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Flow Velocity and Turbulence Structure in Gravel-Bed Rivers: Experimental Investigation in a Straight Laboratory Flume

Author(s): Donatella Termini; Federica Lavignani; Nina Benistati

Linked Author(s): Donatella Termini

Keywords: Rivers; Gravel bed; Velocity field; Turbulence

Abstract: The knowledge of flow hydrodynamics and turbulence structure in natural rough-bed rivers is especially important to identify sediment transport processes and rivers morphological changes. Natural bed rivers are often in the presence of the macro-roughness elements, such as gravels and boulders. Literature indicates that gravel-bed flows are characterized by complex fluid/bed interactions (wake flow downstream of gravels, acceleration over gravel crest and deceleration in interstices between gravels) occurring especially in the near-bed zone and determining spatial flow heterogeneities (Nikora & Goring 2000). Thus, the presence of gravels determines a significant impact on the flow velocity field determining the formation of coherent turbulence structures which evolve periodically along the downstream direction. The turbulent structures make complex the aforementioned interaction processes and affect the lateral diffusion and the stream-wise and vertical transport processes. In recent years, considerable progress has been made in the research in this field. Although the classical procedure consider the time-averaged hydrodynamic quantities to analyze natural bed flows, recent studies (among others Dey & Das 2012; Nikora et al., 2013) indicate that, because of the aforementioned heterogeneity effect, both the temporal and the spatial averaging of the flow quantities provide a better understanding of the turbulence characteristics. In this study, a laboratory experiment in a straight channel has been conducted to measure the three-dimensional instantaneous flow velocity components over a heterogeneous coarse sediments. The experimental results are used to analyse the flow velocity field and the turbulence characteristics. According to previous works (Nikora et al., 2013) double averaging, which includes both the time-and the spatial averaging, has been applied to estimate the mass and momentum conservation terms. References Dey, D., Das, R. (2012). Gravel-bed hydrodynamics: double averaging approach. Journal of Hydraulic Engineering, 138,707–725. Nikora, V., Goring, D. 2000 flow turbulence over fixed and weakly mobile gravel beds. Journal of Hydraulic Engineering 126 (9), 679–690. Nikora, V., Ballio, F., Coleman, S., Pokrajac, D. (2013). Spatially averaged flows over mobile rough beds: definitions, averaging theorems and conservation equations. Journal of Hydraulic Engineering, 139, 803–811.


Year: 2022

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