Abstract
Meandering is a common feature in natural alluvial streams. This study deals with alluvial behaviors of a meander reach subjected to both fresh-water flow and strong tides from the coast. Field measurements are carried out to obtain flow and sediment data. Approximately 95% of the sediment in the river is suspended load of silt and clay. The results indicate that, due to the tidal currents, the flow velocity and sediment concentration are always out of phase with each other. The cross-sectional asymmetry and bi-directional flow result in higher sediment concentration along inner banks than along outer banks of the main stream. For a given location, the near-bed concentration is 2−5 times the surface value. Based on Froude number, a sediment carrying capacity formula is derived for the flood and ebb tides. The tidal flow stirs the sediment and modifies its concentration and transport. A 3D hydrodynamic model of flow and suspended sediment transport is established to compute the flow patterns and morphology changes. Cross-sectional currents, bed shear stress and erosion-deposition patterns are discussed. The flow in cross-section exhibits significant stratification and even an opposite flow direction during the tidal rise and fall; the vertical velocity profile deviates from the logarithmic distribution. During the flow reversal between flood and ebb tides, sediment deposits, which is affected by slack-water durations. The bed deformation is dependent on the meander asymmetry and the interaction between the fresh water flow and tides. The flood tides are attributable to the deposition, while the ebb tides, together with run-offs, lead to slight erosion. The flood tides play a key role in the morphodynamic changes of the meander reach.
Highlights
Meandering is one of the most common shapes formed by river streams, which is especially true for streams in the lowland alluvial plains [1]
At the meander bend apex, its cross-sectional shape is usually asymmetrical, with a deep portion of channel along the outer bank and a broad, shallow section extending towards the inner bank
Changes in both river curvature and bed topography are interrelated, and their geometric shapes need to be considered when assessing the effects of channel configuration on flow and sediment transport
Summary
Meandering is one of the most common shapes formed by river streams, which is especially true for streams in the lowland alluvial plains [1]. The tidal currents interact with the fresh-water run-off and they form a bidirectional flow, which plays an essential role in the fluvial process. Meandering with large curvatures is a characteristic of many streams in low-land coastal regions Changes in both river curvature and bed topography are interrelated, and their geometric shapes need to be considered when assessing the effects of channel configuration on flow and sediment transport. By means of the two approaches, the objective of the study is to provide insight into the interplay between fresh-water flows and tidal currents, to illustrate the circulatory patterns of suspended load transport during the tidal rising and falling, and to make predictions of the bed deformation associated with the meandering properties (curvature, cross-channel asymmetry, etc.).
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