Stage-discharge, resistance and sediment transport relationships for flow in straight compound channels
PhD thesis by Atabay, S., 2001

Mainly experimental work in compound rectangular channels, aimed at obtaining velocity and boundary shear stress data for rigid channels with symmetric and asymmetric floodplains of varying width. A second series of experiments was then undertaken in the same channels, but with a mobile bed replacing the rigid bed in the main channel. Sediment transport rates were examined for smooth floodplains. Later another series of experiments were conducted by Tang (see files) with roughened floodplains.

Abstract: Sediment transport in two stage channels

In order to investigate two-phase flow at high river stage, experiments in straight asymmetric and symmetric compound channels, with either rigid or mobile main channel beds, were performed. Measurements were made of water surface slope, distributions of boundary shear stresses and velocity, sediment transport, bed profile and dune migration velocity, and the results presented graphically and with tabulated summaries.

Single channel and divided channel methods have been used to analyse the experimental data paying particular attention to stage-discharge, division of flow and resistance relationships. Empirical equations have been derived from each type of compound channel and bed category for predicting the discharge, Q, proportion of total flow in sub-areas, %Qi, resistance parameters, n and f, and boundary shear forces, SFi, for a given stage, H. Pavlovskii's composite roughness equation has been used, together with Vanoni & Brooks sidewall correction procedure, to obtain the resistance values on the mobile main channel bed, which were then reduced to their form and grain components. The results do not agree well with two standard alluvial friction formulae: White, Paris & Bettess (1980) and van-Rijn (1984).

The boundary shear stress data were integrated to give boundary shear forces on different elements of the wetted perimeter, SFj. The shear forces carried by the main channel bed and walls for the rectangular main channel, and the apparent shear forces between the main channel and floodplain for the symmetric compound channel, were compared with the results obtained by Knight et al.'s approaches (1994 & 1983). The comparison showed that the experimental data and Knight et al.'s results correlate well.

Sediment bed load data were analysed and empirical equations have been derived for predicting the sediment transport rate, Gs, and sediment concentration, X (in ppm) for a given channel discharge, Q for both inbank and overbank flow cases. It is shown that the trend of the mean sediment concentration for the laboratory data is similar to that for a hypothetical river obtained using the COH method by Ackers (1992a). Sediment bed load data were also compared with the sediment transport predictions based on the theories of White, Paris & Bettess and van-Rijn.

Two one-dimensional (1-D) methods, the weighted divided channel method (WDCM) of Lambert and Myers (1998) and the coherence method (COHM) of Ackers (1991), were applied to three different data sets: the UK FCF data, Knight et al's data (1981-1984) and the 18m flume data of the present study. It is shown that these two methods predicted the total and zonal channel discharges well, especially for the smooth compound channels using the appropriate Manning's n values and an appropriate weighting factor, ξ, in the WDCM. However, the WDCM was modified to eliminate the need for ξ, for smooth compound channels, and better predictions were obtained. The COHM was also applied to the boundary shear stress and sediment transport data. It was proved that reducing the theoretical mean shear stress in the main channel by the factor DISADFC2 did not predict the actual shear stress on the main channel bed well. Therefore predictions of sediment transport rate by the COHM with the Ackers & White equation did not correlate well with the experimental data, despite predicting the main channel discharge very well for the 18m flume.

This study has successfully highlighted the general behaviour of sediment bed-load transport and resistance in compound channels with overbank flow. The simple empirical relationships for predicting the discharge, resistance perimeters, and boundary shear forces for a given stage and the sediment transport rate for a given channel discharge will be of use in benchmarking numerical river simulation models over a wide range of flow conditions.

Data files

Experiments for rigid smooth channels (simple and compound) and sand main channel bed with smooth floodplain (both symmetric and asymmetric compound channels) were carried out by Atabay, based on the flume of Birmingham University, which is a non-tilting 22m long flume with a test length of 18m. The cross-section of flume is shown in Fig.1. These data are presented in the following links:

Details on the notation and programme are given in notes on Birmingham data. Data files are in Microsoft Excel format (raw data + analysis), which are regrouped and listed in the following manner. All files need to be unzipped using WINZIP and then opened with MS Excel.

Data file links

Rigid channels

Mobile Channels (mobile main channel bed and rigid floodplain)

Publications

  • Atabay, S. A. and Knight, D.W., 2002, The influence of floodplain width on the stage-discharge relationship for compound channels, River Flow 2002, Proc. Int. Conf. on Fluvial Hydraulics, Louvain-la-Neuve, Belgium, Sept., Vol. 1, 197-204, Balkema. [C]
  • Atabay, S., Knight, D.W. and Seckin, G., 2004, Influence of a mobile bed on the boundary shear in a compound channel, River Flow 2004, Proc. 2nd Int. Conf. on Fluvial Hydraulics, 23-25 June, Napoli, Italy [Eds M. Greco, A. Carravetta and R.D. Morte], Vol. 1, 337-345. [C]
  • Atabay, S., Knight, D.W. and Seckin, G., 2005, Effects of overbank flow on fluvial sediment transport rates, Water Management, Proceedings of the Instn. of Civil Engineers, London, 158, WM1, March, 25-34. [J]
  • Atabay, S., and Knight, D.W., 2005, 1-D modelling of conveyance, boundary shear and sediment transport in overbank flow, Journal of Hydraulic Research, IAHR, (in press) [J]