Flow development in rough-bed open channels: mean velocities, turbulence statistics, velocity spectra, and secondary currents

Andrea Zampiron* (Corresponding Author), Stuart M. Cameron, Mark T. Stewart, Ivan Marusic, Vladimir I. Nikora

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

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The channel length required for the development of the flow, from the channel entrance to full establishment, is often a prerequisite when designing hydraulic structures or planning research experiments in open channels. However, the information on the flow development length ((Formula presented.)) is scarce, and even its definition remains vague. In hydraulic experiments, this lack of knowledge introduces great uncertainty, often making comparisons of findings from different studies questionable. This paper offers a physics-based definition for (Formula presented.), and reports results of systematic laboratory studies to provide guidance on its quantitative assessment. Our data for uniform flows suggest that up to 100 flow depths ((Formula presented.)) are required for mean velocity field (including sidewall secondary currents), turbulent stresses (except streamwise variance), velocity skewness and kurtosis, and depth-scale large-scale-motions to become essentially independent of the streamwise coordinate. However, very large-scale-motions, streamwise velocity variance, and roughness-induced secondary currents are found to require longer (Formula presented.) of around (Formula presented.).

Original languageEnglish
Pages (from-to)133-144
Number of pages12
JournalJournal of Hydraulic Research
Issue number1
Early online date20 Dec 2022
Publication statusPublished - Jan 2023

Bibliographical note

Acknowledgements: The authors wish to express their gratitude to Roy Gillanders for the help provided in the laboratory and to the School of Engineering of the University of Aberdeen for the support. The comments and suggestions of the Associate Editor and two anonymous reviewers helped to improve the final version of the paper and are much appreciated.

The study has been supported by three Engineering and Physical Sciences Research Council/UK grants: “High-resolution numerical and experimental studies of turbulence-induced sediment erosion and near-bed transport” (EP/G056404/1), “Bed friction in rough-bed free-surface flows: a theoretical framework, roughness regimes, and quantification” (EP/K041088/1) and “Secondary currents in turbulent flows over rough walls” (EP/V002414/1).

Data Availability Statement

Supplemental data: Supplemental data for this article can be accessed at http://dx.doi.org/10.1080/00221686.2022.2132311.


  • Coherent structures
  • hydraulic experiments
  • open-channel flow
  • turbulence
  • velocity spectra


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