SSP3.8

Estuaries, deltas and tidal basins are highly dynamic systems where sand and mud are transported under the complex interactions of bathymetry, currents and waves. A fundamental understanding of the formation of these coastal environments and how they will respond to changes in the future requires a better understanding of natural dynamics at the scale of individual channels and bars. The current research aims to investigate sediment transport of mud and sand mixtures at bar margins under combined waves and currents, with a particular interest in the effect of varying bedforms. To this end, experiments were conducted in an 11m long recirculating flume with an initially transversely sloped bed, representing a side-slope of a coastal sand bar. Wave intensity and mud content were systematically varied between runs. Results showed two significantly different mechanisms of sediment transport depending on the erodibility of the sediment with a clear threshold of mud content and wave intensity. During experiments with only sand, the transverse slope developed towards a flat bed over the cross-section as a result of waves stirring up the sediment and gravity pulling the sediment downslope. Symmetrical ripples formed over the width of the slope and sediment was actively transported downslope along the ripple crests. Additionally, sand waves with a longer wavelength formed in the longitudinal direction. Adding a relatively low volume of cohesive sediment did not have a significant effect on the speed at which the transverse slope decreased towards a flat bed, but there was a slower adaptation of the morphology in longitudinal direction. Ripples were three-dimensional and with highly varying dimensions based on local mud content and location on the transverse slope. With increasing mud content however, the cohesivity of the sediment mixture increased the threshold of sediment motion and only the higher part of the transverse slope experienced shear stresses that were high enough to transport sediment. Here, the mud was winnowed out of the mixture into suspension and only the sand fraction was transported downslope. Future experiments will focus on linking the direction of sediment transport under combined waves and currents to landscape development to study the larger-scale implications of the observed differences in transport mechanisms and bedform dimensions.

How to cite: Baar, A.: Slope-driven sediment transport of sand-mud mixtures in coastal environments, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5303, https://doi.org/10.5194/egusphere-egu22-5303, 2022.

In the marine environment, turbidite supercritical bedforms have been widely reported from channel-axis and overbank wedges. On the contrary, their dominance in the make-up of fans and apron, apart from local areas such as channel mouths, is at present not recognized. However, since it has been postulated that turbidity currents reach the supercritical conditions for slope > 0.5°, submarine slopes should contain abundant supercritical flow deposits. Here, we provide a review of different types of slope fans and aprons dominated by supercritical bedforms, based on examples from the modern seafloor. We compare depositional elements located in different intraslope basins of the Tyrrhenian Sea, through high-resolution bathymetry, chirp subbottom section and, where available cores. The variable geological context results in axial and transvers slope fans with highly variable sizes (few to tens of kilometres) and geometries, dependent upon the erosive and/or depositional processes involved, as well as the seafloor topography of the area. In particular, we have recognized two types of lobe-shaped deposits characterized by supercritical bedforms: channel-attached fans and detached aprons. The first ones are connected to a canyon-channel system and develop on slope gradients of 0.5° to 1.2°, display small-scale bedforms (wavelength of about 150 m and height < 10 m), with upslope asymmetric or symmetric cross-sections, interpreted as cyclic steps and antidunes. According to the amplitude of the reflections, cores, and to the bedform aspect ratio, the channel-attached fans are interpreted to be composed of coarse-grained sediments. Our examples highlight that cyclic steps and antidunes dominate the channel-attached fans both in axial and lateral portion while scours mark topographic changes such as breaks in slope or laterally confined areas. Detached aprons develop from the un-incised shelf edge on steep slopes of about 1.2° to 3° and are composed by large-scale bedforms (wavelength of about 500 m and height of about 5 m) mainly upslope asymmetric, associated with cyclic steps. The low amplitude of the seismic reflections suggests the fine-grained nature of the aprons. This study shows that there are significant differences in the distribution and character of supercritical bedforms in slope settings according to the type of feeding system, the degree of flow confinement and the seafloor topography. The analysis of the downslope evolution of turbidity currents, and of the character of associated bedforms in deep-water systems can contribute new perspectives to refine our models of deep-sea depositions.

How to cite: Scacchia, E., Tinterri, R., and Gamberi, F.: Slope fans and aprons dominated by supercritical bedforms:  topographic and feeding system controls (Southeastern Tyrrhenian Sea), EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9218, https://doi.org/10.5194/egusphere-egu22-9218, 2022.

River dune modelling ranges from linear stability analysis to analyse the initial growth of the dunes (Fredsøe, 1983) up to three dimensional numerical models which can simulate the dune evolution by modelling the sediment transport on particle level (Nabi et al., 2013). For engineering purposes, such as efficient planning of dredging operation or dynamic modelling of dune roughness for water level predictions, a quick and accurate dune development model is needed. Therefore we further develop the model of Paarlberg et al. (2009), in order to accurately model dune shape and migration during high, median and low flow situations.

This model simulates dune development using a flow module in a two dimensional vertical plane and a bed load transport module which calculates the bulk transport. The model solves the flow over the domain of one dune length, using cyclic boundary conditions. The domain length, covering one dune length, is determined using a numerical linear stability analysis. It has been proven to accurately and fairly quickly reproduce the dune height of flume experiments and it is also able to simulate the transition to upper stage plane bed accurately (Duin et al., 2021).

However, for low flow situations it has not been validated yet. One of the main issues during low flow is that the relation between water depth and dune length is not linear and the adaptation of the dune length to new, smaller, water depths and flow velocities is not instantaneous (Lokin et al., 2022). The linear stability routine determines the dune length to which the dunes will grow based on a plane bed with a small disturbance, and directly updates the domain length to this newly determined dune length. In this research we have investigated options to incorporate the lag in the dune length adjustment during the falling stage of a flood wave. Implementing a lag in the dune length adjustment, such that the dune length adapts at a rate that is linked to the depth averaged flow velocity, leads to more realistic dune lengths.

Duin, O. J. M. van, Hulscher, S. J. M. H., & Ribberink, J. S. (2021). Modelling Regime Changes of Dunes to Upper-Stage Plane Bed in Flumes and in Rivers. Applied Sciences 2021, Vol. 11, Page 11212, 11(23), 11212. https://doi.org/10.3390/APP112311212

Fredsøe, J. (1983). Shape and dimensions of ripples and dunes. Mechanics of Sediment Transport. Proc. Euromech 156, Istanbul, July 1982.

Lokin, L. R., Warmink, J. J., Bomers, A., & Hulscher, S. J. M. H. (2022). River dune dynamics during low flows. https://doi.org/submitted for publication

Nabi, M., De Vriend, H. J., Mosselman, E., Sloff, C. J., & Shimizu, Y. (2013). Detailed simulation of morphodynamics: 3. Ripples and dunes. Water Resources Research, 49(9), 5930–5943. https://doi.org/10.1002/wrcr.20457

Paarlberg, A. J., Dohmen-Janssen, C. M., Hulscher, S. J. M. H., & Termes, P. (2009). Modeling river dune evolution using a parameterization of flow separation. Journal of Geophysical Research: Earth Surface, 114(1). https://doi.org/10.1029/2007JF000910

How to cite: Lokin, L., Warmink, J., Bomers, A., and Hulscher, S.: Modelling river dune length adaptation during variable flow conditions, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9040, https://doi.org/10.5194/egusphere-egu22-9040, 2022.

In fluvial systems worldwide, multiple scales of bedforms coexist. Where most research has focused on the larger, primary dunes, recent studies have indicated the importance of the small, secondary bedforms that are superimposed on the primary ones (Galeazzi et al., 2018, Zomer et al., 2021). The secondary bedforms migrate fast and the bedload sediment transport associated with secondary bedform migration equals that associated with the much larger primary dunes. Depending on the primary lee side slope, secondary bedforms disintegrate or persist at the primary dune lee. Secondary bedforms might have large implications for hydraulic roughness, for local flow dynamics and may interact with the development of primary dunes. Current work focusses on understanding the competition and interaction between primary and secondary bedforms in a lowland river, based on a large, multiyear dataset of bed elevation scans as well as a dedicated field campaign that maps the dynamics of both primary and secondary dunes.

A first objective of the study is to understand the competition between primary and secondary bedforms. Previous work has indicated inverse correlations between secondary bedform height  and primary dune lee slope or height. The bed elevation scans indicate a spatial variability in secondary and primary bedform properties and locations where either secondary or primary dunes are dominant. This work aims to map and explain the mechanisms that affect the development and (semi-)equilibrium dune size and shape of both scales as well as the dependence on the discharge and bed grain size distribution.

A second objective is to shed light on the interaction between migrating secondary and primary dunes. Where secondary bedforms disintegrate at the primary lee, the secondary bedform migration contributes to primary dune migration. Secondary bedforms are also observed to persist over the primary dune lee however. Both scales are then actively migrating. Preliminary results suggest that sediment transport associated with secondary dune migration varies depending on the position of the small dunes on the primary dune. Sediment transported by secondary dunes  seems to increase over the primary stoss and decrease on the primary lee. The variability in sediment transport indicates net erosion of the primary dune stoss and net deposition on the primary dune lee, resulting in a downstream migration of the primary dune.

References:

Galeazzi, C. P., Almeida, R. P., Mazoca, C. E., Best, J. L., Freitas, B. T., Ianniruberto, M., ... & Tamura, L. N. (2018). The significance of superimposed dunes in the Amazon River: Implications for how large rivers are identified in the rock record. Sedimentology, 65(7), 2388-2403.

Zomer, J. Y., Naqshband, S., Vermeulen, B., & Hoitink, A. J. F. (2021). Rapidly migrating secondary bedforms can persist on the lee of slowly migrating primary river dunes. Journal of Geophysical Research: Earth Surface, 126(3), e2020JF005918.

How to cite: Zomer, J., Vermeulen, B., and Hoitink, T.: Competition and interaction between two bedform scales in a lowland river, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9579, https://doi.org/10.5194/egusphere-egu22-9579, 2022.

Dunes are ubiquitous features in most sand- and gravel-bed rivers worldwide and are key elements of sediment transport. Their variable height may also interfere with shipping routes and help dictate shipping loads. Knowledge of dune dynamics and spatio-temporal sediment transport is thus essential in understanding river dynamics and for the navigability, sustainable management and maintenance of rivers, especially in times of more extreme floods. To date, most morphodynamic studies of river-beds have been based on either bathymetric time series or sub-bottom profiling data, but not collected at the same time and the sub-bottom data not in time series. As such, these data do not allow for the identification of spatio-temporal variations of sediment storage in, and reactivation of, the shallow sub-surface as related to dune kinematics. Our field study, reported here, sought to address this gap in knowledge by investigating the stratification produced by dunes in the shallow subsurface through sub-bottom profiler time series in combination with bathymetric time series and vibracores.

In three areas of varying grain size in the River Waal, Netherlands, we collected four 7-km long tracks of high-resolution sub-bottom profiler data (Parametric Echo Sounder, PES) and, simultaneously, multibeam echo sounder (MBES) data. In two repeat surveys in areas 2 and 3, and four repeat surveys in area 1, data were acquired to gain insight into the preservation and reactivation of dune deposits over short-term periods of 1 day to 3 weeks. Interpretation of the sub-bottom data is aided using 18 vibracores of 4 – 5 m depth.

Initial analyses show the migration and morphological change of the large dunes, thereby obliterating dunes mapped during the first survey, and the presence of superimposed small dunes. The PES data of large dunes exhibit foresets, reactivation surfaces where superimposed dunes migrated down the lee slopes, and strong near-horizontal reflectors at the base of large dunes, interpreted as the lower bounding surface. The surveys also identified dune stratification preserved below the active dune scour depth, and several horizontal reflectors at depth.

Coupling these sedimentary structures in the bed profile data to both the simultaneous MBES data and a unique longer-term MBES time series, comprising two-weekly surveys (2005-2021) and half-yearly surveys (from 1999), provides an unparalleled opportunity to date these sedimentary structures, (1) to investigate longer-term aggradation and dune preservation and (2) to link these to flood and depositional events over the past decades. Here, we present initial results. This field dataset and approach yield a unique, high-resolution, spatio-temporal reconstruction of sediment preservation that significantly contributes to the insight into sediment storage times and preservation of dune-scale sedimentary structures in river beds. These field data also help to improve data-driven modelling.

How to cite: Van Dijk, T. A. G. P., Best, J., Karaoulis, M., van Rijnsoever, P., van Onselen, E., Lowag, J., and Kleinhans, M. G.: Linking dune dynamics and preservation: a unique approach using multibeam and parametric echo sounding time series, River Waal, Netherlands, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8182, https://doi.org/10.5194/egusphere-egu22-8182, 2022.

During this pivotal time of energy transition, it is of crucial importance to unlock the potential of the seabed for offshore energy conversion and electrical power transport. With the construction of ever larger offshore windfarms plus other coastal infrastructure, a better understanding of the interactions between the infrastructure and the flow, the flow and the seabed, and all the above with marine life has never been more pressing, as they define feasibility and sustainability of the offshore projects. 

To better understand the dynamics of the flow in the wake of a large object, the School of Ocean Sciences at Bangor University deployed a bed frame with an Acoustic Doppler Current profiler in the wake of a 10 m-high and steep-crested sediment wave on a seabed 60 meters deep. Vessel-mounted ADCP data was collected simultaneously in orthogonal transects. Velocity profiles near the seabed diverge from the standard law of the wall. On the flood tides, when the flow interacted with the large bedform, increased turbulence in the water column vertically mixed the suspended sediments (measured via the ADCP) into a vertically uniform suspension. On the ebb tides, without any interactions with the bedform, the backscatter shows a boundary layer bursting structure.  

The enhanced turbulence can affect the sediment composition and bed mobility in these large wakes whether they are natural or anthropogenic, and to numerically model these effects is complex. We discuss the wider impacts of this work, as changes to sediment, seabed and water column properties can affect aggregations of prey that crucially depend on it. These changes can then extend through the food chain and contribute to the ecological impacts of windfarms, both as risks and as opportunities.

How to cite: Van Landeghem, K., Unsworth, C., Austin, M., and Waggitt, J.: Flow changes in the wake of a large sediment wave: helping to understand geological and ecological impacts of seabed infrastructure., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6349, https://doi.org/10.5194/egusphere-egu22-6349, 2022.

Large parts of the sandy seabed of shallow seas are covered with rhythmic bed patterns, such as tidal sand waves. Due to their dynamic nature and size, sand waves may pose a threat to offshore development, such as wind farm construction. Decadal predictions of seabed dynamics are thus required, which are currently determined using data-driven methods. Process-based models could be used to increase the accuracy of bed level predictions in these environments. Moreover, these tools could provide a solution for data scarce areas and show the effects of extreme events and human interventions.

The complex, process-based numerical model Delft3D-4 has been used to model sand wave dynamics in idealized settings (e.g. Borsje et al., 2013) and more recently for realistic cases (Krabbendam et al., 2021).  In the current model set-up, the hydrodynamic boundary conditions are imposed at 20 km from the sand wave area. A flat buffer area is created to enable the flow to adapt and keep boundary effects away from the area of interest. As an undesired consequence of this the hydrodynamic forcing at the boundaries is now different from what is simulated in the sand wave area, making it difficult to define realistic forcing. The newly developed Delft3D Flexible Mesh (FM) model, the successor of Delft3D-4, shows the ability to drastically reduce this buffer area. Through a new, more comprehensive, type of boundary condition more accurate hydrodynamics can be imposed, by defining water level and flow velocity profile over depth simultaneously at inflow boundaries.

In this study the Delft3D FM model is applied to multiple transects in the North Sea, where the accuracy of the hydrodynamics is validated using a large-scale model and measurement data. By splitting the hydrodynamic signal into tidal components and non-tidal currents, the contribution of the various local hydrodynamic components to sand wave dynamics is determined.

This study shows the importance of accurate representation of local hydrodynamics for understanding sand wave dynamics. It is for example found that minor changes in residual currents will significantly alter the bed level changes over the considered time periods. Using the Delft3D FM model more realistic boundary conditions can easily be defined. Combined with a reduction of computation times of over 50%, compared to Delft3D-4, the first steps towards engineering applications of numerical models for predictions of sand wave dynamics are made.

Borsje, B. W., Roos, P. C., Kranenburg, W. M., & Hulscher, S. J. M. (2013). Modeling tidal sand wave formation in a numerical shallow water model: The role of turbulence formulation. Continental Shelf Research, 60,17-27.

Krabbendam, J.M., Nnafie, A., de Swart, H.E., Borsje, B.W., & Perk, L. (2021). Modelling the Past and Future Evolution of Tidal Sand Waves. Journal of Marine Science and Engineering, 9(10), 1071.

How to cite: Overes, P., Borsje, B., Luijendijk, A., and Hulscher, S.: Understanding the Effects of Local Hydrodynamic Components on Sand Wave Dynamics: A North Sea Case Study, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-883, https://doi.org/10.5194/egusphere-egu22-883, 2022.

The global transition towards cleaner energy sources has triggered a tremendous shift of wind energy exploitation to the coastal seas. This threatens the environmental health of the ecosystem in these environments, with (potentially negative) impacts on the ecosystem services they provide. Large parts of the sandy bed of these shallow coastal seas, such as the North Sea, are covered by tidal sand waves. Their large dimensions and dynamic behaviour make them a threat for offshore engineering activities, as, for instance, cables to offshore wind farms can be exposed due to sand wave migration. At the same time, sand waves have been shown to serve as a habitat for large numbers of benthic organisms (Damveld et al, 2018), and should therefore be protected from anthropogenic disturbances. These conflicting interests require an integrated approach in marine spatial planning. To support decision making, process-based models can be applied to gain insight in the processes and mechanisms which control both the morphodynamics of sand waves and the habitat characteristics of the organisms living within, and the interaction between those.

Field evidence shows that the region around the steep slope and the sand wave trough are favourable for benthic organisms. The highest concentrations of organic matter, which serve as an important food source, are also found there. It is hypothesized that organic matter deposits accumulate near the trough and steep slope of sand waves due to the more sheltered hydrodynamic conditions there. The possible presence of a flow separation zone during periods of the tidal cycle may significantly contribute to the sedimentation of organic matter in this region. Unfortunately, current state-of-the-art sand wave models (e.g., van Gerwen et al., 2018) are mainly focused on explaining large-scale hydro- and morphodynamic behaviour. They are not set-up to resolve complex hydrodynamics (e.g., turbulence) which are needed to study small-scale processes near the steep slope of sand waves.

In this work we aim to develop a non-hydrostatic sand wave model in Delft3D, combining earlier work by Lefebvre et al. (2014) and van Gerwen et al. (2018). Using this model, we will systematically investigate the factors that contribute to the possible emergence of a flow separation zone. We are specifically interested in its spatial and temporal extent during a tidal cycle. We expect sand wave shape (e.g., lee slope angle, sharpness of the crest) and tidal current strength to be key parameters for the possible presence of flow separation.

Damveld, J.H., van der Reijden, K.J., Cheng, C., Koop, L., Haaksma, L.R., Walsh, C.A.J., et al. (2018). Video transects reveal that tidal sand waves affect the spatial distribution of benthic organisms and sand ripples. Geophysical Research Letters 45.

Lefebvre, A., Paarlberg, A.J., Ernstsen, V.B., & Winter, C. (2014). Flow separation and roughness lengths over large bedforms in a tidal environment: A numerical investigation. Continental Shelf Research 91.

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.

How to cite: Damveld, J. H., Lefebvre, A., Borsje, B. W., and Hulscher, S. J. M. H.: Complex hydrodynamics over tidal sand waves: the role of flow separation, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13333, https://doi.org/10.5194/egusphere-egu22-13333, 2022.

We study the morphodynamics of reversing dunes on the gravel deposits of the alluvial fan of the Molcha river at the border between the Tibetan Plateau and the Taklamakan Desert (Gao et al., 2021). Independent sets of wind data show that this area of low sand availability is exposed to two prevailing winds from opposite directions and of different strengths. The predicted resultant transport direction of sand particles is westward. Nevertheless, satellite observations combined with field measurements and ground-penetrating radar surveys reveal that isolated dunes a few meters high migrate eastward. This apparent dune migration paradox is resolved using numerical and analytical models that take into account the speed-up effect and the continuous change in dune shape after each wind reversal. When a newly established wind hits what was before the steeper lee slope of the dune, the sand flux at the crest abruptly increases before relaxing back to a constant value as the crest migrates downwind and as the dune reaches a new steady shape. Integrated over the entire wind cycle, we find that this non-linear behavior causes reversing dunes to migrate against the resultant transport direction. This migration reflects the difference in dune slope seen by irregular storm events blowing to the east and the westward wind of the daily cycle. Thus, we explore the impact of extreme events on dune morphodynamics and examine new aspects of the permanent feedback between dune topography and wind speed. We conclude that transient behaviors associated with crest reversals contribute to the observed diversity of dune patterns, even within the same area for dunes of different sizes.

Gao, X., Narteau, C., & Gadal, C. (2021). Migration of reversing dunes against the sand flow path as a singular expression of the speed-up effect. Journal of Geophysical Research: Earth Surface, 126, e2020JF005913. https://doi. org/10.1029/2020JF005913.

How to cite: Gao, X., Narteau, C., and Gadal, C.: Migration of Reversing Dunes Against the Sand Flow Path as a Singular Expression of the Speed-Up Effect, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2487, https://doi.org/10.5194/egusphere-egu22-2487, 2022.

Model predictions of waves, currents, and sediment transport, as well as the acoustic response of the seafloor depend on reliable estimates of seafloor roughness due to both sediment properties and bedform geometry. To predict the spatial and temporal dynamics of seafloor roughness under changing wave conditions, we have developed a modular modeling framework, the Naval Seafloor Evolution Architecture (NSEA). NSEA requires hydrodynamic forcing as input, which can either be directly observed or output from a hydrodynamic model. A nonequilibrium spectral ripple model is driven with this forcing to estimate the power spectrum of the seafloor elevation. Stochastic realizations of seafloor roughness consistent with this power spectrum are generated, which can be used as input to acoustic models to predict the acoustic response of the seafloor. Running ensembles forward through the model allows uncertainty in the hydrodynamic forcing, the sediment properties, and the parameters of the spectral ripple model and acoustic model to be propagated to the model outputs. Bayesian inference can also be applied to solve the inverse problem of estimating the seafloor spectrum and model parameters from observations. We illustrate the features of this model architecture by applying it to estimate seafloor roughness during a field experiment off the coast of Panama City, Florida, USA. We show how NSEA, working in both forward and inverse mode, can use available hydrodynamic models and observations as well as side-scan sonar imagery of the seafloor to estimate changing seafloor roughness with quantified uncertainty.

How to cite: Kearney, W. and Penko, A.: The Naval Seafloor Evolution Architecture: a platform for forecasting dynamic seafloor roughness, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12939, https://doi.org/10.5194/egusphere-egu22-12939, 2022.