Bedform Morphology Research Articles

Late Cretaceous aeolianites in South China: Implications for palaeowind systems and palaeoclimate

ABSTRACTDespite reports of palaeowind from aeolian deposits across several basins in South China, knowledge of the Late Cretaceous palaeowind system and its associated palaeoclimatic conditions in East Asia remains inadequate. To elucidate the palaeowind system, numerous Late Cretaceous sandstones from 15 basins in South China were studied. Sedimentary features such as reddish orange colour, large‐scale and high angle cross‐beddings, good sorting, Fe‐varnish and impact scars demonstrate that the studied sandstones are aeolian in origin. The widespread occurrence of the Late Cretaceous aeolian deposition in South China suggests extensive aridification at this time. Based on the reconstruction of bedform morphology and dune architecture, these aeolian deposits are identified as (superposed) individual barchan, compound barchans and compound transverse dunes. Over 3000 measurements, corrected for structural tilting, together with published data show a primary dip direction of 0° to 200°. These dip directions reflect palaeowind blowing‐towards directions of 40° to 160°, from which two dominant sets of north‐east ca 70° and south‐east ca 110° are identified. These prevailing wind directions are interpreted to reflect a large regional, perhaps continental‐scale, monsoon system set up by the topographic barrier created by the Cretaceous Coastal Mountain in East Asia. The record of aeolian dunes, spanning ca 90 to 80 Ma (Coniacian–early Campanian), provides insights into the transition from the greenhouse conditions of the mid‐Cretaceous into the cool climate of the Late Cretaceous.

A New Approach to Account for Species‐Specific Sand Capture by Plants in an Aeolian Sediment Transport and Coastal Dune Building Model

AbstractVegetation plays a crucial role in coastal dune building. Species‐specific plant characteristics can modulate sediment transport and dune shape, but this factor is absent in most dune building numerical models. Here, we develop a new approach to implement species‐specific vegetation characteristics into a process‐based aeolian sediment transport model. Using a three‐step approach, we incorporated the morphological differences of three dune grass species dominant in the US Pacific Northwest coast (European beachgrass Ammophila arenaria, American beachgrass A. breviligulata, and American dune grass Leymus mollis) into the model AeoLiS. First, we projected the tiller frontal area of each grass species onto a high resolution grid and then re‐scaled the grid to account for the associated vegetation cover for each species. Next, we calibrated the bed shear stress in the numerical model to replicate the actual sand capture efficiency of each species, as measured in a previously published wind tunnel experiment. Simulations were then performed to model sand bedform development within the grass canopies with the same shoot densities for all species and with more realistic average field densities. The species‐specific model shows a significant improvement over the standard model by (a) accurately simulating the sand capture efficiency from the wind tunnel experiment for the grass species and (b) simulating bedform morphology representative of each species' characteristic bedform morphology using realistic field vegetation density. This novel approach to dune modeling will improve spatial and temporal predictions of dune morphologic development and coastal vulnerability under local vegetation conditions and variations in sand delivery.

Submarine Morphological Description of the Ancient Archipelagic Aprons in the Marcus–Wake Seamount Group, Northwestern Pacific Ocean

Herein, the morphological characteristics of submarine archipelagic aprons were presented for five guyots, Suda, Arnold, Lamont, Niulang, and Zhinyv, which are over 80 Ma years old and are located in the Marcus–Wake seamount group, northwestern Pacific Ocean. Nearly 28 landslide deposits were recognized using the bathymetry and backscatter intensity data collected from the studied guyots. Landslides and their deposits that surround seamounts are mostly related to the morphology of debris avalanches, scarps, gullies/channels, and bedforms. The morphology of the archipelagic aprons of the studied guyots indicates mutual landslide processes, including slump and distinct debris avalanches arising from a cohesive or cohesionless landslide material flow. The superimposition of debris flows and sedimentation dominates the recent stages of the studied guyots. The archipelagic aprons corresponding to convex-arc-shaped scarps exhibit larger domains compared to the invagination-arc-shaped scarps with similar lateral lengths. The scarp morphologies of the studied guyots are predominantly of the complex-arc shape, indicating multiple landslide events. Parallel and convergent gullies and channels are mostly found on the elongated landslide deposits, whereas divergent and radial gullies and channels are mostly distributed on the fan-shaped aprons. Ubiquitous sediment waves occurred on the bedforms of the distal archipelagic apron across the studied guyots because of sediment creep. Small-scale sediment waves were only observed in the channels on the aprons of the Suda guyot.

Controls on the Leeside Angle of Dunes in Shallow Unidirectional Flows

AbstractDunes are ubiquitous features in alluvial channels, serve as major agents of sediment transport and contribute significantly to flow resistance. Research in the past decade has illustrated the complexity of dune geometry and widespread occurrence of dunes that have a low leeside angle. However, there is a debate concerning the occurrence of such dunes and their formative processes. This paper seeks to further our understanding of low‐angle dunes by utilizing data from a robust set of shallow flow laboratory experiments detailing equilibrium bedform morphology across a range of sediment transport conditions. Analysis of bedform morphology demonstrates that dunes with low‐angle leesides are generated in shallow laboratory flows and are not restricted to deep rivers. Of the possible processes that have been proposed to explain the formation of low‐angle dunes, this finding unequivocally shows that liquefied leeside avalanches, which rely on deep flows for their generation, are not a controlling mechanism. In addition, dunes formed under suspension‐dominated conditions possess lower leeside angles compared with those formed under bedload‐dominated conditions. However, where bedload transport dominates and sediment suspension is likely of lesser importance, low‐angle dunes are still present, and preliminary analysis shows that bedform superimposition can result in lowering of the dune leeside angle. Low and intermediate angle dunes formed under these various conditions also have a lower potential for large‐scale, permanent, leeside flow separation compared with angle‐of‐repose dunes, confirming the need to account for these differences in predictions of flow resistance associated with dune form roughness.

Bedform evolution along a submarine canyon in the South China Sea: New insights from an autonomous underwater vehicle survey

ABSTRACTTraditional 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.

Large-scale physical experimental study on the evolution of 3D bedform by irregular waves

A large-scale physical experiment was conducted to analyze the morphological seabed evolution in a large-wave flume at the Tianjin Research Institute for Water and Transport Engineering. The morphologies of the bedforms, ranging from two to three dimensions, were analyzed as wave mobility (represented by ѱw) increased. The sand-ripple morphology underwent four stages: straight, staggered, lunate and flat. Furthermore, the three-dimensional (3D) features and flat patterns were enhanced with increasing ѱw. The seabed ripple was processed using a wavelet transform and filter method to eliminate the effects of noise and dunes. The ripple wavelength followed the gamma and lognormal distributions for irregular waves, whereas the distribution of ripple height transitioned from normal to exponential with increasing ѱw. Furthermore, to predict the ripple-geometry statistics more accurately, a modified predictor was developed based on the acquired large-scale experimental data. The findings of this study can aid the development of innovative methods for evaluating bedform morphologies and statistical parameters.

Effects of the spacing between two hydrokinetic turbines on the bedforms by numerical simulations

Accelerating hydrokinetic renewable energy development towards endurance requires investigating interactions between the hydrokinetic turbine and its surrounding physical environment. Interactions between hydrokinetic turbines (HT) and mobile sediment bed are considered as a critical area of assessment, however limited research studies have been published to address this issue. Hill et al. (2015) have shown experimentally that the presence of either single or multiple turbines and the rotation of the blades affect the bed morphology. Musa et al. (2019) have investigated experimentally the local effect of streamwise aligned turbines on the bedload, they found as a result that the geomorphic effects are stronger with increasing shear stress due to the presence of the rotors, inducing an alternating scour-deposition phenomenon. Chen et al. (2017) have investigated the influence of rotor blade tip clearance (distance between blades and seabed) on the scour rate of pile-supported horizontal axis current turbine. The results suggest that the decrease in tip clearance increases the scour depth, hence more sediment transport. Recently, Khaled et al. (2021) have studied the impact of hydrokinetic turbine on erodible sand banks, they showed numerically a significant interaction between the confinement of the turbine and its impact on the near bottom.
 
 In the present issue, we study the impact of two interacted turbines on the near bedform morphology. A modelling framework is derived to predict the significant transport induced by the turbines, such as the Euler-Euler (EE) multiphase model for sediment transport and the Blade Element Method (BEM) to model the forces generated by the turbines, using the open source platform OpenFOAM and the library SedFoam (Chauchat et al. 2017). A phase of validation is presented for the combined model (EE and BEM) using experimental results of Hill et al. (2015). The present study consists in considering one sediment class, sand of diameter of 0.25 mm, and two horizontal axis turbines with an axial flow direction corresponding to the riverine case. The approach is configured with four different axial inter-turbines distances. The wake distribution behind the second turbine is altered by the wake of the upstream turbine in all configurations (fig. 2). This interaction promotes the erosion under the turbines and the deposition along the axis of the turbines in the wake due to relation between the dynamics of ripples generation and the wake effects of turbines (fig. 3).

Plastic pollution in riverbeds fundamentally affects natural sand transport processes

Over the past 50 years, rivers have become increasingly important vectors for plastic pollution. Lowland riverbeds exhibit coherent morphological features including ripple and dune bedforms, which transport sediment downstream via well-understood processes, yet the impact of plastic on sediment transport mechanics is largely unknown. Here we use flume tank experiments to show that when plastic particles are introduced to sandy riverbeds, even at relatively low concentrations, novel bedform morphologies and altered processes emerge, including irregular bedform stoss erosion and dune “washout”, causing topographic bedform amplitudes to decline. We detail (i) new mechanisms of plastic incorporation and transport in riverbed dunes, and (ii) how sedimentary processes are fundamentally influenced. Our laboratory flume tank experiments suggest that plastic is not a passive component of river systems but directly affects bed topography and locally increases the proportion of sand suspended in the water column, which at larger scales, has the potential to impact river ecosystems and wider landscapes. The resulting plastic distribution in the sediment is heterogeneous, highlighting the challenge of representatively sampling plastic concentrations in river sediments. Our insights are part of an ongoing suite of efforts contributing to the establishment of a new branch of process sedimentology: plastic – riverbed sand interactions.

Evidence for Fluctuating Wind in Shaping an Ancient Martian Dune Field: The Stimson Formation at the Greenheugh Pediment, Gale Crater

AbstractTemporal fluctuations of wind strength and direction can influence aeolian bedform morphology and orientation, which can be encoded into the architecture of aeolian deposits. These strata represent a direct record of atmospheric processes and can be used to understand ancient Martian atmospheric processes as well as those on Earth. The strata can: give insight to ancient atmospheric circulation, how the atmosphere evolved in response to global changes in habitability, and how ancient processes differ from modern processes. The Stimson formation at the Greenheugh pediment (Gale crater) records evidence of fluctuating wind across multiple temporal scales. The strata can be subdivided into three intervals–Gleann Beag, Ladder, and Edinburgh intervals. Internally, the intervals record changes of dune morphology and orientation, correlatable to wind fluctuations at multiple temporal scales. The basal Gleann Beag interval comprises compound cross‐strata, deposited by oblique compound dunes. These dunes record a bimodal wind regime, resulting in net sediment transport toward the north. The Ladder interval records a reversal of sediment transport to the south, where straight‐crested simple‐dunes shaped by a seasonally variable winds formed. Finally, the Edinburgh interval records sediment transport to the west, where a unimodal wind formed sinuous‐crested simple dunes. These observations demonstrate active and variable atmospheric circulation in Gale crater during the accumulation of the Stimson dune field, at multiple temporal scales from seasonally driven winds to much longer time‐frames, during the Hesperian. These observations can be used to further understand ancient atmospheric conditions and processes, at a high temporal resolution on Mars.

Streamlined subglacial bedform sensitivity to bed characteristics across the deglaciated Northern Hemisphere

AbstractStreamlined subglacial bedforms observed in deglaciated landscapes provide the opportunity to assess the sensitivity of glacier dynamics to bed characteristics across broader spatiotemporal scales than is possible for contemporary glacial systems. While many studies of streamlined subglacial bedforms rely on manual mapping and qualitative (i.e., visual) assessment, we semi‐automatically identify 11,628 sedimentary and bedrock bedforms, created during and following the Last Glacial Maximum across nine geologically and topographically diverse deglaciated sites in the Northern Hemisphere. Using this large dataset of landforms and associated morphometrics, we empirically test the importance of subglacial terrain on bedform morphology and ice‐flow behavior. A minimum bedform length–width ratio threshold provides a constraint on minimum morphometrics needed for streamlined bedforms to develop. Similarities in bedform metric distribution regardless of bed properties indicate that all bed types may support similar distributions of warm‐based ice flow conditions. Ice flow within valleys with easily erodible beds host the most elongate bedforms yet the widest range in bedform elongation and bedform surface relief. The presence of these highly elongate bedforms suggest high ice‐flow velocities occur within valley settings despite spatially heterogeneous landform‐generating processes. In contrast, lithified sedimentary beds within regions not constrained by topography on the scale of 1–102 km contain bedforms with high density and packing, low change in surface relief and low elongation, indicating spatially uniform and organized interactions at the ice–bed interface and consistency in ice‐flow velocity. Regardless of genesis, we find a sensitivity of bedform elongation (i.e., used to interpret ice‐flow speed or persistence) to topographic conditions on the scale of 1–102 km, while bedform density is sensitive to bed lithology. The findings presented in this study provide analogues for processes of subglacial erosion and deposition, ice–bed interactions and warm‐based ice flow within contemporary glacial systems.

Planetary Aeolian Landforms: An Introduction to the Fifth Planetary Dunes Workshop Special Issue

AbstractAeolian landforms are widespread in our solar system. Understanding the exact nature and processes of formation of these features are challenging tasks necessitating a strong collaboration between scientists with different skills and scientific backgrounds. This paper describes the special issue for the 5th International Planetary Dunes Workshop, which includes 15 research papers and three commentaries. Among the 18 papers included in this collection, 16 cover Martian aeolian science and two Titan aeolian science. The papers presented focus on bedform morphology and dynamics via remote sensing data, modeling, analogs studies and laboratory experiments. Here we put the main results of the papers in their appropriate scientific context and discuss potential future lines of research.

A Field Study on the Lithological Influence on the Interaction Between Weathering and Abrasion Processes in Bedrock Rivers

AbstractLithology is an important control on the efficiency of bedrock incision and thus the pace of landscape evolution. Rock strength is commonly considered the limiting lithologic factor that resists erosion. Yet, rock strength is a dynamic property that oscillates during advection of rock to the channel surface as damaging processes weaken rock and erosion exposes fresh rock. We approach the problem by investigating damage on bedrock surfaces that vary by the frequency they are eroded in channels of different lithologies. Our data set includes measurements of channel slope and width to characterize channel morphology, and Schmidt hammer rebound, P wave velocity, slake durability, and porosity to characterize the mechanical properties of channel surfaces. The average damage accumulation rate of lithologies ranges over 41% of the mean. We find a range of damage patterns among the different lithologies. Local surface damage increases with erosion frequency in channels comprised of coarse‐grained bedrock but decreases with erosion frequency in channels comprised of fine‐grained bedrock. We interpret these patterns to develop from lithological influences on weathering, abrasion, and the threshold of damage to erosion. The cross‐channel damage patterns between channel floors and margins are well correlated with stream power demonstrating links between microstructural rock properties, reach‐scale morphology, and landscape‐scale processes. We conclude that the morphodynamics of bedrock channels are sensitive to the lithological influences on the direction and magnitude of feedback in the coevolution of bedform morphology and the mechanical properties of the surface.

Near‐Bed Structure of Sediment Gravity Flows Measured by Motion‐Sensing “Boulder‐Like” Benthic Event Detectors (BEDs) in Monterey Canyon

AbstractThe near‐bed section of submarine gravity flows travels at the highest and most destructive speeds making direct measurements of this region of the flow difficult. Here results are presented from “boulder‐like” Benthic Event Detectors (BEDs) that measured their own rotation, depth and temperature while carried within the near‐bed region of gravity flows. BEDs were deployed in Monterey Canyon from 200 to 500 m water depth for 18 months (2016–2017) during the Coordinated Canyon Experiment. BEDs moved in 10 out of 14 gravity flows that transited the upper canyon. BEDs moved within the body of the flow because the initial velocities of the BEDs were 66 ± 16% (1 SD) of the flow transit velocities. BEDs rotated freely during most of their first moves and gained depth faster than in later moves, when their motion was more random and wobblier. The differences in BED motions between first and later moves suggest BEDs moved at different depths within the flow. The inferred near‐bed flow structure is strongly stratified with a fast, less‐dense layer moving above a slower and denser layer. Coherent changes between pressure and acceleration indicate that BEDs rode a crescent shaped bedform (CSB) morphology that persisted throughout flow events. The variability in BED speeds while riding the CSB morphology indicates a fluid‐like nature of the near‐bed layer. BED motions ended after being caught in the trough of a CSB. Based on recorded temperature decay rates after flow events, the thickness of redeposited sediment is 2–3 m.

Sedimentary characteristics and morphologic change of till-bedded semi-alluvial streams: Medway Creek, Southern Ontario, Canada

We describe the first detailed reach-scale study of an incisional till-bed river. Our analysis focusses on boundary till characteristics, bare till patch features, annual erosion rates, bedform dimensions and spacing, and grain size distributions of bedforms. Results show that till exposures constitute a relatively small portion of the bed and that till erosion rates are relatively high compared to bedrock rivers, although highly variable between till patches and within patches. The bedforms are not well organized in terms of spacing and show high morphologic variability. The sediment forming the bed is poorly sorted, and grain sizes of the bedforms show high variability ranging from fines to large boulders, although gravel contribution from the till to the alluvium is relatively small. We found evidence of some in situ and transport rounding of till clasts. As expected, riffles and steps are coarser while glides and pools are finer-grained. Sedimentary stability metrics show that riffles are unstable, while pools and glides are more stable. These results indicate that the bedform morphology and sedimentology till-bedded rivers differ substantially from their alluvial and bedrock counterparts in a variety of ways. Consequently, we recommend that semi-alluvial rivers be differentiated from their alluvial and bedrock counterparts in future channel classifications. Such a practice will be useful for the river research and practitioners' community to gain the appropriate research tools needed for assessment, management, and restoration practices for these rivers.

The Influence of Three‐Dimensional Topography on Turbulent Flow Structures Over Dunes in Unidirectional Flows

AbstractDunes are the most prevalent bedform present in sand‐bedded rivers and their morphology typically comprises multiple scales of three‐dimensional topography. However, our understanding of flow over dunes is predicated largely on two‐dimensional models, a condition which is rare in nature. Here, we present results of Large Eddy Simulations over a static, three‐dimensional dune field, using a two‐ and three‐ dimensional topographic realisation, to investigate the interaction between bed topography and turbulent flow structures. We show that flow over two‐dimensional bedforms increases the velocity over the stoss slope and reduces the size of the leeside separation zone as compared to 3D topography. Flow over three‐dimensional bedforms generates twice as many vortices as over two‐dimensional bedforms, and these vortices are longer, wider and taller than flow over their two‐dimensional counterparts. Turbulence is dominated by hairpin‐shaped vortices and Kelvin‐Helmholtz instabilities that interact with the bed in the brink point region of the dune crest and down the lee slope, and generate high shear stresses for long durations. These results are used to propose a new conceptual model showing the differences between flow over two‐ and three‐dimensional bedforms. The findings highlight how the size, morphology and stacking of coherent flow structures into larger flow superstructures may be critical in sediment entrainment, and may dictate the relationship between event duration and magnitude that drives sediment impulses at the bed. This will ultimately lead to an increased in the three‐dimensionality of bedform morphology.

Morphology of estuarine bedforms, Weser Estuary, Germany

AbstractLarge bedforms (dunes) are present in many shallow‐water environments. Knowledge of their dimensions and dynamics is required for river and coastal management. In tidal rivers and estuaries, the interaction among hydrodynamics (which results from the action of river and tidal flows), sediment transport and bedform shape and size is complex. In the present study, the distribution and morphology of bedforms in the Weser Estuary, Germany, were investigated. Bedforms were identified in bathymetric data of monthly multibeam echosounder surveys along the navigation channel during the years 2009 to 2013. Their size and shape were characterized. Bedforms were present along most of the channel, except at the position of the estuarine turbidity maximum and where dredging is carried out. Average bedform length varied between 20 and 60 m and bedform height between 0.3 and 1.6 m. Bedform asymmetry varied spatially and temporally along the estuary. In times of high river discharge, bedforms were generally more ebb‐asymmetric than in times of low discharge. The bedforms were predominantly two‐dimensional low‐angle dunes with their steepest slope situated near the bedform crest. A significant proportion of bedforms possessed a steep face (portion of the lee side steeper than 15°). This implies that they are likely to create flow separation and a turbulent wake, with a strong potential to induce high bedform roughness. However, important variations in steep face area density were noted, both spatially along the estuary and temporally as a function of the tidal phase (ebb or flood) and the seasonal variation in river discharge. The results have wide implications in terms of understanding and modelling hydrodynamics and sediment transport in estuaries.

Bed form-induced hyporheic exchange and geochemical hotspots

Small-scale bed form topographies control hyporheic exchange and biogeochemical processes within aquatic sediments, which ultimately affect water quality and nutrient cycling at the watershed scale. The impact of three-dimensional and small-scale bed form topographies on hyporheic exchange and solute mixing is investigated in the present work. The effect of bed form morphologies on the development of zones of enhanced reaction rates (i.e., hotspots) is also studied. A computational fluid dynamics model to simulate river flow over bed forms is combined with a subsurface flow and multicomponent reactive solute transport model. A wide variety of bed form topographies are generated using geometric models by varying parameters controlling curvature as well as bed form wavelength and amplitude. The results in this research suggest that out-of-phase bed forms generate more complex hyporheic flow patterns which reduce the efficiency of solute transformations. Higher phase shifts in bed form shapes result in overall higher average velocity, larger zones of enhanced pressure and reaction rates, and higher amounts of solute exchange. Moreover, the bed form shapes control the reaction process for a wide range of sediment conductivities. This study advances the understanding of the effects of complex and small scale morphological features on hyporheic exchange processes including the rate and spatio-temporal distribution of reaction hotspots.

Predicting Dominance of Sand Transport by Waves, Tides, and Their Interactions on Sandy Continental Shelves

AbstractWaves and tidal currents resuspend and transport shelf sediments, influencing sediment distributions and bedform morphology with implications for various disciplines including benthic habitats, marine operations, and marine spatial planning. Shelf‐scale assessments of wave‐tide‐dominance of sand transport tend not to fully include wave‐tide interactions, which nonlinearly enhance bed shear stress and apparent roughness, change the current profile, modulate wave forcing, and can dominate net sand transport. Assessment of the contribution of wave‐tide interactions to net sand transport requires computationally/labor intensive coupled numerical modeling, making comparison between regions or climate conditions challenging. Using the Northwest European Shelf, we show the dominant forcing mode and potential magnitude of net sand transport is predictable from readily available, uncoupled wave, tide, and morphological data in a computationally efficient manner using a k‐Nearest Neighbor algorithm. Shelf areas exhibit different dominant forcing modes for similar wave exceedance conditions, related to differences in depth, grain size, tide range, and wave exposure. Wave‐tide interactions dominate across most areas in energetic combined conditions. Meso‐macrotidal areas exhibit tide‐dominance while shallow, fine‐grained, microtidal regions show wave‐dominance over a statistically representative year, with wave‐tide interactions dominating extensively >30 m depth. Sediment transport mode strongly affects seabed morphology. Sand wave geometry varies significantly between predicted dominance classes with increased wave length and asymmetry, and decreased height, for increasing wave‐dominance. This approach efficiently indicates where simple noninteractive wave and tide processes may be sufficient for modeling sediment transport, and enables efficient interregional comparisons and sensitivity testing to changing climate conditions with applications globally.

EVALUATING AN AUTOMATED OBJECT-ORIENTED METHOD TO DELINEATE DRUMLINS FROM BOTH TERRESTRIAL AND SUBMARINE DIGITAL ELEVATION MODELS

Abstract. In the field of geomorphological mapping, the demand for automated delineation of bedforms is growing due to the increasing availability of Digital Elevation Models (DEMs) in small to medium resolutions. This automated technique is not commonly applied in submarine DEMs, where bedform morphology is often subdued due to erosion and part-burial. Here we analyse drumlins in both terrestrial and submarine environments to compare and contrast the set of rules needed for their automated delineation from 3D topographic data. An existing set of rules for automated extraction to delineate the perimeter of terrestrial drumlins was developed in 2011 using object-oriented classification tools, available through eCognition Developer (V.8.7.2). This partly supervised method is evaluated here and subsequently adjusted to be applied to extract drumlins from a submarine DEM with a higher resolution. Several adjustments were needed due to the morphologic differences between the terrestrial and the submarine drumlins. For submarine drumlins, a focus on variation in elevation in the tool is needed, as part-burial and overprinting by other bedforms is common in submarine settings. A Canny Edge Detector filter was used instead of the Sobel Edge detection filter, whilst slope gradient and direction played a larger role in the set of rules. Visual and quantitative comparison with manually delineated drumlin perimeters confirms the success of this revised automated extraction method in both terrestrial and submarine environments. The flexibility and precision of this method thus allow for the future development of object-oriented classification tools to delineate a wide range of bedforms from large-scale DEMs collected from all environments.

Sediment bed destabilization by an isolated vortex: an experimental approach

The destabilization of an erodible sediment bed by a reproducible impulsive phenomenon is studied experimentally. For this, a specific setup is designed to produce a well-controlled isolated vortex, advected over a uniform sand bed. Particle image velocimetry (PIV) measurements are performed to get fluid flow information; suspended particles images are also recorded. A new measurement technique, based on a stereo-correlation method, provides a precise reconstruction of the bedform morphology over time. Strong turbulent structures (namely sweep events) are evidenced in a near-wall region, where particles are mainly dislodged. The sediment plume follows the vortex: in particular, the upward velocity inside maintains the suspension phenomenon. However, the dispersion process seems to be not dependent on the vortex parameters. Then, the bed morphology is investigated: a scour hole is dug as the vortex hits the granular layer. Its dynamics is described by a typical scour law, even if the initial perturbation is transient. Another area, where the particles settled, can be observed: it behaves like a ripple, with a logarithmic growth of its crest. This deposition region has a specific geometry, studied here by a Fourier contour approach. Finally, a pickup rate is also derived from the morphological data: it appears independent of the vortex and bedform features.