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Fluvial cross strata are fundamental sedimentary structures that record past flow and sediment transport conditions. Bedform preservation can be significantly influenced by the presence of larger‐scale topographic features that cause spatial gradients in flow. However, our understanding of the controls on cross strata preservation in the presence of a morphodynamic hierarchy is limited. Here, using high‐resolution bathymetry from a physical experiment, we quantify bedform evolution and cross strata preservation in a zone of flow expansion and deceleration. Results show that the size and celerity of superimposed bedforms decreases along the host‐bedform lee slope, leading to a systematic downstream increase in the sediment accumulation rate relative to bedform celerity. This increase in local bedform climb angle results in the preservation of a larger fraction of formative bedforms. Our results highlight the need to revise current paleohydraulic reconstruction models, and demonstrates that fluvial morphodynamic hierarchy is a fundamental determinant of sedimentary strata. Plain Language Summary: Dune evolution in rivers creates inclined layers of sediment, called cross strata, that are an integral part of the rock record on Earth and Mars. The thickness distribution of cross strata is the primary means of estimating ancient flow and sediment transport conditions. Dunes exist with larger‐scale features, such as bars and larger dunes, in rivers, where a train of dunes responds to flow steering by larger‐scale features through changes in dune size and speed. However, we currently lack data to assess the influence of larger‐scale features on dune evolution and cross strata. Here, we studied dune evolution on the lee side (downstream facing slope) of a larger bedform in an experimental channel, where flow expands and slows down. Using high‐resolution data, we show that the dune size and speed decrease with downstream distance along the host‐bedform lee side. The rate of sediment build‐up relative to dune speed increases downstream, which leads to the preservation of a larger fraction of dunes in cross strata. Results suggest that cross strata preserved in the presence of larger‐scale features are common in the rock record, and we need to revise our current models for estimating past flow conditions from cross strata. Key Points: We characterize bedform evolution and cross strata preservation in a zone of flow expansion and deceleration in a physical experimentBedform size and celerity decrease along the host‐bedform lee slope, causing an increase in aggradation rate relative to bedform celerityA larger fraction of the formative bedforms is preserved as cross strata than typically assumed by paleohydraulic reconstruction models [ABSTRACT FROM AUTHOR]

Morphological characteristics were measured for barchan dunes on Earth (2,686 dunes in 30 barchan fields) and Mars (720 dunes in 10 barchan fields) using satellite images. The data were used to (a) develop a new barchan classification system; (b) compare characteristics of barchans on Earth and Mars; and (c) assess whether barchans, in bulk, display allometric or scale‐invariant characteristics. Dimensional metrics were obtained for the width and length of barchan bodies, the width and length of barchans including the horns, and the length of each horn. Dimensionless metrics were derived for the ratios of the body width to the width between the tips of the horns (width ratio), the length of the entire barchan to the length of the body (length ratio), and the length of the longer horn to the shorter horn (symmetry ratio). The width, length, and symmetry ratios were used to classify barchans into eight types and compare the characteristics of their distributions on the two planets. From this analysis, it was established that, statistically, barchans on Earth are distinctive from those on Mars based on the morphometrics, with terrestrial barchans being, on average, of smaller size and more often symmetrical, while Martian barchans more often have convergent horns that are short relative to the central dune body and are more often asymmetrical. The analysis further reveals that barchan planform morphology can be considered scale‐invariant, and we argue that body width is the most appropriate measure representing barchan size. Plain Language Summary: Crescent‐shaped sand dunes (barchans) are widely found on Earth and Mars. Relative variations in the shape of these bedforms are thought to reflect differences in the planetary sand transport environment in terms of sand flux magnitudes, transport modes, and wind climate (speeds and directional variability). Comparing the shapes of barchans on Mars with those on Earth can generate hypotheses on how and why sand transport conditions and dune formation under the Martian atmosphere may be different. Satellite imagery analysis of 3,406 barchan dunes on both planets yields insights on planform dimensions (width and length of the dune body, lengths and divergence of the horns) as well as three shape indices (Width Ratio, Length Ratio, Symmetry Ratio) that were used to derive a classification of eight barchan types. Analysis shows that average barchan planform geometry displays fixed proportions over several orders of magnitude in size (scale‐invariance). Significant relative differences are apparent between the two planets, with barchans on Mars generally being much larger in absolute size but having shorter, more convergent and asymmetric horns than barchans on Earth. The study further suggests that the transverse width of the dune body is the best measurement to reflect the overall size of a barchan. Key Points: A classification of barchan types is derived by applying three morphometric indices to 3,406 dunes on Earth and MarsBarchan planform morphology can be considered a scale‐invariant property over orders of magnitudeDifferences in relative morphology reflect differences in planetary sand transport conditions and dune formation [ABSTRACT FROM AUTHOR]

The roughness height plays a crucial role, especially in shallow‐water environments with rough‐bed conditions. Specifically, it is a key parameter for predicting flow velocity and bed shear stress. Therefore, an accurate determination of roughness height is essential for precise predictions of hydraulic phenomena. In this study, we propose a method to estimate form roughness height and friction factor using bathymetry data, with a focus on cross‐sectional data in the transverse direction. To overcome the limitations of using lateral direction data, we derived an empirical formula for estimating bedform length from the bed profiles in the streamwise direction. To assess the validity of bedform analysis and estimated form roughness height based on lateral direction data, we compared the characteristics of bedform and form roughness height analysed using lateral direction data with those analysed using the streamwise direction data. Furthermore, we confirmed the importance of considering the form roughness height in estimating bed shear stress through a comparison with bed shear stress calculations considering only grain roughness height. [ABSTRACT FROM AUTHOR]

Recent studies show that near-bed turbulence may be better scaled using a mixed velocity scale u*U , where u* is the shear velocity and U denotes the characteristic bulk flow velocity. This implies that the classical scaling consideration for sediment transport rates could be improved by redefining the Shields number based on the mixed velocity scale. To investigate this potential improvement, a series of analyses was performed with laboratory data from 25 sources of sediment discharge studies. The data were carefully selected by minimizing effects of some factors including sidewall, particle gradation, particle density, and fluid viscosity. The findings indicate that the relationship between sediment discharges and flow conditions can be enhanced significantly when the analysis is carried out based on the redefined Shields number. The new relationship does not depend on the type of bedforms, and therefore facilitates the prediction of sediment discharge. Finally, by comparing with previous studies, it shows that the proposed formula markedly improves the prediction of bed-material sediment discharges. [ABSTRACT FROM AUTHOR]

Accurate approaches for estimating flow resistance in large alluvial rivers are fundamental for simulating discharge, sediment transport, and flood routing. However, methods for estimating riverbed resistance and additional resistance in the channel-bar landscapes remain poorly investigated. In this study, we used in situ river bathymetry, sediment, and hydraulic data from the Shashi Reach in the Yangtze River to develop a semi-empirical approach for calculating flow resistance. Our method quantitatively separates flow resistance into riverbed resistance and additional resistance and shows high accuracy in terms of deviation ratio (∼20%), root-mean-square error (∼0.008), and geometric standard deviation (∼3). Additional resistance plays a dominant role under low-flow conditions but a secondary role under high flows, primarily due to the reduction in momentum exchange in channel-bar regions as discharge increases. Riverbed resistance first decreases and then increases, which might be attributed to bedform changes in the lower and transitional flow regimes as flow velocity increases. Overall, our findings further the understanding of dynamic changes in flow resistance in the channel-bar landscapes of large river systems and have important implications for riverine ecology and flood management. [ABSTRACT FROM AUTHOR]

Relative permeability and capillary pressure are key parameters in multiphase flow modelling. In heterogeneous porous media, flow direction- and flow-rate dependence result from non-uniform saturation distributions that vary with the balance between viscous, gravitational, and capillary forces. Typically, relative permeability is measured using constant inlet fractional-flow—constant outlet fluid pressure conditions on samples mounted between permeable porous plates to avoid capillary end-effects. This setup is replicated in numeric experiments but ignores the extended geologic context beyond the sample size, impacting the saturation distribution and, consequently, the upscaled parameters. Here, we introduce a new workflow for measuring effective relative permeability and capillary pressure at the bedform scale while considering heterogeneities at the lamina scale. We harness the flexibility of numeric modelling to simulate continuum-REV-scale saturation distributions in heterogeneous rocks eliminating boundary artefacts. Periodic fluid flux boundary conditions are applied in combination with arbitrarily oriented, variable-strength pressure gradient fields. The approach is illustrated on a periodic model of cross-bedded sandstone. Stepping saturation while applying variable-strength pressure-gradient fields with different orientations, we cover the capillary-viscous force balance spectrum of interest. The obtained relative permeability and capillary pressure curves differ from ones obtained with traditional approaches highlighting that the definition of force balances needs consideration of flow direction as an additional degree of freedom. In addition, we discuss when the common viscous and the capillary limits are applicable and how they vary with flow direction in the presence of capillary interfaces. [ABSTRACT FROM AUTHOR]

Abstract Fluvial cross strata are fundamental sedimentary structures that record past flow and sediment transport conditions. Bedform preservation can be significantly influenced by the presence of larger‐scale topographic features that cause spatial gradients in flow. However, our understanding of the controls on cross strata preservation in the presence of a morphodynamic hierarchy is limited. Here, using high‐resolution bathymetry from a physical experiment, we quantify bedform evolution and cross strata preservation in a zone of flow expansion and deceleration. Results show that the size and celerity of superimposed bedforms decreases along the host‐bedform lee slope, leading to a systematic downstream increase in the sediment accumulation rate relative to bedform celerity. This increase in local bedform climb angle results in the preservation of a larger fraction of formative bedforms. Our results highlight the need to revise current paleohydraulic reconstruction models, and demonstrates that fluvial morphodynamic hierarchy is a fundamental determinant of sedimentary strata.

Traditional mapping of bedforms in submarine canyons relied on vessel‐mounted and towed sensors, but their fine‐scale geomorphology and shallow structure requires higher resolution datasets. This study utilizes a high‐resolution dataset obtained from an autonomous underwater vehicle, combined with seismic reflection profiles and sediment cores, to analyse bedform sets within a 25.6 km long submarine canyon (canyon C14) in the northern South China Sea. A train of crescent‐shaped axial steps, indicative of cyclic steps formed by supercritical turbidity currents, is imaged along the canyon. Axial steps in the upper course show erosional truncations and sub‐horizontal reflectors on the lee and stoss sides, respectively, pointing to erosional–depositional cyclic steps formed by confined flows with high erosional capacity. This is facilitated by canyon narrowness and steeper axial gradient. After a transition segment, the lower course widens, with a gentler axial gradient, resulting in increased asymmetry and wavelength of axial steps. Backset bed deposits on the stoss sides of these steps indicate depositional cyclic steps with higher aggradation. Sediment filling, almost padding each cyclic step associated scour suggests the reworking of previously formed bedforms by gravity flows fed by destabilization processes on the canyon sidewalls and upstream lee faces and, possibly, by shelf‐edge and uppermost slope spillover into the canyon. At the lowermost course, cyclic steps transition to a furrow field, likely associated with flow velocity reduction facilitated by canyon floor widening and a further decrease in slope gradient. Flow braiding and re‐convergence, related to the erosion of fine‐grained deposits within the canyon floor, should have played a role to produce furrows under supercritical conditions. This work enhances our understanding of the detailed morphology and shallow relief configuration of bedforms in deep‐water submarine canyons, providing insights into their causative processes and evolution. [ABSTRACT FROM AUTHOR]

We present a procedure to systematically estimate the form friction factor through the analysis of spatial data obtained by acoustic Doppler current profiler. The form friction factor is related to the geometric characteristics of the bedform. The zero-crossing method was used to determine the geometric parameters of the bedform, which were then utilized to construct the probability density function for Chezy coefficients. Subsequently, Chezy coefficients were generated from the probability density function and assigned to each grid cell of a numerical model. To evaluate the performance of the presented friction factor estimation method, it was applied to the numerical simulation for the Nakdong River in South Korea. Sensitivity tests were performed to address the potential influence of random extraction from the probability density function, filtering length, and different form predictors during the estimation process. The proposed method showed more robust and accurate results compared to those with Manning's n value. [ABSTRACT FROM AUTHOR]

Strata produced by fluvial dunes can provide insight into the hydrological regime of ancient rivers. Recent experiments indicate that conditions of disequilibrium between bedforms and formative flows may be inferred from the coefficient of variation of preserved dune cross‐set thickness, suggesting that this quantity may act as a proxy for the flashiness of river floods relative to the time required for full bedform translation. To assess whether this idea is applicable to interpretations of the stratigraphic record, this study examines published data relating to more than 2600 cross‐sets from 53 sedimentary units of 19 river systems. The presented analyses must not be over‐interpreted, because the considered rivers span different environmental settings, the data sources are heterogeneous in terms of type and dimensionality, and some variables were established by applying empirical relationships. Yet, significant findings are revealed. Larger rivers exhibit discharge and bedform characteristics that are more conducive to disequilibrium; however, a modest increase in the coefficient of variation of cross‐set thickness, CV(Dst), as opposed to the expected decrease, is seen as a function of river size. Crucially, smaller CV(Dst) values are not systematically associated with conditions that should favour dune disequilibrium. Meanwhile, only ca 25% of the studied examples demonstrate cross‐set thickness statistics compatible with quantitative formulations of the autogenic control by variable dune topography – the notion of 'variability‐dominated' preservation. These findings indicate that the variability in cross‐set thickness may be a poor predictor of discharge variability, perhaps because of the multiplicity of factors controlling dune preservation, such as bedform hierarchy, transport stage and depth‐dependent variations in dune disequilibrium. To improve interpretations of cross‐stratified deposits, further research is needed: (i) to establish the value of process‐to‐product models for reverse product‐to‐process interpretations; and (ii) to define representative samples for preserved dune deposits accounting for temporal and spatial variability in preservation potential. [ABSTRACT FROM AUTHOR]

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ABSTRACTWe present a very high-resolution bathy-morphological map of the offshore sector around the Scoglio d’Affrica islet (northern Tyrrhenian Sea, Italy). The study area covers a sector of 45 km2, between 3 and 85 m depth. Its central part, i.e. the apex of the Ridge, is characterised by a flat or gently sloping seafloor, where three mud volcanoes, and 250 pockmarks are recognised. Differently, the western and eastern Ridge flanks are steeper and characterised by 60 quasi-rectilinear escarpments and small ridges, more than 20 morphological highs, and elongated channels occasionally floored by bedforms. The seafloor shallower than 40 m is covered by Posidonia oceanica, forming compact and continuous or fragmented meadows intermingled with sandy patches. The main map represents the bathy-morphological setting of the area, which is largely affected by fluid seepage, providing insights for habitat mapping and preliminary marine geohazard assessment due to the violent gas outburst from mud volcanoes.

Since the Xiaolangdi Reservoir (XLD) operation, bed materials coarsening and changes in flow resistance have been occurring substantially in the Lower Yellow River (LYR). Thus, to quantitatively describe the effects of XLD on flow resistance in the LYR, the varying characteristics of flow velocity, river width, bed material particle diameter and Manning′s coefficient were analyzed using measured data from the hydrological stations. A movable bed resistance formula incorporating the bedform index was developed based on control factor theory and measured bedform data. The accuracy of the formula was verified using 1 508 data sets measured at the hydrological stations. The results highlight that, first, the bed material is coarser in the upper reaches and finer in the lower reaches of LYR. More developed dunes and higher flow resistance occurred in the wandering reach. Second, in reaches downstream of Gaocun, the bed coarsening and flow resistance reduction are less pronounced than in the wandering reach. The cross-section of the main channel is narrower and deeper, increasing wall resistance obviously during flood periods. XLD operation has facilitated bedforms development and increased flow resistance in the LYR. [ABSTRACT FROM AUTHOR]

Hyporheic exchange (HE), fine particle deposition and clogging are tightly coupled processes that control ecosystem services in rivers. While HE is assumed to be induced primarily by riverbed topography, surface flow turbulence also drives significant exchange. We show that turbulence‐driven HE produces large interfacial fluxes and drives long‐term feedback between HE and fine suspended particles via bed clogging. Turbulence significantly increases total HE fluxes as it rapidly delivers suspended particles into porewater over the entire interface, whereas advective pumping exchange only delivers particles into focused downwelling regions on the upstream side of bedforms. While turbulence is associated with rapid fluctuations and shallow HE, it is key on longer‐timescale outcomes, namely bed clogging. However, beyond the general effect of clogging in attenuating HE, turbulence‐driven HE will also be important for other river‐borne materials that are retained and transformed within hyporheic zones, such as nutrients and organic pollutants. Plain Language Summary: Fine natural sediments like clay particles are transported in rivers and deposited in riverbeds. Clay deposition is important to river ecosystems because it controls habitat conditions within riverbeds. In this research, new computational simulations show that turbulent river flow controls clay deposition in riverbeds. These new simulations incorporate turbulence, and they are able to correctly predict patterns of clay deposits. They also predict how clay deposits clog the riverbed and reduce the exchange of water between the river and bed. These simulation capabilities are important to protect river ecosystems and to manage contaminants that interact with riverbeds, because clogging is a common process that damages river ecosystems. Key Points: Turbulence accounts for a significant fraction of fine particle delivery to the hyporheic zoneTurbulent transport of fine particles contributes substantially to interfacial clogging that attenuates Hyporheic exchange (HE)A numerical model is presented that integrates interfacial turbulence, HE of suspended particles and bed clogging [ABSTRACT FROM AUTHOR]

Abstract Hyporheic exchange (HE), fine particle deposition and clogging are tightly coupled processes that control ecosystem services in rivers. While HE is assumed to be induced primarily by riverbed topography, surface flow turbulence also drives significant exchange. We show that turbulence‐driven HE produces large interfacial fluxes and drives long‐term feedback between HE and fine suspended particles via bed clogging. Turbulence significantly increases total HE fluxes as it rapidly delivers suspended particles into porewater over the entire interface, whereas advective pumping exchange only delivers particles into focused downwelling regions on the upstream side of bedforms. While turbulence is associated with rapid fluctuations and shallow HE, it is key on longer‐timescale outcomes, namely bed clogging. However, beyond the general effect of clogging in attenuating HE, turbulence‐driven HE will also be important for other river‐borne materials that are retained and transformed within hyporheic zones, such as nutrients and organic pollutants.

Initially a matter of intellectual curiosity, but now important for understanding ice‐sheet dynamics, the formation of subglacial bedforms has been a subject of scientific enquiry for over a century. Here, we use a numerical model of the coupled flow of ice, water, and subglacial sediment to explore the formation of subglacial ribs (i.e., ribbed moraine), drumlins and mega‐scale glacial lineations (MSGLs). The model produces instabilities at the ice–bed interface, which result in landforms resembling subglacial ribs and drumlins. We find that a behavioural trajectory is present. Initially subglacial ribs form, which can either develop into fields of organized drumlins, or herringbone‐type structures misaligned with ice flow. We present potential examples of these misaligned bedforms in deglaciated landscapes, the presence of which means caution should be taken when interpreting cross‐cutting bedforms to reconstruct ice flow directions. Under unvarying ice flow parameters, MSGLs failed to appear in our experiments. However, drumlin fields can elongate into MSGLs in our model if low ice–bed coupling conditions are imposed. The conditions under which drumlins elongate into MSGLs are analogous to those found beneath contemporary ice streams, providing the first mechanism, rather than just an association, for linking MSGLs with ice stream flow. We conclude that the instability theory, as realized in this numerical model, is sufficient to explain the fundamental mechanics and process‐interactions that lead to the initiation of subglacial bedforms, the development of the distinctive types of bedform patterns, and their evolutionary trajectories. We therefore suggest that the first part of the longstanding 'drumlin problem' – how and why they come into existence – is now solved. However, much remains to be discovered regarding the exact sedimentary and hydrological processes involved. [ABSTRACT FROM AUTHOR]

Based on the SHI database on the structureless transport of large-particle bedloads, 28 formulas that include bedform height and velocity are validated. These formulas demonstrated an error no higher than 50% and can be recommended for the conditions of active structureless sediment transport in mountain and piedmont rivers. Eight of these formulas can be recommended for a wide range of hydraulic conditions in plain and mountain rivers. [ABSTRACT FROM AUTHOR]

Aeolian deposits form noteworthy reservoirs (for example, Norphlet Formation and Rotliegend Group) in hydrocarbon extraction and carbon capture and storage contexts, but stratigraphic architecture imparts significant heterogeneity. Bounding surfaces result from autogenic and allogenic controls and can represent important changes in dune‐field dynamics. To further evaluate the impacts of facies heterogeneity and flow‐inhibiting bounding surfaces on reservoir performance and reconstruct ancient erg evolution, the stratigraphic architecture of aeolian systems must continue to be studied at multiple scales. This study pairs traditional methods (for example, measured stratigraphic sections) with advanced technologies (for example, drone‐derived outcrop models) to precisely resolve the metre to kilometre‐scale three‐dimensional stratigraphic architecture of wet aeolian Middle Jurassic Entrada Sandstone outcrops located at Rone Bailey Mesa near Moab, Utah, USA. Five facies are identified, primarily based on sedimentary fabrics, and are grouped into three associations named dune, sabkha and sand sheet. Statistical analyses of gamma‐ray spectrometer and automated mineralogy data indicate a distinct mineralogical difference between dune (quartz‐rich) and sabkha (more feldspathic) packages, suggesting that gamma‐ray logs may be used to better predict facies distribution in the subsurface. Seven modelled super bounding surfaces are planar to undulatory, with no perceived spatial trends. Five modelled interdune migration surfaces are undulatory but exhibit an average 0.09° angle of climb roughly parallel to the palaeocurrent direction. Two modelled superposition surfaces are linear to sinuous in plan‐view. Laterally discontinuous sabkha packages observed are interpreted to be remnants of closed, damp, interdune flats located between ca 8.5 to 17.0 m tall, sinuous, transverse bedforms or patches of such bedforms. Based on stratigraphic architecture interpretations, the Entrada Sandstone preserves signals of allogenic forcing and localized autogenic bedform cannibalization of the substrate. The findings of this study, some of which are not commonly recognized in wet aeolian facies models, enhance the understanding of erg evolution and can parameterize static models of aeolian reservoirs. [ABSTRACT FROM AUTHOR]

We present a very high-resolution bathy-morphological map of the offshore sector around the Scoglio d’Affrica islet (northern Tyrrhenian Sea, Italy). The study area covers a sector of 45 km2, between 3 and 85 m depth. Its central part, i.e. the apex of the Ridge, is characterised by a flat or gently sloping seafloor, where three mud volcanoes, and 250 pockmarks are recognised. Differently, the western and eastern Ridge flanks are steeper and characterised by 60 quasi-rectilinear escarpments and small ridges, more than 20 morphological highs, and elongated channels occasionally floored by bedforms. The seafloor shallower than 40 m is covered by Posidonia oceanica, forming compact and continuous or fragmented meadows intermingled with sandy patches. The main map represents the bathy-morphological setting of the area, which is largely affected by fluid seepage, providing insights for habitat mapping and preliminary marine geohazard assessment due to the violent gas outburst from mud volcanoes. [ABSTRACT FROM AUTHOR]

Abstract Sedimentary structures in ancient deposits are clues to reconstruct past geohazards. While parallel lamination formed by plane beds is one of the most common sedimentary structures in event deposits such as turbidites, the formative conditions for plane beds remain unclear. In the literature, two types of plane beds (upper and lower plane beds) exist and are supposed to develop under different shear stresses, particle sizes, and flow regimes. Here, we present new phase diagrams based on the compilation of existing data regarding formative hydraulic conditions for plane beds to clarify the formation processes associated with the two types of plane beds. The diagrams indicated that the data form two separate populations and the gap between them corresponds to the threshold condition of the particle entrainment into suspension. Lower plane beds form when sediment particles move only as bed load. This phase space can be discerned from fine sand to gravel and differs from the conventional view in which the formation of the lower plane bed is limited to grain sizes above 0.7 mm. In addition, our phase diagrams suggest that upper plane beds appear under conditions of the active suspended load. Our analyses demonstrate that the suspended load contributes to the formation of plane beds, whereas other mechanisms can also produce fine-grained plane beds in flows with low bed shear stress. Thus, the results of this study suggest that the existing interpretations on fine-grained parallel lamination such as Bouma’s Td division need to be reconsidered. The bedform phase diagrams newly established in this study will be useful for estimating the flow conditions from the geologic records of event beds.

The Greater Flamingo Phoenicopterus roseus is known as an ecosystem engineer, rearranging sediment in peculiar bedforms as a consequence of its filter-feeding behaviour. In recent decades, the populations of the Greater Flamingo have notably increased, and now the species is one of the most abundant waterbirds in Mediterranean wetlands. Owing to its range expansion, it inhabits and exploits new and suitable foraging sites detectable by foraging structures left on the sediment. There are few images of the foraging morphologies in the literature, possibly due to their ephemeral nature and difficulty in detecting them. In this manuscript, we present a very detailed UAV (Unmanned Aerial Vehicle) image of an aggregate of pit foraging structures of Greater Flamingo discovered on a back-barrier washover fan in the Marano and Grado Lagoon (Northern Adriatic, Italy). [ABSTRACT FROM AUTHOR]

The frequency of droughts and floods is increasing due to the extreme climate. Therefore, water resource planning, allocation, and disaster prevention have become increasingly important. One of the most important kinds of hydrological data in water resources planning and management is discharge. The general way to measure the water depth and discharge is to use the Acoustic Doppler Current Profiler (ADCP), a semi-intrusive instrument. This method would involve many human resources and pose severe hazards by floods and extreme events. In recent years, it has become mainstream to measure hydrological data with nonintrusive methods such as the Large-Scale Particle Image Velocimetry (LSPIV), which is used to measure the surface velocity of rivers and estimate the discharge. However, the unknown water depth is an obstacle for this technique. In this study, a method combined with LSPIV to estimate the bathymetry was proposed. The experiments combining the LSPIV technique and the continuity equation to obtain the bed elevation were conducted in a 27 m long and 1 m wide flume. The flow conditions in the experiments were ensured to be within uniform and subcritical flow, and thermoplastic rubber particles were used as the tracking particles for the velocity measurement. The two-dimensional bathymetry was estimated from the depth-averaged velocity and the continuity equation with the leapfrog scheme in a predefined grid under the constraints of Courant–Friedrichs–Lewy (CFL). The LSPIV results were verified using Acoustic Doppler Velocimetry (ADV) measurements, and the bed elevation data of this study were verified using conventional point gauge measurements. The results indicate that the proposed method effectively estimated the variation of the bed elevation, especially in the shallow water level, with an average accuracy of 90.8%. The experimental results also showed that it is feasible to combine the nonintrusive imaging technique with the numerical calculation in solving the water depth and bed elevation. [ABSTRACT FROM AUTHOR]

An experimental investigation of the entrainment threshold of sediment grains conducted in a non‐tiltable flume is reported and an original processing method based on image correlation suitable for detection of sediment threshold of motion is presented. Unlike existing visual measurement methods, the proposed method is not affected by the individual judgement of the experimentalists, which may strongly impact the estimation of incipient motion. Three entrainment thresholds were exhibited. The method highlighted a first threshold for very low shear stress conditions that was interpreted as a nascent movement threshold. The other two thresholds of motion were related to distinct stages of surface mobility of the bed: one related to bedload threshold and the other to bedform threshold. Series of tests were first performed to study the sensitivity of the method to different parameters: angle of view of the camera, size of the image and acceleration rate of the linear ramp of flow velocity. A ratio of the image surface and the grain surface equal to 3000 was found to be the minimum size to perform reliable estimations of entrainment thresholds for all the sediments in the range of 0.328–2.648 mm tested. It was the only condition for the method to work. Once the method was validated, tests were performed with homogeneous sands and compared to results based on reference‐based and visual methods. The correlation method yields similar values for the bedload threshold to those obtained from reference‐based methods. Thus it could be an alternative to other methodologies by the simplicity of its implementation, precision, no requirement for human interference and direct correlation between threshold velocity and bed mobility without extrapolation. [ABSTRACT FROM AUTHOR]

Atolls in the South China Sea comprise 15% of the world's total in area. In contrast to most reef flats in the Indo-Pacific region, which typically develop up to contemporary sea level, a significant portion of their reef flats in the South China Sea are inundated at depths of 5–20 m. To gain insights into modern processes and determine whether these subtidal reef flats are actively shaped by hydrodynamics, we carried out an in situ observation on a 12 m-deep southwest-facing reef flat over a period of 8 months. The measurements revealed a prevalence of seasonally varying waves and stable tidal currents. While the reef flat remained sheltered from the northeast monsoon (January–May), the southwest monsoon (June–September) led to prolonged reef exposure to substantial waves (mean significant wave height of 1.3 m, with an orbital velocity of 0.22 m/s on average). Such an exposure resulted in the formation of mobile bed ripples and entrainment of coarse-grained coral sands. Estimates of potential bedload transport rate indicated that the combined action of waves and currents caused material loss from the reef flat, with movement into the lagoon or down the fore-reef slope of the atoll. This sediment loss was equivalent to reef bed erosion of up to 28 mm during the observation period. As these potential losses may be compensated by coral reef growth, our measurements implied that modern sediment budgeting has played a significant role in the maintenance of subtidal reef flats, in terms of bed elevation. Hence, the deep reef flat does not necessarily belong to the previously identified give-up pattern; a balance of coral sediment supply and transport-induced loss may result in an equilibrium morphology, or a "lock-up" pattern. • Combination of wave orbital motions and oceanic currents resulted in substantial shear stresses. • Sediment on the reef flat was mobile for over 75% of the 8-month observation period. • The morphology of the subtidal reef flat was maintained by modern processes. • On the global scale, there are "lock-up" atolls, in addition to keep-up, catch-up and give-up ones. [ABSTRACT FROM AUTHOR]

The sedimentology of upper flow regime bedforms represents an important research topic at the present. Deposits interpreted as those of supercritical flows are widely recognized in modern fan systems, but their recovery is challenging. Most of the sedimentological information has come from channel thalwegs but supercritical bedforms are also frequently downslope from the channel mouths. Such an environment has been identified in the Paola basin, where erosive and depositional cyclic steps have been imaged and identified in a sandy submarine lobe of the Amantea Fan. High-resolution sub-bottom profiles provide insight into the bedform internal architecture and their relationships with a frontally-confining ridge. For the first time, supercritical bedforms in a submarine lobe have been interpreted in two distinct positions: in the scour of an erosional cyclic step and in the stoss side of a depositional cyclic step. Coarse to medium-grained massive sand with flame structures, indicating rapid sediment fall-out and frequently associated with the occurrence of hydraulic jumps, has been identified in the scour and at the toe of the ridge. The latter represents an example of topographically induced hydraulic jumps driven by a frontal confinement. Top-cut-out medium to fine sands with tractive structures have been interpreted as the deposits related to the stoss side of a cyclic step or small-scale antidune superimposed on the cyclic step surface. The presented data broaden the understanding of the range of processes that are driven by the interaction between turbidity currents and seafloor topography and the dip of the slope. The recognition that topography influences the density structure and the degree of criticality of the flow and, consequently, the morphodynamics and facies of the relative deposits may help to explain sediment distribution and improve depositional models of fan lobes in confined settings. • Sedimentological information about supercritical bedforms in a frontally confined submarine lobe are provided. • Observations result from the interpretation of high-resolution seismic profiles and the analysis of five gravity cores. • Sedimentary structures, generally associated with hydraulic jumps, have been identified in scour fills and at the base of a ridge. • The topography influences the distribution of supercritical bedforms and the flow criticality. [ABSTRACT FROM AUTHOR]

Sedimentary structures in ancient deposits are clues to reconstruct past geohazards. While parallel lamination formed by plane beds is one of the most common sedimentary structures in event deposits such as turbidites, the formative conditions for plane beds remain unclear. In the literature, two types of plane beds (upper and lower plane beds) exist and are supposed to develop under different shear stresses, particle sizes, and flow regimes. Here, we present new phase diagrams based on the compilation of existing data regarding formative hydraulic conditions for plane beds to clarify the formation processes associated with the two types of plane beds. The diagrams indicated that the data form two separate populations and the gap between them corresponds to the threshold condition of the particle entrainment into suspension. Lower plane beds form when sediment particles move only as bed load. This phase space can be discerned from fine sand to gravel and differs from the conventional view in which the formation of the lower plane bed is limited to grain sizes above 0.7 mm. In addition, our phase diagrams suggest that upper plane beds appear under conditions of the active suspended load. Our analyses demonstrate that the suspended load contributes to the formation of plane beds, whereas other mechanisms can also produce fine-grained plane beds in flows with low bed shear stress. Thus, the results of this study suggest that the existing interpretations on fine-grained parallel lamination such as Bouma's Td division need to be reconsidered. The bedform phase diagrams newly established in this study will be useful for estimating the flow conditions from the geologic records of event beds. [ABSTRACT FROM AUTHOR]

Previous studies have primarily focused on the estimation of equilibrium scour depths around weirs, while overlooking the details of the temporal evolution of the live bed scour process of submerged weirs at the equilibrium stage. This study aims to investigate the relationships between fluctuating frequencies of the approaching bedforms and live bed scour depths around submerged weirs. The average fluctuating frequencies of approaching bedforms, upstream scour depth, and downstream scour depth are quantified from measured topographic time-series data. The frequency of upstream scour depth variations is found to be almost the same as that of bedforms, while the fluctuating frequencies of the scour depth variations downstream of the weir are found to be 0.38–0.68 times the approaching bedform frequencies in the scour equilibrium stage. New empirical equations are proposed to predict the fluctuating frequencies of live bed scour depth variations in the scour equilibrium stage. [ABSTRACT FROM AUTHOR]

Phosphorus is not only an essential nutrient for organisms but also a major contributor to the eutrophication of fluvial ecosystems. Previous studies have focused on the flux of phosphorus at the water‐sediment interface, and very little is known about the transport and distribution of phosphorus in bedforms. In this study, a series of flume experiments and numerical simulations were conducted to investigate phosphorus dynamics in the hyporheic zone. The results show that the phosphorus concentration in the overlying water decreases faster than that of conservative solutes because of sediment adsorption. It is also found that the hyporheic exchange flux of phosphorus is more sensitive to the maximum adsorption capacity than to the equilibrium isotherm distribution coefficient, and the penetration depth is inhibited at higher maximum adsorption capacity. The mass proportion of phosphorus is decreased by 19.5% in the pore water but increased by 95.9% in the sediment with the increase of the maximum adsorption capacity from 0.5 to 1. These findings provide a better understanding of the transport and distribution of phosphorus in the riverbed and the contribution of transient flows such as flood to phosphorus dynamics in the hyporheic zone. Plain Language Summary: Phosphorus is an important nutrient, whose transport and distribution in rivers have a great impact on the ecological environment. Phosphorus can be adsorbed by the sediment when entering the streambed from the water flow. Even if phosphorus pollution is effectively removed from rivers, phosphorus stored in sediments can return to rivers and cause secondary pollution. In order to explore how phosphorus transport and distribute in the river and streambed, indoor physical experiments and numerical simulations were used to describe the whole process. We find that the adsorption effect of sediment makes it easier for phosphorus into the streambed from the river. Stronger sediment adsorption significantly reduces the mass of phosphorus in the water. These findings improve the understanding of the movement of phosphorus and other adsorbable substances in rivers and provide reference and basis for phosphorus pollution control. Key Points: Sediment adsorption leads to a more rapid decrease in the concentration of phosphorus in the overlying waterThe hyporheic exchange flux of phosphorus is more sensitive to the maximum adsorption capacityThe variation of the maximum adsorption capacity leads to significant changes in mass proportions of phosphorus in the sediment and water [ABSTRACT FROM AUTHOR]

Drumlin shape has been hypothesized to correlate with ice‐flow duration and slip speed, but modern‐day analogues and the Coulomb nature of till render the basis of these correlations in question. The evolution of flow‐parallel subglacial landforms is of importance for ice flow because the form drag they provide may be a dominant factor in regulating glacier slip speeds. Here we examine the relationship between drumlin shape and cumulative slip displacement (i.e. time‐integrated slip speed) as a dominant glaciological control on drumlin shape. First, a new method is developed that allows slip speed to be estimated for deformable bedded glaciers along a flow line from an ice surface profile. Then, reconstructed surface profiles for ice margin chronologies of the Green Bay Lobe (GBL) are used to construct and estimate the spatially varying cumulative slip displacement for use in comparison with drumlin elongation ratios. We focus on a sector of the GBL near the central flow line where the geology is simple and glaciological controls are likely to dominate bedform development. Using Bayesian statistical analysis, a positive and statistically robust relationship between cumulative slip displacement and drumlin elongation ratio is found. Our analysis indicates that drumlin shape could be used to infer palaeo glacier slip speeds if time under the ice can be well constrained and geologic influences are minimal. These findings also suggest that drumlin‐supplied drag could decrease with increased cumulative slip displacement in the absence of rigid geologic features that fix drumlin positions. [ABSTRACT FROM AUTHOR]

The Greater Flamingo Phoenicopterus roseus is known as an ecosystem engineer, rearranging sediment in peculiar bedforms as a consequence of its filter-feeding behaviour. In recent decades, the populations of the Greater Flamingo have notably increased, and now the species is one of the most abundant waterbirds in Mediterranean wetlands. Owing to its range expansion, it inhabits and exploits new and suitable foraging sites detectable by foraging structures left on the sediment. There are few images of the foraging morphologies in the literature, possibly due to their ephemeral nature and difficulty in detecting them. In this manuscript, we present a very detailed UAV (Unmanned Aerial Vehicle) image of an aggregate of pit foraging structures of Greater Flamingo discovered on a back-barrier washover fan in the Marano and Grado Lagoon (Northern Adriatic, Italy).

Hyporheic exchange (HE) is the exchange between surface water, including suspended and dissolved matter, and pore water in the sediment surrounding a stream. It is essential to physicochemical and biological processes in both the stream and the surrounding sediment. This study systematically investigated the effects of the stream characteristics (i.e., stream discharge, bedform amplitude and wavelength, stream bed slope and groundwater flow) on the characteristics of HE (i.e., flowrate, scale, and residence time) in a straight pool‐riffle stream with floodplain using laboratory experiments and numerical simulations. The HE in the stream bank and streambed and its controlling factors were evaluated using numerical models. The results show that the HE flowrate and volume in the stream banks (i.e., in the floodplain) and streambed are significant and comparable at a high discharge. The discharge, bedform amplitude and bed slope directly affect the flow state in the pool, and the flow state determines the patterns of the effects of these three factors on the HE flowrate. The HE flowrate shows a linear relationship with discharge and bed slope when the surface flow in the pool remains subcritical. If the flow state in the pool changes from subcritical to supercritical conditions as discharge or bed slope increase, the HE flowrate becomes non‐monotonic with discharge and it first increases and then decreases with increasing bed slope. The flowrate, scale, and flux‐weighted mean residence time of HE linearly increase with bedform wavelength and exponentially decrease with both gaining and losing groundwater flow, independent of discharge. Key Points: A novel laboratory setup and numerical simulations were used to explore hyporheic exchange in a straight pool‐riffle stream with floodplainThe hyporheic exchange in the stream bank and streambed is comparable at a high discharge and the effects of controlling factors are similarThe flow state in the pool determines the patterns of the effects of discharge, bedform amplitude and bed slope on the hyporheic flowrate [ABSTRACT FROM AUTHOR]

Marine and river bedforms are rhythmic features driven by unidirectional or reversing currents and waves. They are ubiquitous on the floors of rivers, estuaries, coastal and marine settings. Despite a considerable history of dedicated studies, many aspects of their origin, development and dynamics are still the subject of scientific debate in various disciplines. The MARID conferences series hosts experts from geosciences, physics, engineering and other disciplines to provide a platform for the interdisciplinary exchange of fundamental and applied knowledge of marine and river dune dynamics. MARID VI took place in April 2019 in Bremen, Germany. Related scientific contributions are presented in this special issue of Earth Surface Processes and Landforms. An overview of the conference series and the specifics of MARID VI, as well as considerations in the context of equity, diversity and inclusion, are given. This introduction highlights the progress made with the papers published in the special issue. [ABSTRACT FROM AUTHOR]

The frequency of droughts and floods is increasing due to the extreme climate. Therefore, water resource planning, allocation, and disaster prevention have become increasingly important. One of the most important kinds of hydrological data in water resources planning and management is discharge. The general way to measure the water depth and discharge is to use the Acoustic Doppler Current Profiler (ADCP), a semi-intrusive instrument. This method would involve many human resources and pose severe hazards by floods and extreme events. In recent years, it has become mainstream to measure hydrological data with nonintrusive methods such as the Large-Scale Particle Image Velocimetry (LSPIV), which is used to measure the surface velocity of rivers and estimate the discharge. However, the unknown water depth is an obstacle for this technique. In this study, a method combined with LSPIV to estimate the bathymetry was proposed. The experiments combining the LSPIV technique and the continuity equation to obtain the bed elevation were conducted in a 27 m long and 1 m wide flume. The flow conditions in the experiments were ensured to be within uniform and subcritical flow, and thermoplastic rubber particles were used as the tracking particles for the velocity measurement. The two-dimensional bathymetry was estimated from the depth-averaged velocity and the continuity equation with the leapfrog scheme in a predefined grid under the constraints of Courant–Friedrichs–Lewy (CFL). The LSPIV results were verified using Acoustic Doppler Velocimetry (ADV) measurements, and the bed elevation data of this study were verified using conventional point gauge measurements. The results indicate that the proposed method effectively estimated the variation of the bed elevation, especially in the shallow water level, with an average accuracy of 90.8%. The experimental results also showed that it is feasible to combine the nonintrusive imaging technique with the numerical calculation in solving the water depth and bed elevation.

Coral reefs are an important source of sediment for reef-lined coasts and help to maintain beaches by providing protection though dissipation of wave energy. Understanding the mechanisms that deliver sediment to the coast from coral reefs and quantifying the total volume of sediment generated at coral reefs are critical for projecting future coastal change. A month-long hydrodynamics and sediment transport study on a fringing reef/lagoon complex in Western Australia indicates that lower frequency wave energy constituents are important to the total bedload transport of sediment across the reef flat and lagoon to the shoreline. The reef flat and the lagoon are characterized by distinctly different transport regimes, resulting in an offset in the timing of bedform migration between the two. Short-term storage of sediment occurs on the reef flat, which is subsequently transported into the lagoon when offshore wave heights increase and strong currents due to wave breaking at the reef crest develop. This sudden influx of sediment is correlated with an increase in bedform migration rates in the lagoon. Infragravity wave energy on the reef flat and lagoon make an important contribution to the migration of bedforms and resultant bedload transport. Given the complexity of the hydrodynamics of fringing reefs, the transfer of energy to lower frequency bands, as well as accurate estimates of sources and sinks of sediment, must but considered in order to correctly model the transport of sediment from the reef to the coast.

Antidunes are fluvial bedforms that form in rivers with supercritical flows. The water surface over antidunes is strongly in phase with the bed surface, and the water surface is amplified to produce large surface waves. Many experimental studies have addressed antidunes; however, the shapes of three‐dimensional antidunes in a wide channel with alternate bars have not yet been appropriately understood. In this study, we experimentally investigated the streamwise and transverse length scales of antidunes under conditions with a large width–depth ratio. Our experimental results provide evidence for the coevolution of antidunes and free alternate bars, and show for the first time that the development of free bars greatly alters the three‐dimensional shape of water surface waves over antidunes. In the absence of free bars in a wide channel, multiple longitudinal wave trains form, and the number of wave trains counted in the transverse direction increases with increases in the width–depth ratio. However, the presence of free bars affects the local flow characteristics, resulting in a decrease of the number of wave trains in the transverse direction. Therefore, we propose a simple model for predicting the reduction in the number of wave trains by combining two previous theories for antidunes and free bars. Results obtained by the model were found to largely agree with experimental observations. © 2020 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd [ABSTRACT FROM AUTHOR]

Evidences of sedimentation affected by oceanic circulation, such as nepheloid layers and contourites are often observed along continental slopes. However, the oceanographic processes controlling sedimentation along continental margins remain poorly understood. Multibeam bathymetry and high‐resolution seismic reflection data revealed a contourite depositional system in the Mozambican upper continental slope composed of a contourite terrace (a surface with a gentle seaward slope dominated by erosion) and a plastered drift (a convex‐shape sedimentary deposit). A continuous alongslope channel and a field of sand dunes (mainly migrating upslope), formed during Holocene, were identified in the contourite terrace at the present seafloor. Seismic reflection data of the water column show internal waves and boluses propagating in the pycnocline near the upper slope. The channel and the dunes are probably the result of the interaction of the observed internal waves with the seafloor under two different conditions. The alongslope channel is located in a zone where intense barotropic tidal currents may arrest internal solitary waves, generating a hydraulic jump and focused erosion. However, upslope migrating dunes may be formed by bottom currents induced by internal solitary waves of elevation propagating landwards in the pycnocline. These small‐scale sedimentary features generated by internal waves are superimposed on large‐scale contouritic deposits, such as plastered drifts and contourite terraces, which are related to geostrophic currents. These findings provide new insights into the oceanographic processes that control sedimentation along continental margins that will help interpretation of palaeoceanographic conditions from the sedimentary record. © 2020 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd [ABSTRACT FROM AUTHOR]

Unit bars are relatively large bedforms that develop in rivers over a wide range of climatic regimes. Unit bars formed within the highly‐variable discharge Burdekin River in Queensland, Australia, were examined over three field campaigns between 2015 and 2017. These bars had complex internal structures, dominated by co‐sets of cross‐stratified and planar‐stratified sets. The cross‐stratified sets tended to down‐climb. The development of complex internal structures was primarily a result of three processes: (i) superimposed bedforms reworking the unit bar avalanche face; (ii) variable discharge triggering reactivation surfaces; and (iii) changes in bar growth direction induced by stage change. Internal structures varied along the length and across the width of unit bars. For the former, down‐climbing cross‐stratified sets tended to pass into single planar cross‐stratified deposits at the downstream end of emergent bars; such variation related to changes in fluvial conditions whilst bars were active. A hierarchy of six categories of fluvial unsteadiness is proposed, with these discussed in relation to their effects on unit bar (and dune) internal structure. Across‐deposit variation was caused by changes in superimposed bedform and bar character along bar crests; such changes related to the three‐dimensionality of the channel and bar geometry when bars were active. Variation in internal structure is likely to be more pronounced in unit bar deposits than in smaller bedform (for example, dune) deposits formed in the same river. This is because smaller bedforms are more easily washed out or modified by changing discharge conditions and their smaller dimensions restrict the variation in flow conditions that occur over their width. In regimes where unit bar deposits are well‐preserved, their architectural variability is a potential aid to their identification. This complex architecture also allows greater resolution in interpreting the conditions before and during bar initiation and development. [ABSTRACT FROM AUTHOR]

The ripple index (RI) is a common metric in the study of bedforms. In this work we review the application of RI to aeolian megaripples and perform a statistical analysis of field measurements. The research highlights ambiguity in how RI is calculated and used to describe and infer bedform morphometry. The statistical analysis demonstrates that although the prevailing application of RI encourages an interpretation as a deterministic quantity, it is in fact a stochastic variable with large variance and several use cases. RI can be used as an individual descriptive statistic, an aggregated descriptive statistic, or a linear predictor. We develop a simple linear model to investigate the notion of a representative RI value for megaripples. We find a value of 17.3 as a linear predictor for individual megaripples and a value of 17.6 as a linear predictor for aggregated megaripples, but show that the large variance observed in megaripple morphology mean that interpreting these values as representative of all megaripples can be misleading. This empirical finding of large variance in megaripple RI values aligns with recent work showing that megaripples are highly variable bedforms. Broadly, we recommend that all research involving RI should use it in an appropriate individual or aggregated data context and present these data with measures of dispersion. [ABSTRACT FROM AUTHOR]

In the present study, among six gravel-bed river reaches, two natural gravel-bed river reaches with approximately similar bedforms and flow conditions have been found, one river reach with vegetation on bedforms and the other without vegetation on bedforms. Based on field measurements, the impacts of vegetation over bedforms on flow characteristics have been investigated. Flow velocity, Reynolds stress and turbulence intensity distributions over bedforms with vegetation have been compared with those over gravel bedforms without vegetation. By using the quadrant analysis, the dominant events of turbulent flow structures have been assessed for flows over gravel bedforms with vegetation and without vegetation. Results show that the effect of vegetation enhances the anisotropy in turbulence and generates strong secondary currents, which lead to the occurrence of dip phenomenon at the stoss section and the crest of vegetated bedforms. The change of bed roughness from gravel bed to vegetated bed causes a development of a new boundary layer. This resulted in the different shape of velocity distributions over bedforms with vegetation from those over bedforms without vegetation, which can affect the lives of aquatic animals. Reynolds shear stress at downstream section of gravel bedform without vegetation is much higher than that of vegetated bedform, which shows that the erosion takes place from downstream of vegetated bedforms, while in the bare beds; the deposition is at downstream of bedforms. It is found that the presence of vegetation causes a chaotic distribution pattern of Reynolds shear stress, while it has more orderly pattern at the downstream section of gravel bedform without vegetation. Besides, the distribution of turbulence intensity in the lee side of vegetated bedform has a concave shape and in the stoss section, turbulence is generated in ejections and sweeps near the vegetation cover. These findings are significant to the practice of river ecological restoration. [ABSTRACT FROM AUTHOR]

Bedforms are ubiquitous features in rivers and shallow seas, as mobile sediment is transported by flowing water. The mutual interaction of hydrodynamics and bedform has been widely studied in the laboratory over two‐dimensional bedforms having an angle‐of‐repose (30°) lee side and a relatively simple shape. However, the influence of bedform natural morphology and three‐dimensionality on the flow is still poorly constrained. The present work looks at how a natural three‐dimensional (3‐D) bedform field influences flow properties through high‐resolution numerical modeling. A 3‐D numerical model is set up with Delft3D and verified against lab experiments of idealized 3‐D bedforms. The model is used to simulate water velocities, turbulence, water levels, and bed shear stress above a natural bedform field from the Río Paraná (Argentina). The presence and size of the flow separation zone and turbulent wake depend on the presence and properties of the slip face (defined here as the portion of the lee side with angles >15°) and not on those of the crest. When present, the flow separation and wake lengths are, for the tested settings, respectively, around 5 and 13 times the slip face height. A slip face orientation of 25° or more compared to the flow increases cross‐stream flow and suppresses flow reversal over the slip face. To understand and predict flow and bedform properties, the slip face rather than the crest position should be identified and analyzed. Plain Language Summary: In rivers and at the coast, the water flows fast and moves sand. This often forms underwater dunes, exactly like the movement of sand by wind forms desert dunes. These underwater dunes can be several meters high and are therefore a danger for navigation. They move and can be a risk for offshore structures (like wind farms). Understanding the movement of water above dunes helps to say how dunes will move. In the past, it has often been done over dunes made in the laboratory. In the work presented here, we look at how water is moving above natural dunes, which were measured in the Paraná River in Argentina. For this, we use a numerical model: A computer calculates how the water is moving above the measured dunes. We found that water movement depends on the part of the dunes which is the steepest, called the slip face. It does not depend on the highest point, the crest, as previously thought. Therefore, when studying a dune, the slip face properties should be looked at in order to say how water will move over the dune. Our work will make it easier to say how dunes and water move. Key Points: The slip face is defined as the portion of a bedform lee side with angles steeper than 15°Reverse flow and turbulent wake properties are linked to the characteristics of the slip face more than to bedform heightThe length of the flow separation zone and the turbulent wake are, for the tested settings, 5 and 13 times the height of the slip face [ABSTRACT FROM AUTHOR]

Subaqueous sand dunes are common bedforms on continental shelves dominated by tidal and geostrophic currents. However, much less is known about sand dunes in deep‐marine settings that are affected by strong bottom currents. In this study, dune fields were identified on drowned isolated carbonate platforms in the Mozambique Channel (south‐west Indian Ocean). The acquired data include multibeam bathymetry, multi‐channel high‐resolution seismic reflection data, sea floor imagery, a sediment sample and current measurements from a moored current meter and hull‐mounted acoustic Doppler current profiler. The dunes are located at water depths ranging from 200 to 600 m on the slope terraces of a modern atoll (Bassas da India Atoll) and within small depressions formed during tectonic deformation of drowned carbonate platforms (Sakalaves Seamount and Jaguar Bank). Dunes are composed of bioclastic medium size sand, and are large to very large, with wavelengths of 40 to 350 m and heights of 0·9 to 9·0 m. Dune migration seems to be unidirectional in each dune field, suggesting a continuous import and export of bioclastic sand, with little sand being recycled. Oceanic currents are very intense in the Mozambique Channel and may be able to erode submerged carbonates, generating carbonate sand at great depths. A mooring located at 463 m water depth on the Hall Bank (30 km west of the Jaguar Bank) showed vigorous bottom currents, with mean speeds of 14 cm sec−1 and maximum speeds of 57 cm sec−1, compatible with sand dune formation. The intensity of currents is highly variable and is related to tidal processes (high‐frequency variability) and to anticyclonic eddies near the seamounts (low‐frequency variability). This study contributes to a better understanding of the formation of dunes in deep‐marine settings and provides valuable information about carbonate preservation after drowning, and the impact of bottom currents on sediment distribution and sea floor morphology. [ABSTRACT FROM AUTHOR]