Abstract
AbstractBifurcations are key geomorphological nodes in anabranching and braided fluvial channels, controlling local bed morphology, the routing of sediment and water, and ultimately defining the stability of their associated diffluence–confluence unit. Recently, numerical modelling of bifurcations has focused on the relationship between flow conditions and the partitioning of sediment between the bifurcate channels. Herein, we report on field observations spanning September 2013 to July 2014 of the three‐dimensional flow structure, bed morphological change and partitioning of both flow discharge and suspended sediment through a large diffluence–confluence unit on the Mekong River, Cambodia, across a range of flow stages (from 13 500 to 27 000 m3 s−1).Analysis of discharge and sediment load throughout the diffluence–confluence unit reveals that during the highest flows (Q = 27 000 m3 s−1), the downstream island complex is a net sink of sediment (losing 2600 ± 2000 kg s−1 between the diffluence and confluence), whereas during the rising limb (Q = 19 500 m3 s−1) and falling limb flows (Q = 13 500 m3 s−1) the sediment balance is in quasi‐equilibrium. We show that the discharge asymmetry of the bifurcation varies with discharge and highlight that the influence of upstream curvature‐induced water surface slope and bed morphological change may be first‐order controls on bifurcation configuration. Comparison of our field data to existing bifurcation stability diagrams reveals that during lower (rising and falling limb) flow the bifurcation may be classified as unstable, yet transitions to a stable condition at high flows. However, over the long term (1959–2013) aerial imagery reveals the diffluence–confluence unit to be fairly stable. We propose, therefore, that the long‐term stability of the bifurcation, as well as the larger channel planform and morphology of the diffluence–confluence unit, may be controlled by the dominant sediment transport regime of the system. © 2017 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd.
Highlights
The passage of water and sediment through fluvial systems controls the evolution of channel planform, defines rates of channel adjustment and, over longer time scales, drives floodplain development and the construction of stratigraphy (Schumm, 1985; Aalto et al, 2003, 2008; Constantine et al, 2014)
570 The results shown in Figure 7 suggest that different regions of the diffluence571 confluence unit become active at different flow stages and that individual links within the unit may display different behaviour at different 573 flow stages
Our data show that the discharge asymmetry ratio declined from a value of Qr* = 0.54 586 during the high flow of the monsoon flood-pulse (September 2013, Q = 27,000 m3 s587 1), to Qr* = 0.44 on the falling limb of the hydrograph (October 2013, Q = 13,500 m3 s588 1), and subsequently rose to a value of 0.59 on the following rising limb of the 589 hydrograph (July 2014; Q =19,500 m3 s-1)
Summary
The passage of water and sediment through fluvial systems controls the evolution of channel planform, defines rates of channel adjustment and, over longer time scales, drives floodplain development and the construction of stratigraphy (Schumm, 1985; Aalto et al, 2003, 2008; Constantine et al, 2014). Analysis of the cross-stream water surface elevations (recorded on average at a ~2.5 m spacing across the channel width) derived from the dGPS elevations recorded whilst conducting the MBES surveys during the different flow discharges (Figure 4A and B) reveals that, during the highest discharges (September 2013), the mean cross-stream water surface slope, calculated as the difference between the water surface elevations at the left and right hand bank divided by the cross-stream distance, is 8x10-5 m m-1, reducing to 3x10-5 m m-1 during the falling limb (October 2013), with a similar value of 4x10-5 m m-1 observed during the rising limb (July 2014) flow. This gain is likely associated with a remobilisation of sediment sequestered into this smaller subsidiary channel during the higher flow flood period
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