellular automata models have enabled researchers to model barchan-barchan interactions. The findings from these studies have been fed into object-based models of entire fields of barchans and used to predict the size distributions. Although there has been some success with these techniques, each model has failed to reproduce certain known properties on the field-scale; for instance, that the mean width is constant with downwind distance. Furthermore, previous attempts have not been based on a theoretical understanding of the role of interactions in determining the dune size distribution, thus limiting their potential as universal models of barchan swarms.
Mean-field models are relatively simple in terms of the mathematics, but have shown some degree of success in the modelling of barchan fields, although previous work has focused only on specific cases of interaction rules. We have developed a more general mean-field model which can include many different forms of interaction, making it applicable to a variety of problems, including socio-economic systems as well as fields of interacting barchans. Despite the generality of our model, we have been able to derive expressions for the dependence of the steady-state size distribution, and its moments, on the choice of interaction rules. This means that, by making a measurement of the size distribution of a barchan field, we are able to infer properties of the interactions at play.
To demonstrate the power of such a model we have measured size distributions of several barchan fields in the area of Tarfaya, Morocco. Measurements were made by recording locations of seven distinct points on each barchan to yield morphometric parameters of each dune and compile the size-distribution. By comparing the distribution and its moments to those predicted by the model, we can infer certain properties of the interaction rules, such as the relative probabilities of the different forms of collision. The results show an example of how our model provides a more comprehensive understanding of the way in which dune-dune interactions determine properties on the scale of the field.
How to cite: Robson, D., Baas, A., and Annibale, A.: Inference of barchan interaction properties from a comparison of theoretical modelling and observation, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-213, https://doi.org/10.5194/egusphere-egu21-213, 2021.
The distribution and morphology of tidal bedforms in the Weser Estuary, Germany, between the tidal limit (Weser-km 0 in Bremen) and the open North Sea (Weser-km 110) has been analysed for a five-year period (2009-2013) based on monthly bathymetric surveys carried out along the main waterway. Bedforms were detected from gridded bathymetry data (2x2 m) and their geometric properties described. In particular, the presence and position of a slip face, here defined as the portion of the lee side steeper than 15°, were determined. In earlier studies, this was shown to be a practical criterion for the presence of a permanent flow separation and a turbulent wake in the lee of bedforms. Here it is used as a simplified indicator of bedform roughness: if a bedform does feature a slip face, it is assumed to be an active roughness element. The results were related to measured river discharge and water levels and modelled flow velocities.
Bedforms properties varied spatially and temporally along the estuary. Along the main bedform field (Weser-km 12 to 55) bedforms were mostly flood-oriented upstream, gradually becoming symmetrical then ebb-oriented downstream. In times of high discharge, all bedforms were more ebb-oriented than in times of low discharge. Bedforms in the Weser Estuary can be described as predominantly low angle dunes and their steepest slope is situated near the bedform crest. The analysed bedforms (in the main navigational channel, which is deepened and constrained) are also very two-dimensional, with little variations of three-dimensionality in time or space.
Although the Weser bedforms are mainly low angle, a significant proportion of bedforms possesses a slip face. This implies that they have a strong potential to induce bed roughness. This roughness is likely to change spatially along the estuary due to the variations of bedform properties, but also vary in time as a function of the tidal phase (ebb and flood) and discharge. This has wide implications in terms of modelling hydrodynamics and sediment transport in estuaries.
How to cite: Lefebvre, A., Herrling, G., Zorndt, A., Krämer, K., Becker, M., and Winter, C.: Morphology of tidal bedforms, Weser Estuary, Germany, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4919, https://doi.org/10.5194/egusphere-egu21-4919, 2021.
Subaqueous bedforms are a fascinating morphological feature that concerns natural scientists and engineers alike. Under certain conditions, the different scales of these natural seafloor patterns merge into compound dunes consisting of large-scale primary and superimposing secondary bedforms. When it comes to the measuring of these composites, however, scholarly opinion varies depending on the investigator’s perspective. Specifically, compound dunes can either be interpreted as a superposition of their respective constituents, whose individual heights are measured independently after mathematical disintegration, or as one coherent bedform with readily measurable extents. Both methodologies, undoubtedly, have fully legitimate scopes of application, but little is written about the actual discrepancy that can result from signal pre-processing or differing geometric height definitions.
We experienced this problem when recently validating a method for the decomposition of compound dunes by comparison with three alternative approaches, of which two relied on detrending the bed elevation profiles before examination, whereas the third approach (similar to the newly proposed one) assessed unfiltered profiles. Although all tools were applied to the same bathymetric raw data, the statistical values of obtained dune dimensions diverged significantly. Even between approaches that generally showed comparable mean dune lengths, the corresponding height values differed by a factor of 2 or so. These results suggest that detrending or band-pass filtering of bed elevation profiles, as it is commonly applied before dune identification, leads to a systematic underestimation of profile amplitudes and thus dune heights. We therefore recommend refraining from these pre-processing steps in all cases where unambiguous absolute heights are needed. Dune identification from unfiltered bed elevation profiles, in return, necessitates that dune dimensions are calculated in consideration of the inherent inclinations. When analyzing the respective behavior of primary and secondary bedforms and their complex interplay, however, mathematical disintegration is the method of choice and, accordingly, dune height remains a matter of perspective.
How to cite: Scheiber, L., Visscher, J., Lojek, O., and Schlurmann, T.: The measuring of compound dunes – when height becomes a matter of perspective, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-258, https://doi.org/10.5194/egusphere-egu21-258, 2021.
Hummocky Cross Stratifications (HCS) are low-angle sedimentary structures found in association to sediments from the offshore transition. They are traditionally interpreted as representing storm-induced bedforms, whereby a combined flow is created including an oscillation component from storm waves and a unidirectional component from a density current, with debate on the intensity of each component.
Here, the lateral evolution of bedsets containing HCS is investigated from field exposures. Drone images were collected from outcrops in the Moroccan Anti-Atlas from the Jbel Bani, a several hundred meters thick succession of shoreface to offshore sandstones and shales deposited during the Late Ordovician. Outcrops were targeted specifically for configurations where a vertical series of HCS sandstone bedsets occurred within silty-shale to flazer background interbeds.
Over a few hundred meters of lateral distance, HCS beds are found to splay out into channel cuts. Outside these channel features, individual bedsets are seemingly discontinuous, either amalgamating into underlying beds or laterally passing into ripple beds. This preliminary study offers new insights into the depositional dynamics of HCS sandstone beds, feeding a long-lasting discussion over the last 50 years.
How to cite: Douillet, G. A. and Harlet, D.: Investigation of the lateral continuity of sandstone bedsets containing hummocky cross stratifications, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16551, https://doi.org/10.5194/egusphere-egu21-16551, 2021.
Estuarine sand dunes are – similar to river dunes and marine sand waves – large-scale rhythmic bed patterns. Their characteristics differ from their riverine and marine counterparts, owing to the complex and dynamic estuarine environment. Using an idealized process-based modelling approach, we investigate the effect of the gravitational circulation on estuarine sand dunes.
The gravitational circulation is a residual current typical to estuaries, as it results from a longitudinal salinity gradient. It constitutes a tide-averaged residual flow with an upstream-directed (landward) component at the bed and a downstream-directed (seaward) component at the water surface (Geyer & MacCready, 2014). Sediment transport primarily depends on the bed shear stress (and thus on the flow near the bed), and therefore this residual flow may well be responsible for upstream migration of these bedforms. Observations of sand dunes in the Gironde estuary, France, suggest that this may indeed be relevant to the migration direction of estuarine sand dunes (Berné et al., 1993).
We incorporated the hydrodynamic features of the gravitational circulation in a morphodynamic model, which is similar to the one of Hulscher (1996). We then perform a so-called linear stability analysis, which shows that bedforms develop as free instabilities of the flat bed.
Results show that a longitudinal salinity gradient may cause upstream migration, provided that the river flow velocity is sufficiently small. During high discharge in the Gironde estuary, the salinity front is pushed outward (van Maanen & Sottolichio, 2018), thus increasing the salinity gradient at the position in the Gironde where the sand wave field is situated. Including this in the model shows that the strengthened gravitational circulation can overpower the increased river flow velocities during high discharge, and thus confirms the observation by Berné et al. (1993). We note that this mechanism is probably limited to estuaries which share similar characteristics as the Gironde estuary, i.e. symmetric tide, well-mixed, little wind and wave influence, and a small residual river flow velocity due to a significant increase in cross-sectional area. Future research will elaborate on the effects of (tidally varying) stratification through implementation of a time- and space dependent eddy viscosity.
Berné, S., Castaing, P., le Drezen, E., & Lericolais, G. (1993). Morphology, Internal Structure, and Reversal of Asymmetry of Large Subtidal Dunes in the Entrance to Gironde Estuary (France). Journal of Sedimentary Petrology, 63(5), 780–793. https://doi.org/10.1306/d4267c03-2b26-11d7-8648000102c1865d
Geyer, W. R., & MacCready, P. (2014). The Estuarine Circulation. Annual Review of Fluid Mechanics, 46, 175–197. https://doi.org/10.1146/annurev-fluid-010313-141302
Hulscher, S. J. M. H. (1996). Tidal-induced large-scale regular bed form patterns in a three-dimensional shallow water model. Journal of Geophysical Research, 101(C9), 727–744. https://doi.org/10.1029/96JC01662
van Maanen, B., & Sottolichio, A. (2018). Hydro- and sediment dynamics in the Gironde estuary (France): Sensitivity to seasonal variations in river inflow and sea level rise. Continental Shelf Research, 165(May), 37–50. https://doi.org/10.1016/j.csr.2018.06.001
How to cite: van der Sande, W. M., Roos, P. C., Gerkema, T., and Hulscher, S. J. M. H.: The influence of the gravitational circulation on estuarine sand dune migration: an idealized modelling approach, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4874, https://doi.org/10.5194/egusphere-egu21-4874, 2021.
Open and data-driven paradigms have allowed to answer fundamental scientific questions in different disciplines such as astronomy, ecology and fluid mechanics, among others. Recently, the need to collaboratively build a large, engineered and freely accessible bed form database has been highlighted as a necessary step to adopt these paradigms in bed form dynamics research.
Most large database architectures have followed the principles of relational databases model solutions (RDBMS). Recently, non-relational (NoSQL) architectures (e.g., key-value store, graph databases, document-oriented, etc.) have been proposed to improve the capabilities and flexibility of RDBMS. Both RDBMS and NoSQL architectures require designing an engineered metadata structure to define the data taxonomy and structure, which are subsequently used to develop a metadata language for data querying. Past research suggests that the development of a metadata language needs a collaborative and iterative approach.
Defining the data taxonomy and structure for bed form data may be challenging because:  there is not a standardized protocol for conducting field and laboratory measurements;  it is expected that existing bed form data have a wide spectrum of data characteristics (e.g. length, format, resolution, structured or non-structured, etc.); and  bedforms are studied by scientists and engineers from different disciplines (e.g., geologists, ecologists, civil and water engineers, etc.).
In recent years, several data repositories have been built to manage large datasets related to the Earth System. One of these repositories is the Earth Science Information Partners, which has proposed standards to promote and improve the preservation, availability and overall quality of Earth System related data. These standards map the roles of participants (e.g., creators, intermediaries and end users) and delivers protocols to ensure proper data distribution and quality control.
This contribution presents the first iteration of a metadata language for subaqueous bed form data, named BedformsML0, which adopts the standards of the Earth Science Information Partners. BedformsML0 may serve as a prototype to describe bed form observations from field and laboratory measurements, model outputs, technical reports, scientific papers, post processed data, etc. Biogeoenvironmental observations associated to bed form dynamics (e.g., hydrodynamics, turbulence, river and coastal morphology, biota density, habitat metrics, sediment transport, sediment properties, land use dynamics, etc.) may also be represented in BedformsML0. It could subsequently be improved in future iterations via the collaboration of professionals from different Earth science fields to also describe subaerial, and extraterrestrial bed form data. Likewise, BedformsML0 can be used as machine search query selection for massive data processing and visualization of bed form observations.
How to cite: Gutierrez, R. R., Escusa, F. E., Lefebvre, A., Gualtieri, C., Nunez-Gonzalez, F., and Roche, M.: BedformsML0, a preliminary metadata language for a large, engineered and freely accessible bed form database, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16081, https://doi.org/10.5194/egusphere-egu21-16081, 2021.
During low flows, river dunes determine the navigable depth of rivers, influencing the maximum draft of ships. Accurate predictions of the height and location of river dunes during low flows is needed to plan shipping operations in rivers. Currently, little data of dune evolution during low flows is available and analyzed, as most research is focused on high flows and most data is retrieved in flume experiments. However, the scaling from flume to full scale river gives issues with the lee slope angle and secondary bed forms.[LL(1] Therefore, research on dune evolution on full scale rivers is lacking.
For this research multibeam echo sounding (MBES) measurements of the Waal river, Netherlands, were used. These measurements were done once per two weeks and cover the fairway. The data was made available by Rijkswaterstaat (Department of public works and water management, Netherlands). We developed a method to analyze dune shape in a large dataset of bed data. This method was applied on a stretch of 2 km Waal river, between the cities of Tiel and St. Andries. The research period covers the whole year 2018. This year is characterized by three separate discharge regimes. High water during January until March, median discharge from April until June and extreme low discharges from July until November.
In the first step of the data analysis locations of the primary dunes were determined using a wavelet analysis. At these locations, the dune crests and troughs were identified. With the crests and troughs, the shape characteristics such as dune length, height lee slope angle and propagation speed were determined. The dune characteristics were eventually related to the governing discharge.
The first results show that the river dunes are mobile during extreme low flows. After a transition period of one month, where the discharge drops from the median value towards the low discharge, the dune length and height become statistically stable. While the dune shape in flow direction becomes stable during these low flows the three dimensionality increases, not only in primary dune shape but also the appearance of secondary dunes and ripples. The dune height near the right bank is smaller than in the middle of the river, towards the left bank the height decreases again. The differences between the banks and the middle of the fairway increase as discharge decreases. Also, by visually inspecting the bed profiles at other locations, a similar trend is observed.
The first results show that the location in the river cross-section influences the dune characteristics. These differences increase as the discharge decreases. In further work we will extend the research area over the full length of the Waal river, to give a quantitative analysis supporting our visual results and to include influence of sediment size. As the differences in the cross-section can be found throughout the river, we will also investigate the influence of shipping on the differences in dune shape.
How to cite: Lokin, L., Warmink, J., Bomers, A., and Hulscher, S.: The variability of river dune shape at high and low flows in a navigable river, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6276, https://doi.org/10.5194/egusphere-egu21-6276, 2021.
Bedforms are thought to be a major cause of hydraulic roughness in channels. The geometry of the river bed, shaped by bars, dunes, and ripples, and the spatial and temporal distribution of these, influence the resulting roughness variations. Roughness is a fundamental parameter for understanding river flow behaviour by influencing sediment transport and water level.
Quantification of roughness is challenging since it is not directly measurable in the field. It is therefore inferred from hydrological characteristics, -including water depth, water surface slope, flow velocity, discharge-, as well as morphological characteristics, -such as bedform height-, or derived from calibration of a hydraulic model.
This study contributes to the elucidation of factors influencing hydraulic roughness, and its quantification from field data. Proper quantification of roughness and its spatiotemporal behavior will increase our knowledge in river behavior and will lead to improvement of river management strategies and operational models.
In this research, three methods will be explored, to quantify the spatial distribution of hydraulic roughness in the field. We aim to state the importance of bed morphology for hydraulic roughness and we pursue the auxiliary aim to explore the spatial distribution of bedforms and roughness in our case study area river Waal, the Netherlands.
Method 1 uses the St. Vernant equations (better known as the Chezy equations) to quantify roughness, with as input among others flow velocity, bed slope and water surface slope. This value is seen as the ‘true’ roughness of the river system. Method 2 is a traditionally often used method, where form roughness is obtained from dune characteristics such as height and length via empirical predictors. Method 3 makes use of characteristics of the bed itself, not strictly related to 2D bedform geometry, specifically the inclination of the streamwise local elevation profile, i.e. local topographic leeside angle. Doing so eliminates the necessity of defining dune characteristics, and therefore taking one, often arbitrary, step out of the procedure to quantify roughness.
The three methodologies show the same general trend and order of magnitude of roughness (C=30-70 m0.5/s, mean 42 m0.5/s) however kilometer-scale variations show contrasting patterns. Nor dune geometry neither local topographic leeside angle manage to fully explain the variations in the roughness as obtain from the st. Vernant equations. From this we conclude that bed morphology does not seem to be the only explaining factor for roughness variations. Possible explanations include the low leeside angle of dunes (mean <10°), the influence of man-made structures such as groynes and longitudinal training dams, the influence of fixed gravel layers in sharp bends, river curvature, and cross-sectional variation in river depth (bars) and flow velocity. Further steps will be made to unravel the contributing factors for spatial variation in roughness.
How to cite: de Lange, S., Naqshband, S., and Hoitink, T.: Quantifying hydraulic roughness from field data: can bed morphology tell the whole story?, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6336, https://doi.org/10.5194/egusphere-egu21-6336, 2021.
Evolution and transition of bedforms in lowland rivers are micro-scale morphological processes that influence river management decisions. This work builds upon our past efforts that include physics-based modelling, physical experiments and the machine learning (ML) approach to predict bedform features, states as well as associated flow resistance. We revisit our past works and efforts on developing and applying numerical models, from simple to sophisticated, starting with a multi-scale shallow-water model with a dual-grid technique. The model incorporates an adjustment of the local bed shear stress by a slope effect and an additional term that influences bedform feature. Furthermore, we review our work on a vertical two-dimensional model with a free surface flow condition. We explore the effects of different sediment transport approaches such as equilibrium transport with bed slope correction and a non-equilibrium transport with pick-up and deposition. We revisit a sophisticated three-dimensional Large Eddy Simulation (LES) model with an improved sediment transport approach that includes sliding, rolling, and jumping based on a Lagrangian framework. Finally, we discuss about bedform states and transition that are studied using laboratory experiments as well as a theory-guided data science approach that assures logical reasoning to analyze physical phenomena with large amounts of data. A theoretical evaluation of parameters that influence bedform development is carried out, followed by classification of bedform type by using a neural network model.
In second part, we focus on practical application, and discuss about large-scale numerical models that are being applied in river engineering and management practices. Such models are found to have noticeable inaccuracies and uncertainties associated with various physical and non-physical reasons. A key physical problem of these large-scale numerical models is related to the prediction of evolution and transition of micro-scale bedforms, and associated flow resistance. The evolution and transition of bedforms during rising and falling stages of a flood wave have a noticeable impact on morphology and flow levels in low-land alluvial rivers. The interaction between flow and micro-scale bedforms cannot be considered in a physics-based manner in large-scale numerical models due to the incompatibility between the resolution of the models and the scale of morphological changes. The dynamics of bedforms and the corresponding changes in flow resistance are not captured. As a way forward, we propse a hydrid approach that includes application of the CFD models, mentioned above, to generate a large amount of data in complement with field and laboratory observations, analysis of their reliability based on which developing a ML model. The CFD models can replicate bedform evolution and transition processes as well as associated flow resistance in physics-based manner under steady and varying flow conditions. The hybrid approach of using CFD and ML models can offer a better prediction of flow resistance that can be coupled with large-scale numerical models to improve their performance. The reseach is in progress.
How to cite: Giri, S., Shakya, A., Nabi, M., Naqshband, S., Iwasaki, T., Yamaguchi, S., Froehlich, D. C., Bhattacharya, B., and Shimizu, Y.: Recent advances on bedform research and application: Process-based to machine learning, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9017, https://doi.org/10.5194/egusphere-egu21-9017, 2021.
Field observations in the Dover Strait (Le Bot and Trentesaux, 2004) show sandy bed patterns in an environment where sand is scarce. Their morphological features closely resemble tidal sand waves, however, these type of bed forms are characterized by a crest-to-crest spacing which is larger than the wavelength of sand waves in the same surveyed area where sand is abundant. Based on stability theory, Porcile et al (2017) developed a morphodynamic model that was able to explain these features. They found that where the motionless substratum is exposed due to the growth of dunes, the lack of sand affects sediment transport, and consequently the morphology of the bed patterns. Their results also showed that the continuous growth leads to a lengthening of the dunes, and an increasing irregularity of the spacing. The found that their results were supported by the field observations.
Since the model by Porcile et al (2017) is partly based on the perturbation principle, the results are only valid for small amplitude patterns. To further understand the nonlinear behaviour of these sand starved dunes (e.g. shape, height), we here apply the fully numerical sand wave model by Damveld et al (2020). We extend this model by accounting for the presence of a hard substrate just below a thin layer of sand. Moreover, we start with a randomly perturbed bed pattern to give the morphodynamic system the freedom of self-organization.
Preliminary results show that the numerical model is able to reproduce the results found by Porcile et al (2017). In situations where sand is less abundant, wavelengths increase, and so does the spacing irregularity. Moreover, it is found that the average height of the sandy dunes is becoming increasingly smaller with decreasing sand availability. Further analysis should reveal dependencies to different environmental parameters, such as grain size, depth and tidal current strength.
Le Bot, S., & Trentesaux, A. (2004). Types of internal structure and external morphology of submarine dunes under the influence of tide- and wind-driven processes (Dover Strait, northern France). Marine Geology, 211(1), 143-168. doi:10.1016/j.margeo.2004.07.002
Damveld, J. H., Borsje, B. W., Roos, P. C., & Hulscher, S. J. M. H. (2020). Horizontal and Vertical Sediment Sorting in Tidal Sand Waves: Modeling the Finite-Amplitude Stage. Journal of Geophysical Research: Earth Surface, 125(10), e2019JF005430. doi:https://doi.org/10.1029/2019JF005430
Porcile, G., Blondeaux, P., & Vittori, G. (2017). On the formation of periodic sandy mounds. Continental Shelf Research, 145(Supplement C), 68-79. doi:10.1016/j.csr.2017.07.011
How to cite: Damveld, J., Porcile, G., Blondeaux, P., and Roos, P.: Nonlinear dynamics of sand waves in sediment scarce environments, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16019, https://doi.org/10.5194/egusphere-egu21-16019, 2021.
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