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Semi-theoretical model for mean sediment resting time of spherical particles: the role of hydrodynamic impulses and sphere size nonuniformity

Wyssmann Micah A., Papanicolaou Athanasios N., Kyriakopoulos Theodoros

Acta Geophysica

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2023

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Vol. 71, no. 6

2883--2904

EN

Prediction of bedload rates in gravel-bed rivers at low-to-moderate flow conditions, where bedload movement is intermittent, remains a challenging problem. While the virtual velocity concept provides a useful approach to bedload rate estimation in the intermittent movement regime, virtual velocity estimation remains hindered by a lack of tools for predicting mean sediment resting time. As a first step toward sediment resting time estimation in gravel beds, the present study develops a semi-theoretical resting time model applicable to nonuniform gravel-sized spherical particles. The model is based on the consideration that interactions of near-bed flow with bed material lead to mobilization of individual resting particles during hydrodynamic momentum transfer events (i.e., impulses). Thus, resting time is affected by impulse magnitude and timing. The primary premise underpinning model development is that an instantaneous velocity time-series generation approach based on the velocity spectrum can be used to mimic hydrodynamic impulses and simulate resting times. Based on past findings, two model parameters are considered important to advancing resting time predictions in gravel beds. First, the relative particle size allows size-fractional resting time predictions for a nonuniform sediment mixture. Second, the hindrance coefficient accounts for hiding effects and enables resting time predictions for different bed structure types. To provide calibration and verification data, laboratory experiments documenting impulse statistics and mean resting times for a range of flow and relative particle size conditions were also performed. The verified model exhibits mean resting times with similar magnitude and trends with increasing stress compared with experimental verification data.

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Flow Characteristics over a Gravel Bedform: Kaj River Case Study

Maddahi M. R., Afzalimehr H., Rowiński P. M.

Acta Geophysica

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2016

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Vol. 64, no. 5

1779--1796

EN

The present study deals with the turbulence structure in order to better understand the interaction of bedform and flow characteristics in a gravel-bed river. Data measured above a bedform is used to analyze the importance of coherent structures in turbulent transfer. The Reynolds stress and turbulence intensity in stream-wise direction illustrate significant difference along the bedform, showing a three-layer distribution at the crest and a convex one at the downstream of bedform. Quadrant analysis technique is used to picture momentum exchange above the considered bedform and to find the dominant event in bursting process of the gravel bedform. Quadrant analysis demonstrates that the mechanisms of bedforms generation in sand and gravel-bed rivers are similar and sweep is the dominant event in both rivers.

3

Flow turbulence over mobile gravel-bed: spectral scaling and coherent structures

Nikora V.

Acta Geophysica Polonica

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2005

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Vol. 53, nr 4

539-552

EN

The turbulence structure of high Reynolds number, quasi-2-D flow with a weakly mobile gravel bed is considered, with particular focus on velocity spectra and bursting events from quadrant analysis. A four-range scaling model for velocity auto-spectra and co-spectra is discussed and tested. It is suggested that eddy clusters revealed using quadrant analysis contribute to the energy production, low wave -number region in velocity spectra, while individual bursting events contribute to the "-1" scaling region of intermediate wave numbers where energy production and cascade energy transfer co-exist. Possible relations between spectral scaling and coherent structures are briefly discussed.