Investigation of sediment behaviour in a channel with flood plains
The objective of this research was to investigate the sediment behaviour in a compound channel and, in particular, the transfer of sediment between a main channel and its flood plain. A review of the literature on compound channels showed that, whilst research on fixed boundaries had been carried out or was in hand, there was no evidence that the loose boundary situation had previously been studied.Experiments were conducted in a loose boundary, outdoor sand channel of symmetrical compound section. The channel was of straight alignment, 50 m long, with an overall width of 3 m. Pumping equipment was available for recirculation of the water and the sediment.Samples of suspended sediment were collected from the shallow and deep sections and analysed by Coulter Counter to obtain the particle size distributions. The distribution curves were found to be very similar for the main channel and the flood plains.Sand from the channel was labelled by fluorescent dye and inserted in the bed of the main channel so as to simulate a point release. Bed samples were collected at sections 5 m and 10 m downstream of the injection point and examined under UV light for their tracer proportions. It was found that the cross-sectional distribution of tracer concentration was approximately Gaussian indicating that there had been some sediment transfer to the flood plains.A two-dimensional diffusion model, which accounts for the movement in the longitudinal and lateral directions, has been applied to describe the transport and dispersion of the tracer particles. A best-fit overlay with the experimental results enabled the longitudinal and lateral dispersion coefficients to be established. The model results, for the distribution of tracer, are depicted in 2 and 3-dimensional form at a sequence of time intervals for up to 2 hours after tracer release.It was concluded that under steady state conditions there would be a constant transfer of sediment from the main channel to the flood plains; in fact, analysis showed that approximately 40% of a continuous tracer injection in the main channel would be transferred to the flood plains.