The fluvial dynamics of confluent meander bends

Riley, James

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https://hdl.handle.net/2142/44346 Copy

Description

Title

The fluvial dynamics of confluent meander bends

Author(s)

Riley, James

Issue Date

2013-05-24T22:08:30Z

Director of Research (if dissertation) or Advisor (if thesis)

Rhoads, Bruce L.

Doctoral Committee Chair(s)

Rhoads, Bruce L.

Committee Member(s)

• Best, James L.
• Parker, Gary
• Thorn, Colin E.

Department of Study

Geography & Geographic InfoSci

Discipline

Geography

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• Fluvial Geomorphology
• Confluences
• Meander Bends
• Flow Structure
• Bed Morphology

Abstract

Planform geometry is a controlling factor that influences flow structure, sediment transport, and bed morphology at river confluences. Previous experimental, field, and modeling studies of confluence dynamics have focused mainly on junctions formed by straight approach channels that meet at angular configurations before entering a straight receiving channel. In contrast, natural rivers often meander and tributaries can enter meandering rivers on the outside of bends to form a junction planform known as a confluent meander bend. Experimental and numerical modeling studies have begun to document flow structure at confluent meander bends with fixed boundaries and rectangular channel cross sections. The research presented here complements this modeling work with field data from natural confluent meander bends to evaluate flow dynamics in channels with erodible boundaries, to document morphologic features of the bed, and to evaluate changes in bed morphology with changing flow conditions. The research is comprised of two field investigations. In the first study, field measurements of three-dimensional velocity components and bed topography at a small confluent meander bend with a 90° junction angle reveal a complex hydrodynamic environment that responds to changes in momentum-flux ratio. Flow from the tributary deflects high-velocity flow and helical motion in the curving main river toward the inside of the bend, inducing bed scour and inhibiting point-bar development. The high junction angle forces the tributary flow to abruptly realign to the orientation of the downstream channel, initiating a counter-rotating helical cell over the outer portion of the bend. Two surface-convergent helical cells persist through the downstream channel, where the combined flows accelerate as the channel cross-sectional area is constricted by a bar along the downstream junction corner, precluding flow separation. Long-term stability of its planform suggests that this confluent meander bend represents a quasi-stable channel configuration. The second field study examines the influence that the angle of entry of the tributary flow has on mutual deflection of confluent flows and the spatial extent of confluence hydro- and morphodynamic features at confluent meander bends. Measurements of three-dimensional flow structure and bed morphology were obtained for high and low momentum-flux ratios at two large-river confluent meander bends with different tributary entry angles. At the high-junction angle confluent meander bend, mutual deflection of converging flows abruptly turns fluid from the tributary into the downstream channel, while flow in the main river is deflected away from the outer bank of the bend where a bar extends downstream of the junction corner from the inner bank of the tributary. Two counter-rotating helical cells inherited from upstream flow curvature flank the mixing interface which overlies a central pool. Substantial morphologic change due to the development of a meander cutoff upstream of the confluence during large, tributary-dominant discharge events results in displacement of the pool inward from the influx of large amounts of sediment into the confluence and substantial erosion of the point bar in the main channel. In contrast, flow deflection is less pronounced at the low-angle junction, where the converging flows almost parallel each other upon entering the confluence. A large helical cell induced by upstream flow curvature in the main river occupies most of the downstream channel for prevailing low momentum-flux ratio conditions and a weak counter-rotating cell forms during infrequent tributary-dominant flow events. Bed morphology remains relatively stable and does not exhibit extensive scour that often occurs at confluences with concordant beds. The mixing interface at both confluences persists through the downstream channel, indicating helical motion does not produce substantial mixing of the flows within the confluence hydrodynamic zone. Together, the results from the two studies indicate that the dynamics of confluent meander bends differ greatly from the dynamics of bends without tributaries and can be characterized as an amalgamation of the dynamics of confluences and meander bends.

Graduation Semester

2013-05

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http://hdl.handle.net/2142/44346

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Copyright 2013 James Riley

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