The effect of asymmetric dune roughness on tidal asymmetry in the Weser estuary

Herrling, Gerald; Becker, Marius; Lefebvre, Alice; Zorndt, Anna; Krämer, Knut

doi:10.1002/esp.5170

Cited by 8 publications

(11 citation statements)

References 59 publications

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“…Bedform morphology, and how it varies in time and space, has some strong implications in terms of the presence and size of flow separation, turbulent wake and bedform roughness. Herrling et al (2021) recently argued that the contrasting effects of dune asymmetry on residual bed load transport may be of similar importance as the controls of seasonal variations of discharge on net sediment fluxes. Furthermore, recent studies (Kwoll et al, 2016; Lefebvre & Winter, 2016) have confirmed the early work of Ogink (1989), who suggested that bedform roughness is not only related to bedform height and length, but also to their lee side angle: bedforms with a steeper lee side produce more roughness than bedforms with a gentler lee side.…”

Section: Discussionmentioning

confidence: 99%

“…2 m) were found to be related to tidal currents and water discharge. Recently, Herrling et al (2021) estimated the effect of asymmetric dune roughness on tidal asymmetry in the Weser Estuary. Based on numerical simulation results, they showed that the spatiotemporal interaction of distinct dune shapes with the main drivers of estuarine sediment and morphodynamics (river discharge and tidal energy) is substantial.…”

Section: Introductionmentioning

confidence: 99%

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confidence: 99%

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Morphology of estuarine bedforms, Weser Estuary, Germany

Lefebvre

Herrling

Becker

et al. 2021

Earth Surf Processes Landf

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

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Section: Discussionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Morphology of estuarine bedforms, Weser Estuary, Germany

Lefebvre

Herrling

Becker

et al. 2021

Earth Surf Processes Landf

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“…Third, we evaluated the hydraulic model by assessing tidal attenuation and water level fluctuations and found minimal impact from local dune geometry. This notwithstanding, incorporating dune roughness could be important for estimation of residual sediment transport (Brakenhoff et al., 2020; Herrling et al., 2021). Local values of shear stresses may be required for morphodynamic modeling, even if they are not needed for modeling tidal propagation in a hydrodynamic model.…”

Section: Discussionmentioning

confidence: 99%

Dune Geometry and the Associated Hydraulic Roughness in the Fluvial to Tidal Transition Zone of the Fraser River at Low River Flow

de Lange,

Bradley,

Schrijvershof

et al. 2024

JGR Earth Surface

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In deltas and estuaries throughout the world, a fluvial‐to‐tidal transition zone (FTTZ) exists where both the river discharge and the tidal motion drive the flow. It is unclear how dune characteristics are impacted by changes in tidal flow strength, and how this is reflected in the hydraulic roughness. To understand dune geometry and variability in the FTTZ and possible impacts on hydraulic roughness, we assess dune variability from multibeam bathymetric surveys, and we use a calibrated 2D hydrodynamic model (Delft3D‐FM) of a sand‐bedded lowland river (Fraser River, Canada). We focus on a period of low river discharge during which tidal impact is strong. We find that the fluvial‐tidal to tidal regime change is not directly reflected in dune height, but local patterns of increasing and decreasing dune height are present. The fluvial‐to‐tidal regime change is reflected in dune shape, where dunes have lower leeside angles and are more symmetrical in the tidal regime. The calibrated model allows to estimate local patterns of dune heights using tidally averaged values of bed shear stress. However, the spatially variable dune morphology hampers local dune height estimation. Changes in dune shape do not significantly impact the reach‐scale roughness, and estimated dune roughness using dune height and length is similar to the dune roughness inferred from model calibration. Hydraulic model performance with a calibrated, constant roughness is not improved by implementing dune‐derived bed roughness. Instead, the data analysis revealed that large‐scale river morphology may explain differences in model roughness and corresponding estimates from dune predictors.

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“…For instance, multiple scales of bedforms can co‐exist in so‐called compound dunes, where smaller bedforms are superimposed on larger primary dunes (Ashley, 1990). Being naturally more abundant than their larger host dunes, these secondary bedforms respond to flow processes of a shorter time scale and have a crucial influence on hydraulic roughness, turbulence and energy dissipation (Herrling et al., 2021; Lefebvre et al., 2013; Zomer et al., 2023). Other cases of complex bathymetric processes are dune amalgamation or calving (Bradley & Venditti, 2021).…”

Section: Introductionmentioning

confidence: 99%

Automated Bedform Identification—A Meta‐Analysis of Current Methods and the Heterogeneity of Their Outputs

Scheiber,

Zomer,

Wang

et al. 2024

JGR Earth Surface

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Ongoing efforts to characterize underwater dunes have led to a considerable number of freely available tools that identify these bedforms in a (semi‐)automated way. However, these tools differ with regard to their research focus and appear to produce results that are far from unequivocal. We scrutinize this assumption by comparing the results of five recently published dune identification tools in a comprehensive meta‐analysis. Specifically, we analyze dune populations identified in three bathymetries under diverse flow conditions and compare the resulting dune characteristics in a quantitative manner. Besides the impact of underlying definitions, it is shown that the main heterogeneity arises from the consideration of a secondary dune scale, which has a significant influence on statistical distributions. Based on the quantitative results, we discuss the individual strengths and limitations of each algorithm, with the aim of outlining adequate fields of application. However, the concerted bedform analysis and subsequent combination of results have another benefit: the creation of a benchmarking data set which is inherently less biased by individual focus and therefore a valuable instrument for future validations. Nevertheless, it is apparent that the available tools are still very specific and that end‐users would profit by their merging into a universal and modular toolbox.

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The effect of asymmetric dune roughness on tidal asymmetry in the Weser estuary

Cited by 8 publications

References 59 publications

Morphology of estuarine bedforms, Weser Estuary, Germany

Morphology of estuarine bedforms, Weser Estuary, Germany

Dune Geometry and the Associated Hydraulic Roughness in the Fluvial to Tidal Transition Zone of the Fraser River at Low River Flow

Automated Bedform Identification—A Meta‐Analysis of Current Methods and the Heterogeneity of Their Outputs

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