Hydraulic aspects of straight-compound channel flow and bed load sediment transport
PhD thesis by Ayyoubzadeh, S.A., 1997

Mainly experimental work in simple ‘regime’ channels, aimed at obtaining geometric dimensions and sediment transport rates for alluvial channels that are in equilibrium.

Comparisons were made with predictions of slope, width, depth and sediment transport rate given by analytical regime theories. Later, a series of experiments was started using rigid compound rectangular channels, aimed at obtaining velocity and boundary shear stress data for such channels with symmetric and asymmetric floodplains. One series of experiments was also undertaken in the same channel, but with a mobile bed replacing the rigid bed in the main channel (asymmetric floodplain case). Sediment transport rates were examined for smooth floodplains only. Later other series of experiments were conducted by Atabay (see files) and Tang (see files) with other geometries and roughnesses.

Abstract

Synopsis

Streamwise bed load sediment transport in a two stage channel has been studied using recent knowledge concerning compound channel flows linked to the principles of fluvial processes. Inbank, bankfull and overbank flow conditions have been investigated.

The rational regime theory of White et al. (1982) has been considered in some detail. A new set of equations is presented for the dimensions of stable channels, covering all the original parameters in the HR Tables as an alternative to the White, et al. optimisation procedure. These are similar to empirical regime equations, but with the coefficients and exponents specified as functions of sediment flow properties.

A number of overbank flow features were studied using the 1-D model of Ackers (1992a), based on the coherence concept. The effect of lateral variations in depth averaged velocity and boundary shear stress were investigated using the analytical 2-D depth averaged model of Shiono & Knight (1990) and the finite element model of Abril (1995). Similar conclusions to Ackers (1992a) were drawn, namely that there is a reduction in sediment transport rate in a compound channel once the flow goes out of bank regardless of which model is used for flow adjustment. This is due to the shear effect between the faster main channel flow and the slower floodplain flow.

In order to investigate and evaluate regime channel theories, 10 straight regime channel tests were carried out using a median sand size of 0.88 mm and d35=0.80 mm, flow discharge ranges from 1 to 6.01s", and bed slopes of 0.002-0.003. These experiments were carried out using a new sediment flume in the Hydraulics Laboratory of the School of Civil Engineering at the University of Birmingham. Some empirical equations were developed for the overall geometrical dimensions of the channels using these experimental results as well as data from additional sources. The time taken for a stable channel to develop is formulated, using a limited number of experiments. A dimensionless function for a stable side bank shape is presented. A comparison between the present experimental results and those calculated by the rational regime channel theory confirmed the well known conclusion that the rational regime theory generally predicts a deeper and narrower channel than the actual one, particularly for small scale alluvial channels. An attempt was made to deal with the effect of the initial bank stability on the dimensions and plan form geometry of a stable channel. A comparison between two well-known sediment transport formulae, namely Ackers-White (1990), and van Rijn (1984), showed that these formulae were not valid for small sized channels. A new method was then developed to predict the sediment transport rate for small scale alluvial channels based on boundary shear stress.

A compound asymmetric channel with one smooth floodplain and a mobile sand bed main channel was used for the inbank and overbank flow studies. A number of tests were carried out below bankfull stage to investigate the sediment channel behaviour under low flow conditions and to identify the threshold of motion. For high inbank flows, similar stages to overbank flows were performed by isolating the floodplains. Seven overbank flow depths covering relative depths up to 0.5 were studied.

Some analysis of the flow division, channel resistance, stage-discharge curves, bed form shapes and sediment transport rate was undertaken.

Data files

Experiments for loose sand bed, simple channel and asymmetric compound channel, were carried out by Ayyoubzadeh, 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.2. These data are shown in the following links.

Other data measured by Atabay (Rigid/Mobile Smooth Channels - Inbank and Overbank Flows) and Tang (Rigid/Mobile Rough Channels - Inbank and Overbank Flows) are also listed here for a view of complete set.

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.

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Datafile links

Rigid channels

  • rough - simple channels (inbank flow)
  • rough - compound channels (overbank flow) - large wire meshes
  • smooth - simple channels (inbank flow) - see Atabay's data
  • smooth - compound channels (overbank flow)

=> Mobile channels (mobile main channel bed and rigid floodplain)

  • inbank flows and asymmetric smooth compound channels
    • inbank flow data (all data + analysis)
    • overbank flow data (all data + analysis)
  • overbank flows (smooth floodplain) - both symmetric and asymmetric - see Atabay's data
  • overbank flows (rough floodplain) - symmetric channels - see Tang's data (l=3m, 1m, 0.5m, 0.25m)
  • overbank flows (smooth floodplain) - symmetric channels, non-equilibrium - see Tang's data

Publications

  • Knight, D.W., Brown F.A., Ayyoubzadeh S.A. and Atabay, S., 1999, Sediment transport in river models with overbank flow, In River Sedimentation [Eds A W Jayawardena, J H W Lee and Z Y Wang], Proc. Seventh International Symposium on River Sedimentation, Hong Kong, 16-18 December 1998, Balkema, 19-25. [C]
  • Ayyoubzadeh, S.A., Samani, J.M.V. and Knight, D.W., 2006, Explicit design method for stable channel, Journal of Hydraulic Engineering,ASCE, (submitted) [J]