Intermittent Suspension and Transport of Fine Sediment Over Natural Tidal Bedforms

Kwoll, Eva; Becker, Marius

doi:10.1002/9781118527221.ch15

Cited by 7 publications

(19 citation statements)

References 45 publications

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“…The structures remain connected to the bed throughout advection, although the heads of the ejections occur progressively higher in the water column through time (Figure a). This could suggest a three‐dimensional hairpin topology, but it is also similar to the topology of large‐scale suspension events, which were not linked to a hairpin shape (e.g., Bradley et al, ; Kostaschuk & Church, ; Kwoll et al, ; Matthes, ). As with suspension events previously observed, the CFS will span the entire water depth and surface as boils at a distance just upstream of the crest.…”

Section: Discussionsupporting

confidence: 87%

“…Wedge‐shaped CFSs occur on the stoss side of all our experimental dunes, albeit shorter and weaker for lower lee slopes. Our findings are supported by field observations of large‐scale, sequenced CFS above low‐angle dunes that may grow to water depth scale and surface as boils above low‐angle dunes (Bradley et al, ; Kostaschuk & Church, ; Kwoll et al, ). This suggests that turbulence production over low‐angle dunes differs significantly from classical boundary layer flow and should be considered in studies examining flow and sediment dynamics over low‐angle dunes.…”

Section: Discussionsupporting

confidence: 85%

“…Almost all current knowledge of dune‐related CFS comes from studies using angle‐of‐repose dunes with permanent flow separation; however, large‐scale CFS can also dominate the flow field over low‐angle dunes (e.g., Bradley et al, ; Kostaschuk & Church, ; Kwoll et al, ) that are the dominant morphology in flows deeper than 2.5 m (Bradley and Venditti, ). Lee side slopes of these low‐angle dunes are typically ≪30°, often ~10° (e.g., Kostaschuk and Villard, , ; Hendershot et al, ; Roden, ; Smith & McLean, ), and do not exhibit permanent flow separation.…”

Section: Introductionmentioning

confidence: 99%

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Observations of Coherent Flow Structures Over Subaqueous High‐ and Low‐ Angle Dunes

2017

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Large‐scale coherent flow structures (CFSs) above dunes are the dominant source of flow resistance and constitute the principal mechanism for sediment transport and mixing in sand bed river and estuarine systems. Based on laboratory observations, CFS formation has been previously linked to flow separation downstream of high‐angle dunes with lee slopes of ~30°. How CFSs form in natural, deep rivers and estuaries where dunes exhibit lower lee slopes and intermittent flow separation is not well understood. Here we present particle image velocimetry measurements from an experiment where dune lee slope was systematically varied (30°, 20°, and 10°), while other geometric and hydraulic parameters were held constant. We show that CFSs form downstream of all three dune geometries from shear layer vortices in the dune lee. The mode of CFS formation undergoes a low‐frequency oscillation with periods of intense vortex shedding interspersed with periods of rare vortex shedding. Streamwise alignment of several vortices during periods of intense shedding results in wedge‐shaped CFSs that are advected above the dune stoss side. Streamwise length scales of wedge‐shaped CFS correspond to large‐scale motions (LSMs). We hypothesize that the advection of LSM over the dune crest triggers the periods of intense shedding in the dune lee. LSMs are weaker and smaller above low‐angle dunes; however, the low‐frequency oscillation in CFS formation periods persists. The formation of smaller and weaker CFS results in a reduction of flow resistance over low‐angle dunes.

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Section: Discussionsupporting

confidence: 87%

Section: Discussionsupporting

confidence: 85%

Section: Introductionmentioning

confidence: 99%

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Observations of Coherent Flow Structures Over Subaqueous High‐ and Low‐ Angle Dunes

2017

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“…These landers bridge the observational gap between longterm monitoring stations, remote sensing applications, and ship-based field campaigns. They are mobile, and can be used to spatially interpolate between monitoring stations and provide data with very high temporal resolution (Kwoll et al, 2013(Kwoll et al, , 2014Oehler et al, 2015a;Ahmerkamp et al, 2017). Lander operations aim at measuring various processes close to the seafloor or in the sediment and are designed to have minimal impact on the environment and quantities that are measured.…”

Section: Landersmentioning

confidence: 99%

“…During some parts of the tidal cycle a periodic stratification of the water column has been observed in shallow areas of the German Bight forming distinct layers that move independently with a decoupled tidal ellipticity (Krämer and Winter, 2016;Kwoll et al, 2013Kwoll et al, , 2014Ahmerkap et al, 2017). The difference in sea bed dynamics between fair weather conditions and storms is also investigated in the research area "Seafloor Dynamics" of the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) Research Center/Cluster of Excellence "The Ocean in the Earth System".…”

Section: Lander Sedobsmentioning

confidence: 99%

The Coastal Observing System for Northern and Arctic Seas (COSYNA)

et al. 2017

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Abstract. The Coastal Observing System for Northern and Arctic Seas (COSYNA) was established in order to better understand the complex interdisciplinary processes of northern seas and the Arctic coasts in a changing environment. Particular focus is given to the German Bight in the North Sea as a prime example of a heavily used coastal area, and Svalbard as an example of an Arctic coast that is under strong pressure due to global change.The COSYNA automated observing and modelling system is designed to monitor real-time conditions and provide short-term forecasts, data, and data products to help assess the impact of anthropogenically induced change. Observations are carried out by combining satellite and radar remote sensing with various in situ platforms. Novel sensors, instruments, and algorithms are developed to further improve the understanding of the interdisciplinary interactions between physics, biogeochemistry, and the ecology of coastal seas. New modelling and data assimilation techniques are used to integrate observations and models in a quasi-operational system providing descriptions and forecasts of key hydrographic variables.

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With or against the tide: The influence of bed form asymmetry on the formation of macroturbulence and suspended sediment patterns

Kwoll

Becker

2014

Water Resources Research

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This study examines tide-dependent variations in the formation and dynamics of suspended sediment patterns coupled to mean flow and turbulence above asymmetric bed forms. In the Danish Knudedyb inlet, very large primary bed forms remain ebb-oriented during a tidal cycle while smaller superimposed bed forms reverse direction with each tidal phase. Hydroacoustic in situ observations reveal pronounced differences in suspended sediment transport patterns between tidal phases caused by the relative orientation of primary bed forms and the mean tidal flow and flow unsteadiness during a single tidal phase. When flow and primary bed form orientation are aligned, water-depth-scale macroturbulence develops in the bed form lee-sides in the presence of flow separation. Macroturbulent flow structures occur at high flow stages and are coupled to increased amounts of sediment in suspension. When flow and bed form orientation are opposed no evidence of flow separation associated with primary bed forms is found. Sediment-laden macroturbulence at high flow velocities is of a smaller scale and attributed to the superimposed secondary bed forms. The flow structures are advected along the primary bed form stoss-side (temporary hydraulic lee-side). The steep primary bed form lee-side (temporary hydraulic stoss-side) however, limits transport capabilities beyond the scale of primary bed forms.

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Intermittent Suspension and Transport of Fine Sediment Over Natural Tidal Bedforms

Cited by 7 publications

References 45 publications

Observations of Coherent Flow Structures Over Subaqueous High‐ and Low‐ Angle Dunes

Observations of Coherent Flow Structures Over Subaqueous High‐ and Low‐ Angle Dunes

The Coastal Observing System for Northern and Arctic Seas (COSYNA)

With or against the tide: The influence of bed form asymmetry on the formation of macroturbulence and suspended sediment patterns

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