Dynamics of the Cross-Circulatory Flow in Meandering Streams Under Variations of Flow Depth and the Morphological Implications

Abstract

Convective redistribution of the downstream velocity and cross-circulatory flow are the two key aspects of the flow dynamic, resulting from the geometrical characteristics of meandering streams. However, morphological consequence of the two flow aspects remains under debate, primarily regarding the role of cross-circulatory flow. The present study delves into the dynamics of the cross-circulatory flow under variation of flow depth and the related morphological process. Series of numerical simulations are performed in 70° sine-generated meandering channel. Variation of the flow depth necessitates a wide range of width-to-depth ratios (B/h), characterizing flume experiments to large scale natural rivers. Classical cross-circulatory flow pattern with one dominant rotating cell persists as B/h > 9. Vertical separation of the cross-circulatory velocity from the overall radial velocity highlights the critical role of the cross-circulatory flow in driving the near-bed radial sediment movements. Using the sediment continuity equation, the bed deformation rate at the initial plain bed is computed, revealing two distinct bed deformation patterns. At field scale (B/h = 100), bed deformation is dominated by the convective redistribution of the downstream velocity that causes longitudinal adjacent erosion-deposition zones. This implies a downstream migration tendency of meander loops. At laboratory scale (B/h = 12), the intensified cross-circulatory flow deflects the sediment trajectory, causing additional laterally adjacent erosion-deposition zones that necessitate the lateral expansion tendency of the loop. Lastly, the flow and sediment dynamics at the outer bank cell flows (B/h < 9) are discussed, serving as an extension of the current findings.

Keywords: Meandering streams, Fluvial Processes, Cross-circulatory flow, Convective redistribution of the flow velocity, Numerical simulation