Flow and Suspended Sediment Division at Two Highly Asymmetric Bifurcations in a River Delta: Implications for Channel Stability

Kästner, K.; Hoitink, A.J.F.

doi:10.1029/2018jf004994

Cited by 23 publications

(40 citation statements)

References 88 publications

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“…The presence of a meander bend in the upstream channel could feed non-uniformly the bifurcation node and favor the downstream channel at the outer bend (Kleinhans et al, 2008;Sassi et al, 2011). Moreover, the downstream channels may have different aggradation and progradation rates (Salter et al, 2018), different lengths (Kästner & Hoitink, 2019;Wagner & Mohrig, 2019), or asymmetric tidal forcing at the mouths. In this work we focus on the two latter conditions.…”

Section: Asymmetrical Downstream Branches and Tidal Forcingmentioning

confidence: 99%

“…This especially occurs among the main water course and the side distributaries. Grain sorting could have thus a strong influence in sediment division (Frings & Kleinhans, 2008;Kästner & Hoitink, 2019). Moreover, despite that our formulation relies on the Engelund and Hansen (1967) formula, which accounts for the total sediment flux, a deeper investigation on the role of suspended load is needed.…”

Section: 1029/2020jf005584mentioning

confidence: 99%

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Effect of Small Tidal Fluctuations on the Stability and Equilibrium Configurations of Bifurcations

2020

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Deltas are fascinating landforms subject to fluvial and marine forcing. Bifurcations are common features in deltas, governing the distribution of water and sediment fluxes among the distributary channel network. Recently, it has been observed that tide‐influenced deltas tend to display less numerous but more stable branches in comparison to their riverine counterparts. River bifurcations subject to unidirectional flow have been widely studied in the last decades. In contrast, the acting physical mechanisms and factors controlling the stability of bifurcations in tide‐influenced deltas are still not well understood, and a theoretical framework is still lacking. In order to fill this gap and understand how the stability and evolution of bifurcations in distributary deltaic systems could be affected by the tides, we investigate, through an analytical model, the equilibrium configurations and stability of tidal bifurcations under the hypothesis of small monochromatic tidal oscillations. In particular, we build on previous works on river bifurcations, incorporating the solution for the equilibrium of a single‐river‐dominated estuary. We find that higher tidal amplitudes and a closer proximity of the junction node to the sea tend to hamper the development of unbalanced solutions, reducing the asymmetries in water and sediment fluxes between branches. This stabilizing effect exerted by the tidal action is associated with the erosive character of the tidal currents that promotes channel deepening and increases the capacity of the system to keep morphodynamically active both bifurcates in comparison with the purely fluvial case. Preliminary field observations of natural deltas corroborate our findings.

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Section: Asymmetrical Downstream Branches and Tidal Forcingmentioning

confidence: 99%

Section: 1029/2020jf005584mentioning

confidence: 99%

Effect of Small Tidal Fluctuations on the Stability and Equilibrium Configurations of Bifurcations

2020

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“…With decreasing width‐to‐depth ratio of a channel, the flow and sediment dynamics becomes evermore three‐dimensional in character, and suspended‐sediment concentration at the surface becomes less strongly correlated to depth‐averaged concentration. In situ observations of channel junctions remain crucial, as they exert a key control in partitioning sediment fluxes over the delta channel network, and the flow and sediment partitioning processes are particularly complex (Buschman et al, ; Buschman et al, ; Kästner & Hoitink, ; Salter et al, ; Sassi, Hoitink, de Brye et al, ).…”

Section: Grand Challengesmentioning

confidence: 99%

“…When seeking nature‐based solutions to create delta resilience, a fundamental understanding of morphodynamic equilibrium conditions remains essential. The last near‐pristine systems on the globe need to be cherished and analyzed, because they may reveal unknown mechanisms of resilience (Kästner & Hoitink, ). Besides those, the stratigraphic record provides an abundant archive of pristine delta behavior, which should further be explored to discover unknown natural mechanisms of morphological resilience.…”

Section: Grand Challengesmentioning

confidence: 99%

Resilience of River Deltas in the Anthropocene

et al. 2020

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At a global scale, delta morphologies are subject to rapid change as a result of direct and indirect effects of human activity. This jeopardizes the ecosystem services of deltas, including protection against flood hazards, facilitation of navigation, and biodiversity. Direct manifestations of delta morphological instability include river bank failure, which may lead to avulsion, persistent channel incision or aggregation, and a change of the sedimentary regime to hyperturbid conditions. Notwithstanding the in‐depth knowledge developed over the past decades about those topics, existing understanding is fragmented, and the predictive capacity of morphodynamic models is limited. The advancement of potential resilience analysis tools may proceed from improved models, continuous observations, and the application of novel analysis techniques. Progress will benefit from synergy between approaches. Empirical and numerical models are built using field observations, and, in turn, model simulations can inform observationists about where to measure. Information theory offers a systematic approach to test the realism of alternative model concepts. Once the key mechanism responsible for a morphodynamic instability phenomenon is understood, concepts from dynamic system theory can be employed to develop early warning indicators. In the development of reliable tools to design resilient deltas, one of the first challenges is to close the sediment balance at multiple scales, such that morphodynamic model predictions match with fully independent measurements. Such a high ambition level is rarely adopted and is urgently needed to address the ongoing global changes causing sea level rise and reduced sediment input by reservoir building.

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“…In gravel‐bed rivers, bifurcations can be stabilized by a transverse slope across the approaching channel, which directs bed material into the larger and deeper branch (Bolla Pittaluga et al, 2003, 2015; Redolfi et al, 2016). However, the transverse slope does not seem to be the factor that stabilizes deltaic bifurcations of large sand‐bed rivers (Buschman et al, 2010; Kästner & Hoitink, 2019; Sassi et al, 2013). Large sand‐bed rivers transport sediment predominantly in suspension, which can reduce the effect of the secondary flow and is known to affect the dynamics of avulsion channels (Slingerland & Smith, 1998).…”

Section: Introductionmentioning

confidence: 99%

Idealized Model for the Deflection of Sediment Into Lateral Branches of Lowland Rivers

Kästner

Hoitink

2020

Water Resources Research

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The division of sediment at river bifurcations is crucial for the morphodynamics of anastamosing rivers and distributary delta channel networks. Many river bifurcations are strongly asymmetric and have a planform where a small channel branches off to the side. Such a configuration is also typical for man‐made diversions of water and sediment into canals. At asymmetric bifurcations, the division of sediment is influenced by the secondary current, which is caused by the turning of the flow toward the side. The secondary currents cause especially water from the lower parts of the water column to be diverted into the side branch. As the sediment concentration close to the bottom is high, side branches can receive a disproportionately large fraction of the incoming sediment load, relative to the water discharge. Lateral diversions have been extensively studied with physical and numerical experiments, with the goal to either mitigate or exploit this effect. However, a systematic mathematical analysis of the parameter space has not yet appeared in the literature. Here, we present a comprehensive analysis by way of an idealized model, revealing how the division of sediment is influenced by the width and depth of the branches. We show that the excess of sediment that is diverted into the side branch is lower when the inlet to the side branch is wider and shallower. This may have larger implications for the stability of delta channel networks, as inlets to side branches tend to be locally wider, which likely contributes to their morphological stability.

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Flow and Suspended Sediment Division at Two Highly Asymmetric Bifurcations in a River Delta: Implications for Channel Stability

Cited by 23 publications

References 88 publications

Effect of Small Tidal Fluctuations on the Stability and Equilibrium Configurations of Bifurcations

Effect of Small Tidal Fluctuations on the Stability and Equilibrium Configurations of Bifurcations

Resilience of River Deltas in the Anthropocene

Idealized Model for the Deflection of Sediment Into Lateral Branches of Lowland Rivers

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