Flow in meander bends with recirculation at the inner bank

Abstract

[1] In highly curved river bends, flow may separate at the inner bank to create a recirculation eddy with weak upstream flow. Very little is known about how recirculation eddies connect with the downstream flow or how the latter is affected by their presence. We investigate these questions using three-dimensional time-averaged computational fluid dynamics models of two natural bends with inner-bank separation. Test measurements of velocity in one bend agree well with the simulation. Common points in the two simulations are that (1) an outer-bank helix is only present in the upstream part of the bend, (2) maximum near-bank velocities are highest here rather than beyond the apex as in most bends, (3) reverse flow extends farther across the channel at the surface than the bed, and (4) flow within the recirculation eddy has a three-dimensional structure, linked with that in the outer-bank free stream. Substantial differences in detail between the two bends appear to be due to differences in upstream planform, manifested through the lateral distribution of inflow velocity.

Numerical simulation of hydrodynamics and bank erosion in a river bend

“…For higher (Q > 40 m 3 s À1 ) flow discharges, the water submerges the entire point bar and the main flow axis shifts to a chute channel across the medial portion of the bar, buffering the eroding bank from the portion of the flow characterized by the highest velocities. A similar outcome (i.e., a reduction of near-bank flow velocity) has been recorded in studies that have highlighted how the main flow can become separated from the channel boundary along the concave (outer) bank of sharply curving river bends [e.g., Hodskinson, 1996;Hodskinson and Ferguson, 1998;Ferguson et al, 2003]. In the case of the Cecina, however, bend curvature decreases as a function of increasing flow stage, and the mechanism of near-bank velocity reduction appears instead to be related to the topographic steering of the flow toward and away from the bank at low and high flow stages, respectively.…”

Flow separation at the inner (convex) and outer (concave) banks of constant‐width and widening open‐channel bends

“…Best and Reid, 1984), downstream of bars in bedrock controlled canyons (Schmidt, 1990;Schmidt et al, 1993;Wright and Kaplinski, 2011) and in river bends (e.g. Bagnold, 1960;Leeder and Bridges, 1975;Hickin, 1977Hickin, , 1978Hickin, , 1986Hickin and Nanson, 1984;Jackson, 1992;Andrle, 1994;Hodskinson and Ferguson, 1998;Ferguson et al, 2003;Kleinhans et al, 2009;Rhoads and Massey, 2012;Parsons, 2003;Vietz et al, 2012;Schnauder and Sukhodolov, 2012).…”

High-resolution numerical modelling of flow—vegetation interactions

“…First, individual point measurements with the same geo-location were compared from numerical and experimental data. This is the most straightforward method of model validation, and is particularly useful for identifying spatial regions of the flow where prediction is particularly good or poor or there is any bias in the data from incorrectly prescribed boundary conditions (Ferguson et al 2003, Lane et al 2004). …”