Shear stresses in beds with large roughness elements

Vahid Etminan Farooji, Mario Conde-Frias, Maryam Abdolahpour, Marco Ghisalberti, Ryan Lowe

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Large bed roughness such as vegetation, gravels, ripples, etc.alters many benthic processes including sediment transport which, in turn, significantly affects the bed stability, channel morphology. Models to predict the onset and rates of sediment transport in environmental systems are conventionally related
to the bed shear stress. Several methods exist to estimate the shear stress on bare beds but none of them are strictly applicable to beds with large roughness, predominantly due to the large impact roughness can have on the near-bed mean and turbulent flow structure. Recently, a Linear Stress Model (LSM) has been proposed to predict the spatially-averaged bed shear stress in vegetated flows. However, while the distribution of shear stress is uniform over bare beds, it is highly spatially-variable in the presence of large roughness elements. A full understanding of this variability is essential for predicting the extent and location of sediment mobilisation. In this study, high resolution computational fluid dynamics simulations were used to describe the spatial variation of shear stress on vegetated beds across a wide range of canopy densities (from 1.6% to 25% by area) and Reynolds numbers (from 200 to 1340). To complement the numerical results, bed shear stress distributions were measured in a laboratory flume using Particle Image Velocimetry (PIV). The numerical and experimental results reveal that, in sparse canopies, bed shear stress distributions
become approximately uniform (and close to that predicted by the LSM) far from the canopy stems. However, the bed shear stress is highly variable throughout denser canopies. In addition, it is shown that presence of vegetation significantly
reduces the spatially-averaged bed shear stress compared to that of a bare bed for given channel and flow conditions. The results of this study provide a foundation for improving predictions of sediment transport in vegetated river beds and currentdominated coastal regions where direct measurement of the full
(spatially-averaged) bed shear stress field is near impossible.
Original languageEnglish
Title of host publicationProceedings of the 21st Australasian Fluid Mechanics Conference
EditorsTimothy C.W. Lau, Richard M. Kelso
PublisherAustralasian Fluid Mechanics Society
ISBN (Electronic)9780646597843
ISBN (Print) 9780646597843
Publication statusPublished - 2018
Event21st Australasian Fluid Mechanics Conference - Adelaide Convention Centre, Adelaide, Australia
Duration: 10 Dec 201813 Dec 2018
Conference number: 21

Publication series

NameProceedings of the 21st Australasian Fluid Mechanics Conference, AFMC 2018


Conference21st Australasian Fluid Mechanics Conference
Abbreviated titleAFMC 2018
Internet address


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