If in natural or anthropized rivers, the river cross-section generally gradually and continuously varies, transversal and discontinuous obstacles either natural or artificial may partially or totally block off floodplains. The flow overbanking in the floodplain is therefore contracted by this obstacle which then promotes two recirculation zones of both sides of the obstacle, resulting in a reduction of the flow section and in the generation of strong mass exchange between channels that superimposes to the classical turbulent interactions. New experiments are conducted in two different compound channels: rapidly varied flows in compound channel with a groyne set on the floodplain. Flows with various groyne lengths and total discharges were investigated. Effects on the hydraulic parameters of the superimposition of the two problems that are (i) flow in compound geometry and (ii) rapidly varied flow in the vicinity of a thin obstacle were analysed. Finally, dominance of physical processes in such flow configuration is discussed
