Hydraulic roughness over simple subaqueous dunes

Bartholdy, Jesper; Flemming, Burghard W.; Ernstsen, Verner Brandbyge; Winter, Christian; ä, Alex Bartholom

doi:10.1007/s00367-009-0153-7

Cited by 35 publications

(31 citation statements)

References 33 publications

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“…The effective shear stress is calculated on the basis of a new method (Bartholdy et al, 2010) that assumes a near bed layer above the mean bed level, twice as high as the dune height, in which the current velocity accelerates towards the bedform crest.…”

Section: Resultsmentioning

confidence: 99%

“…The form-corrected bed shear stress due to skin friction is calculated as the bed shear stress acting at the dune crest, and is based on the method suggested by Bartholdy et al (2010) who rendered the existence of a virtual horizontal boundary layer of height D′ = 2H above the mean bed level, with H being the bedform height. According to this method, water can be regarded as entering the boundary layer at the mean bed level halfway between the trough and the crest with a mean velocity, V′, whichaccording to the logarithmic velocity profi le (e.g.…”

Section: Critical Conditions For the Initiation Of Motionmentioning

confidence: 99%

“…This volume is then directly translatable to bedform migration rate by deviding it by the bedform height, and can be transformed to bedload transport in weight per time and width by means of estimates of the bulk dry density of naturallypacked sand and the bedform shape factor. Based on results from Bartholdy et al (2010) the model can function as a traditional bedload function without the need of knowing the bedform height. The model can be downloaded as supplementary on-line material.…”

Section: Introductionmentioning

confidence: 99%

“…Related to this is the ability to extract the portion of the total bed shear stress which contributes to bedform migration. This is based on the procedure suggested in Bartholdy et al (2010).…”

Section: Introductionmentioning

confidence: 99%

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A simple model of bedform migration

Bartholdy

Ernstsen

Flemming

et al. 2010

Earth Surf Processes Landf

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A model linking subaqueous dune migration to the effective (grain related) shear stress is calibrated by means of fl ume data for bedform dimensions and migration rates. The effective shear stress is calculated on the basis of a new method assuming a near-bed layer above the mean bed level in which the current velocity accelerates towards the bedform crest. As a consequence, the effective bed shear stress corresponds to the shear stress acting directly on top of the bedform. The model operates with the critical Shields stress as a function of grain size, and predicts the deposition (volume per unit time and width) of naturally-packed bed material on the bedform lee side, qb crest . The model is simple, built on a rational description of simplifi ed sediment mechanics, and its calibration constant can be explained in accordance with estimated values of the physical constants on which it is based. Predicted values of qb crest correlate with measured values of bedform height multiplied by bedform migration rate with R 2 = 0·83.

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Section: Resultsmentioning

confidence: 99%

Section: Critical Conditions For the Initiation Of Motionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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A simple model of bedform migration

Bartholdy

Ernstsen

Flemming

et al. 2010

Earth Surf Processes Landf

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“…The presence of bedforms therefore strongly influences the form drag, impacting directly on the overall flow resistance as well as the flow velocity structure and sediment transport pattern (e.g. Venditti 2007;Bartholdy et al 2010;Lefebvre et al 2016). Detailed characterisation of bedform roughness reflected in bedform parameters of their spatial distribution and orientation is therefore vital for constructing numerical models of flow field, hydrology and sediment transport.…”

Section: Introductionmentioning

confidence: 99%

Estimation of dimensions and orientation of multiple riverine dune generations using spectral moments

Lisimenka

Kubicki²

2016

Geo-Mar Lett

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A new spectral analysis technique is proposed for rhythmic bedform quantification, based on the 2D Fourier transform involving the calculation of a set of low-order spectral moments. The approach provides a tool for efficient quantification of bedform length and height as well as spatial crestline alignment. Contrary to the conventional method, it not only describes the most energetic component of an undulating seabed surface but also retrieves information on its secondary structure without application of any band-pass filter of which the upper and lower cut-off frequencies are a priori unknown. Validation is based on bathymetric data collected in the main Vistula River mouth area (Przekop Wisły), Poland. This revealed two generations (distinct groups) of dunes which are migrating seawards along distinct paths, probably related to the hydrological regime of the river. The data enable the identification of dune divergence and convergence zones. The approach proved successful in the parameterisation of topographic roughness, an essential aspect in numerical modelling studies.

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Flow structure and resistance over subaquaeous high‐ and low‐angle dunes

et al. 2016

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A prominent control on the flow over subaqueous dunes is the slope of the downstream leeside. While previous work has focused on steep (~30°), asymmetric dunes with permanent flow separation, little is known about dunes with lower lee slope angles for which flow separation is absent or intermittent. Here we present a laboratory investigation where we systematically varied the dune lee slope, holding other geometric parameters and flow hydraulics constant, to explore effects on the turbulent flow field and flow resistance. Three sets of fixed dunes (lee slopes of 10°, 20°, and 30°) were separately installed in a 15 m long and 1 m wide flume and subjected to 0.20 m deep flow. Measurements consisted of high‐frequency, vertical profiles collected with a Laser Doppler Velocimeter. We show that the temporal and spatial occurrence of flow separation decreases with dune lee slope. Velocity gradients in the dune leeside depict a free shear layer downstream of the 30° dunes and a weaker shear layer closer to the bed for the 20° and 10° dunes. The decrease in velocity gradients leads to lower magnitude of turbulence production for gentle lee slopes. Aperiodic, strong ejection events dominate the shear layer but decrease in strength and frequency for low‐angle dunes. Flow resistance of dunes decreases with lee slope; the transition being nonlinear. Over the 10°, 20°, and 30° dunes, shear stress is 8%, 33%, and 90% greater than a flat bed, respectively. Our results demonstrate that dune lee slope plays an important but often ignored role in flow resistance.

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Hydraulic roughness over simple subaqueous dunes

Cited by 35 publications

References 33 publications

A simple model of bedform migration

A simple model of bedform migration

Estimation of dimensions and orientation of multiple riverine dune generations using spectral moments

Flow structure and resistance over subaquaeous high‐ and low‐angle dunes

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