Measurement of momentum transfer caused by a groyne in a compound channel
Résumé
The present study investigates flow in an asymmetrical compound channel with a thin obstacle set on the floodplain, perpendicular to the main flow direction. Six flow configurations were investigated. Two flows were carried out under uniform flow conditions and four flows were carried out with a thin obstacle of variable length (d). Depending on the investigated relative depths (Hr) and aspect ratios d/Bfp (where Bfp = floodplain width), depth-averaged velocity and distribution of water depth across the channel were first examined. Streamwise profile of discharge distribution and mass exchanges were then deduced. Using a one dimensional momentum equation in each subsection, momentum balances were finally worked out. In addition to the classical turbulent exchange located at the vertical interface between the main channel and the floodplain, a significant momentum flux related to mass exchanges between subsections was observed when flow occurred near the groyne. In the floodplain, analysis emphasizes that the momentum transfer due to mass exchanges is (1) several orders of magnitude larger than momentum transfer due to turbulent exchange and (2) have the same magnitude order as friction on solid walls, whatever the groyne length and the relative depth are. In the main channel, solid wall friction is equivalent or larger than momentum transfers.
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