Flow fields, bed shear stresses, and suspended bed sediment dynamics in bifurcations of a large river

Abstract

Channel bifurcations associated with bars and islands are important nodes in braided rivers and may control flow partitioning and thus affect downstream confluences, as well as the formation and dynamics of bars. However, the morphodynamic processes associated with bar formation are poorly understood, and previous studies have largely concerned laboratory experiments, small natural streams, or numerical analyses with large Froude numbers, high slopes, and low Shields stresses. In these cases, the morphologic changes at bifurcations are relatively rapid, with predominant bed load transport and the suspended load playing a minor role. In this paper, the evolution of the flow structure and suspended bed sediment transport along four expansion‐diffluence units in the Rio Paraná, Argentina, are described. The Rio Paraná is a large multichannel river with a bed composed of medium and fine sands and possesses low Froude numbers and high suspended bed material transport. Primary and secondary flow velocity components were measured with an acoustic Doppler current profiler (ADCP) along the expansion‐diffluence units, and the backscatter signal of the ADCP was calibrated to allow simultaneous measurements of suspended bed sediment concentrations. The interactions between these variables show that the cores of primary flow velocity and suspended bed sediment concentration do not necessarily follow the thalweg at the bifurcation and that inertial effects on the suspended bed sediment may influence the morphodynamics of bar formation. It is suggested that changes in flow stage, as well as the presence of vegetation, may further increase the deposition of suspended bed sediment at the bar head. This study suggests that the ratio of suspended bed material to bed load is an important factor controlling the morphodynamics of bifurcations in large sand bed braided rivers.

Extremes in dune preservation: Controls on the completeness of fluvial deposits

“…Numerical simulations of dune migration (Jerolmack and Mohrig, 2005) indicate that a steady rain of sediment may cause dune sets to 'climb'. Although their study does not specify the spatial limits or physical causes of decreased migration and/or increased aggradation, it highlights the importance of local bypassing and the sediment transport paths that control local sediment transport rates as essential factors in set climbing (Ghienne et al, 2010;Szupiany et al, 2012;Naqshband et al, 2014). Observations from modern channels indicate that Fig.…”

“…1E). However, the variability in preservation that is a logical consequence of bar-scale gradients in sediment transport (Szupiany et al, 2012) remains largely unknown. Experiments with ripples (Storms et al, 1999) and dunes (Leclair, 2002(Leclair, , p. 1159) indicate that high rates of upstream sediment feed cause the development of a bar-scale bedform over which smaller bedforms migrate (Fig.…”

“…investigate the processes and magnitudes of sediment fallout during waning flows, the spatial geometry of the flow field and sediment transport paths, and their relation to varying flow depth and water surface slope. Local bypassing and deposition of sediment is proportional to the absolute sediment transport rate and hence greater during larger discharges, and is promoted by a greater propensity for sediment to be suspended (Ws/U*; where Ws is the settling velocity of sediment and U* is the fluid shear velocity; Szupiany et al, 2012;Nicholas, 2013;Naqshband et al, 2014). Significant time may be involved in the burial of dunes under a thick cohesive clay layer because the settling velocity of clay is typically low and suspended sediment concentrations are relatively low in most river systems.…”

Estimating sand concentrations using ADCP‐based acoustic inversion in a large fluvial system characterized by bi‐modal suspended‐sediment distributions

“…This knowledge could then support prediction of river channel morphodynamics (see for example Szupiany et al [2012] and Hackney et al [2018]), sedimentation and scour patterns near in-channel structures, reservoir infilling and sustainability, suitability of aquatic habitat, and transport of sediment-associated pollutants. These results are relevant for various research disciplines and water resources management agencies trying to weigh sediment sampling efforts and associated costs in medium and large rivers.…”

Evaluation of aDcp processing options for secondary flow identification at river junctions

“…Parsons et al, 2005;Kostaschuk et al, 2009;Shugar et al, 2010), bends (e.g. Parsons et al, 2007;Szupiany et al, 2012), canyons (e.g. Parsons et al, 2007;Lane et al, 2008;Szupiany et al, 2009;Riley and Rhoads, 2012;Riley et al, 2015;Gualtieri et al, 2017), bifurcations (e.g.…”