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SSP3.5

Sedimentary features are the result of a complex interplay between the erosion, transport and deposition of grains under the action of a current - unidirectional, oscillatory, combined or multidirectional. Each sedimentary structure represents a palaeo-surface expression, and therefore they contain a record of the geomorphology driven by the flow conditions, provided that one understands how to invert and read this history. Evidence for sedimentary processes have been identified on Earth but also on other planetary bodies, based on observations of geomorphic features and stratigraphy.

Bedforms and other sedimentary features are generated in a wide variety of environments, including: aeolian wind-driven transport, rivers, estuarine, lacustrine and deltaic settings, pyroclastic currents, sub- and pro-glacial environments, shorelines and continental shelves, offshore storms, turbidity currents and subaqueous mass flows, deep-sea currents and extra-terrestrial bodies.

This session will host contributions regarding many aspects of the complex interaction between flow, bedforms and sedimentary structures on Earth and planetary surfaces, from their description to interpretation, and from modelling to experiments to field quantification, with studies ranging across differing spatial and temporal scales, from large-scale organisation patterns down to the grain-scale, as well as the palaeo-dynamic and morphodynamic aspects of control and feedback between flow, sediment transport and bedform evolution, on Earth and on Mars.

The varied contributions from field, laboratory, theoretical, and numerical approaches are intended to advance our knowledge of how to decipher the information contained in terrestrial and extra-terrestrial sedimentary bedforms, and help foster fruitful discussions between sedimentologists, geomorphologists, hydrologists, physicists and all researchers working on understanding bedform dynamics and their sedimentary products.

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Co-organized by GM5/HS13, co-sponsored by IAS
Convener: Anne BaarECSECS | Co-conveners: Maria Azpiroz-ZabalaECSECS, Guilhem Amin DouilletECSECS, Alice Lefebvre, Thaiënne van Dijk, Francesco SaleseECSECS, Steven BanhamECSECS
Displays
| Fri, 08 May, 08:30–10:15 (CEST)

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Chat time: Friday, 8 May 2020, 08:30–10:15

D861 |
EGU2020-6583
Zhongyuan Wang and Yongqiu Wu

Desert (sandland) margin is the transition region from inner aeolian landforms  to other landforms outside, while it remains as an ambiguous conception in previous researches. Accurately delineating its boundary line and realizing the characteristics of the particle size distribution of surface aeolian sands in margin area can help us understand the formation of modern boundary of desert (sandland). In this research, the criteria of identification of the boundary were proposed and the boundary line was extracted quantitative. Then systematic analyses of grain size of aeolian sand in margin were conducted. Together with the morphologic type, activity and the geomorphological location of collected dunes, the factors controlled the particle-size distributions had also been analyzed. The results reveal the following: (1) There is notable difference in grain size characteristics of aeolian sand between inside and outside of Mu Us sandland. The outside samples are finer than inside. Additionally, the aeolian sand covering on loess is always more poorly sorted and with different grain size fraction composition. (2) The controlling factors on particle size distribution are different in different downwind margins. In southwest margin, the grain size characteristics of aeolian sand are influenced by time and degree of stabilization of sampled dune and locally topographic relief; From the estuary of Lu River to Yuxi River, sediment transport by wind is affected by topographic obstacles including both valley and loess gully. Meanwhile, the small dunefields in Loess Plateau outside of Mu Us sandland may originate from a local alluvial source; In northeast downwind margin, the grain size characteristics of aeolian sand covering on loess are determined by regional gully erosion after its deposition.

How to cite: Wang, Z. and Wu, Y.: Grain size characteristics of surface aeolian sands in the downwind margin of modern Mu Us Sandland, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6583, https://doi.org/10.5194/egusphere-egu2020-6583, 2020

D862 |
EGU2020-9998
Lieke Lokin, Jord Warmink, and Suzanne Hulscher

In the near future river discharges are expected to become more extreme due to climate change. Both high discharges and low discharges will occur more frequently and become more extreme (Klijn et al., 2015). While during high discharges bedforms grow and result in increased bed roughness resulting in higher flood levels. During extreme low discharges bed material may become immobile. Remaining bedforms are obstacles for waterborne transport, reducing the maximum load each vessel can transport.

Current dune models can describe the growth of the dunes and representative bedform related roughness under high flows and flood waves (Paarlberg et al., 2009). Also steps have been made to implement the processes leading to upper stage plane bed by adding suspended load transport (Naqshband et al., 2016). However, a model resolving the dune evolution though a full flood wave, including a falling stage towards extreme low flows with partly immobile bed, are not available yet.

The evolution of dunes during the falling stage of a flood wave, towards extremely low discharges, is not well understood and therefore cannot be properly predicted. Predictions of dune heights during periods of extreme low discharge can help fairway managers to maintain sufficient depth. To obtain this understanding first a fast dataset of bed level measurements, made by COVADEM on the Rhine river, will be analyzed with special focus on the growth and decrease of bedforms. This analysis will produce a set of parameters, valid for circumstances where immobile bed can occur and will lead to immobile bedforms. This new understanding of bedform development will be combined with the current knowledge on bedform development into an integrated model which can predict dune development from lower stage immobile dunes or flat-bed toward upper stage flat bed and vice versa.

References

Klijn, F, Hegnauer, M., Beersma, J. and Sperna-Weiland, F. (2015). Wat betekenen de nieuwe klimaatscenario’s voor de rivierafvoeren van Rijn en Maas? Samenvatting van onderzoek met GRADE naar implicaties van nieuwe klimaatprojecties voor rivierafvoeren. Deltares, KNMI, Ministerie van Infrastructuur en Milieuw (In Dutch)

Naqshband, S., van Duin, O., Ribberink, J., and Hulscher, S. (2016). Modeling river dune development and dune transition to upper stage plane bed. Earth Surf. Process. Landforms, 41: 323– 335. doi: 10.1002/esp.3789.

Paarlberg, A.J., Dohmen-Janssen, C.M., Hulscher, S.J.M.H. and Termes, A.P.P. (2009). Modeling river dune development using a parameterization of flow separation. Journal of Geophysical Research 114: F01014. DOI:2002JB001785/2007JF000910

How to cite: Lokin, L., Warmink, J., and Hulscher, S.: River dunes under extreme high and low flows: outline of a research project, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9998, https://doi.org/10.5194/egusphere-egu2020-9998, 2020

D863 |
EGU2020-10561
Simone Silvestro, Francesco Salese, David Vaz, Joel Davis, Hezi yizhaq, Gabriele Franzese, and Francesca Esposito

Aeolian bed forms such as dark dunes and ripples are abundant and widespread on Mars and can be used to constrain present-day wind conditions at the surface. Fossils aeolian bed forms are usually fractured, cemented and useful to constrain paleo wind conditions. Here we describe active dark dunes and fossil megaripples from an area in Arabia Terra and we discuss theirs climate implications. This area shows dark-toned domes and barchans dunes 1.5 – 10.5 m in height. Dunes slip faces, dipping SW, suggest NE dominant winds. Dunes were targeted in 2006 and 2016 (ΔT = 9.37 Earth years) by the HiRISE camera onboard of the NASA Mars Reconnaissance Orbiter (MRO). By tracking the position of the dunes in the 2006 and 2016 images, we measured an average SW displacement of 1.1 m (0.12 m yr-1). This translates to an average flux of 0.82 m3 m-1 yr-1 (median 0.78 m3 m-1 yr-1), which is almost three times the median dune flux in the MSL Curiosity landing site but ¼ of the flux measured in McLaughlin and Nili Fossae, areas where active megaripple migration were measured for the first time. Flux distribution (dune by dune) in the study area provides insights on the topographic effect, with the dunes located in depressed areas showing the lower fluxes. The dunes monitored over the 9.37 Earth years’ time-span migrated on the top of light toned layered deposit, which show a stair-stepped pattern of bright and dark layers showing different resistance to erosion. The different albedo and erosional pattern may represent different cementation/lithology, chemical composition and/or different grain sizes (bimodal). Eroded mounds 50 – 400 m-large, are the remnants of the widespread-layered unit in the studied area and are surrounded by a set of NW-SE trending linear ridges 10 – 20 m spaced. The morphology and regular spacing of the ridges suggest they are aeolian in origin. The ridges show a clear sinuous morphology that is typical of terrestrial megaripples. Megaripples are a particular type of ripples forming in bimodal sand which have coarse grains (> 1 mm) accumulating over the crest. In this scenario, the light-toned unit erosion could result in the production of bimodal sediment then re-organized in megaripples by the blowing winds and finally fossilized as suggested by the presence of fractures cutting through the megaripple crestlines. The capability of the winds to move coarse grains give hints on the transport capacity of the flows blowing in the past. The trend of the sinuous megaripples, matching the orientation of the dunes, suggests that the wind regime was consistent through time. The results reported here show how different aeolian features both active and fossils can be used to better constrain Martian climate and sedimentology.

How to cite: Silvestro, S., Salese, F., Vaz, D., Davis, J., yizhaq, H., Franzese, G., and Esposito, F.: Active and fossil aeolian bedforms in Arabia Terra (Mars): climate and sedimentological implications, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10561, https://doi.org/10.5194/egusphere-egu2020-10561, 2020

D864 |
EGU2020-6371
Xiaochuan Ma, Yu Gan, and Jun Yan

Dunes and dune-fields, being indicators and recorders of environmental conditions, have attracted extensive attentions and are massively studied on the individual behaviors and dune-dune interactions. However, the processes of field-field interaction are still elusive. Here, using the latest bathymetric datasets, we presented the new-found dune fields developing under reversing tidal currents on a shallow shelf, northwest South China Sea. The dune fields separately had dunes with opposite inclination and were head-to head colliding with a coarse-coarse pattern and a coarse-fine pattern in term of the sediment character. The dune-field fronts defined by the transition of dune asymmetry were outlined, where convergent bed load transports coexisted with divergent suspended load transports. The dunes had apparent spatial variability in their scale and morphology across the dune fields. Dunes obtained steeper shapes towards the dune-field fronts due to the different responses of height and length when dune-fields met together, which were benefited from the bidirectional sand supply and the comparable reversing current speed. Dune scale also exhibited distinct variations towards the dune-field fronts, suggesting the past dominant southward migration of the north fields and the resistance of the south fields. From 2014 to 2016, dunes inside the dune fields mostly moved to their inclining direction while some dunes in the dune-field fronts migrated to oppositely. The dune-field fronts shifted oppositely in various regions because of the rebalance of sand transports, which are inferred to essentially result from the regional flow changes. The migrating rates of the fronts are also influenced by the magnitude of grain size, water depth, dune height, and current speed. The behaviors of dune-field fronts can possibly record the interactions between dune-field and local environmental changes. More studies are still required on the internal structures of dunes near dune-field fronts and the modelling of local effects of regional environment modification.

How to cite: Ma, X., Gan, Y., and Yan, J.: Head-to-head encountering dune-fields under reversing flows in the Beibu Gulf, South China Sea, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6371, https://doi.org/10.5194/egusphere-egu2020-6371, 2020

D865 |
EGU2020-10439
Maria Azpiroz-Zabala, Joep Storms, Joris T. Eggenhuissen, Helena van der Vegt, Bert Jagers, Dirk-Jan Walstra, Arnau Obradors-Latre, and Anna Pontén

Turbidity currents are powerful submarine density flows that travel towards the deep-sea carrying huge amounts of suspended sediment. The flow capacity of keeping sediment suspended in turbidity currents controls the flow duration, which can last from minutes to days. Sediment in turbidity currents is entrained when the flows are triggered, or when the flows erode the seafloor and suspend additional sediment in their downslope path. Eventually, suspended sediment settles to form deposits on the seafloor. Therefore, the composition of the seafloor after the passage of turbidity current depends on the initial composition of the seafloor and the erosion, reworking of the bed and deposition of the flow-entrained sediment. Can the comparison of seafloor before and after turbidity currents provide information about the initial flow and seafloor parameters? Can the seafloor composition after a turbidity current passage modify next flow behaviour and to what extent?

We set up numerical models of multiple consecutive turbidity currents in Delft3D4 to study the evolution of both the flows themselves as well as their interaction with the seafloor. Some simulated flows run into clear water while other simulated flows run into the tail of the previous turbidity current. We analyse the influence of the initial flow sediment grain size composition on the flow behaviour and on the bed sediment composition. We investigate the influence on the flow structure of the sediment kept in suspension between consecutive flows. We analyse the evolution of the stratigraphy in the deposit formed by multiple consecutive turbidity currents. The aim of this work is enhancing the knowledge on the evolution of both flows and seafloor. The combination of the findings of these numerical models with field and experimental measurements and interpretations add to the prediction of the characteristics of turbidity currents and the distribution of the flow sediment.

How to cite: Azpiroz-Zabala, M., Storms, J., Eggenhuissen, J. T., van der Vegt, H., Jagers, B., Walstra, D.-J., Obradors-Latre, A., and Pontén, A.: Turbidity current signature on consecutive turbidity current: analysis through numerical simulations of multiple consecutive submarine flows, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10439, https://doi.org/10.5194/egusphere-egu2020-10439, 2020

D866 |
EGU2020-8948
Katrien Van Landeghem, Irinios Yiannoukos, Connor McCarron, Jacob Morgan, and Barney Clayton-Smith

Coarse and bimodal sediment mixtures like sand and gravel are common in palaeo-glaciated shelf seas and in coastal environments. Their presence leads to more complex sediment transport and morphodynamic processes, depending on the ratio of sand to gravel in the bed. With increased pressure on our near-and offshore sea beds, there is a growing need to more accurately model sediment transport and bedform dynamics with an increasing focus on bimodal sand-gravel sediment mixtures. Revisiting the quantification of the hiding-exposure (HE) effect highlighted how differently sized grains in a bimodal mixture modify each other’s threshold of motion. The critical shear stress needed to mobilise the sand and gravel fractions increased by up to 75% and decreased by up to 64% respectively, compared to that needed to mobilise well-sorted sediment of similar size. Implementation of this newly quantified HE correction in current-and wave-driven models illustrated that its influence on bedload transport rates and bed morphodynamics was greatest for mixtures where gravel percentage ranges between 10 and 20 %. Laboratory experiments were therefore conducted to investigate ripple formation and bed dynamics in mixtures with gravel percentage between 0 and 25%. The development of initial bedforms was quicker in sand-gravel mixtures compared to those developed in pure sand, whilst final heights and migration rates of the developed ripples decreased with increasing fraction of gravel in the bed. During this presentation, a full comparison will be made of the morphology and “down-core” sedimentary properties of ripples formed at different flow speeds. If we want to use our seabeds cost-effectively and sustainably, we need a better understanding on the influence of a decreased mobilisation of the finer fractions and an increased mobilisation of the coarser fraction on the dynamics of beds with a bimodal sediment composition.

How to cite: Van Landeghem, K., Yiannoukos, I., McCarron, C., Morgan, J., and Clayton-Smith, B.: The influence of gravel mixed with sand on the formation and development of ripples., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8948, https://doi.org/10.5194/egusphere-egu2020-8948, 2020

D867 |
EGU2020-17914
Daniel Papa and Christophe Ancey

Braided rivers are highly dynamical systems characterized by varying network-like structures even under quasi-steady conditions. Understanding their dynamics is crucial in geomorphology and river engineering (e.g., river restoration in Alpine and piedmond streams). Open questions about these dynamics include the definition and quantitative description of bed equilibrium. Here we propose to tackle this problem using a new method based on graph theory. This algorithm, called low-path allows one to extract the network structure of a braided river from its Digital Elevation Model (DEM). It is then possible to quantify and analyse the dynamics of the braided system, and not just the bed evolution, as has been done in earlier studies. To assess the dynamics and equilibrium of a braided river, we study two runs representing two distinct phases of the same braided river: the transition from a single channel to a braided river (run 1) and the equilibrium state of this river (run 2). A set of control parameters was used to characterise both runs and supplement the low-path method. We find that although a clear distinction can be made between straight channel and braided channel for both methods, it is more difficult to distinguish between transitional braided and equilibrium braided rivers. Finally we propose a set of dimensionless numbers that specify the braided network and can be used with numerical or stochastic simulations of a braided network. To illustrate their utility, we apply the Low Path method to a real Alpine braided river (the River Navisence, Wallis, Switzerland) and compare the results to our experimental data.

How to cite: Papa, D. and Ancey, C.: Braided rivers networks dynamics: analyzing topographic data from a large flume experiment, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17914, https://doi.org/10.5194/egusphere-egu2020-17914, 2020

D868 |
EGU2020-7802
Tjalling de Haas, Brian McArdell, Susan Conway, Jim McElwaine, Maarten Kleinhans, Francesco Salese, and Peter Grindrod

Understanding the initial and flow conditions of contemporary flows in Martian gullies, generally believed to be triggered and fluidized by CO2 sublimation, is crucial for deciphering climate conditions needed to trigger and sustain them. We employ the RAMMS (RApid Mass Movement Simulation) debris flow and avalanche model to back-calculate initial and flow conditions of recent flows in three gullies in Hale crater. We infer minimum release depths of 1.0–1.5 m and initial release volumes of 100–200 m3. Entrainment leads to final flow volumes that are 2.5–5.5 times larger than initially released, and entrainment is found necessary to match the observed flow deposits. Simulated mean cross-channel flow velocities decrease from 3–4 m s-1 to ~1 m s-1 from release area to flow terminus, while flow depths generally decrease from 0.5–1 m to 0.1–0.2 m. The mean cross-channel erosion depth and deposition thicknesses are _0.1–0.3 m. Back-calculated dry-Coulomb friction ranges from 0.1 to 0.25 and viscous turbulent friction between 100–200 m s-2, which are values similar to those of granular debris flows on Earth. These results suggest that recent flows in gullies are fluidized to a similar degree as are granular debris flows on Earth. Using a novel model for mass-flow fluidization by CO2 sublimation we are able to show that under Martian atmospheric conditions very small volumetric fractions of CO2 of ~1% within mass flows may indeed yield sufficiently large gas fluxes to cause fluidization and enhance flow mobility.

How to cite: de Haas, T., McArdell, B., Conway, S., McElwaine, J., Kleinhans, M., Salese, F., and Grindrod, P.: Initiation and flow conditions of contemporary flows in Martian gullies, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7802, https://doi.org/10.5194/egusphere-egu2020-7802, 2020

D869 |
EGU2020-7920
Laura Brakenhoff, Reinier Schrijvershof, Bart Grasmeijer, Jebbe van der Werf, Gerben Ruessink, and Maarten van der Vegt

To contribute to solving scientific and practice-inspired questions, the morphological development of coastal systems is predicted using numerical morphodynamic models like Delft3D. In such models, many of the processes are parameterized, for which various assumptions have to be made. One of the estimated variables is the bedform-related hydraulic roughness, which affects the magnitude and vertical structure of the flow and consequently also the magnitude of the sediment transport. A comparison is missing between model-predicted and observed hydraulic roughness values and it is unknown how this affects the hydrodynamics and sediment transport. Furthermore, the roughness is often used as a calibration parameter. The calibrated value might be very different from observed values and models might possibly do a good job for the wrong reasons.

The aim of this study is to determine the effect of the roughness caused by small-scale ripples (length ≈ 10 cm, height ≈ 1.5 cm) on hydrodynamics and sediment transport computed by a high-resolution, fully-coupled Delft3D model that is forced by waves, tides, wind, and atmospheric pressure. The study site is the wave-current dominated environment of the Ameland ebb-tidal delta in the north of the Netherlands. In 2017, a six-week field campaign was executed here, in which bedform heights and lengths, water levels, wave orbital velocity and direction, and current velocity and direction were measured.

The model was run for the duration of the field campaign with various bedform roughness scenarios, in which the roughness was either coupled to the hydrodynamics (thus varying over space and time), or it was set to a constant and spatially uniform value based on the observed mean ripple heights. Of all scenarios, we compared the predicted ripple heights, wave orbital velocities, depth-averaged current velocities and sediment transport magnitudes and directions. In addition, we compared the modelled and observed ripple heights, wave heights and flow velocities.

A previous study focused on the field campaign showed that observed ripple heights were much more constant than the ones computed by the default ripple predictor in Delft3D. Ripple heights were found to be related to orbital velocity and no other relations between ripple characteristics and hydrodynamics were found. However, first results of the present study indicate that the predicted roughness used to calibrate Delft3D to the water levels and currents is quite similar to the measured roughness. The main difference is that the predicted roughness is highly variable through time, which is not observed in the field. The simulations also show that the ripple-related roughness especially affects the magnitude of the depth-averaged current velocity, while its effect on the wave-orbital velocity is negligible. This also affects the sediment transport magnitude, while its direction is not affected. The cumulative suspended load transport magnitude can increase with more than 50% when a constant roughness is used instead of a spatio-temporally variable roughness.

How to cite: Brakenhoff, L., Schrijvershof, R., Grasmeijer, B., van der Werf, J., Ruessink, G., and van der Vegt, M.: The effects of bedform-related roughness on hydrodynamics and sediment transport patterns in Delft3D, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7920, https://doi.org/10.5194/egusphere-egu2020-7920, 2020

D870 |
EGU2020-19118
Joshua Ahmed, Jeffrey Peakall, Anne W. Baar, Na Yan, Daniel R. Parsons, and Matthew R. Balme

Sedimentary deposits and geomorphic landforms preserved on the surface of Mars afford scientists a possible insight into the formative conditions of these features and of the planetary climate in the deep past. We use a high-resolution digital elevation model (~1 m/pixel) created from imagery captured using the High Resolution Imaging Science Experiment (HiRISE) camera on board the Mars Reconnaissance Orbiter to examine the characteristics of 17 meanders from a channel in the Aeolis Dorsa region of Mars. We extracted the topographic signatures from 17 meanders that reveal a characteristic vertically stacked sequence of preserved point bars. These bars increase in elevation by up to 10 m and exhibit incline angles of between one and eight degrees. Each of the meanders is observed to grow vertically while contemporaneously undergoing lateral migration, creating sedimentary architecture not characteristic of fluvial meandering rivers. These landforms are commonly described as inverted channels; however, the preserved architecture differs from other so-called inverted channel topography on Mars, which fail to display such prevalent preserved bar topography. On Earth, vertically stacked deposits are observed in aggradational environments such as in tidally-affected backwater river sections, coastal wetlands, submarine settings, and sediment-rich fluvial systems. Given the pronounced vertical aggradation, and observed in-channel topography, we propose that this channel was fed with an exceptional sediment load – potentially comprised of a high concentration of fine-grained sediment – confined within strong cohesive banks that limited lateral migration and promoted vertical bedform growth. The channel may have also experienced base level controls from a downstream water body. These findings suggest that liquid water must have been persistent over a considerable timescale in Mars’ history.

How to cite: Ahmed, J., Peakall, J., Baar, A. W., Yan, N., Parsons, D. R., and Balme, M. R.: Aggradational Channels on Mars, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19118, https://doi.org/10.5194/egusphere-egu2020-19118, 2020

D871 |
EGU2020-1554
Olga Borisova and Alexei Sidorchuk

There are two main types of movement of bedforms in the river channel. Active bedforms are three-dimensional, symmetrical, with gentle slopes. They move without significant change in shape, since all parts of their surface move at the same celerity. Passive bedforms are two-dimensional, asymmetric, with steep leeward slope. Bedform top moves faster than hollow and bedform deform, skew.

Bedforms are usually organised into hierarchical complexes in the river channels, where smaller bedforms move along the surface of larger ones. With active movement, the morphology and dynamics of bedforms of different orders in the hierarchy are relatively independent. The relationships between bedforms of different orders is increasing in the case of passive movement.

Bed load transport in the river channel depends on the type of bedforms movement. In the case of active bedforms, bedload transport rate, computed with their morphology and celerity, is different for different bedform orders. The total bedload transport rate is equal then to the sum of bedload transportation by bedforms of different orders, plus sediments transit. In the case of passive movement, the total bedload transport rate is equal to bedload transportation by bedforms with steep leeward slopes and complete deposition there of all incoming sediments. Then it is possible to use Exner’s equation of deformation for estimating of bedload transport rate.

This study was carried out under the project: “Evolution and Transformation of Erosion-Channel Systems under Changing Environment and Human Impact”

How to cite: Borisova, O. and Sidorchuk, A.: The main types of river channel bedforms movement, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1554, https://doi.org/10.5194/egusphere-egu2020-1554, 2019

D872 |
EGU2020-16500
Joohee Jo, Dohyeong Kim, and Kyungsik Choi

Intertidal dune morphodynamics is closely tied to bedload transport that is variable in time and space due to the interplay between tide, wave and runoff discharge. Surprisingly the control of intertidal channel morphology on the dune morphodynamics and related bedload transport is scarcely documented. Actively migrating dunes are widely developed in the lower intertidal zone of Yeochari tidal flat in the northern Gyeonggi Bay, west coast of Korea. High-resolution aerial images, high-precision transect profiles, and hydrodynamic dataset were repeatedly obtained and analyzed to quantify the intertidal dune morphodynamics and associated bedload transport, and to address the role of channel morphodynamics. During the research period, the intertidal channel became more sinuous and an ebb barb arose concurrently at the upstream of the channel point bar. The ebb barb exerted a key role in the downstream delivery of fine-grained sediments onto the areas covered by dunes and the intertidal channel by reinforcing ebb currents with a pronounced time-velocity asymmetry. The presence of the ebb barb resulted in a rapid decrease of the width/depth ratio of the channel that had migrated laterally 130 m in six years. After the ebb-barb development, the heights and steepness (height/wavelength) of dunes on the point bar and near the ebb barb decreased notably. Simultaneously dune migration rate had increased from 0.5 m/day to 2.5 m/day, which decreases away from the channel. Bedload transport estimated by using Meyer-Peter and Muller (MPM) equation and Dune-Tracking Method (DTM) also decreases away from the channel. Bedload transport calculated by DTM (qbDTM, 0.03-0.38 m2/day) is much smaller than that estimated by MPM (qbMPM, 0.10-4.17 m2/day) by a factor of 1.5 to 62. The discrepancy ratio between the two bedload estimates (qbMPM/qbDTM) increases toward the channel and the ebb barb. Downslope flow toward the channel during the late stage of ebb tide may account for the underestimation of qbDTM by facilitating downslope sediment transport that reduced the dune steepness with the infilling of dune trough. The present study showcased a dynamic response of the dune morphodynamics and associated bedload transport in the open-coast tidal flats to the changes in the channel morphodynamics that is controlled by seasonal runoff discharge as well as tidal currents.

How to cite: Jo, J., Kim, D., and Choi, K.: Controls of channel morphodynamics on the intertidal dune morphodynamics and associated bedload transport in the open-coast macrotidal flats, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-16500, https://doi.org/10.5194/egusphere-egu2020-16500, 2020

D873 |
EGU2020-2600
Geert Campmans, Pieter Roos, Thaiënne Van Dijk, and Suzanne Hulscher

Tidal sand waves are dynamic large-scale bed forms occurring in tide-dominated, sandy shelf seas such as the North Sea. Since they may interfere with various activities, understanding sand wave dynamics is important from a practical point of view. Recently, two process-based model studies were carried out to investigate the influence of storm processes on sand wave dynamics (Campmans et al., CSR2017; JGR2018). While this type of model gives insight in the morphodynamic mechanisms, quantitative comparison with field observations remains a challenge.

 

Here we present a systematic validation of the afore mentioned linear and nonlinear models, against a wide range of sand wave observations from the entire Netherlands Continental Shelf (Damen et al., JGR2018). Specifically, from the available locations with sand wave observations and environmental characteristics, we have chosen a grid for calibration and, staggered to that, a grid for validation. For the so-called calibration locations, we tuned the linear model (using local environmental conditions) in order to minimize the difference between observed and modelled wavelengths. Next, on the validation locations, we used the thus obtained parameter settings (location-independent values of slip parameter and effective wave period) to test our model performance, both in the linear and nonlinear regime. First results demonstrate fair agreement for the wavelengths from the linear model and indicate a systematic overestimation of sand wave heights by the nonlinear model.

 

References

Campmans, G.H.P., Roos, P.C., De Vriend, H.J., Hulscher, S.J.M.H., 2017.  Modeling the influence of storms on sand wave formation: A linear stability approach. Continental Shelf Research 137, 103–116.

Campmans, G.H.P., Roos, P.C., De Vriend, H.J., Hulscher, S.J.M.H., 2018. The influence of storms on sand wave evolution: a nonlinear idealized modeling approach. Journal of Geophysical Research: Earth Surface 123, 2070-2086.

Damen, J.M., Van Dijk, T.A.G.P., Hulscher, S.J.M.H., 2018. Spatially varying environmental properties controlling observed sand wave morphology. Journal of Geophysical Research: Earth Surface 123, 262-280.

How to cite: Campmans, G., Roos, P., Van Dijk, T., and Hulscher, S.: Validation of process-based sand wave models: applying a linear and nonlinear sand wave model to the Netherlands Continental Shelf, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2600, https://doi.org/10.5194/egusphere-egu2020-2600, 2020

D874 |
EGU2020-5281
Wessel M. van der Sande, Pieter C. Roos, and Suzanne J.M.H. Hulscher

Estuaries are hydrodynamically complex regions where a river meets saline water. In many estuaries, sand dunes can be found; these are large-scale rhythmic bedforms. Observational studies have revealed several estuarine processes that affect sand dune dimensions and dynamics. These are for instance sand-mud interactions and tidal amplification. Here, we build upon an observational study in the Gironde Estuary, France, which indicated that the gravitational circulation – present in many estuaries due to the interaction between (heavy) seawater and (light) freshwater – is significant enough to affect sand dunes (Berne et al., 1993). Our aim is to understand the effect of this circulation on bedform dimensions and dynamics, and to explain the underlying mechanisms.

To this end, we develop an idealized process-based model which contains descriptions for the motion of water and non-cohesive sediment transport within a local section of a generic estuary. On this geometry, we impose a steady river discharge, superimposed on an oscillatory tidal flow. Furthermore, we include the effect of salinity-induced density differences by following the model as presented by MacCready (2004). In here, we adopt a diagnostic approach, meaning that the along-estuarine salinity gradient is imposed on the domain instead of being an unknown which interacts with the flow. The alternative, a so-called prognostic approach, is also explored.

This model is analyzed using a so-called linear stability analysis, as applied earlier to e.g. marine sand waves (Hulscher, 1996) but not yet to estuarine dunes. Within this analysis, the reference state with a flat bed is slightly perturbed, and the model shows whether these perturbations decay (the flat bed is stable) or grow (it is unstable). The model results provide a generic insight into the role of the gravitational circulation on bedform dimensions and dynamics, particularly growth and migration; the latter possibly directed opposite to the river discharge. To test our model, it is then applied to the specific settings of the Gironde. Furthermore, a systematic sensitivity analysis shows the effect of environmental parameters on bedform development when subject to the gravitational circulation. Including this estuarine-specific process is a novel and first step in obtaining a solid understanding of the behavior of estuarine sand dunes.

 

References

Berne, S., Castaing, P., le Drezen, E., & Lericolais, G. (1993). Morphology, Internal Structure, and Reversal of Asymmetry of Large Subtidal Dunes in the Entrance to Gironde Estuary (France). Journal of Sedimentary Petrology, 63(5), 780–793. https://doi.org/10.1306/d4267c03-2b26-11d7-8648000102c1865d

Hulscher, S. J. M. H. (1996). Tidal-induced large-scale regular bed form patterns in a three-dimensional shallow water model. Journal of Geophysical Research, 101(C9), 727–744. https://doi.org/10.1029/96JC01662

MacCready, P. (2004). Toward a unified theory of tidally-averaged estuarine salinity structure. Estuaries, 27(4), 561–570. https://doi.org/10.1007/BF02907644

 

How to cite: van der Sande, W. M., Roos, P. C., and Hulscher, S. J. M. H.: Modeling the influence of the gravitational circulation on estuarine sand dunes, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5281, https://doi.org/10.5194/egusphere-egu2020-5281, 2020

D875 |
EGU2020-9261
Jonathan Wilkin, Alan Cuthbertson, Sue Dawson, Dorrik Stow, Karl Stephen, Uisdean Nicholson, and Nadia Penna

Results are presented from the current experimental campaign which aims to observe the character of supercritical turbidity currents and other supercritical sediment gravity flows (SGFs) as they respond to morphological transition zones, e.g. slope breaks and losses of lateral confinement. This experimental setup aims to reproduce lower slope, channel-lobe transition zone, and, proximal lobe conditions, in order to be analogous to conditions found within deep-marine sedimentary environments such as those found within foreland basins, and on passive margins. Of particular interest is the sedimentological expression of these systems, how sedimentological variability arises in the form of sediment waves and scour fields, and how does an understanding of current dynamics help in the prediction of the internal structures of these features. Thus, this study will yield new data on how turbidity currents impact multi-layered sedimentary beds and determine parametric controls on erosion, deposition and bed restructuring processes. Turbidity currents are scaled via dimensionless parameters representing prevalent flow (e.g. Reynolds, Densimetric Froude Number, and Shields Numbers) and sedimentary (e.g. Rouse and Reynolds Particle Numbers) conditions, following the scaling techniques of de Leeuw et al., (2016) which have now been tested in numerous experimental studies e.g. Pohl et al., 2019.

 

Investigating how varying experimental conditions such as current parameters, severity of breaks in-slope, and, losses of lateral confinement impact the resulting depositional signature of lower slope, and channel-lobe transition zones. Of particular interest is the impact of previously developed bedforms upon current dynamics which will be studied via UVP and ADV measurements, as well as through the application of digital elevation models (DEM), which will be used to understand how systems evolve over multiple runs. DEM models will be generated using a photogrammetry technique capable of producing a high-resolution model (±2mm). The results of which will then be linked to synchronous sedimentological packages – both on the modern seafloor and preserved within ancient geological outcrops – with the aim of enhancing the predictive sedimentological concepts applied to these systems when being interpreted within the subsurface.

 

A seafloor study will focus upon supercritical bedforms generated by SGFs upon a deep-water slope and terrace located offshore of Senegal, West Africa. Combining seafloor seismic images, high-resolution sparker data, and drop cores taken from deep water channels, and overbanks. Through the interpretation of this dataset, it will be possible to understand the sedimentological variability of bedforms present on this slope system and allude to the flow conditions that led to their formation.

 

References

de Leeuw, J., Eggenhuisen, J.T., Cartigny, M.J.B., 2016. Morphodynamics of submarine channel inception revealed by new experimental approach. Nat. Commun. 7. https://doi.org/10.1038/ncomms10886

Pohl, F., Eggenhuisen, J.T., Cartigny, M.J.B., Tilston, M., de Leeuw, J. & Hermidas, N. (in review). The influence of a slope break on turbidite deposits: an experimental investigation. Marine Geology.

How to cite: Wilkin, J., Cuthbertson, A., Dawson, S., Stow, D., Stephen, K., Nicholson, U., and Penna, N.: New insights into the internal structure of Turbidite deposits from physical modelling of relevant erosional and depositional processes, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9261, https://doi.org/10.5194/egusphere-egu2020-9261, 2020

D876 |
EGU2020-20538
Kartikeya S. Sangwan, Sanjeev Gupta, Robert Barnes, and Steven G. Banham

Noachian-Hesperian terrains on Mars host multiple geomorphic and stratigraphic signatures of ancient water flow on early Mars. Sinuous, branching systems of ridges are one such geomorphic landform and have been interpreted as topographically-inverted ancient river channel systems. Characterising the internal sedimentary architecture of such systems is important to constraining the evolution and duration of ancient fluvial flows on early Mars. For example, such ridges are present at the Western delta at Jezero crater, the landing site for the NASA Mars 2020 rover mission. Some studies have used these ridges as time-frozen snapshots of ancient channels and focused on the distribution and geometries of these ridges. However, recent works have characterised these ridges as exhumed channel-belt deposits, composed of multiple channel deposits based on comparisons with similar deposits on Earth. Detailed sedimentary architectural analysis of Terrestrial ridges is needed to provide models for interpreting the earliest fluvial flows on Mars.

Here we present a fine-scale sedimentological analysis of sinuous ridges from the Caspe Formation, Ebro Basin, Spain as potential Terrestrial analogues for sinuous ridge networks on Mars. The Caspe Formation comprises of Oligo-Miocene fluvial deposits of the Guadalope-Matarranya fan system deposited within the endorheic Ebro Basin. The sandstone ridges of the formation are dissected by a number of roads and recent road cuts present a unique opportunity to analyse the fluvial stratigraphy in cross-section. We used traditional field methods and photo-panel interpretations to identify the internal architecture of the ridges and the analysis was complemented with observations from sections parallel to the ridge axes and Unmanned Aerial Vehicles.

Our results provide identification of complete suite of elements preserved within an exhumed channel belt deposits such as channel scour surfaces, channel deposits in the form of lateral and downstream accreting bedforms and barforms, coarser gravel lags and finer-grained deposits from overbank and splay deposits. The ridges preserve a complex continuum of these elements suggesting deposition in an amalgamated channel-fill complex. We also record multi-storey depositional structures with stacked channel elements suggesting an aggrading fluvial system that experienced frequent local avulsion and reoccupation of previous channel positions. The internal architecture of these ridges suggests that Martian sinuous ridges are likely to comprise multiple stacked channel units. If correct this would indicate long lived fluvial activity on early Mars as opposed to short episodes of water flow.

How to cite: Sangwan, K. S., Gupta, S., Barnes, R., and Banham, S. G.: The internal sedimentary architecture of Terrestrial sinuous ridges: clues to understanding sinuous ridges on Mars, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20538, https://doi.org/10.5194/egusphere-egu2020-20538, 2020

D877 |
EGU2020-9753
Jianyu Li and Andrew Tien-Shun Lin

The Loshui Sandstone, a Miocene turbidite succession accumulated in the northern slope of the rifted continental margin of the South China Sea, is exposed in the Hengchun Peninsula, Taiwan. We conduct lithofacies analysis to understand the depositional processes and mechanisms of the gigantic-thick turbidite succession.

Several features can be recognized from outcrops: (1) the Loshui Sandstone is of around 1,000 m thick with turbidite units stacked vertically; (2) high net-to-gross ratio (> 0.9) with dominant fine-to-medium grained sandstones, amalgamated beds are commonly found in the very thickly-bedded turbidites; (3) thick individual turbidite beds with a nominal thickness of 70 cm, which is thicker than classical Bouma sequence; and (4) limited deep scouring surfaces and thick mud are found. Two end-member lithofacies of high-density turbidites and low-density turbidites, respectively, are identified. High-density turbidites are thicker (more than 1 m thick) and coarser in grain size (mostly medium sands) with abundant massive intervals, dewatering structures and/or climbing ripples. Low-density turbidites tend to be thinner in thickness and finer in grain size (mostly fine sands) with parallel bedding and/or normal ripples. In addition to the above two lithofacies, chaotic deposits of mass transport deposits (MTDs) are also widespread within the studied succession.

Sand-rich, vertically aggrading succession, but lack of deep-scouring surfaces and levee deposits, indicates that turbidites are laid down by unconfined turbidity currents in a sand-rich deepwater lobe. In addition, gigantic thick turbidite unit stacked continuously up to 1,000 m, implying that the lobe is confined within a rapidly subsiding basin. We interpreted that the Loshui Sandstone is vertically stacked and accumulated within a fault-bounded trough in the deepwater area of the rifted continental margin of the South China Sea.

 

How to cite: Li, J. and Lin, A. T.-S.: Gigantic-thick turbidite succession in a Miocene fault-bounded trough of the South China Sea rifted continental margin: Examples from Taiwan, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9753, https://doi.org/10.5194/egusphere-egu2020-9753, 2020

D878 |
EGU2020-6423
Dursun Acar, M. Sinan Ozeren, Nazmi Postacioglu, Sebnem Onder, Ulku Ulusoy, K. Kadir Eris, Serdar Akyuz, Namık Cagatay, and Bedri Alpar

During the co-seismic development of a fault in lithological environments, regions containing cavities may form momentarily or permanently. In the tectonic shift zones, these pressure gaps lead to the formation of irregular new intermediate sediment zones, as infiltrate in to the gap, if the pressure perturbations are large. The semi-fluid sediment material and sea water enter through opening fault sector's surrounding sediments at the far place from dispersing fault energy burst. But pore water infiltration is independent about place of vomited energy burst. In some cases hard material which detached from fault wall or top sediment material, provide isolation lids, as obstacling on 'cell type empty interlaying gaps' at tectonic line. They can collapse again or stay as gap form for a long time with suction force after seismic activities by effects of gravitation or pressure perturbations. For durable gaps, pore water is capable to infiltrate in to the gap with long lasting suction forces.  In these regions, in contrast to gravitational folding or collapse structures, the partial sediment sequence may be drawn and folded into the area of the material with different or close lithological density value. Deformational variety of the displaced materials are related with physical properties of seismic event at opening sector such as friction, displacement parameters (velocity, time), dimensional parameters of gap, and water depth.  The main objective of the paper is to figure out all interference mechanisms about these zones (created by pressure perturbations), which develop rapidly during earthquake fractures (or in some cases fractures generated by impulsive pressure changes such as those created by volcanoes). Fracture of fault segments forms a complex mechanical system associated with bedrock, upper sedimentary sequence, and aquatic environment, depending on the location where they occur, even the atmosphere. Therefore, the displacement may be bi-directional to the lower slit or upward from the seabed during the opening or closing stages of the cavity, depending on the nature with variations of the atmosphere & water-sediment mixture. The strong (pulling or impulsive) pressure perturbation effect associated with permanent cavities caused by rapid breakage pulls the material that may form a sludge volcano or water outlet under deformation and brings the environment to near pressure equilibrium. This simple explanation can help to find real additional effective reason for the different formations of assumed collapse or folding structures created by gravitational movements in geology. The hypothesis after main objective at above mentioned in this article is based on the fact that the emergence of  escapes as squeezed fluid form  of water & sediment from compacted secondary irregularities in the previously broken fault segment will help to understand the next seismic mobility in other tectonic segments by identifying source depth cues through physical and chemical analysis. Geophysical instrumentation and applications are still need further developments of compact reflection line information, because the vertical thin anomalies mentioned in this paper are the most difficult structures for detection.

How to cite: Acar, D., Ozeren, M. S., Postacioglu, N., Onder, S., Ulusoy, U., Eris, K. K., Akyuz, S., Cagatay, N., and Alpar, B.: A new hypothesis that contributes to the formation of cold sludge volcanoes and fluid outlets in tectonic seabed & terrestrial regions; with its helpful interpretation for time fracture sequence of fault segments., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6423, https://doi.org/10.5194/egusphere-egu2020-6423, 2020

D879 |
EGU2020-4577
Gwénaël Caravaca, Nicolas Mangold, Stéphane Le Mouélic, Laetitia Le Deit, and Marion Massé

Since 2012, the Mars Science Laboratory Curiosity rover has studied the sedimentary deposits within the Gale Crater, leading to the description of varying lacustrine to fluviatile and fluvio-deltaic environments. Here, we focus on the sedimentary record of the Kimberley outcrop traversed by Curiosity between sols 603 and 630. This section presents siliciclastic rocks with an unusually high potassic content (Le Deit et al., 2016, JGR-Planets). However, poorly constrained architecture and stratigraphic relations between the series of the Kimberley Formation and their local to regional surroundings still prevent further understanding of the exact extent of these accumulations and their significance within the broader Gale Crater paleoenvironmental scheme.
Such questions highlight the need for a new finer mapping of the area to characterize the contacts observed on the outcrop itself and in its immediate vicinity, but also for a new assessment of the precise nature and morphology of the sedimentary structures and their spatio-temporal distribution throughout the outcrop and beyond.
We therefore propose to use a true color highly resolved Digital Outcrop Model (DOM) of the Kimberley outcrop, obtained using Mars Science Laboratory imagery, integrated into a Virtual Reality (VR) environment (Caravaca et al., in press, PSS). Taking advantage of this “in situ” geological analysis of the DOM, we were able to observe and characterize such sedimentary structures and contacts, as well as their spatial extension throughout the reconstructed area of Kimberley with an unprecedented precision. We notably observe and describe both conformable and unconformable contacts over the entire outcrop, but also several sets of varying scale cross-stratifications (from cm- to pluri-meter scale). These results are in accordance with a fluviatile hydrodynamically active system. They tend to corroborate the idea of a complex yet diachroneous evolution of the area, with the possibility of laterally evolving depositional settings, spanning a significant amount of time.

How to cite: Caravaca, G., Mangold, N., Le Mouélic, S., Le Deit, L., and Massé, M.: “In situ” characterization of the sedimentary record and structures using Virtual Reality: new insights from the Kimberley outcrop (Gale Crater, Mars), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4577, https://doi.org/10.5194/egusphere-egu2020-4577, 2020

D880 |
EGU2020-13679
Johan Damveld, Bas Borsje, Pieter Roos, and Suzanne Hulscher

Tidal sand waves are rhythmic bed forms found on coastal shelves all around the world. An important property of sand waves is their mobility, as they display migration rates of several meters per year. Insight in these dynamics is of practical relevance, as this behaviour may interfere with offshore engineering activities. State-of-the-art morphodynamic models are used to predict sand wave dynamics, but they still overestimate dimensions such as their height (Van Gerwen et al, 2018). Moreover, these models often assume a uniform grain size distribution, whereas field observations indicate a clear sorting of sediments along sand waves. Previous modelling studies found that a combination of sediment mobility effects and tidal current strength may explain these sorting patterns (e.g. van Oyen and Blondeaux, 2009). However, as these models were limited to the early stage of sand wave formation, they did not account for the nonlinear effects of increasing sand wave amplitudes. Our goal is to include these nonlinear effects in order to further unravel sorting processes, in particular the internal sand wave structure.

Hereto we extend the work by van Gerwen et al (2018), allowing for an arbitrary number of sediment fractions, and we adopt the active layer approach of Hirano (1971) to account for bed stratigraphy. To investigate the role of asymmetry in hydrodynamic forcing, we include a residual current superimposed on the dominant tidal component.

Results show that in general the crests of sand waves are coarser than the troughs. In the case of an asymmetrical forcing, larger sediments are found on the upper lee slope, whereas the smaller grains are deposited on the lower lee slope. Due to migration, also the internal structure of the sand wave is revealed over time, showing the same pattern as found on the lee slope surface. Many field studies have shown that these model results qualitatively agree with observations on surficial sorting patterns (e.g. Cheng et al, 2018). However, as field data on the internal sediment structure is scarce, it is difficult to validate this model output.

Hence, the question remains whether the results on the internal sorting are a true representation of the substrate of sand waves. Nonetheless, the model results give insight in the processes governing grain size sorting over and in sand waves, which could be a valuable element in developing future coastal management strategies, such as sand extraction.

Cheng, C.H., Soetaert, K., & Borsje, B.W. (2018). Small-scale variations in sediment characteristics over the different morphological units of tidal sand waves offshore of Texel. NCK Days 2018.
Hirano, M. (1971). River bed degradation with armouring. Trans. Jpn. Soc. Civ. Eng, 3, 194-195.
Van Gerwen, W., Borsje, B.W., Damveld, J.H., & Hulscher, S.J.M.H. (2018). Modelling the effect of suspended load transport and tidal asymmetry on the equilibrium tidal sand wave height. Coastal Engineering, 136, 56-64.
Van Oyen, T., & Blondeaux, P. (2009). Tidal sand wave formation: Influence of graded suspended sediment transport. Journal of Geophysical Research: Oceans, 114(C7).

How to cite: Damveld, J., Borsje, B., Roos, P., and Hulscher, S.: Sediment sorting in tidal sand waves fields: the internal structure revealed?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13679, https://doi.org/10.5194/egusphere-egu2020-13679, 2020

D881 |
EGU2020-13499
Alice Lefebvre, Gerald Herrling, Anna Zorndt, Knut Krämer, Marius Becker, and Christian Winter

The distribution, morphology and dynamics of tidal bedforms in the Weser estuary, Germany, between the tidal limit (river-km 0 at the tidal weir in Bremen) and the open North Sea (river-km 111 in the Outer Weser) has been analysed for a five-year period based on monthly bathymetric surveys carried out along the main waterway. For the years 2009 to 2014, bedforms were detected from gridded bathymetry data (2x2 m) and their geometric properties described. In particular, the presence and position of a slip face, here defined as the portion of the lee side steeper than 15°, was traced. This was shown to be a practical criterion for the presence of permanent flow separation and turbulent wake in the lee of bedforms. Here it is used as a simplified indicator of bedform roughness: if a bedform does feature a slip face, it is assumed to be an active roughness element. The results were related to river discharge, water levels, and flow velocities.

Bedforms were present along most of the river channel, apart from a large section between river-km 55 and 75. There, muddy cohesive sediment in the estuarine turbidity maximum zone hindered the formation of bedforms. Along the channel and throughout the years, bedform lengths varied between 20 and 60 m and heights between 0.3 and 1.6 m.

During times of high fluvial discharge, in winter and spring, ebb velocities were stronger than flood velocities. The bedforms then were small, long and ebb-oriented (i.e. the ebb lee side was shorter than the flood lee side) and many bedforms featured an ebb slip face but no flood slip face. This suggests that throughout the survey area, bedforms were active roughness elements during the ebb phase only.

In summer and autumn, when the discharge was low, bedforms in the upper reach (ca. river-km 15 to 30) gradually became flood-oriented and many bedforms there developed a flood slip face, implying that these bedforms were active roughness elements during the flood. Between km 30 and 55, bedforms were predominantly ebb-oriented, and many bedforms had an ebb slip face but only few had a flood slip face, so most bedforms were only active during the ebb phase.

The annual variations of bedform dimensions and shapes reveal an intricate feedback between river and tidal flows, channel morphology, sediment dynamics and bedforms. The results have implications for bedform research, river management and numerical modelling.

How to cite: Lefebvre, A., Herrling, G., Zorndt, A., Krämer, K., Becker, M., and Winter, C.: Tidal bedforms dynamics, Weser River, Germany, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13499, https://doi.org/10.5194/egusphere-egu2020-13499, 2020

D882 |
EGU2020-3321
Xiao-Cheng Zhu and Wen-Shan Chen

In northwestern Taiwan, Cholan Formation in Dahan river is about 1400 m thick that contains high-frequency sequence stratigraphy (6th-order) and detail of facies architecture which indicates evolution of the foreland basin. In late Miocene (6 Ma), the Taiwan orogeny belt is formed by the arc-continental collision (the Luzon Volcanic Arc and the Eurasian plate). During Pliocene-Pleistocene, uplift of the Hsueshan Range and the Western Foothill created by a series of the fold-thrust belt formed the foreland basin. Most importantly, high subsidence rate and high sedimentation rate are critical that glacio-eustasy (6th-order) could be correlated to parasequences in Cholan Formation. It provides a precise age model to discuss different stages of foreland basin.

Parasequences in Cholan Formation could be divided into three types of depositional systems including siliciclastic shallow marine (Type 1), margin marine (Type 2) and nonmarine (Type 3) that are a typical sequence of foreland basins. Type 1, which is tidal-dominated open coast, shows 10-30 m coarsening-upward succession. Type 2, which is tidal-dominated delta, shows two different parts. The lower part is 10-50 m coarsening-upward succession which unconformity contact with Type 1. The upper part changes to 20-50 m fining-upward succession. Type 3, which is alluvial system, shows 30-70 m fining-upward succession that is conformable with Type 2. From shallow marine to nonmarine, the thickness of parasequence is growing thicker that indicates long-term tectonic subsidence rate is getting higher with more sediment deposits in the basin. In more detail, in marine setting, sea level change is the main considered factor to identify sequence boundary (SB) and maximum flooding surface (MFS), while in nonmarine setting, precipitation change in glacial and inter-glacial may be a critical factor to impact the formation of SB. However, MFS is complicated to define because some parasequences show tidal signal, but some don’t. It could be influenced by degree of sea level uplift or paleotopography. In Cholan Formation, the signal of sea level, tectonic and climate is sensitive to reflect in stratigraphy architecture.

Keywords: Foreland basin, High-order sequence stratigraphy, Marine to nonmarine facies architecture

How to cite: Zhu, X.-C. and Chen, W.-S.: High-frequency Sequence stratigraphy and facies architecture in Cholan Formation (Pleistocene), northwestern Taiwan: the evolution of a foreland basin, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3321, https://doi.org/10.5194/egusphere-egu2020-3321, 2020

D883 |
EGU2020-20680
Suleyman Naqshband and Ton Hoitink

Dunes dominate the bed of sandy rivers and they are of central importance in predicting sediment transport and flow resistance. Using a novel acoustic instrument over migrating dunes in a laboratory setting, we quantify a number of dynamical properties that are crucial in our understanding and modeling of dune response to changes in flow and sediment transport. Measured sediment transport distribution during the initial stage of dune growth reveals a negative spatial lag between dune crest and maximum sediment transport rate. In absence of this spatial lag, the dune field is observed to grow by merging of smaller, faster migrating dunes.

How to cite: Naqshband, S. and Hoitink, T.: The growth process of river dunes, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20680, https://doi.org/10.5194/egusphere-egu2020-20680, 2020

D884 |
EGU2020-19409
Timo C. Gaida, Thaiënne A.G.P. Van Dijk, Mirjam Snellen, and Dick G. Simons

Grain-size sorting in bedforms is well known in river dunes. On continental shelves, however, datasets aimed at grain-size sorting over bedforms, are limited. More extensive observations of sediment sorting over bedforms may help to understand their morphodynamic processes, and are key in habitat mapping, since grain-size is a main control on the composition of benthic fauna. A time series of seven multibeam (MBES) bathymetry and backscatter measurements and box cores were collected for the monitoring of a coastal nourishment in a tidal inlet at Ameland, Netherlands. Prior to the nourishment (April 2017), 10-15 m long and 1.5 m high megaripples occurred. The time series shows the rapid development of high and steep megaripples in the newly replenished sediment, with a wavelength of 40 m and height of 2.5 m within three months (during-nourishment; October 2017), which then grew into 120 m long and 3 m high sand waves in relatively shallow water (10 - 14 m) within 5 months (post-nourishment; March 2018). 
Relative backscatter (BS) strengths, which are corrected for, among others, transmission losses and bed morphology, represent seabed sediment characteristics. Bed classification of BS strengths, using an unsupervised Bayesian method, resulted in a high-resolution map of 5 acoustic classes (ACs), to which sediment types were assigned using the box cores as ground truthing. These box cores, however, were not taken at the detailed level of sand wave crests and troughs. 
The acoustic sediment classes (ASCs) exhibit a repetitive pattern, indicating horizontal sediment sorting over bedforms, that shifted and intensified during the growth of the megaripples into sand waves. The ASC megaripple pattern is less consistent, but generally comprises finer sediments (ASC2-3: sand) on the stoss sides and coarser sediments on the lee sides (ASC3-4: sand to slightly gravelly sand). The sand wave pattern is very consistent and comprises coarse sediments on the stoss sides (ASC5: gravel- and shell-containing sands), finer sediments towards the crests (ASC2-3: sand) and even finer sediments (ASC1: sandy mud) in the troughs. In the course of one year, both the morphological and sorting patterns seem to repeat itself. A similar sorting evolution was observed during the growth of megaripples just farther offshore. 
In a different data set, farther offshore on the Netherlands Continental Shelf and built up over several years, grab samples were collected in transects, specifically at crests and troughs of sand waves and long bed waves, and were analysed for grain size, organic matter and CaCO3 contents. Median grain sizes in the troughs of bedforms are consistently finer than at the crests, and reveal significant signatures between sand wave fields, with crest-trough differences among sites ranging between 10 and 85 micrometer. Unfortunately, MBES-BS data are not available for establishing large-scaled, spatial sorting patterns.
This evolution of horizontal sediment-sorting patterns during the growth of marine bedforms may support modelling studies of hydrodynamic responses of flow over undulating beds and may explain the morphodynamic evolution of marine bedforms, as relevant in marine ecology. However, coherent empirical datasets are required. 

How to cite: Gaida, T. C., Van Dijk, T. A. G. P., Snellen, M., and Simons, D. G.: Observations of sediment sorting over rapidly developed marine bedforms, using multibeam backscatter, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19409, https://doi.org/10.5194/egusphere-egu2020-19409, 2020

D885 |
EGU2020-6486
Huixian Chen, Jianhua Wang, Nicole S. Khan, Jiaxue Wu, and Benjamin P. Horton

Proxy reconstructions of estuarine evolution provide perspectives on regional to global environmental changes, including relative sea-level changes, climatic changes, and agricultural developments. Although there are studies of the Holocene sedimentary processes in the Pearl River estuary, the understanding of early Holocene sedimentation in unknown due to limited preservation.

Here, we present a new record of lithological, benthic foraminiferal, and geochemical (δ13C and C/N) change from a sediment core in the west shoal of the modern Lingding Bay along a paleo-valley. The lithologic and foraminiferal record reveal the transgressive evolution from fluvial, inner estuary to middle estuary in the early Holocene between 11300 and 8100 cal a BP in response to rapid sea-level rise. δ13C and C/N data indicate high freshwater discharge from 10500 to 8100 cal a BP driven by a strong Asian monsoon. The middle Holocene (8100 - 3300 cal a BP) sediment is absent in this core and others in the northward of the Lingding Bay. Seismic profiles reveal a tidal ravinement surface across Lingding Bay, which contributed to subaqueous erosion on the mid-Holocene sedimentation hiatus, might be resulted from unique geomorphology of the Pearl River Delta. In the late Holocene (3300 cal a BP to the present), the lithology and foraminiferal assemblages suggest further regressive evolution from outer estuary, middle estuary channel, to middle estuary shoal due to deltaic progradation under stable relative sea levels. In the last 2000 years, δ13C and C/N values reveal the intensive development of agriculture coupled with the reduction of freshwater input derived from a weakening Asian monsoon. Our study illustrates the interaction of Asian monsoon and sea-level changes within the Pearl River estuary landform and their impact on Holocene sedimentary processes.

How to cite: Chen, H., Wang, J., Khan, N. S., Wu, J., and Horton, B. P.: Early and late Holocene paleoenvironmental reconstruction of the Pearl River estuary, South China Sea, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6486, https://doi.org/10.5194/egusphere-egu2020-6486, 2020