Boundary shear stress and velocity distribution in differentially roughened trapezoidal open channels
PhD thesis by Alhamid, A.A.I., 1991

Mainly experimental work in heterogeneously roughened trapezoidal channels, aimed at obtaining velocity and boundary shear stress data, mixing coefficients, etc. Two sizes of gravel were used, d84 = 9.3 mm and 18.0 mm.

Abstract

Several series of experiments were undertaken in simple trapezoidal open channels with both differentially and uniformly roughened boundaries for uniform, steady and fully developed turbulent flow. Two types of gravel distributions were used for roughening the channel boundaries (ie walls only or walls and bed) in order to vary the ksw/ksb ratio. Experiments were conducted within the ranges of aspect ratio, 0.85 < B/H < 10.0, Reynolds number, 3.4xl04 < Re < 1.6x105, and Froude number, 0.39 < Fr < 0.89, for channel bed slopes 0.00392 , 0.00403 and 0.001935 with l :1 wall side slopes.

Primary velocity distributions, boundary shear stress distributions, shear forces, the mean and maximum shear stresses, the resistance coefficients for uniform and differentially roughened channels and the eddy viscosity were studied. Boundary shear stress and velocity distributions were found to be more affected by the secondary currents and boundary roughness in differentially roughened channels than in uniformly roughened ones. An empirical equation for evaluating the percentage shear force carried by the walls, %SFw, as a function of the channels geometry, ie Pb/Pw, was obtained for smooth and uniformly roughened channels having different cross section shapes. For differentially roughened trapezoidal channels a similar equation was developed using the ksw/ksb ratio as a representative of the transverse heterogeneity of the roughening materials. Equations for estimating the mean and maximum wall and bed shear stresses were also developed, based on the %SFw and the channel geometry using either B/H or Pb/Pw ratios. The sidewall correction procedures together with some equations for evaluating the effective flow resistance coefficient in differentially roughened channels were also examined and found to break down due to the existence of secondary flows and the transfer of momentum from the fast moving fluid in the central region to the slow moving fluid in the wall regions.

The zero datum plane for boundary shear stress computations was found to be different from that for discharge computations. The geometric mean datum plane was found to be adequate for discharge computations whereas for boundary shear stresses computations another datum plane was found at 17% below the geometric mean datum plane.

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Data files

Introduction

Table 3-1 shows the main experimental programme, using 3 channels with different types of roughness R1 (d84 = 18.0 mm) and R2 (d84 = 9.3 mm).The physical characteristics of roughening materials is given in Table 4.1. Channel cross-section is shown in Fig.3.4. In all some 38 experiments were conducted in 3 channels covering a range of aspect ratios from 0.85 to 10.0. See Figs 4.1 for gravel size distribution curves and Figs 4.2 - 4.9 for cross-section profiles of the channels. Channels 1-3 were labeled as ‘narrow’, ‘intermediate’ and ‘wide channels respectively, as indicated in Table 4.2. For further details see the pictures and PhD thesis.

Some stage-discharge data are shown in Table 4.3 and Figs 4.11 and 4.12. A summary of the key parameters from the experimental results are shown in Table 4.4. The results are shown in the various published papers related to this project. Some external data utilized in the analysis are shown in Appendix C. It should be noted that n0 is the geometric mean datum plane and n1 the shear stress zero datum plane.

Data file links

Channel profiles:

Some results:

Publications

  • Knight, D.W., Al-Hamid, A.A.I. and Yuen, K.W.H., 1992, Boundary shear in differentially roughened trapezoidal channels, In Hydraulic and Environmental Modelling: Estuarine and River Waters (Eds R.A. Falconer, K. Shiono and R.G.S. Matthew), Ashgate Press,3-14. [C]
  • Knight, D.W., Yuen, K.W.H. and Alhamid, A.A.I., 1994, Boundary shear stress distributions in open channel flow, in Physical Mechanisms of Mixing and Transport in the Environment, (Eds K. Beven, P. Chatwin and J. Millbank), J. Wiley, Chapter 4, 51-87. [B]