An experimental investigation of the mean flow structure in wide ducts of simple rectangular and compound trapezoidal cross-section in particular zones of high lateral shear
PhD thesis by Rhodes, D.G., 1991

Mainly experimental work in a large purpose-built wind tunnel with a rectangular cross-section, later modified to an asymmetric compound cross-section to investigate the shear layer zone of mixing at the 'main channel/floodplain' interface. The work was aimed at obtaining very detailed velocity and boundary shear stress data, using air as the medium. Contains some very high quality data on rectangular and compound asymmetric ducts of varying size, with the side slope, s, of the main channel varying from s = 0, 1 & 2 (1:s, vertical : horizontal).


Distributions of primary velocity and boundary shear stress were measured in a wide closed duct for a range of rectangular and asymmetric compound sections. At the interface between the shallow and deep subsections, the compound geometries included three different wall slopes, vertical, 1:1 and 1:2 (vertical:horizontal). The results will provide a data base for mathematical modelling, especially relevant to the field of single and two-stage open channel flows.

The analysis is largely presented as the relationship between selected flow variables and two geometric parameters, relative depth (H-h)/H for the compound geometry and aspect ratio b/h for the rectangular. The flow variables include the measured distributions, and distributions of depth mean velocity, momentum and kinetic energy flux, apparent shear stress, local friction factor and lateral eddy viscosity. Also included are momentum and kinetic energy coefficients, cross-section friction factors and the width of lateral shear layers.

In the compound section, the very detailed measurements indicate the presence of sec­ondary flow cells for which there is little previous evidence. Secondary flow is discussed in relation to apparent shear stress and shear layer widths, and on the flood plain is shown to significantly influence the flow field beyond the range indicated by the primary velocity and bed shear stress distributions.

For the rectangular geometry, new empirical equations are presented relating per cent shear force on the walls to aspect ratio.


  • Rhodes, D.G. and Knight, D.W., 1994, Anomalous measurements in a compound duct, ASCE Symposium on Fundamentals and Advancements in Hydraulic Measurements and Experimentation, Buffalo, New York, August, Vol. 2, 311-320. [C]
  • Rhodes, D.G. and Knight, D.W., 1994, Velocity and boundary shear in a wide compound duct, Journal of Hydraulic Research, IAHR, Vol. 32, No. 5, October, 743-764. [J]
  • Rhodes, D.G. and Knight, D.W., 1995, Lateral shear in a compound duct, 26th IAHR Congress, London, International Association for Hydraulic Research, September, HYDRA 2000, Vol.1, Thomas Telford, 51-56. [C]
  • Rhodes, D.G. and Knight, D.W., 1995, Lateral shear in a wide compound duct, Journal of Hydraulic Engineering, ASCE, Vol. 121, No. 11, November, 829-832. [J]
  • Rhodes, D.G. and Knight, D.W., 1996, Distribution of local friction factor in a compound duct, Proc. 10th Asian and Pacific Division IAHR-APD Congress, [Eds Lee, S.C., Hiew, K.L. and Ong, S.H.], National Hydraulic Research Institute, Malaysia, 26-29 August, Vol. 2, 220 -227. [C]