HR: 17:30h
AN: H54A-07    [Abstracts]
TI: The Effect of Alternating Bars Migration on River Bifurcation Dynamics
AU: * Miori, S
EM: stefano.miori@ing.unitn.it
AF: University of Trento - Department of Civil and Environmental Engineering, via Mesiano, 77, Trento, 38100, Italy
AU: Bertoldi, W
EM: walter.bertoldi@ing.unitn.it
AF: University of Trento - Department of Civil and Environmental Engineering, via Mesiano, 77, Trento, 38100, Italy
AU: Repetto, R
EM: rodolfo.repetto@gmail.com
AF: Imperial College of London - Department of Bioengineering, South Kensington Campus, London, SW7 2AZ, United Kingdom
AU: Zanoni, L
EM: luca.zanoni@ing.unitn.it
AF: University of Trento - Department of Civil and Environmental Engineering, via Mesiano, 77, Trento, 38100, Italy
AU: Tubino, M
EM: marco.tubino@ing.unitn.it
AF: University of Trento - Department of Civil and Environmental Engineering, via Mesiano, 77, Trento, 38100, Italy
AB: Recent theoretical analysis, field and laboratory observations pointed out that fluvial bifurcation show an intrinsic instability, leading to the establishment of an unbalanced flow and sediments distribution in the downstream branches. The existence of equilibrium configurations has been proved, which mainly depend on the hydraulic and morphologic conditions of the upstream flow. However, flow and sediment transport in braided networks are highly unsteady, so that the bifurcation can hardly reach an equilibrium configuration. One of the main causes of temporal fluctuations is the migration of alternate bars in the upstream channel, that can affect and control the flow partition in the distributaries. We analysed the bar – bifurcation interactions by experimental and analytical investigations. We performed a set of flume experiments on a Y shaped fixed banks and movable bed bifurcation. Laboratory results show that bar formation in the upstream channel perturbs the discharge distribution with a series of fluctuations strictly related to the period of bar migration. Four different behaviours have been identified, characterised by small perturbations of the equilibrium state (balanced or unbalanced), by the occurrence of large fluctuations or by the closure of one of the distributaries. The character of the bifurcation is controlled by the amplitude and speed of alternate bars that directly influence the amplitude and period of discharge oscillations. Consequently, at large values of the aspect ratio (high bars) and low sediment mobility (slow bars) the bifurcation dynamics is likely to be dominated by bars migration. Extending the one-dimensional model proposed by Bolla Pittaluga et al. (2003), we introduce the effect of bars migrating in the upstream channel. In the present model, the bifurcation is forced with spatial crosswise fluctuations of feeding conditions, in order to reproduce the transverse distribution of sediment and water of an alternate bar pattern as predicted by the weakly nonlinear theory of Colombini et al. (1987). In this way, the model reproduces bed perturbation and discharge fluctuations at the inlet of the downstream channels. For different bar characteristics, correspondent to different flow conditions in the upstream channel, the model is able to qualitatively reproduce the four behaviours detected in the experiments. The obtained results underline the complexity of the bar-bifurcation interaction, due to their similar evolution time scales.
DE: 1825 Geomorphology: fluvial (1625)
DE: 1856 River channels (0483, 0744)
DE: 1862 Sediment transport (4558)
SC: Hydrology [H]
MN: 2007 Fall Meeting