Field investigation of bedform morphodynamics under combined flow

Ebb‐tidal deltas are highly dynamic environments affected by both waves and currents that approach the coast under various angles. Among other bedforms of various scales, these hydrodynamics create small‐scale bedforms (ripples), which increase the bed roughness and will therefore affect hydrodynamics and sediment transport. In morphodynamic models, sediment transport predictions depend on the roughness height, but the accuracy of these predictors has not been tested for field conditions with strongly mixed (wave–current dominated) forcing. In this study, small‐scale bedforms were observed in the field with a 3D Profiling Sonar at five locations on the Ameland ebb‐tidal delta, the Netherlands. Hydrodynamic conditions ranged from wave dominated to current dominated, but were mixed most of the time. Small‐scale ripples were found on all studied parts of the delta, superimposed on megaripples. Even though a large range of hydrodynamic conditions was encountered, the spatio‐temporal variations in small‐scale ripple dimensions were relatively small (height η≈ 0.015 m, length λ≈ 0.11 m). Also, the ripples were always highly three‐dimensional. These small dimensions are probably caused by the fact that the bed consists of relatively fine sediment. Five bedform height predictors were tested, but they all overestimated the ripple heights, partly because they were not created for small grain sizes. Furthermore, the predictors all have a strong dependence on wave‐ and current‐related velocities, whereas the ripple heights measured here were only related to the near‐bed orbital velocity. Therefore, ripple heights and lengths in wave–current‐dominated, fine‐grained coastal areas ( D50<0.25 mm) may be best estimated by constant values rather than values dependent on the hydrodynamics. In the case of the Ameland ebb‐tidal delta, these values were found to be η=0.015 m and λ=0.11 m. ©2019 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd.

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“…; Wengrove et al. ; Wu and Parsons, ) and on continental shelves (e.g. Li and Amos, ; Krämer and Winter, ).…”

Section: Introductionmentioning

confidence: 99%

“…Studies do exist on wave-current ripples in shallow water along straight coasts or on nearshore bars (e.g. Meirelles et al, 2016;Wengrove et al, 2018;Wu and Parsons, 2019) and on continental shelves (e.g. Li and Amos, 1999;Krämer and Winter, 2016).…”

Section: Introductionmentioning

confidence: 99%

Spatio‐temporal characteristics of small‐scale wave–current ripples on the Ameland ebb‐tidal delta

Brakenhoff

Kleinhans

Ruessink

et al. 2020

Earth Surf Processes Landf

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“…Sedimentary structures have traditionally been investigated by means of visual field observations (van den Berg et al, 2017;Grundvåg et al, 2019), ground penetrating radar (Lang et al, 2017), seismology (Kiswaga & Felix, 2020), core samples (Hammer & Webb, 2010;Wu & Parsons, 2019), x-ray imaging (Gyøry et al, 2018) and experimental laboratory flumes (Alexander et al, 2001;Leclair, 2002;Baas et al, 2016;Yokokawa et al, 2010). Many studies start with outcrop investigations (Fielding, 2006;Lord et al, 2017), where depositional signatures are classified according to established systems (Campbell, 1967;Collinson & Mountney, 2019).…”

Section: Introductionmentioning

confidence: 99%

Explaining the formation of sedimentary structures under antidunes using a 2D width-averaged numerical model

Olsen¹

2022

NJG

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A new numerical model for computing the formation of sedimentary structures in a fluvial environment is presented. The model has been used to explain the strata-forming processes under antidunes in detail, including water velocities, turbulence, free surface location, erosion, sedimentation and formation of each sediment layer. The Reynolds-Averaged Navier-Stokes equations were solved together with the k-epsilon turbulence model to determine the water flow field and the bed shear stress on a width-averaged two-dimensional grid. The sediment transport was computed by solving convectiondiffusion equations for multiple sediment sizes. An empirical formula for resuspension at the bed was used, enabling the computation of erosion and deposition for each sediment fraction. Five sediment sizes were used, from 0.5 to 2.5 mm. The bed sediment stratification was recorded using a very fine grid, resolving the bedding with up to 1000 cells in the vertical direction. A novel algorithm was used to vertically adapt the bed grid, enabling the computation of very large gradients of the grain size distribution, including thin laminae. The model was tested by computing sediment deposition in a channel with a Froude number of 1.21. The hydraulic parameters and the average sediment size were selected similar to an earlier laboratory flume study where downstream-migrating antidunes were observed.The antidunes started as small bed irregularities and increased in size as they moved in the downstream direction. Recirculation zones were observed behind bedforms with large slopes on the lee side. Here, sediments deposited in slides. Smaller sediment waves moving on top of the bedforms, depositing layers of a fine and a coarse material simultaneously. After the initial phase, stationary antidunes formed in the upstream and middle part of the flume, producing wavy sedimentary structures. Sinusoidal crosslamination formed in deposits under bedforms of different downstream velocities. The antidunes in the downstream part of the flume moved in the streamwise direction. They were affected by occasional upstream-moving surface waves, causing complex sedimentary structures to form. Figures and videos show the strata-forming process in detail, including velocity vectors, turbulence and layers of different particle diameters.

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“…In contrast to stability diagrams for current-and wave-generated bedforms, stability diagrams for combined-flow bedforms of the basic form shown in Figure 1 (e.g., Arnott and Southard, 1990;Dumas et al, 2005;Kleinhans, 2005;Perillo et al, 2014) cover such a small fraction of the parameter space that their application is challenging. This is because laboratory flume investigations typically do not allow for the evaluation of more than two or three variables, and field studies of more complex combined-flow bedform dynamics are poorly integrated into these stability diagrams (Amos et al, 1988(Amos et al, , 1999Gallagher et al, 1998;Li and Amos, 1998;Gallagher, 2003;Hay and Mudge, 2005;Smyth and Li, 2005;Larsen et al, 2015;Wengrove et al, 2018Wengrove et al, , 2019Wu and Parsons, 2019;Cuadrado, 2020). For example, despite being the most comprehensive flume study available in the literature, the experimental dataset of Perillo et al (2014) is limited to co-linear waves and currents, a single sand size, and three closely spaced wave periods.…”

Section: Introductionmentioning

confidence: 99%

Current- and Wave-Generated Bedforms on Mixed Sand–Clay Intertidal Flats: A New Bedform Phase Diagram and Implications for Bed Roughness and Preservation Potential

Baas

2021

Front. Earth Sci.

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Field investigation of bedform morphodynamics under combined flow

Cited by 10 publications

References 39 publications

Spatio‐temporal characteristics of small‐scale wave–current ripples on the Ameland ebb‐tidal delta

Spatio‐temporal characteristics of small‐scale wave–current ripples on the Ameland ebb‐tidal delta

Explaining the formation of sedimentary structures under antidunes using a 2D width-averaged numerical model

Current- and Wave-Generated Bedforms on Mixed Sand–Clay Intertidal Flats: A New Bedform Phase Diagram and Implications for Bed Roughness and Preservation Potential

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