HR: 0800h
AN: H51A-0172    [Abstracts]
TI: Bifurcation Evolution in Meandering Rivers With Adapting Widths
AU: * Kleinhans, M G
EM: m.kleinhans@geo.uu.nl
AB: Bifurcations in meandering rivers are stable for a surprisingly variable duration despite similar conditions: avulsion duration (when one channel becomes dominant) is between a decade up to within millenia. I present a 1D morphodynamic model with results for bifurcations in a channel network of the lower River Rhine (the Netherlands) for the past 1000 years. The results are compared to detailed geological data and historical data and maps since 1595AD. The model accounts for gradient differences between the bifurcates (well-known to trigger avulsions), a meander bend just upstream of the bifurcation, and the adjustment of bifurcate channel width in response to changing discharge. The model component for sediment division over the bifurcates is validated with 3D morphodynamic model results (accepted by WRR). An upstream meander favours one bifurcate with more sediment and the other with more water depending on the bend radius, leading to destabilisation which agrees with the historical data. The effect of a gradient advantage and an upstream bend may amplify or nearly balance each other, causing an order of magnitude increase in modelled avulsion duration and bifurcation stability. Critical combinations of slope advantage counteracted by an upstream bend did not lead to stable bifurcations. The adjustment of channel width in changing discharge affects the sediment balance for the channel bed, leading to a much longer and more realistical adaptation time and avulsion duration. Which bifurcate increases in size depends on the initial width for certain realistic gradient advantage and bend radius combinations. This implies that avulsion and bifurcation evolution differs in pristine versus re-occupied channels. Idealised modelling yields realistic retrodictions of the Rhine delta evolution.
DE: 1825 Geomorphology: fluvial (1625)
DE: 1856 River channels (0483, 0744)
DE: 1862 Sediment transport (4558)
SC: Hydrology [H]
MN: 2007 Fall Meeting