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Abstract
The reported experimental study assesses the effects of flow non-uniformity on the momentum flux in straight compound channels. Two flumes were used, featuring vertical and sloping banks. Starting with uniform flow condition, various imbalances in the upstream discharge distribution were introduced. This resulted in a time-averaged lateral flow and advective transport of momentum, which interacted with the shear-layer turbulence generated by the compound geometry. To investigate this interaction, the three contributions to transverse momentum flux (depth-averaged flow, shear-layer turbulence and dispersive term of spanwise velocity) are assessed. The first two contributions were strengthened by the sloping banks, while the third becomes important for the case of the vertical bank. With a lateral flow towards the main channel, the first contribution rises at the expense of the second. With a lateral flow towards the floodplain, the first two contributions have the same order of magnitude, and the Boussinesq approach is invalidated.
Acknowledgements
Travel costs of J. Leal, J. Fernandes and S. Proust were supported by a Hubert Curien Project Pessoa, funded by EGIDE, France, and by FCT, Portugal. The authors are grateful to Fabien Thollet, Mickaël Lagouy and Pedro Duarte, for their assistance during the experiments. They are also grateful to Roger Bettess for his corrections.
Notation
Superscript u refers to uniform flows
Subscripts m and f refer to main channel and floodplain, respectively
Subscript d refers to a depth averaging
| A = | = | compound-channel cross-section area (m2) |
| Bf = | = | width of one floodplain (m) |
| f = | = | Darcy–Weisbach friction coefficient (–) |
| h = | = | local water depth (m) |
| hf, hm = | = | mean water depths on the floodplain and in the main channel (m) |
| hb = | = | bank full height in the main channel (m) |
| Mxy = | = | depth-averaged value of transverse momentum flux |
| Q = | = | total discharge (m3/s) |
| Qf = | = | floodplain discharge (m3/s) |
| Txy = | = | depth-averaged value of lateral Reynolds shear stress |
| UA = | = | bulk velocity, Q/A (m/s) |
| u, v = | = | instantaneous longitudinal and lateral velocity components (m/s) |
| = | time-averaged longitudinal and lateral velocity components (m/s) | |
| = | lateral Reynolds shear stress (N/m2) | |
| Ud, Vd = | = | depth-averaged, time-averaged longitudinal and lateral velocity (m/s) |
| Uf, Um = | = | mean longitudinal velocity in the floodplain and main channel (m/s) |
| x, y, z = | = | longitudinal, lateral and vertical distances (m) |
| = | local transverse eddy viscosity (m2/s) | |
| = | depth-averaged transverse eddy viscosity (m2/s) |