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

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 (L-D) 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 L-D, 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 (H) 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 L-D of around 150H.

Automated summary

The channel length required for the development of the flow is important in designing hydraulic structures or planning research experiments. However, information on flow development length (L-D) is scarce and its definition remains unclear. This paper offers a physics-based definition for L-D and presents results of laboratory studies to provide guidance on its quantitative assessment. The results suggest that different flow characteristics require different lengths of L-D, with some requiring up to 150 flow depths.