H51A-0169
Flow Structure and Secondary Flows in Fine-grained Bifurcate Channels
Bifurcations form vital nodes within many fluvio\-deltaic channels and their stability is critical in determining long- term channel behavior. Although there are an increasing number of laboratory and numerical models detailing flow and morphology within such bifurcations, relatively few field studies exist on the nature of flow within such sites by which to test and validate these models. In particular, the presence and nature of any secondary flow cells is seen as central in controlling fluid and sediment partitioning within the bifurcation, and contributing to long term stability or instability. Here we present results of a field study in the Cumberland Marshes, Saskatchewan, Canada, which sought to document the mean flow fields and patterns of secondary flow at two asymmetric bifurcations within the Mossy River delta. The study used a Teledyne RDI 1200 kHz acoustic Doppler profiler to record three\-dimensional flow velocities along a series of flow\-transverse cross\-sections within these bifurcations. These surveys document the partitioning of main downstream flow through the bifurcation and reveal that bed topographic forcing and secondary flows can be important processes as the flows divide at the bifurcation. These results will be discussed in relation to the functioning of bifurcations and their sediment transport pathways.
H51A-0170
Flow structures and controls at river bifurcations: a laboratory flume experiment
River bifurcations are key nodes within fluvio-deltaic systems and in braided rivers, the mechanics controlling the flow and sediment partitioning at such sites being the key controls on dynamics of river braiding and distributary systems. Recent research has shown that certain geometrical configurations induce instabilities that lead to downstream mid-channel bar formation. However, to date, we have a poor process understanding of flow dynamics within and through channel bifurcations and their downstream distributary channels and we know very little on the flow division process and its overall controls. This paper presents results of a series of experiments from a flume laboratory study that were conducted in a dedicated facility in the Sorby Environmental Flow Dynamics Laboratory at the University of Leeds. A series of detailed three-dimensional velocity measurements were made at controlled cross sections through the bifurcation model. These measurements were made at equal input discharges but with different downstream weir conditions in the bifurcate channels, essentially controlling water surface slopes through the system. The results indicate that flow division occurs close to the point of bifurcation in all cases. Results also indicate that the influence of the downstream water elevations are very significant; dividing the flow farther upstream than in the equal weir height case and altering the flow structures through this zone. Results also indicate that secondary flows induced at ethe bifurcation and just upstream play an important role in the downstream bifurcate channels, which will have implications for overall system dynamics.
H51A-0171
A stability diagram for fine-grained, cohesive fluvial-channel bifurcations
Although the evolution of fine-grained fluvial distributive networks depends upon the stability of channel bifurcations, neither the stability field nor the stabilizing processes of bifurcations are currently well-known. Here we define the theoretical stability field for fine-grained bifurcations using Delft3D, a morphodynamic numerical model, and test the model predictions using field data collected on eight natural bifurcations in the Mossy River Delta, Saskatchewan, Canada. The numerical experiments start with a generic, preformed, bifurcation that is symmetrical about the channel centerline, has fixed walls, and constant bathymetry in each bifurcate. Upstream boundary conditions include a constant incoming water and sediment discharge with equilibrium sediment concentrations and both steady and equal water surface elevations downstream. Computations proceed until the discharge ratio between bifurcate channels ( Qr) does not change for one morphological time unit (defined as cross-sectional area divided by sediment transport rate per unit width), at which time the system is defined as stable. Similar to the stability diagram for coarse-grained distributive networks (Bolla Pittaluga et al., 2003; Miori et al., 2006), stable, equilibrium, fine-grained bifurcation configurations are asymmetrical in their Qr. However, unlike coarse-grained systems, Qr becomes more asymmetrical with increasing Shields thetaƒ|ƒn When the eight natural bifurcations are plotted on the stability diagram, three plot in ¡§unstable¡¨ space. Even though these ¡§unstable¡¨ bifurcations have been active for 40 years, analyses of their evolution from serial maps of the Mossy Delta shows that there is significant bar growth and change in channel area compared to the five ¡§stable¡¨ bifurcations. The implications of these findings for our understanding of channel bifurcation dynamics, evolution and avulsion will be discussed.
H51A-0172
Bifurcation Evolution in Meandering Rivers With Adapting Widths
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.
H51A-0173
Formation and maintenance of bifurcations by avulsion
In order to better understand branching distributaries in anastomosed rivers and deltas, recent research has focused on the formation and stability of bifurcations from a hydrodynamic perspective. Such formulations begin with some kind of diverging flow inducing bar deposition, leading to enhanced flow divergence around the bar and formation of a bifurcation. Increasingly sophisticated transport models have attempted to determine which bifurcation geometries are stable, where stable means no significant erosion or deposition. However, there is a class of channel bifurcations - avulsions- that are created by a process that is independent of the details of in- channel transport . A scaling analysis employing field and laboratory data shows that avulsion is a large-scale gravitational instability arising from the interaction of a channel with its floodplain. The Niobrara River, Nebraska, is in transition to a branching pattern due to rapid aggradation forced by a dam built downstream. Historical data demonstrate that bifurcations have formed by avulsion and partial abandonment of channels during the last 30 years. Aerial photographs show that individual bifurcations persist for many decades. This apparent stability in planform, however, belies the dynamic nature of these bifurcations. Data collected over the past four years show repeated cutting and filling of individual branches. Bifurcations of the Niobrara River over this time do not appear to achieve stability, but rather oscillate over a wide range of configurations as the channel avulses during aggradation. A simple theoretical analysis suggests that many anastomosed and deltaic bifurcations are created and maintained by avulsion. Development of simplified, large-scale models of avulsion-driven bifurcations is needed to complement the growing body of hydrodynamic models.